# The Three-Dipole Kicker Injection Scheme for the ALS-U Accumulator Ring

M. Ehrlichman, T. Hellert, S.C. Leemann, G. Penn, C. Steier, C. Sun, M.
Venturini, and D. Wang  Lawrence Berkeley National Laboratory, Berkeley, CA

###### Abstract

The ALS-U light source will implement on-axis single-train swap-out injection
employing an accumulator between the booster and storage rings. The
accumulator ring design is a twelve period triple-bend achromat that will be
installed along the inner circumference of the storage-ring tunnel. A non-
conventional injection scheme will be utilized for top-off off-axis injection
from the booster into the accumulator ring meant to accommodate a large $\sim
300$ nm emittance beam into a vacuum-chamber with a limiting horizontal
aperture radius as small as $8$ mm. The scheme incorporates three dipole
kickers distributed over three sectors, with two kickers perturbing the stored
beam and the third affecting both the stored and the injected beam
trajectories. This paper describes this “3DK” injection scheme and how it fits
the accumulator ring’s particular requirements. We describe the design and
optimization process, and how we evaluated its fitness as a solution for
booster-to-accumulator ring injection.

##  I Introduction

The ALS-U is the Advanced Light Source upgrade to a 4th generation
diffraction-limited soft x-ray light source Steier  _et al._ (2018). Achieving
the ALS-U high brightness goal requires strong focusing-magnet gradients; the
strong gradients necessitate strong chromatic sextupoles and these will shrink
the dynamic aperture of the storage ring (SR) to about $0.5$ mm radius once
lattice imperfections are taken into account.

To inject into such a small dynamic aperture, ALS-U will implement on-axis
single-train swap-out injection utilizing an accumulator ring (AR) housed
along the inner wall of the SR tunnel. See Fig. 1 for an overview of the ALS-U
accelerator complex. A small $2$ nm-rad natural emittance, much smaller than
the approximately $300$ nm-rad emittance of the beam delivered by the existing
booster, is required to inject into the storage ring with near-$100$%
efficiency and sufficient margin. In addition to its function as a damping
ring, the AR is intended to act as a beam-charge recycler in between swap-
outs. The SR average current is $500$ mA, distributed evenly over a 284-bunch
beam consisting of eleven trains of $26$ (or $25$) bunches each. The AR is
designed to carry a single train at a time; this is swapped with one of the SR
trains once every about half a minute and replenished with top-off injection
from the booster before the next swap-out.

Table 1:  Parameter list of the ALS-U Accumulator Ring and existing ALS. | AR | ALS111Emittance and energy loss per turn reported for the ALS are for the lattice without superbends and without insertion devices.
---|---|---
Beam energy | $2.0\text{\,}\mathrm{GeV}$ | $1.9\text{\,}\mathrm{GeV}$
Circumference | $182.122\text{\,}\mathrm{m}$ | $196.8\text{\,}\mathrm{m}$
Tune $x/y$ |  $16.221$/$8.328$ |  $16.165$/$9.25$
Natural chrom. $x/y$ |  $-43$/$-36$ |  $-46.8$/$-39.6$
Mom. compaction | $1.04\text{\times}{10}^{-3}\text{\,}$ | $0.9\text{\times}{10}^{-3}\text{\,}$
Emittance | $1.8\text{\,}\mathrm{nm}$ | $1.8\text{\,}\mathrm{nm}$
Dispersion in straight | $11.6\text{\,}\mathrm{cm}$ | $15.0\text{\,}\mathrm{cm}$
Charge per bunch | $1.15\text{\,}\mathrm{nC}$ | $1.1\text{\,}\mathrm{nC}$
Energy spread | $8.5\text{\times}{10}^{-4}\text{\,}$ | $9.6\text{\times}{10}^{-4}\text{\,}$
Energy loss per turn | $269\text{\,}\mathrm{k}\mathrm{e}\mathrm{V}$ | $228\text{\,}\mathrm{k}\mathrm{e}\mathrm{V}$
Damping time $x/y/z$ |  $6.2$/$8.5$/$5.2$ $\text{\,}\mathrm{ms}$ |  $7.7$/$8.9$/$5.0$ $\text{\,}\mathrm{ms}$
Harmonic number | $304\text{\,}$ | $328\text{\,}$
Main RF frequency, | $500.417\text{\,}\mathrm{M}\mathrm{H}\mathrm{z}$ | $499.642\text{\,}\mathrm{M}\mathrm{H}\mathrm{z}$
Main RF voltage, | $1.0\text{\,}\mathrm{M}\mathrm{V}$ | $1.2\text{\,}\mathrm{M}\mathrm{V}$
Synchrotron tune | $4.9\text{\times}{10}^{-3}\text{\,}$ | $5.4\text{\times}{10}^{-3}\text{\,}$

The AR design has to fulfill two competing demands on the vacuum-chamber
aperture: it should be wide enough to accept the large emittance beam from the
booster, with a goal of $\gtrsim 95\%$ injection efficiency, but as narrow as
possible to minimize the magnets’ aperture and thus their weight and volume so
that the AR can fit in the same tunnel as the SR. Consideration of these
demands has guided the choice of injection scheme while recognizing that
unlike light sources the AR can tolerate significant injection transients.
Fully exploiting the latitude offered by the latter observation, we have
developed a simple and nearly 100% efficient injection scheme, coined “3DK”,
that utilizes three dipole kickers distributed over three sectors in
combination with a pair of thick and thin pulsed septa.

In short, the first two dipole kickers (“prekickers”) kick only the stored
beam and place it on a trajectory that partly compensates the main injection
kick placed further downstream, which kicks both the stored and injected
particles onto stable (but not closed) trajectories. The features of this
scheme include  1) the reduced straight length is accommodated by distributing
the kickers among $3$ straights,  2) near-100% injection efficiency,  3) a
reduced septum aperture that eases vacuum and magnet engineering in the
injection straight,  4) flexibility to trade between residual oscillations of
the stored and injected beams, and  5) reliance on only conventional power
supply and kicker technology.

An important aspect of our study is the demonstration that potential drawbacks
resulting from the combination of relatively large stored-beam oscillations
and collective effects are manageable.

Figure 1:  The Advanced Light Source Upgrade (ALS-U) complex. The $3$ dipole
kickers and injection septum of the 3DK booster-to-accumulator injection
scheme are highlighted. Every $1.4\text{\,}\mathrm{s}$ up to four bunches are
generated and accelerated to $2\text{\,}\mathrm{GeV}$ through the existing
linac and booster and then injected through the BTA into the new AR to fill or
top-off bunches in a single 26- (or 25-) bunch train. Every
$\sim$$30\text{\,}\mathrm{s}$ this train is swapped with one of the eleven
trains circulating in the Storage Ring (SR).

Other schemes that were considered and ultimately discarded included a non-
linear kicker (NLK), a conventional $4$-kicker single-straight closed-bump
injection scheme, as well as a two-dipole kicker variant of the 3DK schemes.
These will be briefly discussed in this paper as well.

Section II is a brief introduction to the ALS-U AR. The layout and design of
the 3DK injection scheme is described in Sec. III. For completeness Sec. IV
briefly describes the design of the booster-to-AR transfer line. The
robustness of the injection scheme against lattice imperfections is discussed
in Sec. V. Impedance and collective effects, as well as multi-bunch feedback
issues are addressed in Sec. VII. Alternatives to the 3DK injection scheme are
discussed in the appendix.

##  II ALS-U Accumulator Ring

The ALS-U AR is essentially a slightly smaller version of the existing ALS
twelve period triple-bend acromat. Its basic parameters, along with those of
the existing ALS, are shown in Table 1. The optics and layout through one arc
are shown in Fig. 2. To adapt the ALS layout to the AR layout, the length of
the straight sections was shrunk from $9.386$ m to $8.762$ m and the arcs were
shrunk from $7.014$ m to $6.415$ m by reducing the magnet spacing. Nominal
bend, quadrupole, and sextupole magnet lengths are the same between the ALS
and AR. These adjustments bring the total circumference down from $196.805$ m
to $182.122$ m which makes the AR fit neatly along the inner tunnel wall. The
AR utilizes a non-swept gradient dipole design, as does the ALS. The beam
energy is increased from $1.9$ GeV to $2.0$ GeV to match that of the ALS-U
storage ring. The AR emittance of $1.8$ nm is comparable to that of the ALS.

To preserve egress, the AR will be mounted close to the tunnel ceiling, about
$2$ m above the floor and about $0.6$ m above the plane of the storage ring.
The AR magnet stands are a combination of floor and wall attachments. The
elevated installation and limited space inside the tunnel places a premium on
reducing the size and weight of the magnets, and so the vacuum chamber
dimensions were shrunk, bringing the pole tips closer to beam, achieving the
same gradients with less bulky magnets. The arc and straight chambers are
round and $14.2$ mm in radius. The dipole chambers are elliptical with a $20$
mm axis in the horizontal and $7.28$ mm axis in the vertical.

Figure 2:  Lattice and magnet distribution of an arc in the ALS-U AR. The ring
is composed of $12$ such arcs. Shown are the beta and the dispersion functions
(top), the aperture model and the distribution of magnets (bottom).

The AR-to-SR injection swap is expected to occur approximately every $30$
seconds. A long beam lifetime in the AR is not necessary and so dynamic
aperture and injection efficiency are prioritized when optimizing the AR. The
AR dynamic aperture thus extends to the physical apertures. Simulations taking
imperfections and correction algorithms into account bring the expected usable
dynamic aperture close to linear acceptance.

