SUPERCONDUCTING CAVITY BEAM TEST IN THE TRISTAN ACCUMULATION RING
Shuichi NOGUCHI, Takaaki FURUYA, Kazufumi HARA, Kenji HOSOYAMA, Yuzo KOJIMAShinji MITSUNOBU, Toshiharu NAKAZATO and Kenji SAITONational Laboratory for High Energy Physics
Oho-machi, Tsukuba-gun, Ibaraki-ken, 305, Japan
Horst HEINRICHS
University of Wuppertal, Departments of Physics
D-5600, Wuppertal, West Germany
INTRODUCTION

EXPERIMENTAL SETUP

A three-cell 508 MHz superconducting niobium accelerating structN'e has been tested in the TRISTAN
Accumulation Ring 1 . The objective of this beam test
was to demonstrate the feasibility of using superconducting cavities in the TRISTAN Main Ring. One of themost difficult problems associated with the use ofsuperconducting cavities is to dump beam induced higher
order modes (HOM's) sufficiently so that these HOM's do
not cause instability and additonal He loss due tomulti-turn effect. Although the number of the cavity
installed in the TRISTAN Hairr Ring will be several
tens, the difference in the machine parameter andoperating condition is such that this test places morethan 10 times severe constaints on the HOM damping
Twocomparing with the operation in the Main Ring.
other important components in practical use, a tuner
and an input coupler, were not designed satisfactorily
but tentative ones were used.
Although some technical diffeculties were encountered, the feasibility of using the superconducting
cavities in the Main Ring has been demonstrated by thisbeam test.

CavityThe fabrication and the performance of the cavity
is reported by Furuya in this proceedings. The cavity
was fully equipped with an input coupler, three HOM
Thecouplers and a monitor coupler for each cell.
cavity Q value was measured in the beam line at several
field gradient (Eacc) by measuring the He consumption.
For example Qo of 7.9 x 108 at Eacc of 3.7 MV/m was observed. The maximum Eacc was 4.3 MV/m and was limitedby heating around the input coupler port.
Input coupler
The input coupler (Fig. 1) is set on the equator
The input power is suppliedof the center cell.
Inner andthrough a waveguide and a coaxial T-stub.
outer conductor are mostly made of copper plated stainless steel to reduce heat flow from the outside of thecryostat, and cooled by He gas flow. Unfortunately thecrack was formed at the head of the coupling loop
during the first beam test, then it was changed to acopper pipe. As a result Q value of the input coupler
became 1.3 x 10 7 from the former optimum value of 2.2 x
106.

HOM coupler
Two types of couplers made of niobium were used.
A loop type coupler (Fig. 2) with a filter for the accelerating mode was set on the equator of one end celland two antenna type coupler (Fig. 3) were on the otherend cell. They are oriented to ± 45° from the verticaldirection. The polarization of dipole modes are determined by the input coupler port or by the antenna
couplers. Q values of the input and HOM couplers were
measured for longitudinal modes of frequency up to 1.5
GHz and 1.3 GHz for the other modes. The measured frequency and the loaded Q are listed in Table I and II,
where two modes of low coupling impedance, TE !21 andTM112• are neglected. Q value of the loop HOM coupler
for the accelerating mode was 1.5 x 10 9 •B

Ceramic window
N2 gas cooling pipe
Upper plate of cryostantHe gas cooled coupling
loop
CavitycTuner
A mechanical and two piezo electric tuners wereBoth are such types as change the total
prepared.

H. 0. M. Antenna

H. 0. M. Loop Coupler

Fig. 177 D input coupler.

Fig. 239 D loop coupler.
124

CouplerFig. 339 D antenna coupler.

length of the cavity. The mechanical tuner can change
the total length by± 1.7 mm corresponding to the frequency change of ± 250 KHz with smallest step of 7. Hz.
But the system has backlash of 0. 5 to 2 KHz dependingon the tuner position, so it was used only for pretuning. The piezo tuner is made by stacking 100 disksof piezo electric ceramics and is 72 mm in length and16 mm in diameter. The maximum displacement is 37 ~mwith rated voltage of 400 V corresponding to the frequency change of 3 KHz.

