# Injection and capture of antiprotons in a Penning-Malmberg trap using a
drift tube accelerator and degrader foil

C. Amsler  H. Breuker  M. Bumbar  S. Chesnevskaya  G. Costantini  R. Ferragut
M. Giammarchi  A. Gligorova  G. Gosta  H. Higaki  M. Hori  E.D.
Hunter111Present address: Experimental Physics Department, CERN, 1211 Geneva,
Switzerland C. Killian  V. Kraxberger  N. Kuroda<EMAIL_ADDRESS>A. Lanz222Present address: University College London, Gower St, London WC1E
6BT, United Kingdom M. Leali  G. Maero  C. Malbrunot333Permanent address:
TRIUMF, Vancouver, BC V6T 2A3, Canada V. Mascagna  Y. Matsuda  V.
Mäckel444Present address: INFICON GmbH, Bonner Str. 498, 50968 Köln, Germany
S. Migliorati  D.J. Murtagh  Y. Nagata  A. Nanda  L. Nowak  F. Parnefjord
Gustafsson  S. Rheinfrank  M. Romé  M.C. Simon  M. Tajima
<EMAIL_ADDRESS>V. Toso  S. Ulmer  L. Venturelli  A. Weiser  E.
Widmann  Y. Yamazaki  and J. Zmeskal  Stefan Meyer Institute for Subatomic
Physics, Austrian Academy of Sciences, Dominikanerbastei 16, 1010 Vienna,
Austria  Ulmer Fundamental Symmetries Laboratory, RIKEN, 2-1 Hirosawa, Wako,
351-0198 Saitama, Japan  Experimental Physics Department, CERN, 1211 Geneva,
Switzerland  Vienna Doctoral School in Physics, University of Vienna,
Universitätsring 1, 1010 Vienna, Austria  Dipartimento di Ingegneria
dell’Informazione, Università degli Studi di Brescia, via Branze 38, 25123
Brescia, Italy  INFN - Sezione di Pavia, via Agostino Bassi 6, 27100 Pavia,
Italy  L-NESS and Department of Physics, Politecnico di Milano, via Anzani 42,
22100 Como, Italy  INFN - Sezione di Milano, via Giovanni Celoria 16, 20133
Milan, Italy  Graduate School of Advanced Science and Engineering, Hiroshima
University, 1-4-1, Kagamiyama, Higashihiroshima, 739-8527 Hiroshima, Japan
Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom
QUANTUM, Institut für Physik, Johannes Gutenberg-Universität Mainz, 55128
Mainz, Germany  Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße
1, 85748 Garching, Germany  Institute of Physics, Graduate School of Arts and
Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, 153-8902 Tokyo,
Japan  Dipartimento di Fisica, Università degli Studi di Milano, via Giovanni
Celoria 16, 20133 Milan, Italy  Department of Physics, Tokyo University of
Science, 1-3 Kagurazaka, Shinjuku-ku, 162-8601 Tokyo, Japan  RIKEN Nishina
Center for Accelerator-Based Science, 2-1 Hirosawa, Wako, 351-0198 Saitama,
Japan  Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho,
Sayo-gun, 679-5198 Hyogo, Japan

###### Abstract

The Antiproton Decelerator (AD) at CERN provides antiproton bunches with a
kinetic energy of 5.3 MeV. The Extra-Low ENergy Antiproton ring at CERN,
commissioned at the AD in 2018, now supplies a bunch of electron-cooled
antiprotons at a fixed energy of 100 keV. The MUSASHI antiproton trap was
upgraded by replacing the radio-frequency quadrupole decelerator with a pulsed
drift tube to re-accelerate antiprotons and optimize the injection energy into
the degrader foils. By increasing the beam energy to 119 keV, a cooled
antiproton accumulation efficiency of $(26\pm 6)$% was achieved.

###### keywords:

Drift tube , Antiproton , Penning-Malmberg trap , Antihydrogen

††journal: Nuclear Inst. and Methods in Physics Research A

##  1 Introduction

Fundamental research activities using low-energy antiprotons have been
performed at the CERN Antiproton Decelerator (AD) [1] and Extra-Low ENergy
Antiproton ring (ELENA) [2] for spectroscopic studies of exotic atoms such as
antihydrogen [3, 4, 5, 6, 7] and other antiprotonic atoms [8], measurements of
antiproton properties [9, 10], testing the weak equivalence principle [11, 12,
13, 14], and studies of antiproton–matter interaction [15, 16, 17, 18, 19].