Transfer lines intersect the AR along $3$ consecutive straights. Sector $12$
is the take-off of the accumulator-to-storage ring (ATS) transfer line; the
straight section has to accommodate the dipole kicker and pulsed thin septum
required for extraction from the AR into the ATS in addition to the first pre-
kicker for AR injection from the booster. Sector $1$ houses the pair of
booster-to-accumulator (BTA) injection septa, which are depicted in Fig. 3;
immediately upstream in the same straight section is the second pre-kicker for
AR injection from the booster. The sector $2$ straight contains the landing
point of the storage ring-to-accumulator (STA) transfer line. It has to
accommodate the STA pulsed thin septum, as well as both the dipole kicker for
injection from the SR and the main kicker for AR injection from the booster.
The close proximity of the three transfer lines and injection/extraction
components posed a design challenge, requiring careful consideration of the
layout and close interaction between the beam physics and engineering groups.

Figure 3:  The layout diagram of the BTA thick and thin septa in sector 1. The
separation between the injected beam and unkicked stored beam is $14$ mm and
the thickness of the thin septum blade is $1$ mm.

The arrival of the injected bunches from the booster is timed to replenish the
bunches of the depleted train in a top-off manner. In normal ALS-U operations
we expect that a train circulating in the SR will lose about $10\%$ of its
charge before being swapped out. The beam coming out of the booster is $4$
pulses long, with a separation of $3$ empty buckets between each pulse. The
total charge in the $4$ pulses is approximately $0.5$ nC, or about $10$% of
the charge that needs to be replenished, implying that about 10 injection
shots will be needed to replenish the circulating train before swapping it
back to the SR.

On-axis injection from the SR into the AR is more straightforward than
injection from the booster; this and extraction from the AR into the SR are
not in the scope of this paper.

##  III 3DK Injection Scheme

Figure 4:  Trajectories of the stored (green) and injected (red) beams during
an injection cycle for the “balanced” mode kicker strength settings, also
referred to as “Mode A”. Solid black profile indicates horizontal physical
aperture. Dashed lines delineate $3\sigma$ of the beam size. Kickers are
labeled and represented by orange bow-ties. The first pre-kicker is located at
the start of sector $12$. The second pre-kicker and injection septum are
located in sector $1$. The main injection kicker in sector $2$ kicks both the
stored and injected particles.

The AR layout along with injection elements and beam trajectories during an
injection cycle are depicted in Fig. 4. At the start of a BTA injection cycle,
the stored bunch train is given an outward “pre-kick” at the upstream end of
the sector $12$ straight. A second outward pre-kick is applied at the upstream
end of the next straight, which is sector $1$. The injection septum is at the
downstream end of sector $1$. Injected particles make a large excursion
through the sector $1$ arc. The main injection kicker, located at the
downstream end of sector $2$, applies an outward kick to both the stored and
injected particles, leaving both on trajectories that damp with very little
particle loss. The strengths of the $3$ kickers and injection septum are
adjusted to control the injection transients and are shown in Table 2. The
kickers’ flat tops span the entire bunch train and are timed to the train
head, irrespective of which buckets are being injected into.

Table 2: Injection modes achieved by adjusting kickers and septum strength. Mode A achieves a high injection efficiency with a small stored beam transient. Mode B reduces the stored beam transient to mitigate short range wake fields, at the expense of greater losses on the BTA-side septum sheet and due to injected particles exceeding the acceptance. Mode C recovers high injection efficiency by shrinking (mismatching) the horizontal $\beta$ of the injected beam. |  |  On-axis injection |  Closed stored-beam bump |  Mode A |  Mode B |  Mode C
---|---|---|---|---|---|---
Injection element strengths | Thin septum (mrad) | $35.82$ | $35.85$ | $35.85$ | $35.72$ | $35.67$
Pre-kicker 1 (mrad) | $0.365$ | $0.489$ | $0.383$ | $0.417$ | $0.417$
Pre-kicker 2 (mrad) | $1.09$ | $0.759$ | $1.04$ | $0.933$ | $0.933$
Inj. kicker (mrad) | $1.21$ | $0.613$ | $1.11$ | $0.987$ | $0.987$
Post-injection cycle residuals | Stored beam rms (mm) | $2.88$ | $0.0$ | $2.44$ | $1.86$ | $1.86$
Stored beam max (mm) | $6.32$ | $0.0$ | $5.39$ | $4.18$ | $4.18$
Injected beam rms (mm) | $0.0$ | $3.74$ | $0.734$ | $1.02$ | $0.996$
Injected beam max (mm) | $0.0$ | $8.16$ | $1.59$ | $2.21$ | $2.17$
Optics at septum exit |  $\beta_{x}$ (m) | $14.82$ | $14.82$ | $14.82$ | $14.82$ | $11.33$
$\alpha_{x}$ | $-0.139$ | $-0.139$ | $-0.139$ | $-0.139$ | $-0.275$
| BTA-side septum losses (%) | $0.89$ | $0.89$ | $0.89$ | $1.05$ | $0.48$
| Capture losses (%) |  |  |  |  |
|  no wakes | $-$ | $66.3$ | $0.36$ | $0.91$ | $0.46$
|  with wakes | $-$ | $-$ | $16.4$ | $6.8$ | $6.9$
|  with wake mitigations | $-$ | $-$ | $0.7$ | $1.1$ | $1.0$
| Stored Beam Survival (%) | $99.9$ | $99.9$ | $99.9$ | $99.9$ | $99.9$

The vacuum chamber between the injection point and the second downstream
dipole is horizontally widened to accommodate the large excursion of the
injected-beam trajectory but no special-aperture magnets are required.

The injected bunches arriving from the pulsed thin septum are offset $14$ mm
outward from the nominal chamber center, as depicted in Fig. 3. The transverse
position of the septum was arrived at by considering both the limitation the
septum places on dynamic aperture and the constraint it imposes on the beam-
trajectory oscillation amplitude during the post-injection transient. The
distance of the injected beam centroid to the septum sheet was arrived at by
considering particle loss on the septum sheet and the amplitude of the
injected particles.

The starting point for determining the optimal septum angle is found by
adjusting the angle of the injected particles to create a zero-crossing at the
main injection kicker. The septum angle is refined along with the strengths of
the $3$ kickers by optimizing amplitudes of the stored and injected transients
in action/angle coordinates. The optimum is that which minimizes the action
while preserving minimum distances from the limiting physical apertures and
tolerating no losses out to $3\sigma$ of stored beam.

The optimum does not result in a zero-crossing at the injection kicker. In
fact, placing either the injected beam or the stored beam onto the reference
orbit at the end of the injection kicker results in unacceptably large
trajectories of the off-axis beam. Instead, the optimum involves a trade-off
between reducing the transient amplitudes of the injected particles and the
stored beam.

These options are explored in Table 2, which presents the thin septum and
kicker strengths for different injection modes, as well as the maximum
amplitude and rms of the horizontal injection transient for the stored and
injected beams. The “on-axis injection mode,” as was done at MAX IV Leemann
(2012a), is expected to be useful during early commissioning. It will place
the injected beam on-axis, but the stored beam, if present, will be left with
a large oscillation amplitude that limits imperfection tolerance and generates
large short-range wake fields. The “closed stored-beam orbit bump mode”
eliminates the stored beam injection transient, but achieves only $63$%
injection efficiency. It is included here to illuminate the bounds of the
parameter space. The remaining three modes indicate different strategies to
optimize performance for small but non-zero residual amplitudes of the stored
and injected beams. Mode A, or “balanced mode,” represents a compromise
solution between the two previous modes and maintains near-$100$% injection
efficiency while reducing the amplitude of the stored beam oscillations, with
an injected beam that is matched to the storage ring. For this mode, the
transient of the stored beam is allowed to be larger than that of the injected
beam as it has a much smaller beam size than the injected particles. The
trajectories in Fig. 4 are based on Mode A.

The parameters of the injection elements are further refined after considering
short-range transverse wake fields. Because of the non-zero injection
transient, the stored beam generates relatively strong wakes which can sweep
some of the injected particles into the vacuum chamber. This is described in
more detail in Sec. VI. When taking short range wakes into account, it is
found that a smaller perturbation to the stored beam improves the overall
injection efficiency, even though the injected bunch has a larger initial
offset and suffers slightly larger losses initially.

Mode B brings the injected beam closer to the septum as it enters from the BTA
thus requiring a weaker main injection kick and allowing for a smaller stored
beam transient at the expense of slightly reduced injection efficiency. The
injected beam losses to the machine acceptance increase from $0.36$% to
$0.91$%, while the losses due to wake fields decrease from $16.4$% to $6.8$%.
BTA-side septum sheet losses also increase slightly from $0.89$% to $1.05$%.

Mode C has the same kicker settings as Mode B but decreases $\beta_{x}$ to
better fit the injected beam into the AR acceptance available after taking the
septum sheet into account, similar to the technique applied in Streun (2005).
This differs from Modes A and B where the beam distributions are matched to
the ring. Reducing $\beta_{x}$ shrinks the injected beam horizontally,
reducing losses along the septum sheet. After adjusting the septum strength to
re-center the injected beam within the available machine acceptance, the
centroid is brought closer to the septum, thereby reducing the injection
transient. BTA-side septum sheet losses are reduced to $0.48$%, and losses to
the acceptance are reduced to $0.46$%.

Figure 5: Injected and stored beam profiles out to $3\sigma$ at septum exit
during an injection cycle for the three injection modes. The white region is
the acceptance in $x$-$x^{\prime}$ space at the septum exit. The septum is
drawn in black and is $1$ mm thick with its inward edge $8$ mm from the stored
beam reference trajectory. The septum image was obtained by tracking. The
parameters of the three modes are detailed in Table 2.

Fig. 5 shows the DA of the latter three modes viewed from the septum exit
during an injection cycle, along with the injected and stored particle
distributions, and also the thin septum. Both the inward and outward profiles
of the thin septum were determined by particle tracking. Comparing Mode B to
Mode A, the acceptance appears to move since the acceptance itself relative to
the septum is affected by the choice of main injection kick.

In producing the nominal operational mode and its two variants, we conclude
that the 3DK scheme appears to have enough flexibility to accommodate
perturbations and offers various optimization opportunities.

