JAERI-Conf 95-021

PA14
COLD TEST OF A 25. 5 MHz DOUBLE-COAXIAL.A. /4RESONATOR

Y.Hashimoto, H.Masuda, K.Yoshida, S.Arai and K.Niki
Institute for Nuclear Study, University of Tokyo
3-2-1 Midori-cho, Tanashi-shi, Tokyo 188, Japan

Abstract
A 25. 5 MHz A I 4 coaxial line resonator has been
constructed. The resonator has a double-coaxial structure
and the total length is ·as short as 1. 3 m. Drift tubes
mounted at the open end of the central conductor compose
six acceleration gaps. This resonator is to be used as arebuncher in a linac complex for the acceleration ofunstable nuclei. The cold test on the completed resonatorhas been made and the result is consistent with the halfscale model tests.

1. Introduction
A linac complex for the acceleration of the unstable
nuclei produced by cyclotron beam bombardment is underconstruction at Institute for Nuclear Study, University ofTokyo [1]. The ions from ISOL with charge-:to-mass ratio
1130 at minimum will be accelerated first by 25.5 MHz RFQ
linac of split-coaxial type [2] up to 170 keVIu and, after
charge stripping, will be further accelerated by 51 MHz
interdigital-H linac [3] up to 800 keYlu. The reason of
the doubled operation frequency for the interdigi tal-H
linac is to make the tank size small and to obtain higher
shunt impedance. This configuration, however, requires a
rebuncher between two linacs to adjust the longitudinalbeam-erili ttance to be efficiently accelerated by the rearlinac [4]. The rebuncher works also to compensate theenergy loss at the charge stripper just behind the RFQ
linac, about 7 keVIu. Since a linearly varying electricfield is desirable for the emittance adjustment, theoperation frequency of the rebuncher should be 25. 5 MHz
rather than 51 MHz. The necessary peak voltage at therebuncher is about 200 kV [4].
The rebuncher with these specifications may beprepared based upon the AI 4 transmission line resonatorwith a few acceleration gaps. The natural length of the A
I 4 resonator for the frequency of 25. 5 MHz, however, isvery long, 2. 94 m. To make the tank size compact, we canemploy the so-called spiral resonator, even in which thelength of the inner conductor stays to be the same, and wecan not expect the mechanical stability of the drift tube
to be installed at the open end of the conductor. Thus wehave developed a double-coaxial resonator, which is afolded coaxial line, and in which the outer conductor of
the inner coaxial line is concurrently the inner conductorof the outer coaxial line. Thus the length is, inprinciple, almost the half of the natural line length.
Furthermore it is known that the resonant line length of
the transmission line which is composed of two parts
having different characteristic impedances becomes shorter
than the one for uniform transmission line [5-61 The
capacitance accompanying the drift tubes also contributes
to shorten the resonant line length.
By using a half-scale model, we have determined thedetailed design dimensions of the resonator [5], and haveconstructed a real resonator. The length from the bottomof the resonator to the center of the drift tube is about

1. 04 m, which is short enough to install the resonatorbetween the floor and the beam level.
The outline of the completed rebuncher is describedin section 2 and the result of the cold tests is given in
section 3.

2. Construction of theA. I 4 Resonator

In Fig. 1 is shown the cross section of the 25. 5 MHz
double-coaxial it I 4 resonator constructed as a rebuncher.
The dimensions of the resonator and the drift tube
parameters are given in Tables 1 and 2, respectively. Dueto the floating capacitance around the drift tubes, about
37 pF, the resonant transmission line length is shortenedby 30%. Further reduction of the length is realized byincreasing the difference in the impedances between theinner and the outer coaxial lines. Since the diameter ofthe outer coaxial line is limited to the extent of thesix-cell drift tube length. we have to shorten thedistance between the intermediate and the centralconductor. To lighten possible multipactoring, the central
conductor is a square· pillar while the intermediate
conductor is a cylinder. Thus the impedance of the outercoaxial line is 1.8 times as large as that of the innerconductor, which brings 19% reduction of the resonanttransmission line lengt~ These effects in addition to thefolding of the transmission line enabled us to construct
a rebuncher with a limited lengt~

A-A'

8-8'

Fig. 1 Cross section of the 25.5 MHz rebuncher.
The tank is composed of an upper lid, an outerconductor cylinder and a bottom flange. The intermediate
conductor is supported between the cylinder and the lid.
They are assembled by use of sealing gaskets and rfcontactors. Drift tubes of earth potential are mounted
inside the upper lid, on which are also furnished anevacuating port, a capacitive tuner, a vacuum gauge port
and a viewing port in addition to entrance and exit beam-61-JAERI-Conf 95-021

