JAERI-Conf 95-021

2a2-4
ACCELERATION TESTS OF THE JAERI TANDEM
SUPERCONDUCTING BOOSTER
Suehiro TAKEUCHI, Tetsuro ISHII, Makoto MATSUDA, ZHANG Yan* and Tadashi YOSHIDA
Japan Atomic Energy Research Institute,
Tokai Research Establishment
Tokai-mura, Naka-gun, lbaraki-ken 319-11 Japan
A~stract

An independently phased heavy ion linac composed of46 superconducting quarter wave resonators has been built forthe booster of the tandem accelerator at Japan Atomic Energy
Research Institute, Tokai. Several kinds of heavy ions of Sito Au have been accelerated. The performances of theresonators and the results of the beam acceleration aredescribed.

The booster is equipped with two identical refrigerators ofaaude cycle with two expansion turbines. Each system has
two loops, a liquid helium loop and a 80K gaseous helium forthe radiation shielding in the transfer line and in thecryostats.

~

10 LINAC UNITS

fJ8 ~:t a;n :1l. Introduction
An upgrade project of the JAERI tandem accelerator with asuperconducting booster has been completed after a R&D of asuperconducting quarter wave resonator from 1984[1],fabrications and testing -of prototype units composed of two
superconducting resonators from 1986 and a full scaleconstruction of the booster from 1988[2-4]. Heavy ion
beams of Si to Au have been accelerated by the tandem andthe booster for the commissioning test in 1994.
For heavy ion beams up to around Au, a bombarding
energy higher than nuclear reaction threshold has become
available at the new target room.

130MHz QWR x~ x10~
DOMHr: OWRx2

REFR1GERA1llRS

Fig. 1 Diagram of the JAERI tandem booster.

3. Cryogenics
2. Outline of the Tandem Booster
The diagram of the JAERI tandem booster is shown inFig.l. The continuous beam from the tandem is injected into
the booster. The booster starts off with a double drift
bunching system composed of two 129.8MHz QWRs and two
259.6MHz QWRs. One of the two for each frequency is used,while the other stands by.
The linac comprises ten
acceleration units, each of which contains four 129.8 MHz
QWRs in a cryostat and a quadrupole doublet lens outside.
· After the linac, there is a debunching unit composed of two
129.8MHz QWRs. The debunched beams are analyzed by adouble focusing bending magnet . The beams obtained fromthe booster are, then, 129.8 MHz CWbeams.
With respect to the beam transport system, the beam
waists should be located at the middle of the two bunchingunits, at the entrance of the linac, in the middle of the linac,
at the exit of the linac, at the object and image points of theanalyzing magnet and finally the target position in a target
beam line. At or near the waist points,· apertures or slits,
beam profile monitors and Faraday cups are placed. Beambaffle apertures of 16mm in diameter are put at the entrancesof all the acceleration units to protect resonator surfaces fromstray beams.
The longitudinal beam diagnostics are important to anindependently phased linac. Energy and time detectors areplaced before and after the linac and after the debuncher.
Those are compact scattering chambers that ions scatteredfrom a Au foil are detected by a solid state detector. Threebeam bunch phase detectors, 129.8 MHz normallyconducting QWRs, are used for the phase setting of the linac
resonators.

* On leave from China Institute of Atomic Energy

The flow diagram is shown in Fig. 2. liquid helium
passes through heat exchangers cooling four resonators,flows into the Dewar and inside the resonators and returns tothe transfer line. The pressure in the Dewar was about 0.35
kg/cm2 during operation.
Each cryostat is connected to a valve box through four
retractable transfer tubes. Remotely variable valves are usedin liquid supply lines to cryostats, and locally variable
valves in gas supply lines.
The refrigeration power necessary for the booster was
estimated to be more than 300W for the liquid loops if an rf
power of 4 W is dissipated in a resonator and 2500 W for the80 K gas loops. The refrigeration power designed for one
system is, then, 250 W for the liquid loop and 1.5 kW for the80K gas loop. In the cold box, there is a small sump of a
volume of about 10 litters. The liquid level is controlled by
the heater in the sump. The helium liquefied by the heatexchanger in the sump is supplied to the resonators. The
returning gas and liquid from the resonators flow into thesump. We obtained the refrigeration powers 10% more thanthose in the commissioning test.
The operation can be done semi-automatically. We have
COMPRESSOR
MODE,
severaloperationmodes,
QRCUIATION MODE, OOlD BOX MODE, CRYOSTAT
MODE, RECOVERY MODE and WARM-UP MODE. The cold
box and 80K radiation shields are cooled in OOID BOX
MODE before the resonators. The CRYOSTAT MODE starts
to cool the resonators when the radiation shields are cooled
to 150K. It took 2.5 days from a start of OOID BOX MODE
to cool the resonators and 12 to 15 hours to fill all theDewars with liquid.
liquid filling was excellent except anacceleration unit, no.5. The unit needed to open the variable
valve a lot more than others. The rf power dissipation waslimited to about 80W/system(4 W per resonator) for the-194-