##  IV BTA layout design and optics matching

Transport from the booster to the AR is through the Booster-to-Accumulator
(BTA) transfer line. The BTA utilizes about 2/3 of the existing booster-to-ALS
storage ring (BTS) transfer line. For early commissioning a slow bending
switch will be installed for on-demand steering of the beam into the new BTA
branch while ALS can continue normal operation.

Figure 6:  Twiss functions (a) and layout(c) of the BTA transfer line.

To accommodate the different elevation between booster and AR (see Fig. 1),
the BTA includes a vertical two-bend achromatic dogleg Sun  _et al._ (2019),
where the achromatic condition is met with two quadrupoles between the two
dogleg bends to generate the required vertical $\pi$ phase advance. The BTA
terminates into a pair of pulsed thin and thick septa providing a combined 12
deg (horizontal) bending. Since the space is limited, thick and thin septa are
located at the far end of the injection straight (straight 1) as shown in Fig.
3 to increase the distance from the last bend of the BTA to the septa and
lower the incoming angle into the septa, therefore reducing the required kick
angles from septa.

The main design difficulty posed by the BTA is the avoidance of interference
between the BTA and other systems’ components in the tight and congested
injection region while meeting the layout and lattice functions’ matching
boundary conditions. Linear-optics matching was performed with constraints on
beam sizes along the transfer line to minimize vacuum chamber and magnets’
aperture. The decoupling of horizontal and vertical planes provided by the
dogleg simplifies the matching problem but we found it useful to deploy a
Multi-Objective Genetic Algorithm (MOGA) Deb  _et al._ (2002) to optimize the
design. The optics functions and beam-size envelopes of the baseline lattice
are shown in Fig. 6. The desired Twiss functions at the end of the transfer
line are obtained and the beam sizes are below $4$ mm throughout, which
satisfies the design requirements.

##  V Injection efficiency analysis and robustness against lattice errors

The injection efficiency is evaluated using AR and BTA lattices obtained after
undergoing a process of simulated commissioning Hellert  _et al._ (2019).

Optimizing the injection efficiency in the presence of errors requires small
adjustments to the nominal kick angles, with the exact value depending on the
error realization. In simulations and eventually in operation, the
optimization can be accomplished by running parameter scans in a neighborhood
of the kick-angle nominal settings.

While one would have the freedom to adjust all three DKs independently, in the
simulations we used a simplified setup in which both pre-DKs are varied by the
same amount, thus resulting in a 2D optimization problem. We found that this
setup gave satisfying results while significantly reducing the computation
time compared to a full 3-parameter scan.

Figure 7:  Top: particle losses in the injected (left) and stored (center and
right) beam vs. kickers’ angles for the ideal lattice. The pre-kickers’ kick
(vertical axis) is reported relative to Mode-A settings (red lines). Bottom:
for each one of 100 lattice-error realizations, a DK scan is carried out on a
grid to identify the setting yielding the highest injection efficiency. The
ensemble of the best DK settings is reported in the right figure. The left
figure is the injection efficiency CDF.

In a preliminary study we took a first cut at the parameter space by
delimiting the region that exhibits no or minimal losses in both injected and
stored beam in the absence of errors: see the top images in Fig. 7. In the
simulations the injected and stored bunches are represented with $1000$
particles and tracked for $1000$ turns.

Table 3:  Imperfections assigned to injection elements in the commissioning simulations. The error distributions are Gaussian and truncated at $2\sigma$. The stability errors are relative to the set point, while the roll error is absolute. Error Type | Distribution Width
---|---
Septum stability | $2.5\cdot 10^{-4}$
Dipole kicker stability | $1\cdot 10^{-3}$
Dipole kicker roll | $4\text{\,}\mathrm{mrad}$
Figure 8:  Temporal and spatial profiles of the thin-septum leakage field as
included in the injection efficiency studies. The left plot shows the
integrated kick at various horizontal coordinates as a function of turns.
Conversely, the right plot shows the integrated kick at various times as a
function of a particle horizontal coordinate.

As the best combination of kicker settings may vary slightly depending on the
specific random error realization and correction, a 2D kicker scan was carried
out for each of the $100$ random lattice-error realizations considered. For
each lattice-error realization, the kickers’ settings that induce no stored-
beam particle losses were selected and among these were identified the one
kickers’ setting that gives the highest injection efficiency. The set of those
best 100 settings (one per lattice) is shown in the bottom-right image of Fig.
7 (only $15$ distinct data points appear, as a single kickers’ setting will in
general represent the optimum for several lattice-error realizations.) The
bottom-left picture in Fig. 9 is the CDF of this set, showing that about half
of the lattices have injection efficiency larger than $98.5$%.

We then proceeded by refining the simulations to include a more complete set
of errors and perturbations. Specifically, to the AR lattice errors we added:
septa and kickers shot-to-shot variations (Table 3), non-uniformity of the
kicker-pulse temporal profile, and septum leakage fields.

The non-uniformity of the kicker pulse is simulated by assigning a linear
slope to the pulse profile. The slope is defined as the difference between the
kicks on the first and last bunch of the bunch train relative to the nominal
kick. The simulation of the leakage fields is done by calculating the kick map
associated with the field accounting for both the temporal and spatial field
extension, to be applied at every turn before the field has died off, Fig. 8.
A result from these more complete simulations is shown in Fig. 9 demonstrating
the sensitivity of injection efficiency to the sloping of the kickers’ pulse.
As before, for each lattice-error realization, the injection efficiency was
determined after performing a 2D scan of the kicker’s amplitude to identify
the optimum. Among other things, from these results we concluded that a $4$%
slope would be acceptable.

Figure 9:  Sensitivity of the injection efficiency to the non-uniformity of
the injection kickers’ pulse profile. The data sets correspond to three
amounts of pulse profile slope indicated in the plot key. The simulation is
conducted with a complete set of errors including shot-to-shot variation of
the kick and septa strengths, as well as septum leakage fields.

##  VI Single bunch (short-range wake fields)

During injection both the electrons from the booster and the stored bunch onto
which they are added undergo several millimeter transverse oscillations and
this has the potential to induce potentially harmful transverse collective
effects. The investigation of these effects it the topic of this section.

The short-range transverse wake fields have been calculated for each vacuum
chamber element using a detailed model of its geometry. Longitudinal wakes are
not included in these results, however only the horizontal wakes have been
observed to have a significant impact on the injection process. The wakes are
applied close to each source point, although simulations are in good agreement
with simpler calculations that use beta-function weighting to group together
the impact of wake fields in every sector or even at a single location in the
ring. The threshold for the Transverse Mode-Coupling Instability (TMCI) is
$5.8$ nC, about $8$ times the nominal charge for a single bunch. Fig. 10 shows
the beta-weighted wakefields from different components in both horizontal and
vertical planes, along with their total. Wake fields are calculated for a
Gaussian beam with rms bunch length of $1$ mm, and serve as pseudo-Green
functions in the following beam dynamics simulations.

Figure 10:  Transverse short-range wakefields in the AR from individual
components and their combined total. The wake potentials are calculated for a
Gaussian beam with rms bunch length of $1$ mm and serve as pseudo-Green
functions in beam dynamics simulations. Left: horizontal plane, right:
vertical plane

The resulting transverse oscillations, especially of the stored bunch (because
it contains about $90$% of the charge), induce short-range wake fields.
Despite the fact that the total bunch charge is expected to be far from the
threshold for instabilities driven by short-range wakes, the wake fields do
build up in intensity over a time scale of order $1$ ms (roughly $1600$ passes
around the ring). Simulations show that the wake fields are strong enough to
confine the stored bunch and suppress phase decoherence, as shown in Fig. 11.
At the same time, the more diffuse injected electrons increase in both
oscillation amplitude and transverse width.

Figure 11:  Horizontal phase space of the stored (red) and injected (green)
bunches after $2000$ turns, without wakes (left) and with nominal wakes
(right). The upper figures show the results for Mode A, the lower Mode C.

The 3DK ‘balanced’ configuration (Mode A) was chosen by optimizing for
injection efficiency, but without considering the impact of wakes. Less than
$1$% beam losses are predicted in this case. However, when the expected wake
fields are included, the injection efficiency drops to $83.6$%. As shown in
Fig. 12, most of the losses occur between $0.5$ ms and $1$ ms after injection,
roughly between $1000$ and $2000$ passes around the ring.

An important aspect of the 3DK scheme is that it is flexible enough to allow
for additional adjustments to accommodate the impact of wake fields. An
alternate set of all three kicker magnet amplitudes, referred to as ‘Mode B’,
was found that reduced the displacement of the trajectory of the stored beam
by $20$%, while increasing losses by a modest amount when wake fields are
neglected. These additional losses occur in the first 50 passes around the
ring. The capture efficiency remains above $99$%. For the nominal wake fields
the capture efficiency is $93.2$%, a significant improvement compared to
$83.6$% for Mode A. In none of these cases are there losses in the stored
bunch.

Mode C is similar to Mode B except that the septum field is weaker and the
horizontal beta function at the end of the septum is lower than the matched
value so as to reduce transfer-line losses on the injection septum. These
adjustments also reduce the injected beam transient slightly, though when wake
fields are taken into account the capture efficiency is comparable to that of
Mode B. The performance of Modes B and C are much more similar to each other
than they are to Mode A, and comparisons will focus on the differences between
Mode A and Mode C.

Figure 12 shows the evolution of the size and horizontal offset of the
injected bunch when short-range wake fields are either included or ignored.
Results for Mode A are shown on the left-side plots, and for Mode C on the
right-side plots. In Mode A, the width of the injected bunch steadily decays
without wake fields, while with the nominal wakes the width of the bunch grows
until it peaks after $1800$ turns around the ring with an rms of $3.5$ mm, up
from $2.2$ mm. The centroid motion rapidly damps from $1.5$ mm without wakes,
but with the nominal wakes it increases to $3.5$ mm amplitude, followed by
envelope oscillations which slowly decay over thousands of passes around the
ring. For Mode C, there is similar behavior in the width of the injected bunch
but the amplitude of the centroid motion starts out at $2$ mm and only grows
to $3.0$ mm. The centroid motion again has continued envelope oscillations in
the presence of wakes, but at a lower amplitude than for Mode A.