Table 1The dimensions of the 25.5 1Hz rebuncher
Central Conductor
60x60x905 mm
Inner Diameter of the Intermediate Conductor
150 mmThickness of the Intermediate Conductor
5mm
Inner Diameter of the Outer Conductor
690 mmCharacteristic Impedance of the Inner Coaxial Line 48 Ohm
Characteristic Impedance of the Outer Coaxial Line 88 Ohm
Length of the Double-coaxial Structure
850 mmTotal Inside Length of the tank
1170 mm
Height of the Basement
108 mmDistance from the Floor the Drift Tube Center
1175 mmof these tuners into the lower tank reduces the tankvolume and will increase the resonant frequency. Thecapacitive tuner is also a cylinder block having smaller
diameter of 56. 8 mm. Insertion of this tuner towards adrift tube with high potential increases the capacitanceof the drift tube and will decrease the resonant frequency.
When possible resonant frequency deviation of completed
resonator does not exceed the capability of the adjustable
reactive-tune, we need no other tuners. After the cold
test on the completed resonator, the insertion length for
any of the fixed-type tuner will be specified if necessary.

3. Cold Tests of the Resonator
Table 2The drift tube parameters of the rebuncherNumber of Cells
6Unit Cell Length112.3 mm
Gap Distance
20 mm
60 mm
Inner Diameter
Outer Diameter
100 mmDrift Tube Edge
10 R
Fabrication and Setting Accuracy
0.1 mmTransit Time Factor
0. 84

ports. The outer conductor cylinder is equipped with an rf
coupler, an rf monitor, a fixed and an ·adjustable
Since thereactive-tuners and vacuum gauge port.
evacuation is made only from upper part of the tank, manyholes are drilled in the disk supporting the intermediate
conductor and separating the tank volume.
Three drift tubes mounted at the open end of thecentral conductor together with the ones attac~ed to theupper lid compose six acceleration gaps.
Thisconfiguration has following significance: (1)Moderatecapacitance is introduced to shorten the resonanttransmission line length, (2)Necessary voltage of 200 kV
in total is shared to six gaps and the voltage required at
the open end of the central conductor is only 33 kV, (3)60mm bore of the drift tube allows for the beam to satisfy
the beam optics requirement without focusing element
inside drift tubes. Measurement on the half-scale model
proves that the electric field distribution in each gap. isalmost equal in spite of rather asymmetric structure ofthe drift tube support.
Measurement on the model resonator indicates that thenecessary rf power to generate 33 kV at the drift tube
will be about 1 kW. The power loss distribution in theresonator has been studied by use of the computer code
SUPERFISH. The result of the calculation shows that thepower loss is concentrated on the central and theintermediate conductors, especially at their lower parts.
Thus one cooling water channel is equipped inside thecentral conductor pillar. Since the power loss at thedrift tubes and their support is quite small, the water
channel runs only in the pillar. Another cooling water
channel is wound on the intermediate conductor getting inand out through the supporting disk edge with a thicknessof 22 mm. The third water channel trails on the tanksurface whose purpose is to keep the temperature of thetank constant.
The capacitive tuner and two reactive tuners, fixed
and adjustable, will work as follows: The reactive tuners
are both cylinder blocks of 188 mm in diameters. Insertion

A. Resonant Frequency
We expected for the completed resonator to have theresonant frequency lower than 25. 5 MHz by some tens kHz,
which may be brought to proper frequency only by use ofadjustable reactive-tuner. The resonant frequency without
any tuner, however, resulted in rather high frequency,
25.52 MHz. This may be partly due to the installation of
the cooling water channel inside the tank, which reduces
the tank volume and increases the resonant frequency. Thus
we have to use the capacitive tuner to bring the resonant
frequency into the range covered by the adjustable
reactive-tuner, which is shown in Fig. 2. As is seen in
the figure, the tunable range is about 100 kHz. The
characteristics of the capacitive tuner is shown in Fig.3.
The resonant frequency drastically decreases when thetuner approaches the drift tube closer than 100 mm. We-62-

25.62 I"''"'I""'""'......"''"'T'.......,.,..,...,..,..,..,.......T"''""'I""'""'......"''"'T'.........,

•

25.6
25.58
......
N::J::::E........

...

~

25.56
25.54
25.52

········t········j···········t··········:··········i··········j·········t········

11: II !I I--~--t---------r------·t----------~----------~----------~----------t--------

25.5L.&..,..............""""-........"""-'....................L.&..,j.................................
~
0
W ~ W 00 1001W1~

Insertion

Length[mm]

Fig.2 Characteristics of the adjustable reactive-tuner.
2 5. 55 r"""''........T"'"".......'T"'".......'T"".,..,..PT"'.......'I"'T"......."''T...........,.

