CERN decelerating RFQ for antiprotons
A new decelerating radio frequency quadrupole (RFQD) has been put in operation at CERN. It decelerates either protons or antiprotons from a momentum of 100 MeV/c (kinetic energy 5.33 MeV) down to a kinetic energy variable between ~10 keV and 120 keV. A novel feature is the implementation of a floati...
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irk-123456789-789852015-03-25T03:01:44Z CERN decelerating RFQ for antiprotons Bylinsky, Yu. Lombardi, A.M. Pirkl, W. A new decelerating radio frequency quadrupole (RFQD) has been put in operation at CERN. It decelerates either protons or antiprotons from a momentum of 100 MeV/c (kinetic energy 5.33 MeV) down to a kinetic energy variable between ~10 keV and 120 keV. A novel feature is the implementation of a floating internal RF structure, mounted on HV insulators. It allows continuous post-deceleration or acceleration from the nominal energy of 63 keV by a high tension DC bias applied to accelerating electrodes. A description of the system is given, followed by reports on the first operating experience with the ASACUSA experiment, dedicated performance measurements and consolidation progress. 2001 Article CERN decelerating RFQ for antiprotons / Yu. Bylinsky, A.M. Lombardi, W. Pirkl // Вопросы атомной науки и техники. — 2001. — № 5. — С. 63-65. — Бібліогр.: 8 назв. — англ. 1562-6016 PACS number: 29.17.+w http://dspace.nbuv.gov.ua/handle/123456789/78985 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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A new decelerating radio frequency quadrupole (RFQD) has been put in operation at CERN. It decelerates either protons or antiprotons from a momentum of 100 MeV/c (kinetic energy 5.33 MeV) down to a kinetic energy variable between ~10 keV and 120 keV. A novel feature is the implementation of a floating internal RF structure, mounted on HV insulators. It allows continuous post-deceleration or acceleration from the nominal energy of 63 keV by a high tension DC bias applied to accelerating electrodes. A description of the system is given, followed by reports on the first operating experience with the ASACUSA experiment, dedicated performance measurements and consolidation progress. |
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Bylinsky, Yu. Lombardi, A.M. Pirkl, W. CERN decelerating RFQ for antiprotons Вопросы атомной науки и техники |
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CERN decelerating RFQ for antiprotons |
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CERN decelerating RFQ for antiprotons |
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CERN decelerating RFQ for antiprotons |
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CERN decelerating RFQ for antiprotons |
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CERN decelerating RFQ for antiprotons |
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cern decelerating rfq for antiprotons |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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CERN decelerating RFQ for antiprotons / Yu. Bylinsky, A.M. Lombardi, W. Pirkl // Вопросы атомной науки и техники. — 2001. — № 5. — С. 63-65. — Бібліогр.: 8 назв. — англ. |
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Вопросы атомной науки и техники |
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CERN DECELERATING RFQ FOR ANTIPROTONS
Yu. Bylinsky, A.M. Lombardi1, W. Pirkl1
TRIUMF, 4004 Wesbrook Mall, Vancouver V6T 2A3, Canada
e-mail: bylinsky@triumf.ca
1CERN, Geneva 23, CH-1211, Switzerland
A new decelerating radio frequency quadrupole (RFQD) has been put in operation at CERN. It decelerates either
protons or antiprotons from a momentum of 100 MeV/c (kinetic energy 5.33 MeV) down to a kinetic energy vari-
able between ~10 keV and 120 keV. A novel feature is the implementation of a floating internal RF structure,
mounted on HV insulators. It allows continuous post-deceleration or acceleration from the nominal energy of
63 keV by a high tension DC bias applied to accelerating electrodes. A description of the system is given, followed
by reports on the first operating experience with the ASACUSA experiment, dedicated performance measurements
and consolidation progress.
PACS number: 29.17.+w
1 DESCRIPTION OF THE SYSTEM
The RFQ decelerates the beam ejected from the An-
tiproton Decelerator ring AD [1] at CERN. The key pa-
rameters are given in Table 1 below.