Figure 12:  Comparison of the evolution of the size (top) and centroid motion
(bottom) of the injection bunch for the cases without wake fields (red) and
with nominal wake fields (green). The left figures show results for the 3DK
settings referred to as Mode A (‘balanced’), and right figures show Mode C,
which is tuned for better performance in the presence of wake fields. The
corresponding injection efficiencies for the two cases are 83.6% and 93.1%.

For the nominal wake fields, the goal of at least $95$% injection efficiency
is not met even for Mode C. Thus, other strategies to mitigate the impact of
short-range wake fields have been examined. The nominal chromaticity of $1$ in
both planes can easily be increased and will drive more rapid phase
decoherence of the bunches. Both horizontal and vertical chromaticities
contribute to this effect at high amplitudes. A small temporal offset of the
injected bunch relative to the stored bunch directly reduces the wake field
forces experienced by the injected electrons.

Increasing the chromaticity from $1$ to $1.2$ increases the capture efficiency
to $93.5$% for Mode A and $97.7$% for Mode C. Similarly, keeping the
chromaticity at unity and delaying the injected bunch by $100$ ps relative to
the stored bunch leads to an capture efficiency of $98.5$% for Mode A nd
$98.9$% for Mode C. Applying both changes brings the capture efficiency above
$99$% in both modes.

To exercise a measure of caution, we considered the possibility that wake
fields would be twice as high as expected (“enhanced wakes”) In this case, the
capture efficiency drops to $64.8$% for Mode A and $78.3$% for Mode C without
additional mitigations. The evolution of the capture efficiency as a function
of number of passes after injection is shown in Fig. 13 for both Mode A and
Mode C. The wake field levels are varied among no wakes, nominal wakes and
enhanced wakes. In addition, some examples include using a combined mitigation
of setting both chromaticities to $1.2$ and shifting the injected bunch by
$100$ ps. For the enhanced wakes, this brings the capture efficiency to
$97.8$% for Mode A and $98.3$% for Mode C. Creating a temporal offset between
the stored and injected bunches is particularly effective at reducing the
impact of wake fields. For Mode C with the above mitigations, the capture
efficiency is 97.2% when the strength of the wake fields is multiplied by a
factor of three. The corresponding efficiency for Mode A is $95.9$%.

Figure 13:  Capture efficiency as a function of number of passes around the
ring after injection, comparing Mode A, where the kicker settings are
optimized without including wake field effects, to Mode C, which uses an
alternative set of kicker strengths which is more tolerant to wake fields and
also changes the injection beta function. Results are shown with and without
wake fields, for enhanced wake fields, and for additional mitigations.

##  VII Multi-bunch stability and interference with TFB system

The transverse bunch-by-bunch feedback system (TFB) will detect the transient
of the stored beam and apply kicks to damp it. Unfortunately, those kicks will
anti-damp the injected particles since the stored beam and injected particles
are out of phase. The injected particles carry relatively little charge and
will offset only slightly the center-of-charge that the feedback system
detects. To prevent the loss of injected particles as the TFB damps the
injection transient, the TFB will be masked for those buckets into which
charge is injected. A single injection shot delivers charge into $4$ buckets,
spaced $4$ buckets apart, for example, buckets $10$, $14$, $18$, and $22$.
Masking the buckets significantly diminishes the ability of the TFB to damp
multibunch instabilities and so we investigate here the injection transient
and TFB interaction using a tracking simulation.

Under nominal conditions, the growth rates of multibunch impedance
instabilities in the AR are less than that of the radiation damping and
multibunch feedback system. The accumulator-ring HOM wakes have been evaluated
with theory and tracking simulations. The strongest cavity HOM growth rate is
$0.03$ ms-1, and the radiation damping rate is $0.16$ ms-1. The growth rates
of the resistive wall modes are shown in Fig. 14. Except for the two highest
order modes, all are below the radiation damping time, and the highest order
modes are well-within the capabilities of the TFB.

Following an injection cycle the stored beam is left with a large transverse
offset and the $4$ buckets into which charge is injected are masked from the
multi-bunch feedback. Since the resistive wall modes are near or exceed the
radiation damping rate, a tracking simulation is used to evaluate their
stability during the injection transient when the effectiveness of the TFB is
diminished.

The simulation models the long-range resistive wall wakes at any position in
the ring using a fit of $15$ pseudo-modes to the $1/\sqrt{t}$ dependence of
the long-range wake fields and follows closely the technique described in Wang
_et al._ (2017). The long-range resistive wall wake is,

$W\left(t\right)=\frac{\sqrt{c}}{\pi
b^{3}}\sqrt{\frac{Z_{0}}{\pi\sigma}}\frac{L}{\sqrt{t}},$ (1)

where $b$ is the chamber dimension and $\sigma$ its conductivity; $c$ and
$Z_{0}$ are the speed of light and impedance of free space; $L$ is the length
of the chamber and $t$ is time since the passage of the source particleChao
(1993). The basis of pseudo-modes is

$W_{i}\left(t\right)=\frac{A_{i}^{2}}{S_{i}^{2}}\exp\left(-\frac{d_{i}^{2}}{S_{i}^{2}}t\right),$
(2)

where $A_{i}$, $S_{i}$, and $d_{i}$ are fit parameters. Here, $S_{i}$ is
redundant but including it as a fit parameter has been found to help the
fitter converge consistently and quickly with simply default parameters. The
fitter is Mathematica’s NonlinearModelFit Inc. (2021). The log of the basis is
fit to $\log{\left(1/\sqrt{t}\right)}$ sampled with a logarithmic distribution
of $t$. The residuals of the fit are less than $0.1\%$ from $1$ ns to $44,000$
turns, and reliably decay to zero beyond the ends of the fit. The same basis
is used for all chambers, but is scaled according to the local chamber
material and dimension shown in Eq. (1). Resistive wall wakes are represented
at $216$ locations in the ring.

For each horizontal mode $n$, each bunch $i$ in the train is seeded with an
appropriate offset,

$\displaystyle x_{i}=$
$\displaystyle\sqrt{J\beta_{x}}\cos\left(\frac{\pi}{2}+\frac{2\pi
ni}{N_{bunches}}\right)$ (3) $\displaystyle x^{\prime}_{i}=$
$\displaystyle\sqrt{\frac{J}{\beta_{x}}}\Big{(}\alpha_{x}\cos\left(\frac{\pi}{2}+\frac{2\pi
ni}{N_{bunches}}\right)$ $\displaystyle+\sin\left(\frac{\pi}{2}+\frac{2\pi
ni}{N_{bunches}}\right)\Big{)}.$ (4)

and tracked for $5000$ turns.

The multi-bunch tracking simulation is developed using Bmad Sagan (2006),
which keeps track of the pseudo-modes as they build up and influence the beam
turn-by-turn. Radiation damping is implemented by a per-turn decrement to the
action of each macro-particle. The multibunch feedback is simulated using a
pickup, FIR filter, and kicker.

Each turn the normal mode coordinates of each bunch are recorded and a Fourier
transform is taken along the bunch train. For the seeded mode, the growth rate
is obtained from the fit of an exponential to the the height of the spectral
peak versus turn number. This is done similarly for the vertical modes. The
growth rates of each of the $25$ modes are shown in Fig. 14.

Figure 14:  Per-mode growth rates of a $25$ bunch train in the accumulator
ring determined by multi-bunch tracking with resistive wall wake fields.
During steady-state, all RW modes are safely damped with a total growth rate
of about $-2.5$ ms-1. During the injection transient the effectiveness of the
TFB is significantly diminished by the necessity of masking the $4$ buckets
into which charge is injected, though all modes remain damped.

From Fig. 14 we conclude that while masking will significantly reduce the
damping obtained from the multibunch feedback system, all resistive wall will
modes remain stable by comfortable margin during the transient.

Following an injection cycle, the bunch train is offset with a large uniform
transient. However, the study just described used small amplitudes and
particular offsets given by Eqs. (3) and (4). To simulate absolute stability
following an injection cycle, the beam is given a uniform $5$ mm horizontal
offset and tracked for $5000$ turns. Without masking, after $1702$ turns the
bunch oscillating with the largest amplitude has an action that is $50$% of
the initial condition. With $4$ bunches masked, $50$% is achieved after $3266$
turns. All $15$ RW pseudo-modes are reliably decaying in strength throughout
the simulation.

##  VIII Conclusion

The three-dipole kicker injection scheme is well-fit to the particular
constraints and allowances of the ALS-U accumulator ring. It reliably injects
the relatively large beam coming out of the booster with nearly $100\%$
efficiency using conventional technology. It meets the accumulator ring’s
tight space constraints by distributing the injection kickers across multiple
straights. It fits the large beam coming in from the booster into the
relatively small vacuum chamber by allowing a transient on the stored beam so
as to shrink the transient of the injected beam.

The injection transient has consequences for long- and short-range wake
fields, but several techniques have been prepared to effectively mitigate
these consequences. The scheme is robust in the presence of lattice and pulsed
element imperfections. Both the wake field issues and imperfection tolerances
benefit from the ability of the scheme to tune the amplitudes of the stored
and injected transients.

In the appendix, the merits of three alternative injection schemes are
compared to those of the 3DK scheme.

##  Appendix A Alternate Injection Options

###  A.1 Nonlinear Kicker

Figure 15:  NLK-design concept. Top: main conductors’ transverse positions
(the return conductors are not shown). Bottom: magnetic-field profile in the
mid plane.

Unlike conventional dipole-kicker magnets, a nonlinear kicker (NLK) generates
a transverse nonlinear magnetic-field profile with a maximum located off-axis
at the injected-beam arrival point and zero in the center. The injected beam
is kicked while the stored beam at the center is minimally perturbed. In
addition, a single NLK device is sufficient for injection, thus easing issues
of space and synchronization. Because of these reasons, several light source
facilities have adopted NLK in alternative to more conventional closed-orbit
bump schemes Leemann (2012b); Rast  _et al._ (2011); Liu  _et al._ (2016);
Alexandre  _et al._ (2021).