The maximum field gradient was 4.3 MV/m and thewas accelerated to 5.2 GeV by the superconducting
cav1ty alone. The field was limited not by the cavity
itself but by slow heating at somewhere around theinput coupler port. No processing was needed to get
this maximum field. This field gradient is highest in
those of the sup')rconducting cavities tested in thestorage rings2•3•4 .
b) The maximum single bunch current stored at 2. 5GeV was lOrnA, which was limited by heating of therubber gaskets of the gate values at both ends of thecryostat.
The maximum power transferred to the beam wasc)
measured by accelerating the beam to 4 GeV. The maximum current held was 4 rnA corresponding to the powerof 4 KW. This was also limited by the heating aroundthe input coupler port.
d) HOM coupler worked well but the development of
the input coupler is needed.
In spite of highe) The piezo tuner worked well.
loaded Q value, a single bunch of 8. 7 rnA was stored
using the piezo tuner and the fixed driving frequency.
f) Additional He loss by the beam was measured withthe beam of 10 rnA held at 4 GeV by normal cavities. It
was not detected.
a)

bea~

Cryogenics
Figure 4 shows the horizontal cryostat for thethree cell cavity. The design was especially restricted by the limited height of the ceiling of the tunnel.
Static loss of the cryostat is 12 W and the loss of thecavity is for example 40 W at Eacc = 4 MV /m, so thetotal refrigeration power of about 65 W is needed
including the heat loss of the He transfer system:
Cool ~own from the room temperature to the He temperature 1s done by the refrigeration mode in which therefrigerator is directly connected to the cryostat, it
takes ~ 36 hours. During the beam test, liquid He istransfered from a 1000 liter reservoir. The pressure
fluctuation in the He vessel was about 1 mb. A detailed description of the cryogenic system will be reportedelsewhere.

HOM study
HOM modes were studied with a single bunch beam
stored by normal cavities at 2.5 GeV. Many modes werescanned by the mechanical tuner according to the diagram shown in Fig. 5 and 6. To confirm the resonancecondition, the spectrum of the HOM coupler output wasmonitored. The beam instability was monitored by theTV image of the synchrotron radiation light. Attenuation of cables and other RF components was calibrated
as a function of freqeuncy.

BEAM TESTGeneral
At the end of May 1984, an electron bunch of 4. 2rnA was captured and stored at 2.5 GeV using superconducting cavity alone. Then the beam was acceelrated to5 GeV with the field gradient of 3.5 MV/m. The field
gradient was increased up to 4.1 MV/m. But in waiting
the beam, a vacuum leak developed at the input coupler.
The He gas flow to cool the inner conductor was insufficient and the coupling loop was heated up and got
a crack. While the input coupler was being repaired,the cavity was rinsed by HF, pure water and methanol.
The second beam test was done at the beginning of July
1984. First of all, the field gradient was calibrated
at 5 field gradient by measuring the synchrotron freThe calibrated field gradient was 3 ± 1 %quency.
higher than that evaluated using the transmitted powerof monitor couplers and their external Q value from low
power measurements. Normally the tuning was not doneby the tuner but the master oscillator followed theresonance freqeuncy.
The results obtained during the beam test aresummarized as follows.
VACUUM RELIEF

Longitudinal modes
Six longitudinal modes were brought to revolution
harmonics. The measured HOM losses normalized to 10 rnA
are listed in Table I. The calculation of HOM losses
are done with R/Q given by SUPERFISH, the measured QL
They agree
and assuming a bunch length of 1. 2 em.
The current dependence of HOM loss
within factor 2.
was measured with TMo2o-TI/2 mode and showed good I2
dependence.
Robinson instability was also examined at theupper fs sidebands of the revolution harmonics. The
instability was not observed except for the accelerating mode. The bunch lengthening was not measured.

770 COUPlER......
Fig. 4

Horizontal cryostat.

125Transverse modes
The beam instability were examined with a single
bunch of 6 rnA and the accelerating field of 2 MV. As
is shown in Fig. 6, many modes were brought to thelower betatron sidebands of revolution harmonics. The
results are listed in Table II. The beam instability
was observed only by TE 111 -rr mode (721.3 MHz) with vertical oscillation of the TV image without beam loss.
This instability was also observed at a current of 3 rnA
but the dependence on beam current and accelerating
field was not measured. The spectrum of the HOM coupler output is shown in Fig. 7, where the lower verticalbetatron sideband is much enhanced. The threshold currents for the transverse coherent dipole mode werecalculayed using the formula given by Suzuki an1
Yokoya5 , wher the measured damping time of 15 msec 6
The transverse
for betatron oscillation was used.
coupling impedance was calculated by URMEL and wasassumed to be reduced by factor cos45° for the modes
The disagreement ispolarized to the HOM coupolers.
rather large (or small ?) and more systematic measurement is needed, for example, the measurement of thetransverse impedance and damping effect due to othertransvers modes.