Table 1. RFQ key parameters
Operating frequency 202.5 MHz
Shunt impedance 13.8 kΩ
Vane voltage 167 kV
Maximum electric field 33 MV/m
Dissipated power 1.1 MW
Minimum aperture radius 0.4 cm
Max. vane modulation factor 2.9
Input energy 5.314 MeV
Output energy with internal
DC post-deceleration
63 keV (RFQ )
± 60 keV (DC)
Transverse acceptance 15π mm mrad
Phase/energy acceptance ± 10°/ ± 0.9×10-3
Decelerating efficiency (theory) 45%
Number of particles at input 2.4⋅× 107
Nominal pulse rate (test) 1/2.4 s-1
1.1 Basic system concept
Since the output energy of an RFQ is essentially
fixed, some other kind of variable post-acceleration or
deceleration has to be provided. Variation by RF (e.g. by
a cavity) cannot provide sufficient range due to the lim-
ited flat-top of a single sinusoid relative to the large
beam length at the output. Constant energy variation
over the full RF cycle can be provided by a DC bias.
However, mounting the physics apparatus or the RFQ
on a floating HV platform is excluded for practical rea-
sons. A novel solution is to mount the internal RF struc-
ture of the RFQ on insulators to apply the energy cor-
recting DC voltage (see Fig. 1). The bias on the struc-
ture acts not only on the output energy but also shifts
the input energy in the opposite direction. To compen-
sate for this unwanted effect, an energy corrector cavity
is provided directly at the input flange of the RFQ.
In a decelerating RFQ, smooth bunching of the beam
by the same approach as in accelerating RFQs leads to
prohibitive overall length [2]. External bunching had
therefore to be foreseen using a separate buncher cavity
located 6.15 m upstream of the RFQ. This limits the
overall decelerating efficiency to less than 50%. The
RFQ acts, nevertheless, as an efficient beam transport
also for particles outside the longitudinal acceptance.
Therefore virtually the full beam intensity within the
transverse acceptance appears at the output, albeit with
only partial or no deceleration.
Fig. 1. Cross-section of the RFQ.
1.2 Short description of the hardware
A complete description of the system is found in [3],
of which the main points are repeated in the following.
The RFQ is of the 4-rod type, where the "rods" are
rectangular electrodes mounted at ± 45° in a symmetric,
ladder-like RF resonating and supporting structure. SPI-
RASHIELD RF contacts of 1.2 mm diameter are pro-
vided at each connection interface. The ladder is com-
posed of 34 RF cells with a total length of 3415 mm,
held in place by 5 vertical ceramic insulators, 2 on top
and 3 on the bottom. The assembly is housed in a cylin-
drical tank made of stainless steel, copper plated on the
inside to reduce the RF losses.
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 63-65.
63
Fig. 2. RFQ girder with MEBT (left), longitudinal section of the RFQ (mid), LEBT (right).
An aluminium girder of 6 m length carries the last
part of the medium-energy beam transport "MEBT"
(two diagnostic boxes and the energy corrector cavity),
the RFQ tank and the low-energy beam transport
"LEBT" (two solenoids, two steerers and a SEM moni-
tor), see Fig. 2.
1.3 Beam Diagnostics
The beam at the input of the RFQ is monitored by:
-the "watchdog": two perforated caesium-iodide scintil-
lator screens, 1mm thick, 543 mm apart, with holes of
24 mm and 17 mm respectively, which allow a 15π mm
mrad beam of the nominal divergence to pass unper-
turbed. The signals of beam tails outside this limit are
captured by two synchronised cameras and frame grab-
bers:
-two full scintillator screens that can be moved over the
watchdog screens, thus providing (destructively) infor-
mation on the transverse beam parameters,
- a beam transformer.
One secondary emission (SEM) monitor called
"HORI-box" is permanently mounted in the LEBT. It is
95% transparent and consists of a horizontal and a verti-
cal grid of 32 gold-plated tungsten wires, 20 µm diame-
ter, read out by charge-sensitive preamplifiers [4]. A
second HORI-box is provided either at the interface to
the physics experiment or at the end of a dedicated mea-
surement line.