The ASACUSA collaboration developed a slow antiproton beam source, the
Monoenergetic UltraSlow Antiproton Source for High-precision Investigation
(MUSASHI) [20], to perform ground-state hyperfine spectroscopy of antihydrogen
and other interaction studies with the combination of the former radio-
frequency quadrupole decelerator (RFQD) [21] and the AD ring. The production
of antihydrogen atoms in the so-called Cusp trap [22] with beams from MUSASHI
has been demonstrated [23]. They were extracted to a field-free region
downstream of the positron-antiproton recombination location [24], and their
atomic quantum number distribution was measured [25]. Since then, we have been
working on increasing the antihydrogen beam intensity for high-precision
spectroscopy by reducing the plasma temperature [26]. A measurement of
annihilation fragments from antiproton-nucleus scattering using ultraslow
antiprotons from MUSASHI and thin targets was performed, and further studies
are planned [16].

Before the commissioning of ELENA, a bunch of $3\times 10^{7}$ antiprotons
supplied from the AD at 5.3 MeV was decelerated by the RFQD down to 120 keV
which was found to be optimal, and injected into the Penning-Malmberg type
MUSASHI trap. The antiprotons were further decelerated to less than 10 keV by
degrader foils at the entrance of the MUSASHI trap. The foils maintained an
ultra-high vacuum within the trap of $\leq 10^{-10}$ Pa, compared to $10^{-7}$
Pa in the RFQD, and also acted as profile monitors for the injected
antiprotons [27]. The ultra-high vacuum is essential to suppress antiproton
annihilations with the residual gas and to achieve efficient trapping,
especially for stacking multiple antiproton bunches. The antiproton injection
point was steered by monitoring the beam profile on the foils, as described in
Section 2. Precise antiproton injection on the axis of the MUSASHI trap is
required for stable confinement of non-neutral plasmas. The MUSASHI trap
previously trapped up to $10^{7}$ antiprotons from 7 antiproton bunches with
the RFQD at the AD facility [20].

ELENA, commissioned at the AD in 2018, supplies a bunch of antiprotons at
fixed 100 keV energy simultaneously to 4 users. The first beam from ELENA to
ASACUSA was delivered in 2021. A thinner foil than the current degrader foils
is one option to decelerate antiprotons from ELENA. In this case, a mechanism
is needed to optimize the energy of the emitted antiprotons from the foil
since the energy of the antiprotons from ELENA is fixed at 100 keV. A remotely
controlled degrader holder of thin foils with different thicknesses could be
used. It is however not easy to adjust the foil thickness with a hundred
nanometer-scale precision, which is needed to optimize the antiproton energy
in these experiments, and the thin foils can be damaged by the movable
mechanism [28].

An accelerating drift tube is another option. A pulsed drift tube to
accelerate ion bunches has been used in many nuclear physics facilities after
buffer-gas cooling of high-energy particles, see e.g. [29, 30]. Since
antiprotons annihilate in the buffer-gas, making this type of cooling
unfeasible, a pulsed drift tube to directly decelerate antiprotons from 100
keV to 1 keV was developed by GBAR [31]. Electrostatic optics for good
focusing and insulating components to avoid discharges were carefully
designed, which is important for high-voltage switching at 100 kV. The
resulting energy is adjustable by changing the switching voltage of the drift
tube.

We decided to use a pulsed drift tube to adjust the energy by up to +20 keV,
as used previously with the RFQD, and to decelerate the antiprotons using the
current degrader foils. This is an efficient solution for ASACUSA to work with
ELENA since the discharge problem is less serious with 20 kV and we could keep
the current degrader foils without modifications. Therefore, the drift tube
accelerator was constructed, replacing the RFQD, and commissioned with ELENA.
The magnet, Penning trap, foil detector, and antiproton annihilation detectors
used in this work have been described elsewhere [20]. In this paper we
restrict ourselves to describing the upgrades and modifications made to the
apparatus to allow its operation with the 100 keV antiproton beam from ELENA.

Figure 1:  Experimental setup including the drift tube accelerator and the
MUSASHI trap. A bunch of antiprotons is injected from the left.