JAERI-Conf 95-021

stable resonator operation that the helium pressure was stableor the liquid levels were kept full during the operation of all
Power of as much as 120 W was
the resonators.
supplementary dissipated by the heater in the sump forstabilizing the system.
CRYOSTATS

VALVE BOX

severe that field gradients higher than 5 MV/m were obtained
within 4 W.
The Q-degradation can be reduced by increasing thecooling rate at the precipitating temperature, around 120K.
The cooling rate from 130K to 90K was increased to about
15K/h in a test that we split the16 resonators into twogroups and used the whole gas from the cold box to cool
down one of two groups at a time over the temperature range.
The increases are also shown in Fig. 4. An increase of about
0.5 MV/m was obtained. The acceleration voltage summed
over all the resonators finally passed 30 MV.
With respect to the frequency stability, the frequency
oscillation was only a few Hz when the cryogenic system
was stable. A frequency deviation of about 10 Hz or more
happened at the time that the liquid levels fell down from
100% or came back to 100 %, and the pressure deviated byabout 0.05 kg/cm2. The rf input coupling was set to give aband width of more than 20 Hz for stable phase lock.

a:~~~~~--~~~~----,
a:
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8w-+lllnrlrH--+---.fli--fl}+f!l-fMM><I---1'0g

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lll

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108

0Fig.2 Diagram of gas and liquid flows in a cold box and avalve box.

107 w_~~~~~~~~~~~~~~~~~~
10205301502535404. Resonator Performances

Resonator number

The resonator performances were measured in an off-line
test cryostat. Fields of about 7 MV/m were obtained at an rf
input of 4 W[4, 5]. For most of the resonators, the Qdecrease due to electron field emission at high fields was not
so much that high maximum fields were obtained withouthigh power pulse conditioning. This result is due to that theresonators were cleaned well in the final surface treatment.
We have found the Q-degradation with the resonators
which absorbed much of hydrogen (an order of a few wppm)
in the electro-polishing and were cooled slowly around
This phenomenon is understood as· a120K[4,5].
precipitation of a niobium-hydride weak superconductor onto
the niobium surface[6]. The cooling rate at 120K was 10K/h
when the resonators on line were cooled by the refrigerators.
The rate is approximately one-fourth of that in the off-line
test cryostat. The Q-values at low fields measured for theresonators in off-line and on-line are shown in Fig.3. A
strong Q-degradation happened to most of the resonatorsfrom no.1 to no.16. Hydrogen absorption was not prevented
enough for them. But, the following improvement wasgiven to the resonators from no.17 in the surface treatment.
The hydrogen gas coming out during electro-polishing was
brought away as much as possible from the polishingsolution by passing nitrogen gas bubbles. The bubbling
seemed to be efficient for preventing hydrogen absorption
into the niobium for the closed cavity structure.
Fields gradients obtained for the on-line resonators areshown in Fig.4. The resonators of no.1 to no.16 needed thepresent allowable rf power input of about 4W per resonatormentioned above to obtain their maximum field gradients.
Many of them were lower than the design value of 5 MV/m.
For those from no.17 to no.40, the degradation was not soFig.3

Resonator Q-values at low fields measured for theresonators in off-line and on-line.
76E'>
6.

c54Q)

'6lll
....
(.!J'032(iiu:01 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

Resonator NumberFig.4 Accelerating field gradients obtained for the on-lineresonators. Open bars· indicate the increase of thefields after a faster precooling around 120K.

5. Resonator Control with Beam Diagnostics
Each resonator is controlled in a self-excited resonant
loop which is composed of a resonator with a variable rf
input coupler and a signal pick-up probe, a resonator control
circuit and a 120 W rf power amplifier. The rf signal and therf power are transmitted through heavy doubly shielded
coaxial cables. We use control stations made by Applied
Superconductivity Inc. , in each of which control circuits for
8 resonators are assembled.

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JAER1-Conf 95-021

For setting the buncher resonators of a1m (129.8 MHz)QWR and a 2m (259.6 MHz) QWR, the energy and time
detector located at the entrance of the linac was mainly usedin the beam acceleration tests. Next to the detector, a beam
bunch phase detector, a 2m normally conducting QWR, isplaced.
The phases of the beam bunches respectively
bunched by the 1m QWR and the 2m QWR were measured byusing the phase detector and a vector volt meter. This wasuseful for quick phase setting.
For the phase setting of the linac resonators, three beam
bunch phase detectors, 1m normally conducting QWRs arelocated after the 3rd acceleration unit, after the 6th unit andafter the last unit. Their optimum j3s are 0.08, 0.1 and 0.11
and their sensitivities are 7, 10 and 12mV/nA, respectively.
The signals are amplified by about 40dB and inputted tovector volt meters. Computer aided measurements were done
to display a curve of beam bunch phase as. a function ofresonator phase in an instant.
A beam bunch phase shift due to the change of time offlight is given as