Although injection perturbations to the stored beam are not a major concern in
the AR (the stored beam does not serve user experiments), NLK injection
represents an attractive option because of its compactness and simplicity,
while a relaxed tolerance to stored-beam perturbation can be exploited to
optimize the design in ways that are not suitable for light-source
applications.

The NLK design configuration we considered consisted of $8$ main conductors
symmetrically positioned around the vacuum chamber in the direction of the
beam as shown in Fig. 15. The NLK was located in the same lattice position as
the main kicker in the 3DK scheme. The conductor placement was optimized using
MOGA methods Deb  _et al._ (2002) in start-to-end macroparticle injection
simulations; for a detailed description of the optimization setup and results
see Ref. Sun  _et al._ (2020). With an optimized NLK design, injection
tracking studies in the presence of lattice errors and including orbit/optics
correction showed injection efficiency above 95% (not including transverse
wakefield effects).

While these simulations were encouraging, an NLK-solution for the AR injection
was eventually abandoned after the R&D effort initiated at ALS Sun  _et al._
(2020) encountered technical difficulty with the prototyping of the ceramic
vacuum-chamber.

###  A.2 Conventional closed orbit bump 4DK injection in one sector

The existing ALS utilizes a conventional $4$-kicker orbit-bump injection
scheme with all four kickers in the same straight section. A similar scheme
was explored for the AR with all $4$ injection kickers placed in Sector $2$.
Potential advantages of this scheme are:  1) it is conventional,  2) residual
oscillations of the stored beam following the injection cycle are eliminated,
3) it eliminates the need to carefully control the phase advance between the
pre-kicker and main injection kicker.

Figure 16: Injection efficiency for the conventional 4DK scheme evaluated over
a population of 100 lattice-error realizations including orbit and optics
corrections. Results are for the septum positioned at $8\text{\,}\mathrm{mm}$
(left) and $13\text{\,}\mathrm{mm}$ (right) from the vacuum chamber center.
Tracking simulations were done with $10$k particles over 10k machine turns.

The straights in the AR are $0.624$ m shorter than those in the ALS. This
shorter space drives up the the strength required from the dipole kickers.
Additionally, because the orbit bump is closed for the stored beam, the
injected particles oscillate with larger amplitude. To avoid loss of injected
particles, the injection septum aperture would need to be increased to the
nominal vacuum chamber aperture of about $14$ mm. This complicates engineering
in the injection straight and further increases the necessary dipole kick to
about $10$ mrad. Such a large dipole kick requires some combination of kicker
R&D, a multi-turn ramp, or placing the $4$ kickers on a single power supply
string. A multi-turn ramp would constrain the machine tune, thus negating one
potential advantage of this scheme. Placing the $4$ kickers on a single string
would make it difficult to trade off between stored beam and injected beam
oscillation amplitudes, which is a disadvantage compared to the 3DK scheme. An
additional drawback is that the septa would need to be moved upstream (toward
the center of the straight) to make room for the 4 kickers. This has the
consequence of forcing a stronger bending requirement in the thick septum.

Even with a $14$ mm septum aperture, a conventional $4$-kicker design delivers
less than $90$% injection efficiency, as shown in Fig. 16. In contrast, the
3DK scheme approaches $100$% injection efficiency. The 3DK scheme also
compares well in requiring only an $8$ mm septum aperture, and its kicker
technology requirements are modest: three $0.6$ m long ferrite-loaded dipole
kickers delivering $\sim 1$ mrad kick.

###  A.3 Two dipole kicker (2DK)

It is possible to implement an injection scheme similar to 3DK while using
only two dipole kickers. Coined “2DK” this scheme saves costs by requiring one
less kicker. With 3DK, the two pre-kickers allow adjustment of the position
and angle of the stored beam at the main injection kicker. With 2DK, only one
pre-kicker is employed and its location in the storage ring is chosen to have
a horizontal phase advance from the pre-kicker to the main injection kicker
such that the stored beam has a near-zero crossing at the main injection
kicker. The pre-kicker is placed in sector $7$, which is six sectors upstream
of the main injection kicker.

2DK is competitive as far as injection efficiency goes, but the lack of
flexibility limits the ability to accommodate difficult-to-predict
consequences of imperfections and wake fields. It also constrains the ring
tune, as the pre-kicker to main injection kicker phase advance must be
precisely set. Generally, the phase advance along a single straight is too
little to allow for significant adjustment. It was judged that the versatility
of the 3DK scheme justified the cost.

## References

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# Unraveling p-type and n-type interfaces in Superconducting
Infinite-Layer Nickelate thin films

###### Abstract

After decades of research, superconductivity was finally found in nickel-based
analogs of superconducting cuprates, with infinite-layer (IL) structure. These
results are so far restricted to thin films in the case of IL-nickelates.
Therefore, the nature of the interface with the substrate, and how it couples
with the thin film properties is still an open question. Here, using scanning
transmission electron microscopy (STEM)- electron energy loss spectroscopy
(EELS) and four-dimensional (4D)-STEM, a novel chemically sharp p-type
interface is observed in a series of superconducting IL-praseodymium nickelate
samples, and a comparative study is carried out with the previously reported
n-type interface obtained in other samples. Both interfaces have strong
differences, with the p-type interface being highly polar. In combination with
ab-initio calculations, we find that the influence of the interface on the
electronic structure is local, and does not extend beyond 2-3 unit cells into
the thin film. This decouples the direct influence of the interface in driving
the superconductivity, and indicates that the IL-nickelate thin films do not
have a universal interface model. Insights into the spatial hole-distribution
in SC samples, provided by monochromated EELS and total reflection-hard x-ray
photoemission spectroscopy, suggest that this particular distribution might be
directly influencing superconductivity.

###### keywords:

infinite-layer nickelates, interface, thin-film, quantum material,
superconductivity, STEM-EELS, HAXPES

Aravind Raji*, Araceli Gutiérrez-Llorente, Dongxin Zhang, Xiaoyan Li, Manuel
Bibes, Lucia Iglesias, Jean-Pascal Rueff*, Alexandre Gloter*

Aravind Raji
Laboratoire de Physique des Solides, CNRS, Université Paris-Saclay, 91405
Orsay, France
Synchrotron SOLEIL, L’Orme des Merisiers, BP 48 St Aubin, 91192 Gif sur
Yvette, France
Email Address<EMAIL_ADDRESS>
Araceli Gutiérrez-Llorente
Universidad Rey Juan Carlos, Escuela Superior de Ciencias Experimentales y
Tecnología, Madrid 28933, Spain
Dongxin Zhang, Manuel Bibes, Lucia Iglesias
Laboratoire Albert Fert, CNRS, Thales, Université Paris-Saclay, 91767
Palaiseau, France

Xiaoyan Li
Laboratoire de Physique des Solides, CNRS, Université Paris-Saclay, 91405
Orsay, France
Jean-Pascal Rueff
Synchrotron SOLEIL, L’Orme des Merisiers, BP 48 St Aubin, 91192 Gif sur
Yvette, France
LCPMR, Sorbonne Université, CNRS, 75005 Paris, France
Email Address<EMAIL_ADDRESS>
Alexandre Gloter
Laboratoire de Physique des Solides, CNRS, Université Paris-Saclay, 91405
Orsay, France
Email Address<EMAIL_ADDRESS>
##  1 Introduction

The synthesis of superconducting (SC) nickelate thin films [1] with infinite-
layer (IL) ABO2 structure is one of the most significant milestones in recent
years in the field of unconventional superconductivity. A puzzling fact
however is the absence of superconductivity in bulk IL-nickelate compounds
with the same nominal composition [2, 3, 4], perhaps due to the presence of
extended defects. Yet, the properties of oxide thin films are known to
strongly depend on the interface of the film with the substrate, so one may
wonder whether there is not something specific to thin films in realizing the
superconducting state. Indeed, in oxide thin film heterostructures the
interface often determines the physics of the samples [5, 6, 7] including
superconductivity.[8, 9]. Although, there are recent reports of
superconductivity on IL-nickelate films grown on (LaAlO3)0.3(Sr2TaAlO6)0.7
(LSAT) substrates [10, 11], most results are on films grown on SrTiO3 (STO)
substrates. Intrhinguinly, superconducting IL-nickelate films grown on STO
present only one particular interface termination type to date [12]. This
raises questions on structural differences between bulk and films, and on the
role of the interface with the substrate in achieving the zero-resistance
state.

Regarding the interface aspect, previous scanning transmission electron
microscopy (STEM) experiments reported one interface model in doped and un-
doped thin films [13, 14, 15] and in superlattices [16]. This was represented
by a B-site intermixed perovskite layer, considered as a buffer layer that
directly quenched the two-dimensional electron gas (2DEG) expected to form at
the otherwise highly polar NdNiO2/STO interface [13]. This B-site intermixed
interface, has a reduced polarity mismatch coming from NdO(+1
charge)-(Ni/Ti)O2(0 charge) layer sequence, typical of a n-type interface
[17]. There, it was concluded from several SC and non-SC samples that this
interface model is somewhat universal, and it decoupled the possible
interfacial 2DEG from superconductivity in IL nickelates. Such a concept of
the universal B-site intermixed interface in IL nickelates was challenged by
the recent reports on synthesizing SC IL nickelate thin films with a
chemically sharp interface by molecular beam epitaxy that avoided such a Ni/Ti
intermixing [18, 19]. In such chemically sharp interfaces the polarity
mismatch could be stronger, and how it influences the crystal and electronic
structure, and thereby the superconductivity is still an open question. A
comparative study on these different interfaces by advanced characterization
techniques possessing high spatial and energy resolution that can isolate the
interface structural and electronic effects is so far lacking in the domain of
nickelate superconductivity. Notably, ab-initio calculations on IL nickelates
with different substrates previously discussed the more probable formation of
interfaces with n-type band alignment on STO [17], and with p-type band
alignment on a NdGaO3 substrate, here driven by +3/-1 interface polarity [20].
As discussed before, the n-type interface was previously reported, but so far
there are no experimental reports of a p-type interface in IL nickelates. In
this direction, we investigate different praseodymium IL nickelate samples, SC
and non-SC, all being 20$\%$ Sr doped. By topotactic CaH2 reduction of the
parent perovskite, we obtain the SC Pr0.8Sr0.2NiO2(PSNO)/STO system, that
structurally involves a direct transition from the perovskite substrate to the
IL without displaying a nickelate perovskite unit cell (u.c.) at the interface
[21]. Through a spectro-microscopic analysis with STEM, 4D-STEM, and
monochromated electron energy loss spectroscopy (EELS) in combination with ab-
initio calculations, we identify a novel chemically sharp p-type interface, in
contrast to the B-site intermixed (n-type) interface that we obtained in other
samples reduced in-situ by aluminium.