Table 1 Longitudinal modes and HOM loss
Freq.
(MHz)

MODE
™o1oorr/2

Tr™o11orr/2

Tr™020 0rr/2
Tr™o21 orr/2
Tr

R/Q
(Q)

504.82 < 1. 3
507.34 < 0.006
395
508.58109953.0613.4
946.992.6
941.600.13
1079.900.13
1088.570.05
1096.240.01
1421.831. 50
1384.652.94
1369.84

Table II

Ploss
meas. (w)'V

1 Xl, 3Xl, 8l, 4X
X

7,4 X
l, 3X

4,2 X
9.9 X

Transverse higher order modes
Freq.
(MHz)

R /Q
(Q/m)

TE111 0+ 45°

689.74689.78702.09702.43721.40721.68

40

- 45°
- 45°
+ 45°
- 45°
+ 45°
Ver.
Hor.
Ver.
Hor.
Ver.
Hor.

749.67750.66746.01746.25733.70734.4373+ 45°
- 45°
Ver.
Hor.
- 45°
+ 45°

1034.26
1034.561039.94
1040.481057.36
1058.05

Hor.
Ver.
Ver.
Hor.
Ver.
Hor.

1140.34
1141. 101122.901123.391161.901063.44™110 0rr/2
rr™lll 0rr/2
TrTEl12 0rr/2rr***

0.68 off reson.
0.17 off reson.
18.3
31.7
24.4
45.8
17.9
17.5
0.001 off reson.
0.11
1.0
5.57
4.3
26.0
17.9

3.3 X
3.2 X
7,0 X

Polari.

Tr(w)

5,6 X

Mode

rr/2

Peale,Thresholdcurrent (rnA)

4,3 X 105
3.2 X 10 5
9,3 X 10 4
2,3 X 10 5
1.8 X 10 5l, 1 X 10 5

4.1 *
4.0 *
1. 8 *
1. 6 *
1.8 **
6.2 *
2.5
3.3
3.0 *
1.2
4.3
7.2

10 5
2,1 X 105
6,8 X 10 4
7,3 X 10 4l, 1 X 105
1.2 X 10 5

0.5
1.1 *136 *125
26.6
23.86,6 X 10 5
6,8 X 10 5
2,1 X 105
2,4 X 105
2,8 X 105
8,2 X 10 4290
21227499

4,45118.739X

8.2 X 10 3
8.1 X 10 3
2,8 X 10 42, 8 X 10 4l, 1 X 105
8,4 X 10 4

1.2
7.3
58

ACKNOWLEDGEMENTS
The authors wish to thank members of machine shop
for the construction of cryogenic system and making
models of couplers, members of RF group for their help
in tuning the RF system and members of Vacuum group forWe also wish to thank Professors T.
their support,Nishikawa, T. Kamei, S. Ozaki and Y. Kimura for theircontinuous encouragements.

REFERENCES1)
2)
3)
4)

400
13.3
16.8

***5)
6)

Brought to betatron sidebands
Vertical oscillation without beam loss

Y. Kojima, Proc. 2nd Workshop on RF-Superconductivity, CERN (July 1984).
R. Sundelin et al., IEEE Trans., NS-30 (1983) 3336.
W. Bauer et al., Nucl. Instr. Meth., 214 (1983)
189.
P. Bernard et al., Proc. 12th Intern. Conf. on HighEnergy Accelerators, Fermilab (1983) 244.
T. Suzuki and K. Yokoya, Nucl. Instr. Meth., 203(1982) 45.
T. Ieiri and K. Yokoya, TRISTAN DESIGN NOTE,TN-84-005 (1984), Internal Note in Japanese.

Tuner Effect on f 0 ILDngitudinall

CTR:

721.3MH~

SPAN:

REF:- 23dBm

182kHz/lOdB/

InI
I~Jj.ll j ll
RBW:

10kHz@t

907 x frev.

VBW:

30kHz@r

IIIn

SWP:

tt ~.
lOmS/@.n
ATT:lOdB@

908 x frev.

TE111 - rr Mode·200o-roo
Tuner Position CounterFig. 5Longitudinal modes.

roo·200

Fig. 6-roo
Tuner Position Counter

Transverse modes.
126

rooFig, 7 Spectrum of a HOM coupler whenvertical oscillation was observed.