##  2 Experimental setup

Figure 1 shows the experimental setup. The drift tube accelerator is connected
to the existing MUSASHI trap detailed in [20]. The MUSASHI trap is mainly
composed of a superconducting solenoid of 2.5 T and Multi-ring electrodes
(MRE) [32, 33] in a UHV bore. The degrader foils at the entrance of the
MUSASHI trap isolate the vacuum of $10^{-7}$ Pa inside the drift tube. The
foils are double-layered and made of biaxially oriented polyethylene
terephthalate (Bo-PET) with a mass thickness of 90 $\mu$g cm-2 each. Silver
strips of 25 nm thickness deposited onto the surface of each foil act as
anodes readout by a peak-sensing ADC (CAEN, V785) or a digitiser (NI,
PXI-6224), see [27] for details. There is a retractable electron gun
downstream, which is used to supply electrons prepared for antiproton cooling.
A gate valve, located at the downstream end, is closed and acts as a beam dump
during the measurements of time of flight and the energy distribution of
antiprotons. The Cherenkov detector is composed of an acrylic plate
(Mitsubishi Rayon, Acrylite-000) with a refractive index $n=1.49$. Charged
particles from annihilation (mainly pions) produce Cherenkov light in the
acrylic plate, which is read out by a fine-mesh photo multiplier tube
(Hamamatsu Photonics, R5505GX-ASSY), see [34] for details. One is attached to
the downstream edge of the magnetic shield of the MUSASHI (detector #1) and
the other is attached to the side of the gate valve (detector #2). Two
scintillator bars ($4\times 6\times 200$ cm3) are located parallelly to the
axis of the MUSASHI trap with photomultiplier tubes (PMT Hamamatsu Photonics,
H1949-50) connected to both ends of each bar. The distances between the trap
axis and the scintillator bars are 82 cm and 150 cm, respectively. A Time-to-
Digital Converter (Agilent, U1051A) is used to record coincidence events of
four PMTs. The threshold of the energy deposit is set at approximately 5 MeV
so that the event is mainly due to charged pions according to GEANT4 [35]
simulation, see [32] for details.

The beam energy adjuster of the drift tube consists of a 1.5 m long electrode
made of aluminum alloy with an inner diameter of 31 mm. It is mounted within a
stainless steel chamber with an inner diameter of 101 mm. The magnetic field
in the drift tube is less than 1 mT everywhere. The tube length of 1.5 m is
designed to cover 95% of the injected bunch length from ELENA with
$4\sigma_{\rm rms}=300$ ns [2]. The upstream side of the chamber is first
evacuated by a turbomolecular pump (Shimadzu, TMP-303), and then isolated by a
gate valve, to be further evacuated by an ionization pump (Varian, VacIon Plus
300 Triode). A typical pressure of $6\times 10^{-8}$ Pa is achieved,
sufficiently low to connect to the upstream LNE05 beamline of $2\times
10^{-8}$ Pa pressure. A set of four electrostatic deflectors (rectangular
plates) are installed in the downstream side of the vacuum chamber to optimize
the beam direction after acceleration by the drift tube. The acceleration
voltage is applied via a high-voltage feedthrough (CeramTec, 21144-01-CF) at
the center of the long chamber of the drift tube, connected to a custom-made
control circuit.

Figure 2:  Diagram of the control circuit for the drift tube accelerator. A
negative HV power supply charges the capacitor bank (Cb). A HV switch made of
silicon carbide is used for fast switching triggered by external TTL input. Rd
is adjusted to suppress DC offset voltage of the drift tube caused by the leak
current of the switch. A snubber circuit composed of Rs and Cs is included in
order to suppress voltage surges when the switch is triggered, which could
damage the switch by a reversed current (see also Tab. 1).  Table 1:
Specifications of circuit components.

Symbol | Value | Description
---|---|---
Cb | 10.5 nF | three TDK, FHV-6AN
R1 | 1 M$\Omega$ | KOA, GS 7LC 105K
RL | 100 $\Omega$ | KOA, PN-1 M F 101 J
Rd | 7.7 M$\Omega$ | three KOA, GS 12LC 107K (100 M$\Omega$) and
|  | one Ohmite, MOX95021005FVE (10 M$\Omega$)
|  | in parallel
Rs | 200 $\Omega$ | KOA, PN-1 M F 101 and two PN-0.5 C F 500 J
Cs | 50 pF | Comet PCT, MINI-Cap CFMN-50EAC/35-DH-G

Figure 3:  Time structure of the applied acceleration voltage monitored by an
attenuation probe. It takes 340 ns for antiprotons at 100 keV to go through
the drift tube electrode.  Figure 4:  Output signals from the Cherenkov
detectors when the acceleration voltage of the drift tube is (a) 0 kV, (b) 10
kV, and (c) 20 kV. The green and magenta lines correspond to detectors #1 and
#2, respectively. The broad peak at $t>0.6$ $\mu$s corresponds to antiprotons
decelerated by the foils (with mean energy $\leq 10$ keV). With increasing
acceleration voltage, more annihilations at the gate valve are observed at
earlier times. This implies that the antiprotons are successfully accelerated
by the drift tube.