Apart from the interface aspect, to date, there are no studies on the spatial
hole distribution in SC IL nickelates, notwithstanding the fact that
superconductivity arises in nickelates upon hole-doping. Competing orders with
superconductivity, such as the 3$a_{o}$ charge order [22, 23, 24], was found
to be emerging from a peculiar oxygen vacancy ordering in IL nickelates [25,
26, 27]. On similar grounds, there are theoretical studies on how doping
suppresses this electronic instability, and paves the way for phonon-mediated
superconductivity from bond disproportionation in nickelates [28]. To address
this, we also carried out a spatial mapping of the hole-doping in the SC PSNO
thin-film using depth resolved total reflection hard x-ray photoemission
spectroscopy (TR-HAXPES), that also sheds some light on the IL nickelate
superconductivity.

##  2 Results

###  2.1 The Interface dichotomy

Figure 1:  STEM-EELS interface elemental analysis of SC STO-PSNO-STO and Al-
PSNO-STO samples. (a) HAADF image of STO-PSNO-STO thin film. (b) The EELS
element profile of maps is shown in (c,d). A slight Ni/Pr off-ratio is seen in
the top and bottom of the thin-film, as highlighted. (c) Combined element map
of all four Sr, Pr, Ni and Ti elements. (d) Separated element maps from the
Sr-L2,3, Pr-M4,5, Ni-L2,3, and Ti-L2,3 edges. (e-g) The novel p-type SrO
terminated interface in SC STO-PSNO-STO sample; (e) The magnified HAADF image
at the bottom interface, (f) The combined element map of this region, (g) The
element map intensity profile showing Sr-termination. (h-j) The n-type
interface in Al-PSNO-STO sample; (h) HAADF image of Al-PSNO-STO thin film’s
substrate interface, (i) EELS combined element map in this region, (j) The
element map intensity profile showing the Ni/Ti mixed interface model
suggested in [13], which we identify as n-type.

Here, we report the new p-type interface model in the SC PSNO/STO system, and
compare with the n-type model discussed so far in IL nickelate thin-films,
with an all-cation element map including Sr, using STEM-EELS. Our studies are
on a STO/PSNO/STO sample with p-type interface obtained through topotactic
reduction with CaH2 and an AlOx/PSNO/STO sample with n-type interface obtained
by in-situ chemical reduction from an Al overlayer. This in-situ reduction
using aluminium has been previously reported as an efficient technique to
synthesize superconducting IL nickelate samples [29, 30]. See methods for more
details on the synthesis. Considering the presence of Sr in the substrate, and
it being the dopant, mapping the Sr gives advanced insights into the interface
in comparison with previous studies, where an element map of Sr is absent [13,
15]. Figure 1 displays the interface comparison using STEM-EELS, between the
novel chemically sharp p-type interface and the B-site intermixed n-type.
Figure 1a shows the STEM-high angle annular dark field (HAADF) image of the SC
STO-capped PSNO/STO sample reduced using CaH2 [21]. Figure 1b shows the
intensity profile of the combined element map in Figure 1c. The isolated maps
of each element from their respective EELS edges is given in Figure 1d. A zoom
into the substrate interface region is given in Figure 1e-g, which evidences
the chemically sharp interface, with a Sr-layer separating the Ni and Ti on
either sides, as indicated by the arrow. The Sr-layer interfacial layer was
also observed in the perovskite phase before reduction, as shown in Supporting
Information, Figure S1. This interface is chemically sharp and essentially
avoids the B-site intermixing. Moreover, we observe Ni/Pr ratio slightly
larger than 1 near both top and bottom interfaces as highlighted in Figure 1b.
This can be read as the distribution of Sr, and hence the distribution of
holes. This will be discussed in detail in a later section on spatial mapping
of hole-distribution. We also observe the same chemically sharp interface in
another SC uncapped PSNO/STO sample prepared the same way (Supporting
Information, Figure S2). In contrast, we observe the B-site intermixed
interface in our in-situ reduced PSNO/STO sample, shown in Figure 1h-j. The
element map intensity profile in Figure 1j shows the strong Ni/Ti intermixed
interface, as indicated by the arrow, with the neighboring layers on either
side being Pr. Although this specific sample is not SC, this n-type interface
is essentially similar to those reported up to now [13, 14, 16]. Similar
results are found on another in-situ reduced sample with the same n-type
interface, with resistivity plot showing a SC transition is given in
Supporting Information, Figure S3. From now on we will refer to the chemically
sharp interface as p-type and the B-site intermixed as n-type due to their
respective interface polarity.

Figure 2:  4D-STEM and Monochromated EELS analysis of SC STO-PSNO-STO and Al-
PSNO-STO samples near the interface. (a) 4D-STEM dCOM and HAADF image with the
interface model of STO-PSNO-STO thin film. (b) Ni-L3 edge EELS at the u.c. as
numbered from 0 to 4 near the interface. (c) Ti-L2,3 edge EELS at the u.c. as
numbered from -1 to -3 near the interface, and from bulk STO. One bulk is
around 4 nm from the interface, and other 6nm, they are similar. (d) 4D-STEM
dCOM and HAADF image with the interface model of Al-PSNO-STO thin film. (e)
Ni-L3 edge EELS at the u.c. as numbered from 0 to 4 near the interface. (f)
Ti-L2,3 edges EELS at the u.c. 0 to -3 near the interface, and from bulk STO.
Notably, the STO-PSNO-STO sample do not have Ni2+ at the interface, rather a
doublet feature might be arising from orbital effects of the pyramidal Ni at
the interface. There is a clear Ni2+ at the interface u.c. in the Al-PSNO-STO
sample. In both samples, there is no clear signature of Ti3+.

To evaluate the associated structural and electronic evolution of these two
interfaces, we employed 4D-STEM and monochromated EELS with atomic resolution.
Figure 2a shows the divergence of the center of mass (dCOM) obtained by
4D-STEM measurements, which qualitatively approximates to a projected charge-
density image and indicates the distribution of oxygen across the interface,
along with the associated HAADF image of this p-type interface. Here, the
final STO u.c. labelled as u.c. -1 in HAADF, includes Ti in an octahedral
coordination, as in the bulk STO. The interface u.c. (defined as u.c. 0) hosts
Ni in a pyramidal coordination, and with a highly asymmetric A-cation
geometry, with Sr from the STO side and Pr from the PSNO side. From u.c. 1, Ni
starts to be in square-planar coordination with symmetric Pr on either side.
This essentially evidences a direct transition from the perovskite STO to the
IL PSNO, without having any structural reconstruction at the interface. This
makes the interface highly polar, with SrO[0 charge] - NiO2[-3 charge] - Pr[+3
charge], which has the potential to host a strong electric field. The
electronic structure of this interface is evaluated from the u.c. resolved
EELS fine structure shown in Figure 2b,c. The Ni-L3 edge EELS shown in Figure
2b reveals significant differences between Ni at the interface (u.c. 0) and
those above. All the Ni exhibits primarily Ni1+ character indicated the
intense low energy edge, but with a doped shoulder at a higher energy loss,
similar to the ones reported for a doped Nd0.8Sr0.2NiO2 [31]. However, one
significant difference is observed for the Ni in u.c. 0, which is in a
pyramidal coordination. Its Ni-L3 edge exhibits a doublet feature, which could
come from additional hole or some orbital ordering at the interface. Along
with this, the u.c. resolved Ti-L2,3 edges shows Ti4+ character throughout,
but with a more broadened spectral shape in the u.c. -1. Since it is a p-type
interface, this small broadening is not expected to arise from mobile
electrons contributed by the Ti 3d conduction band. This could arise from
localized state and defect, or from the influence of the strong electric field
at the interface, as it has been previously shown to broaden the Ti-L2,3 edges
at a nickelate-titanate interface [32]. Such an influence of the interface
polarity will be discussed in detail in the coming section.

On the other hand, Figure 2d-f shows a similar characterization done on the
n-type interface, which displays a very different structural and electronic
landscape. Figure 2d shows the dCOM map and the HAADF image of the interface
region in this system. As shown, the final STO u.c. is labeled as -1 in the
HAADF image, with Ti in octahedral coordination, and with an asymmetric
A-cation distribution with Sr from the STO side and the Pr on top. The u.c. 0
consists of the Ni/Ti mixed layer, these cations being located in octahedral
coordination and symmetric Pr-sites on either side. The u.c. 1 hosts Ni in a
pyramidal coordination, and a symmetric A-site with Pr on either sides. The
NiO2 square planar begins from the u.c. above it. Such an interface
encompassing the perovskite-intermixed nickelate layer is less polar, and it
is expected to have a smaller interface electric field compared to the p-type
discussed before. The u.c.-resolved Ni-L3 edge EELS shown in Figure 2e
indicates a strong Ni2+ character for the Ni at the u.c. 0, and a bit weaker
Ni2+ character for the Ni at u.c. 1. Ni exists in octahedral coordination in
the former, and in pyramidal in the latter. In the following unit cells, the
Ni exhibits strong Ni1+ character, but not with a strong hole-doped shoulder
as in Figure 2b. Also, as shown in Figure 2f, the Ti-L2,3 edges does not
exhibit notable differences from the bulk to the intermixed interface u.c. 0,
and this indicates the absence of Ti3+ as previously suggested for such an
interface [13]. The absence of any spectral broadening of this Ti-L2,3 edges
at the interface is also consistent with a weaker electric field here.