Additional devices such as a Faraday cup or a silicon
strip detector can be mounted in place of blind flanges
that are provided in the measurement boxes or pumping
ports in the beam lines.
2 BEAM TESTS
2.1 Tests with Protons at Aarhus University
The use of a proton beam is very attractive for the
running in of the decelerating RFQ (fast repetition rate,
high intensity, and absence of annihilation, so simplify-
ing the diagnostics). Since no suitable proton beam was
available at CERN, it was decided to follow an invita-
tion to perform the initial tests at the Tandem accelerator
of the University of Aarhus/Denmark.
A dedicated measurement line consisting of a spec-
trometer magnet, two steerers and a quadrupole triplet
had been constructed. The diagnostics consisted of a
measurement box for a Faraday cup together with a
"HORI-box".
Two weeks of beam time were allocated for the RFQ
and associated systems to be run-in. Procedures for the
proper adjustment of RF settings were devised and
recipes for systematic exploration of the parameter
space were tested. Energy variation up to 120 keV was
demonstrated by bias of positive polarity on the ladder.
Owing to the deceleration, the physical dimensions
of the input beam are increased by about a factor of 3 at
the 63 keV output; the same factor applies for output/in-
put variations. In the absence of "single-shot" instru-
mentation the reliability of the measurement results de-
pends critically on the stability of subsequent pulses:
transmission measurements compare different shots in
the (destructive) Faraday cups, emittance reconstruction
relies on a series of data during a quadrupole scan. In-
stabilities of the beam and multipactoring in the buncher
prevented fine tuning and precise performance measure-
ments. An antiproton content of about 25% could never-
theless be demonstrated at the output of the RFQ (ratio
of the Faraday-cup readings at the end of the measure-
ment line and in the LEBT, respectively). Also a vertical
transverse emittance of about 60 π mm mrad at 63 keV
was measured, although for uncertain input beam condi-
tions.
2.2 Antiproton Tests at CERN
Following the positive outcome of the proton tests,
the RFQ was installed in the ASACUSA experimental
area of the AD to provide the experiment ESA [5] with
antiprotons. Useful beam was delivered quickly, thanks
to the experience previously gained with the protons.
The testline with the spectrometer magnet was initially
kept between the RFQ and ESA to allow further assess-
ment of the system performance.
Considerable pulse-to-pulse variations caused by in-
stabilities of the input beam were observed, further am-
plified by the cascading of two sensitive spectrometers
in the measurement line and in the physics apparatus.
The test line was therefore dismantled and the experi-
ment connected directly at the end of the LEBT. This
resulted in a very noticeable stability improvement, and
"publishable" results could be obtained by ESA. A low-
energy antiproton content of at least 37% was measured
at the output of the RFQ (see Fig. 3).
64
Fig. 3. Display of two HORI-boxes with antiprotons be-
hind RFQ (left) and end of test line (right).
The test period was interrupted by the 4-months
winter-shutdown of the CERN machines. Several im-
provements of the system were implemented, e.g. modi-
fication of the timing system and installation of a Fara-
day cage around the RF power amplifiers to eliminate
any risk of RF interference.
In the buncher and energy-corrector cavities, multi-
pactoring prevented stable operation in certain ampli-
tude ranges. Continued conditioning cured this problem,
although strong electron emission still persists which in-
terferes with the nearby beam diagnostics. Improved
vacuum conditions were prepared as ad-hoc prevention
of this effect.
In the RFQ, positive bias on the ladder could be ap-
plied without restriction, but negative polarity led to ex-
cessive DC current and disruption of the power supply
at nominal RF level. This was identified as multipactor-
ing, in accordance with theory predicting significantly
wider multipactoring bands for negative DC bias com-
bined with RF [6]. The HV supply was modified to cope
with high pulse currents. Peak currents of up to 5A were
initially drawn by the multipactoring at -40 kV DC, but
conditioning during one night reduced this by a factor of
six. The remaining DC bias variation is negligible dur-
ing the beam duration of a fraction of a microsecond,
therefore operation is assured over the full range of ± 60
kV ladder bias.