where Lis the flight length from the resonator in operation
to the phase detector, E 0 the incident beam energy, 13 = v0/c
the incident beam velocity, q the charge state, Eacc the field
gradient, La the acceleration length, f(l3) the transit time
factor and cjl the synchronous phase. By measuring a curve of
the phase shift as a function of resonator phase, one can findthe value of the resonator phase which corresponds to f=O or
the synchronous phase to be set.
The measurements were so quickly done that this method
was suitable to repeat 40 times of resonator phase setting.
At each resonator setting, the beam energy was checked byusing the energy detector located after the linac.
With respect to the debuncher, one of the two QWRs wasenough. Phase and field gradient were set, after bending
beams 90", by looking at the beam profile at the image ofthe analyzing magnet.
6. Acceleration Test Results

We have accelerated various heavy ions of 28Si, 35Cl,
SBNi, 74Ge, 107Ag, 127I and 197Au from ~he tandem
accelerator. The results are shown in table 1. The injection
beam intensities were not the maximum.
In many cases, the total acceleration voltage was about 28
MV because a few resonators were not · used because ofproblems in control circuits. The beams of I and Au in thelast two lines were accelerated after the improvement of field
gradients by the fast precooling mentioned above. For Siand Ge, which were the latest cases, field gradients were set at
3 MV/m for nearly all the resonator. The synchronous
phases were set to the values calculated to give a goodcondition for debunching. The values were the same among
all the resonators from no. 1 to no. 40. The final energies
were in good agreement with calculated ones.
According to the beam optics[3,7], the ideal beamtransmission effiCiency is about 60 %. A satisfying beamtransmission was obtained in the cases of 58Ni20+ and
74Qe11+ but not in other cases. It seemed partly due to ourskill of the beam transport, because we had to transport thebeams through many small apertures using many quadrupole
lenses and steering magnets. For example, in the adjustmentof the 9 quadrupole doublets, the transmission could not be

improved by adjusting one by one. We finally obtained agood feasibility in the case of 74Gel1+
as a result of
simultaneously varying the field parameters of all thedoublets. There could be partly other possibilities such thatsome resonators were out of alignment.
As long as the rfload to the cryogenic systems was within
the limit and the liquid levels in the Dewars were kept full, all
the resonators were locked in phase and stable beams wereobtained for a long time with a phase stability of about:t0.5".
Table.1 Results of beam acceleration tests
Ions

Energy(MeV) Current(nA) Total
Synchronous
initial final
in outacceleration
phase
voltage(MV)(deg)

28Sjl0+ 180 327
35CJ10+ 164 351
35CJ14+ 164 446
58Ni20+ 190 628
58Ni20+ 190 658
74Qell+ 180 326
107Ag25+ 231 798
127}27+ . 225 812
127J27+ 225 880
197Au25+ 340 912

80 20
130 4080 1680 30
170 100
110 6045 15204
100 23
193

17.3
24.6
27.6
28.2
27.7
17.3
27.6
28.3
30.2
30.7

-22-30-25-30-18-35-21-25-18-227. Conclusion

The superconducting booster for the JAERI tandem
accelerator was tested with many species of heavy ion
beams.
A total acceleration voltage of 30 MV was obtained asexpected, although a severe Q-degradation occurred withmany resonators. The cryogenic systems worked well. The
rf load was limited to about 4 W per resonator in stable
operation. The measurement of beam bunch phases wassuccessfully done by using beam bunch phase detecting
resonators. The beam transmission was satisfactory in a few
cases. It is promising to improve the transmission forothers.
Various heavy ions from C to Au can be available withenough energy for nuclear reactions from the tandem booster
from now on.
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Superconductivity,Argonne, 1988, pp429-434.
[2] S. Takeuchi, T. Ishii and H. Ikezoe, Nucl. lnstr. andMeth. A281(1989)426.
[3] S. Takeuchi, T. Ishii, H. Ikezoe andY. Tomita, Nucl.
Instr. and Meth. A287(1990)257.
[4] T. Ishii, M. Shibata and S. Takeuchi, Nucl. Instr. andMeth. A328(1993)231.
(5] M. Shibata, S. Takeuchi, T. Ishii, S. Kanazawa, T.
Yoshida and Y. Sugiyama, Proc. of 6th Workshop on
RF Superconductivity, CEBAFNewportNews, 1993,
p895-899.
[6] B. Aune, B. Bonin, J .J. Cavedon, M. Juillard, A. Godin,
C. Henriot, Ph. Leconte, H. Safa, A. Veyssiere and C.
Zjlberajch, Proc. 1990 linear Accelerator Conf.,
Albuquerque, IA-12004-C, pp253-255.
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Minehara, JAERI-M 88-155.

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