###  2.2 Interface polarity

Figure 3:  Polar displacements at the interface and electronic structure of
p-type (a-f) and n-type (g-l) interfaces. (a-c) 4D-STEM dCOM and the intensity
profiles showing polar-like displacement of oxygen at the p-type interface.
(d) DFT relaxed interface structure, reproducing the similar displacements of
oxygen atoms as in the 4D-STEM dCOM. (e) Layer resolved PDOS of the structural
model in (d). (f) k-resolved PDOS of Ti 3d and Ni 3d orbitals from u.c. -2 to
1. (g-i) 4D-STEM dCOM and the intensity profiles indicating the absence of any
relative displacement of oxygen at the n-type interface. (j) DFT relaxed
interface structure, reproducing relatively smaller displacements between
atoms at the interface as in the 4D-STEM dCOM. (k) Layer resolved PDOS of the
structural model in (j). (l) k-resolved PDOS of Ti 3d and Ni 3d orbitals from
u.c. -1 to 1.

The spectroscopic signatures in Figure 2 thus indicate that, compared to the
n-type interface, the p-type interface is highly polar and is expected to host
a strong electric field. To make this more evident, we analyzed the atomic
displacements at this interface with 4D-STEM dCOM data, also comparing with
ab-initio calculation models of such an interface. Figure 3a-c shows such an
analysis of the atomic displacements. The Ni-Sr distance is labeled as A, and
the O-Sr distance is labeled as B for the u.c. 0. Corresponding to the arrows
labeled in the dCOM map in Figure 3a, the distance A (Ni-Sr) is estimated as
0.185 nm in Figure 3b and the distance B (O-Sr) is estimated as 0.199 nm in
Figure 3c. This indicates a relative difference of 14 pm. Such atomic
displacements are qualitatively observed in the relaxed structural model in
Figure 3d, that further corroborates the strong interface polarity and the
emergent electric field here. The layer-resolved partial density of states
(PDOS) shown in Figure 3e, also clearly identifies it as the p-type interface
with the Fermi level close to the valence band. The k-resolved PDOS shown in
Figure 3f indicates that the Ti 3d bands in u.c. -2 and u.c. -1 are very far
from the Fermi-level, at energies larger than 2 eV. This is consistent with
the absence of any strong signature of Ti3+ near the interface. As expected
from the EELS analysis, the pyramidal Ni at the u.c. 0 has a very different
electronic structure in comparison with the square-planar Ni at the u.c. 1.
The Ni 3d bands in the pyramidal Ni at u.c. 0 are under the influence of a
strong electric field due to the high polarity mismatch as discussed before.
On combining with the PDOS shown in Supporting Information, Figure S4, it
seems that the occupied 3d${}_{z^{2}}$ bands get shifted to higher energy, as
it was also discussed in [33]. This modified orbital hierarchy where
3d${}_{z^{2}}$ bands get closer to the Fermi level, could potentially be
causing the doublet of Ni L3 edge observed in EELS fine structure. On similar
grounds, a complex orbital rearrangement has been reported in a related
LaNiO2/LaGaO3 interface with a pyramidal Ni layer [34]. There the extra holes
were getting accommodated with a multi-orbital character at this site, instead
of getting evenly distributed across the layers [34]. The Ni 3d bands in the
square-planar u.c. 1 experience a lower electric field, as can be seen in the
k-resolved PDOS plot for u.c. 1. It is quite similar to the Ni 3d bands in the
bulk IL as reported in previous studies [20], and also for a single u.c.
PrNiO2, as shown in Supporting Information, Figure S5.

In comparison, the n-type interface does not have any strong atomic
displacements as shown in Figure 3g-i. This matches very well with the ab-
initio calculation of the relaxed interface model shown in Figure 3j. The
n-type nature of this interface is confirmed by the layer-resolved density of
states shown in Figure 3k, where the Fermi level is closer to the conduction
band. This also goes in hand with the previous theoretical studies [20].
Notably, from the k-resolved PDOS shown in Figure 3l, the Ti 3d states in the
STO seem to be located directly above, and cross the Fermi level a bit (u.c.
-1), in contrast to the p-type interface. Interestingly, the Ti 3d bands in
the u.c. 0, that is the Ni/Ti mixed u.c., show a bit of difference with Ti 3d
in STO, and clearly do not have any bands near or crossing the Fermi level.
This is consistent with the absence of any Ti3+ inferred from EELS at the
interface. On the other hand, in the octahedra at this u.c. 0, the Ni
3d${}_{z^{2}}$ becomes unoccupied, in line with the demonstration of Ni2+ at
this u.c. by EELS. As already reported in the previous studies [20, 17] and
the PDOS shown in Supporting Information, Figure S4, the Ni
3d${}_{x^{2}-y^{2}}$ stays near the Fermi-level, but with reduced bandwidth.
Interestingly, the Ni 3d bands in the pyramidal u.c. 1 seem very different
from the Ni 3d bands in the pyramidal u.c. 0 of the p-type interface in Figure
3f. However, it is similar to the Ni 3d bands in the square-planar u.c. 1 of
the p-type interface. This difference might arise from the fact that the
pyramidal Ni (u.c. 0) of the p-type is under the influence of the strong
electric field since the interface is highly polar. However, such a strong
electric field is absent for the n-type interface, and thus the pyramidal Ni
in u.c. 1 renders an orbital energetic hierarchy similar to the square-planar
Ni [26]. From these results, one can understand that the two interfaces have a
very different electronic nature that might influence the electronic structure
of neighbouring layers. However, it is to be noted here that in both cases,
the influence of the interface vanishes beyond 2-3 unit cells into the
nickelate layer.

###  2.3 Spatial hole distribution

Figure 4:  Macroscopic mapping of the electronic structure of SC STO-PSNO-STO
using STEM-EELS and Total Reflection(TR)-HAXPES. (a) HAADF image of STO-PSNO-
STO thin film. (b,c) EELS Ni-L3 and Pr-M4,5 edges at different regions of the
thin film shown in (a). (d) HAADF image of STO-PSNO-STO thin film. (e) EELS
O-K edge at different regions shown in (d). (f) The Electric field at the
total reflection condition using YXRO code [35], for an STO/PSNO/STO system
with around 1 nm of carbon on top. (g) Ni 2p core level PES at two different
angles around the total reflection condition. (h) Rocking curve with the angle
of the two components of Ni-2p at the total reflection condition. The Ni shows
more hole doping near the top and bottom interface.

One hallmark of the IL nickelate superconductivity is its SC dome upon varying
the hole doping from 10 to 25 $\%$ [36]. It is thus important to study the
distribution of this doped holes throughout the nickelate layer, and in
combination with local structural and chemical effects, this could be one key
factor dictating superconductivity in this system. Here using monochromated
EELS over the whole thin film, and with depth-resolved total
reflection(TR)-HAXPES, we carried out a spatial mapping of the charge/hole
distribution in a superconducting STO-capped PSNO/STO sample with the p-type
interface. Figure 4 a-e shows the evolution of Ni-L3, Pr-M4,5, and O-K EELS
edges across the whole thin film of the superconducting STO-PSNO-STO sample.
The Ni-L3 edge shown in Figure 4b forms a representative map of the hole
distribution, as the higher energy loss shoulder emerges with Sr-doping as
reported in previous studies with EELS and XAS [31, 37]. Notably, this feature
is spatially varying and it emerges near the top and bottom interfaces.
Especially in region 4, the hole-doped signature is weaker. To further
corroborate this, as shown in Supporting Information Figure S6, we did a
separate spatial map of the ratio of a pure Ni1+ component (from reference
[25]) and Ni -hole doped component in this thin-film, and it shows the same
trend. The Ni-hole doped component exhibits an increase concentration near the
top and bottom interface, except the middle. But it always stays within the
superconducting dome [12], as represented in Supporting Information Figure
S6b. This goes along with the Ni/Pr off ratio seen in the element map in
Figure 1b, which suggested that the doped 20$\%$ Sr gets distributed near both
interfaces. Thus this spatial difference in the spectral shape of Ni-L3 edge
can be directly coupled with the Sr-distribution in the thin film, and having
the ideal hole doping coupled with the perfect IL structure at the regions
near the interfaces could be core requirement for IL nickelate
superconductivity. In addition, as shown in Figure 4c, the Pr-M4,5 edges does
not show any difference spatially and this is expected because it is quite
insensitive to local electronic or chemical changes [25]. On the other hand,
the spatial mapping of the O-K edge shown in Figure 4e, displays strong
differences throughout the thin film. The spectra from the regions 3 and 6,7
that are near the bottom interface and top of the thin film exhibit a reduced
pre-edge and it is representative of the spectra of a doped Ni1+ [31]. For the
region 5, that is in the middle of the thin film, the O-K edge exhibits a
stronger pre-edge and a spectral feature representative of a defect/non-
stoichiometric region as reported in previous studies [25, 31].