After the shutdown the measurement line was re-in-
stalled for a short dedicated RFQ test period. In addition
to the HORI-boxes a Lucite Cherenkov counter was a
helpful diagnostic tool to compare intensities of the
beam that was deliberately stopped at different spots of
the installation. It was shown that the two modes of AD
ejection, with short or long bunch (respectively with
high or low energy spread), had virtually no effect on
RFQ transmission.
A first attempt was undertaken to measure the inte-
grated number of low-energy particles at the end of the
spectrometer line via 24Na production by activation of a
thin Al foil [4],[7]. An average overall transmission of
about 20% was found as the ratio between the number
of particles at the end of the spectrometer line compared
to the number of particles in the AD just before ejection
at 100 MeV/c. The momentary ratio fluctuated from
pulse to pulse due to variations of the input beam and
attained 25% according to the real-time reading of the
Cherenkov counter.
The unexpectedly low transmission can be caused by
losses in the spectrometer line (only the lower part of
the aluminium foil had been found activated) and
losses/mismatches in the AD ejection line (the beam
transformer readings showed a substantially lower in-
tensity at the RFQ input than expected).
From the end of the test period till the end of May
2001 the ESA experiment regularly took beam, at RFQ
output energies down to 15 keV. Fully satisfactory re-
sults were reported [8] despite of considerable fluctua-
tions of the input beam. The RMS energy spread mea-
sured in the ESA electrostatic analyser was 5.7 ±
0.5 keV at 61.7 keV.
3 FUTURE PLANS AND CONCLUSION
Work remains to be done to reduce the electron
emission in the small cavities and to improve the input
diagnostics. Also, precise transmission data are not yet
available, and should be measured together with other
parameters as soon as the operational schedule permits.
A new challenge for the RFQ is the imminent installa-
tion of a superconducting solenoid in the physics experi-
ment. The resulting strong stray field in the LEBT, to-
gether with different requirements for spot size and
beam length, may necessitate additional studies and/or
hardware modifications.
However the validity of the decelerating RFQ con-
figuration with DC output energy variation has been
demonstrated. The possibility to compensate for energy
variations of the input beam by the energy corrector
cavity at the input of the RFQ has emerged as an addi-
tional bonus of the scheme. It can be considered as a
very promising tool for low-energy antiproton physics.
4 ACKNOWLEDGEMENTS
The realization of the project would not have been
possible without the support of the ASACUSA Collabo-
ration and the help of many different groups at CERN.
The hospitality of the Aarhus group during the beam
tests was exceptional and memorable. Special thanks go
to M. Hori for his contribution of the HORI-boxes and
continued help and interest.
REFERENCES
1. S.Maury et al. Commissioning and First Operation
of the Antiproton Decelerator (AD) // Proc. PAC-
2001 Conference, Chicago, 2001.
2. A.M.Lombardi. Accelerating and Decelerating
RFQs // Proc. Linear Accelerator Conference
Chicago 1998, p. 377-381.
3. Y.Bylinsky, A.M.Lombardi, W.Pirkl. RFQD - A
decelerating Radio-Frequency Quadrupole for the
CERN Antiproton Facility // Proc. Linac 2000 Con-
ference, Monterey, p. 554-556.
4. M.Hori, private communication (paper in prepara-
tion).
5. H.Knudsen et al. Construction of an Electrostatic
Analyzer (ESA) for Energy Loss Measurements in
the CERN AD. CERN PS/CA Note 99-23.
6. P.Ylä-Oijala. Suppressing Electron Multipacting in
Coaxial Lines. TESLA 97-21, November 1997.
7. P.Lubinski et al. // Phys. Rev. Lett. 1994, v. 73,
p. 3199.
8. H.Knudsen, private communication (paper in
preparation)
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 65-65.
65
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