Figure 2 shows a diagram of the control circuit (see also Table 1). It is
designed to apply a HV pulse with an amplitude of $\leq 25$ kV and a rise time
of 20 ns, assuming that the drift tube has an effective load inductance of 513
nH, capacitance of 85 pF, and resistance of 0.06 $\Omega$. A negative high-
voltage power supply (Matsusada, HGR30-30N) adjusted by an external DC control
voltage is used to charge the capacitor bank (Cb, three TDK, FHV-6AN). A
commercial high-voltage switch made of silicon carbide (Behlke, HTS
301-60-SiC) is used for fast switching, triggered by external TTL input. Three
resistances R1, RL, and Rd are selected not to exceed the current limit. Rd is
adjusted to suppress DC offset voltage of the drift tube caused by the leak
current of the switch. A snubber circuit composed of resistance Rs and
capacitance Cs suppresses voltage surges and reverse currents when the switch
is triggered, which can otherwise damage the switch. The entire setup is
covered by a cage of perforated metal sheets for safety. Figure 3 shows the
typical time structure of the applied pulse, monitored by an attenuation probe
(1/1000, Tektronix, P6015A). It takes 340 ns for antiprotons at 100 keV to
traverse the drift tube electrode.

##  3 Test of the drift tube accelerator

The antiproton bunch is detected by the Cherenkov detectors in synchronization
with the injection trigger signal supplied by the ELENA facility. Figure
4(a)-(c) shows the PMT signals observed by the Cherenkov detectors for various
acceleration voltages applied to the drift tube. The green and magenta lines
correspond to detectors #1 and #2, respectively. The earlier sharp peak at
$t=0$, mainly observed by detector #1 which is closer to the degrader foils,
corresponds to antiprotons accelerated by the drift tube (with a mean energy
between 100 and 120 keV) annihilating in the foils. The delayed broad peak at
$t>0.6$ $\mu$s corresponds to antiprotons decelerated by the foils (with mean
energy $\leq 10$ keV). They annihilate on the surface of the closed gate valve
located 1.6 m downstream of the foils. With increasing acceleration voltage,
more annihilations at the gate valve are observed at earlier times. This
implies that the antiprotons are successfully accelerated by the drift tube
such that more antiprotons traverse the degrader foils and emerge from the
downstream surface with a higher energy. The small peak at $t=0.4$ $\mu$s,
mainly observed by detector #2 which is positioned at the gate valve,
corresponds to fast antiprotons at 100 keV passing through some pin holes on
the foils.

Figure 5:  A typical time evolution of the cumulative annihilation counts
measured by the two scintillator bars during one accumulation cycle.
Antiproton injection occurs at $t=36$ s, followed by electron cooling for 40
s, and slow extraction of the cooled antiprotons to the foils for 10 s after
$t=95$ s.

##  4 Antiproton capture with ELENA

In the following, we describe the procedure to accumulate antiprotons from
ELENA in the MUSASHI trap using the drift tube. Before antiproton capture,
electrons are injected from the electron gun into the MUSASHI trap. The
electron plasma is then radially expanded to increase the overlap with the
incoming antiproton bunch. Figure 5 shows a typical time evolution of the
cumulative annihilation counts measured by the two scintillator bars during
one accumulation cycle. The rise at $t=36$ s corresponds to antiproton
injection. The typical beam profiles observed by the Bo-PET foil detector at
injection are shown in Fig. 6. The root-mean-square widths are
$\sigma_{x}=1.0$ mm horizontally and $\sigma_{y}=1.6$ mm vertically by
Gaussian fitting. A negative high voltage of -12 kV is applied on the ring
electrodes UCE and DCE (Fig. 1) to confine antiprotons decelerated by the
foils. A fraction of the high-energy antiprotons have annihilated at this
time, but only a few of them are counted due to pile-up. Antiprotons are then
electron-cooled for 40 s to an energy of less than 1 eV [36]. The negative
high voltage is then switched off at $t=76$ s. Hot antiprotons (due to the
poor overlap with electrons at high radius) annihilate at this time. The
cooled antiprotons are extracted to the Bo-PET foils upstream of the MRE after
$t=95$ s by electrostatic potential manipulations. By slowly extracting the
antiprotons for 10 s the annihilation rate does not saturate the detector.
Hence all annihilations are counted to estimate the number of trapped
antiprotons. The antiprotons are extracted downstream for antihydrogen
production with a beam energy of 1–1000 eV. Both a pulsed extraction of the
antiprotons from the trap over a duration of $10^{-5}$ s or a slow extraction
over several seconds are possible.