,. • .

•.

•!,.

•:i•

25 -5 r-- -------r·-~···r--------·r·----------r----------r-----------:·--------!·''··' rJ i···i·······i l·l·······
25.4---------r---------T----------r--------r·--------r--------r-------•i i i ii

Gap Width[mm]

i25.35 ........................................................o.~..~..........................................
0.
50
100 150 200 250 300 350

Fig.3 Characteristics of the capacitive tuner.

JAERI-Conf 95-021

decided the distance between the tuner edge and the drift
tube to be 50 mm, the resonant frequency corresponding towhich comes into the middle of the tunable range by theadjustable reactive-tuner.
B. Q-values
The unloaded Q-value is about 6,000, which isconsistent with the value expected from the measurement on
the half-scale model made of Brass [5]. We have measured
Q-values under the. various conditions both for thecapacitive and reactive tuners. The result shows there isno significant change in Q-value.
C. Longitudinal Electric-field Distribution
The longitudinal electric-field distribution along
the drift tube axis as measured by bead perturbation
method is shown in Fig. 4. The bead is made of aluminum,
the diameter of which is 10 mm. One can see that thefield strength in each gap is almost constant, as expectedfrom the measurement on the model.

::A:·········A·········[ ······+ft·····-\-··ft···+··-1,-··[······A··r··.f.\. . . . .!......... -!h. . . :. :..,. . -! . . ,.l..t. .... Il.~n ·:••: ••· •!•i •::••.: :···::' • : ' • •.. , ........ .
:. •• I• •:: • •• •4. Conclusion••:: :::f : : : -: ; : 1 · f -::• ;1: I•·•i :i : I • • i : I : : I : : : .
• ..,...........' .. ....,.,, .. ,l -~· ····'I ,:I=···=·I ' •= ,.....
I 1:
•

..

1· · · ·-~~·····'\··i······'~··-i--··~/:~···f·.I ·:· · -I : · ..•.j. · ' .,,.,J_.......J ···\:,· · ·0·t···· ···:::100zoo'300

400-500:'600

700800[mm]Position

Shunt impedances on and off the axis are shown inFig. 5 to show horizontal and vertical position
dependence. The shunt impedance on the axis is 20. 3 MQ/mwhich is close to the value expected from the model
measurement. The reason why the shunt impedances .become
higher at the positions far from the center is that theelectric field including its radial component is higher
there than on the axis. Our purpose of this series ofmeasurement is to examine possible asymmetry of the field
distribution in the drift tube bore. One-sided capacitive
tuner may affects the field ·symmetry along the horizontal
direction, and the vertically asymmetric structure of thedrift tube support may introduce asymmetry in verticaldirection. As can be seen in the figure, there is no
asymmetry in the horizontal direction, and we can say thatthe capacitive tuner gives no effect on the field
distribution in the drift tube bore. On the other hand,
obvious asymmetry in the vertical direction is seen. The
origins of this asymmetry may include the effect of thedip of the bead trajectory due to the gravitation. Further
examination will be made by using computer code MAFIA.

In constructing 25.5 MHz A/4 resonator, we have
employed a double-coaxial structure, which enabled us tomake the total resonator length as short as 1. 3 m. The
results of the cold tests ·on the resonator are almost
consistent with the measurements on the half-scale model
made of brass. Preparation of both capacitive and reactive
tuners is quite efficient to adjust the unexpectedresonant-frequency shift. Next step will be to excite theresonator with high rf power. Since multipactoring is moreor less expected, a pulsive aging system is prepared. BeamacceleFation test is scheduled at the end of 1995 fiscalyear.
REFERENCES

Fig. 4 Longitudinal electric-field distribution.

IeD

VerticalHorizontal

30~~~~~~r~~~~~~~~~~

25

(/)

N.. ------------r---------- ----+---------------{---r---------[1] S.Arai et al .• Proceedings of the 1995 Particle
Accelerator Conference, Dallas, Texas. U.S.A.
[2] S.Arai, et al., Proceedings of the 1994 International
Linac Conference P. 725.
[3] M. Tomizawa et al .• Proceedings of This Conference.
[4] K.Niki et al., Proceedings of the 1994 International
Linac Conference P. 725.
[5] K. Yoshida et al .. Proceedings of the 1994
International Linac Conference P. 771.
[6] 0. Kamigaito et al., Proceedings of the 9th Symposiumon Accelerator Science and Technology P.220.

......•..............! . . . . . . . L. . . . . .

20-20f::

01020

Position[mm]-1030The
Fig.5 On- and off-axis shunt-impedances.
horizontal and the vertical positions are measured
from the drift tube center to the capacitive tuner.
and to the upper direction, respectively.

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