To address the uniformity of such an epitaxial distribution of holes over the
whole sample, we employed a macroscopic depth resolved analysis using TR-
HAXPES. In this method, in addition to obtaining a bulk sensitive
photoemission spectra, one can tune the incidence angle in a way to have
selective probing of the top or bulk of the thin film with good spatial
resolution [38]. Figure 4f shows the depth-resolved simulated electric field
that will be generated in this STO/PSNO/STO system from x-rays when one varies
the incidence angle in photoemission near the total reflection angle.
Depending on the depth of the electric field, the photoemission signal will be
enhanced up to that depth. This is the principle of TR-HAXPES, and it is given
in detail in Methods. Figure 4g shows the Ni 2p3/2 photoemission spectrum for
two different incident angles. It shows a Ni1+ character, but with a component
at a higher binding energy labeled as B, representative of the hole doping. On
par with the EELS analysis, the B-component gets stronger at 0 deg, that is
when the electric field enhances the photoemission signal from the top of the
nickelate layer. At a higher angle 0.6 deg, when the signal probes the entire
nickelate layer, the relative intensity of the B-component becomes smaller
compared to that at 0 degrees. When compared with the rocking curve in Figure
4h shows that the B-component is relatively stronger for the incidence angles
between -0.2 and 0.2 deg, that is when the electric field is stronger at the
top of the nickelate layer as illustrated in Figure 4f. It has to be
considered that some of the regions between the middle of the nickelate layer
to the bottom interface lacks Sr-doping as evidenced by EELS. As shown in
Figure 4f, on angles higher than 0.25 deg, the electric field is almost
homogeneous and strong in the whole nickelate layer. This indicates that up to
0.25 deg, we can selectively enhance photoemission from top of the nickelate
layer, after that, the signal gets averaged out in the whole layer. This gives
us the constrain that we can only isolate the signal from top nickelate
region, and not the middle or bottom region from the whole signal. In this
way, the photoemission signal over the whole sample at higher incidence
angles, the signal from the Sr-rich and Sr-poor regions gets averaged out.
Thus, the hole-doping component B, emerging from Sr-doping gets almost the
same intensity as the main Ni1+ component A. This concomitantly proposes more
doped holes in the thin film top than in the middle. On combining with the
microscopic results, this can be read as doped Sr-getting bit enriched near
both interfaces, and such a trend might be present throughout the sample as
probed macroscopically by TR-HAXPES.

##  3 Conclusion

It is now demonstrated that both p-type and n-type interfaces can form in IL
nickelate thin films. While from previous works, the n-type is
superconducting, here we show the p-type is superconducting as well. It
directly questions the role of the interface as a primary factor in
controlling superconductivity. In particular, our u.c. resolved EELS fine
structure and ab-initio calculations indicates that after 2-3 unit cells, the
interface influence is weaker on the nickelate. On the other hand, intrinsic
doping arising from Sr, along with self-doping effects from local off-
stoichiometry are prone to modulate the hole-doping within the layer. Along
with this, different interface reconstructions can act as a secondary hole
source, however, the occurrence of superconductivity is not bound to a
specific interface model. These results unravels the non-universality of the
interface in SC IL nickelates.

##  4 Experimental Section

###  4.1 Sample preparation

Perovskite Pr0.8Sr0.2NiO3 nickelate thin films of thicknesses around 8 nm were
grown in a PLD system equipped with a KrF excimer laser (248 nm) focused onto
the target [21]. The laser pulse rate was fixed at 4 Hz. During the growth,
oxygen was supplied in the PLD chamber yielding a background pressure up to
0.5 mbar. The laser fluence was 1.6 $\mbox{J}\,\mbox{cm}^{-2}$, with a laser
spot size of $2\pm 0.2\;\mbox{mm}^{2}$. The substrate temperature was set at
640 ∘C, and the oxygen pressure at 0.33 mbar. The films were cooled down to
room temperature at a rate of 5 ${}^{\circ}\mbox{C}\,\mbox{min}^{-1}$ at the
growth pressure. Single crystals of (001)SrTiO3 (STO) were used as substrates.
Prior to the growth, they were etched in buffered HF and annealed at 1000 ∘C
for 3 h to obtain stepped surfaces with TiO2 termination. No further annealing
under vacuum was carried out before the film growth. We sintered the PLD
target from a mixture of Pr2O3 (99.99%), Nickel(II) oxide (99.99%) and SrCO3
with controlled cation stoichiometry ([Ni]/([Pr]+[Sr])=1.1; [Pr]/[Sr]=4) by a
solid state reaction. These mixtures were ground in an agate mortar and, after
initial decarbonation at 1200 ∘C for 12 h, pressed into pellets, and heated in
a box furnace at 1300 ∘C for 24 h. To get high-density PLD targets, the
powders were reground and repressed, and then fired at 1300 ∘C for further 24
h.

Reduction of the perovskite phase thin films into the IL phase was carried out
by two different methods: CaH2 reduction and in-situ Al reduction. Regarding
the CaH2 reduction, the process was performed in evacuated glass tubes. The
tubes were filled with 0.1 g of CaH2 powder in an N2-filled glovebox. Samples
were wrapped in aluminum foil and inserted into the glass tubes. The samples
are separated from the CaH2 powder (Alfa Aesar A16242) by means of a lump of
glass wool. The tubes were evacuated by a rotary pump ($<10^{-3}$ mbar) and
sealed. A horizontal tube furnace, ventilated with N2 for increased
temperature homogeneity was used for the process, with temperature accuracy
$\pm 1\;^{\circ}$C. The reference temperature is measured at the center of the
tube furnace, where the sealed ampoule is placed. Heating and cooling rates in
the oven were 5 ${}^{\circ}\mbox{C}\;\mbox{min}^{-1}$.

In the case of in-situ reduced Al/PSNO/STO samples, the deposition of 3.5 nm
of aluminum metal overlayer was carried out inside sputtering chamber (from
PLASSYS) at temperature of 380 ${}^{\circ}\mbox{C}$ and pressure of 6 ×
10-4mbar. The argon flow was set to 5.2 sccm,the current was set to 15 mA,
power at 5 W and voltage at 320V. Before Al depositon the target was pre-
sputtered for 10 minutes to remove possible oxidized layer. After Al
deposition, the samples are kept at same temperature as deposition (380∘C) for
2h before cooling. The heating and cooling rates were $\pm$ 10
${}^{\circ}\mbox{C}$/min.

###  4.2 High resolution STEM-EELS and 4D-STEM

Cross-sectional transition electron microscopy (TEM) lamellae were prepared
using a focused ion beam (FIB) technique at C2N, University of Paris-Saclay,
France). Before FIB lamellae preparation, around 20 nm of amorphous carbon was
deposited on top for protection. The HAADF imaging and 4D-STEM were carried
out in a NION UltraSTEM 200 C3/C5-corrected scanning transmission electron
microscope (STEM). The experiments were done at 200 keV with a probe current
of approximately 14 pA and convergence semi-angles of 30 mrad. A MerlinEM
(Quantum Detectors Ltd) in a 4×1 configuration (1024 × 256) has been installed
on a Gatan ENFINA spectrometer mounted on the microscope [39]. The EELS
spectra are obtained using the full 4 × 1 configuration and the 4D-STEM by
selecting only one of the chips (256 × 256 pixels). For 4D-STEM, the EELS
spectrometer is set into non-energy dispersive trajectories and we have used a
6-bit detector mode that gives a diffraction pattern with a good signal-to-
noise ratio without compromising much on the scanning speed. The monochromated
EELS have been done using a NION CHROMATEM STEM at 100 keV with a probe
current of approximately 30 pA, convergence semi-angles of 25 mrad and an
energy resolution of around 70 meV. The EELS detection was also done with a
MerlinEM in a 4×1 configuration (1024 × 256) that has been installed on a Nion
IRIS spectrometer mounted on the microscope.

###  4.3 Ab-initio Calculations

The first principles calculation were performed using the density functional
theory [40] as implemented in the Quantum ESPRESSO package [41, 42]. The
exchange-correlation functional was approximated by the generalized gradient
approach [43]. Planewave cutoffs of 51 and 526 Ry were used basis-set and
charge density expansions, respectively. Structural relaxations and electronic
structure calculations were done with a Hubbard U parameter of 4eV for Ni and
Ti elements.The p-type and n-type interface models were created in
superlattice geometry with in-plane $\sqrt{2}$a × $\sqrt{2}$a and out-of-plane
c. This gives two transition metal atoms per layer. By constraining this to
the substrate STO, it gives in-plane lattice parameters of a = b = 5.52 Å . To
understand the band folding that might arise, we also did the calculation for
a single u.c. of PrNiO2 with in plane 1a x 1a and and in-plane $\sqrt{2}$a ×
$\sqrt{2}$a. This is shown in Supporting Information, Figure S5.

###  4.4 HAXPES measurements

The measurements were carried out at the GALAXIES beamline at the SOLEIL
synchrotron [44] on the HAXPES endstation [45] using a photon energy of 6000
eV, with varying incidence angles from -1∘ to 80∘. The electric field at TR-
condition was simulated from the YXRO code [35]. The synchrotron operated with
a ring current of 450 mA, giving an intensity of 3.4 × 1013 photons/s at 3000
eV, which was then reduced using a built-in filter to 5% of the original
intensity. The photoelectrons were detected using a SCIENTA Omicron EW4000
HAXPES hemispherical analyzer, and a Shirley background [46] was removed
before fitting the core levels spectra.

##  5 Supporting Information

Supporting Information is available from the Wiley Online Library or from the
author.

##  6 Acknowledgements

A.R. acknowledges financing from LABEX NanoSaclay and H2020 for the doctoral
funding. A.G.L. acknowledges financial support through a research grant from
the Next Generation EU plan 2021, European Union. Nion CHROMATEM at LPS Orsay
and the FIB at C2N, University of Paris-Saclay was accessed in the TEMPOS
project framework (ANR 10-EQPX-0050). This work is supported from the
framework of the joint ANR-RGC ImagingQM project
(RGC,A-CityU102/23;ANR,ANR-23-CE42-0027). We acknowledge SOLEIL Synchrotron
for provision of beamtime under proposals 20221574 and 20231845. We thank
Alberto Zobelli, Daniele Preziosi, and Danfeng Li for fruitful discussions.

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##  Appendix A Supporting Information

See pages - of Interface_PSNO_supp_vfin.pdf