Figure 6:  Beam profiles observed by the foils at the entrance of MUSASHI when
19 kV is applied to the drift tube. The rms widths are $\sigma_{x}=1.0$ mm and
$\sigma_{y}=1.6$ mm by Gaussian fitting.  Figure 7: The number of trapped
antiprotons as a function of acceleration voltage of the drift tube,
normalized to the number of injected antiprotons from ELENA. The error bars
below 16 kV are smaller than the dots. Figure 8:  (a) The signal intensity of
the antiproton beam that traversed the trap and annihilated in the beam dump
as a function of the voltage barrier applied to DCE for beam pulses
accelerated by the drift tube by 0 kV, 10 kV, and 20 kV. (b) The longitudinal
component of the energy distributions of transmitted antiprotons through the
degrader foils. The black dotted line shows the normalized simulation results
[37].

Figure 7 shows the number of trapped antiprotons observed by the two
scintillator bars as a function of the acceleration voltage of the drift tube.
By applying 19 kV, the number is increased by a factor of four, compared to
the case without acceleration. According to the detection efficiency of
approximately 0.5% of the two scintillator bars (estimated by GEANT4
simulation), the trapped number of antiprotons in MUSASHI corresponds to
$(1.4\pm 0.2)\times 10^{6}$. The number of antiprotons supplied from ELENA at
the time of these experiments is about $(5.5\pm 0.9)\times 10^{6}$ per bunch
every 2 minutes. Thus, the efficiency is $(26\pm 6)$%. With the previous setup
of the AD and the RFQD, $3.5\times 10^{7}$ antiprotons were supplied from the
AD and 20% of them were decelerated by the RFQD [21]. They were injected into
the MUSASHI trap and the typical trapped number was $1.5\times 10^{6}$ [20].
The efficiency of the trapped number divided by the decelerated fraction was
$1.5/(35\times 0.2)=21$%, which is comparable to the value obtained with the
drift tube.

##  5 Discussion

Figure 8(a) shows the signal intensity of the antiproton beam that traversed
the trap and annihilated in the beam dump as a function of the voltage barrier
applied to DCE electrode of the MUSASHI, for beam pulses that were accelerated
by the drift tube by 0 kV, 10 kV, and 20 kV. It is measured by integrating the
signal of Cherenkov detector #2 from $t=0$ to $t=10$ $\mu$s. In the absence of
a detailed physics analysis, we model the distribution of pbar energy due to
scattering in the foil as a Poisson process, which in the limit of large
numbers gives a Gaussian. We therefore model the signals in Fig. 8(a) using
the integral of a gaussian, namely the error function. Although the momentum
spread $\Delta p/p\approx 10^{-3}$ [2] of the beam provided by ELENA is 10
times smaller compared to the beam of RFQD, the longitudinal energy of the
transmitted antiprotons remained distributed over more than 10 keV [20]. The
observed trapping efficiency was 26%.

Molecular dynamics simulations in the recoil interaction approximation (MD-
RIA) were recently carried out to study the transmission of antiprotons
through degrader foils [37]. The simulations showed that nuclear scattering of
antiprotons on atomic targets into large scattering angles occurs with a high
probability, especially when the antiprotons are slowed down to keV-scale
energies. This increases the effective path length of the antiprotons in the
foils and causes a significant fraction to annihilate. In ref.[37], the
simulation shows an example when antiprotons with a kinetic energy of 111.5
keV with an energy spread of 5 keV (standard deviation) traversed a 1900 nm
thick Bo-PET foil with 25 nm thick Ag coatings on both surfaces. Only $\approx
40$% of the antiprotons were predicted to emerge from the foil that may be
trapped, with a kinetic energy and angle corresponding to a Larmor radius of
less than 5 mm. Further rejecting the transmitted antiprotons, that emerged
with an angle larger than 50 degrees relative to the normal of the foil
surface, further reduced the trappable fraction to $\approx$30%. These values
roughly agree with the experimental trapping efficiency of 26% within the
uncertainties of the thicknesses of the foils, despite the fact that the
simulation does not include the losses that practically occur in the trap
after capture. The beam energy of 111.5 keV used in these simulations
corresponds to the estimated mean value utilized in the previous experiment
reported in ref.[20]. The energy distribution of the transmitted antiprotons
(indicated by the dotted line in Fig. 8) shows almost no particles at $<1$
keV, as nuclear scattering and atomic capture cause them to annihilate. In
these simulations, the processes that occur at such low energies are not fully
understood. The simulated and experimental distributions cannot be directly
compared since they correspond to the total and longitudinal kinetic energies,
respectively. The experiment may also be affected by pinholes that developed
on the surfaces of the foils.

There are several possible reasons for the broad energy distribution which was
observed. First, the temporal ringing of the high voltage pulse applied to the
drift tube (Fig. 3). It caused modulations in the energy of the antiproton
pulse prior to its arrival at the degrader foils. The timing of the high
voltage pulse was optimized by varying it in 50 ns steps so that the number of
trapped antiprotons was maximized. The timing jitter of this trigger signal
provided by the ELENA control system was about 10 ns relative to the arrival
of the antiprotons [38]. Assuming that the antiprotons are distributed within
the pulse following a Gaussian distribution, we estimate that some 15% of the
antiprotons may not have reached the nominal acceleration energy. Second, MD-
RIA simulations [37] show that when a mono-energetic antiproton beam at 111.5
keV traverses a Bo-PET foil with a uniform thickness of 1800 nm, nuclear
scattering effects causes the transmitted antiprotons to acquire an energy
spread of 5 keV (FWHM). Third, the simulations indicate that a $\pm 100$ nm
thickness variation over the surface of the Bo-PET foils leads to 8 keV energy
spread, whereas our foils have a spatial thickness uniformity of not better
than 10%.

The simulation also suggests that nuclear scattering in the antiproton energy
range of a few keV is more significant when materials with a higher atomic
number (such as silver) are used. This leads to a reduction in the number of
transmitted antiprotons and to an expansion of the beam size after traversing
the foils. This reduces the fraction of antiprotons that overlap with pre-
confined electrons in the trap, so that the trapping efficiency deteriorates.
The simulation suggests that the transmitted fraction would be higher if a
light material such as aluminum were used instead of the silver electrodes
[37]. In the future, we will replace the silver strips with aluminum ones,
thereby improving the trapping efficiency.

##  6 Summary

The ASACUSA collaboration has developed a drift tube accelerator to optimize
the injection energy of antiprotons into the degrader foils at the entrance of
the MUSASHI trap, which is an efficient solution to work with ELENA supplying
antiprotons at a fixed energy of 100 keV. Commissioning has been performed,
which shows that the drift tube operated successfully and that $(1.4\pm
0.2)\times 10^{6}$ antiprotons per bunch are confined by biasing the drift
tube at 19 kV which corresponds to a trapping efficiency of $(26\pm 6)$%. The
trapping efficiency may be improved by replacing the silver strips on the
surface of the degrader foils with aluminum, as suggested by MD-RIA
simulations.

## Acknowledgments

The authors are grateful to the AD and ELENA groups for their contributions on
the smooth and successful operations of ELENA after the long shutdown 2 at
CERN. This work is supported by JSPS KAKENHI Fostering Joint International
Research No. B 19KK0075, Grant-in-Aid for Scientific Research No. B 20H01930;
the Austrian Science Fund (FWF) Grant Nos. P 32468, W1252-N27, and P 34438;
Special Research Projects for Basic Science of RIKEN; Università di Brescia
and Istituto Nazionale di Fisica Nucleare; the European Union’s Horizon 2020
research and innovation program under the Marie Sklodowska-Curie Grant
Agreement No. 721559; Heisenberg Program and Individual Grant Program of the
Deutsche Forschungsgemeinschaft; and Research Grants Program of the Royal
Society.

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