SPLIT COAXIAL RFQ LINAC WITII t".ODULATED VANE

S. Arai, T. Fujino, T. Fukushima, E. Tojyo, N. Tokuda
Institute for Nuclear Study, University of Tokyo,Tanashi-shi, Tokyo 188, Japan
T. HattoriTokyo Institute of Technology,Ohokayama, Meguro-ku, Tokyo 152, Japan
electrodes is of advantage with respect to decreasing
the resonant frequency but is of disadvantage withrespect to increasing the rf power loss.

ABSTRACTA new type split coaxial RFQ structure withmodulated vanes is developed for a very heavy ion RFQ
linac which can accelerate ions with charge to massratio more than 1/60 from 1.68 keV/u to 45 keV/u atfrequency of 12.5 MHz. This structure is reduced to ascale of 1/4 the very heavy ion RFQ linac and isoperated at 50 MHz. The structure consists of four
cavity modules divided by three sterns supporting
horizontalandvertical
vanes
periodically andalternately. The structure is 2m in length and 0.4 min diameter.

Split Coaxial Cavity

2xf..../4 TEM Cavity

INTRODUCTION
A split coaxial ( S.C. ) RFQ linac operated at
frequency of 50 MHz is in course of development atInstitute for
Nuclear Study ( INS ) in order toconstruct the very heavy ion RFQ linac. While theoperating frequency is required to be as low as 12.5 MHz
in the case of acceleration of uranium ions of energy as
low as a few keV/u , it is expected that the basic andtechnical problems in the S.C.
RFQ accelerating
structure can be solved by the fablication and tests of50 MHz structure.
In order to reduce the tank diameter of very heavy
ion accelerating structure, S.C. RFQ structures haveFrankfurt 2 ,been proposed and developed at GSI 1
Argonne 3andso on. Compared with these RFQstructures, the structure developed at INS hasafollowing feature: In the S.C. RFQ structure proposedand developed so far, the opposite electrodes are fixedand electrically grounded at one end of cavity and otheropposite electrodes are fixed and grounded at other endof cavity. That is, all electrodes are supported atonly one point. On the other hand, in INS
typestructure, all electrodes are supported at several
points by adopting the multi-module cavity structure 4 .
In the present work, the modulated vane is to befabricated as a structure of electrode for the reason
that the accelerating and quadrupole fields produced by
the electrode are expressed exactly by a simple formula
5 and that effective use of our experience in a fourvane type RFQ linac · LITL " construction is made.
The main purpose of this developement is to verify
whether good mechanical stability and ease of the vanesetting are obtained by supporting the modulated vanes
at several points and whether voltage flattness along
the beam axis is satisfactry. Furthermore, that is toestablish accelerator techniques to be able to construct
S.C. RFQ linac of 12.5 MHz at the next step.

~U(z)

C/'"
Fig. 1. Principle of the split coaxial resonant cavity.
BEAM DYNAMICS DESIGN

Supposing to inject the beam into synchrotron, aprofile of vane has been designed. Since the beam is
not required to be so intensive, 238u4• has been chosen
as a heaviest ion to be accelerated.
Table 1
RFQ Parameters of 238tf .. RFQ and Its 1/4-Scaled Machine

Charge to mass ratio

Frequency (f)0.06723
50t'fu1.68- 45

keV/u

1.8

1.295

Intervane voltage (V)164.641.1

Focusing strength (B)4.5-Q.100

4.5

Normalized emit lance (£N)

Max. defocusing strength (4,)
Synchronous phase ( cp s )--oo- -30

Max. modulation (m.ax )1.83

Number of cells
Vane length

Mean bore radius (ro)Min. bore radius (a.in )Margin of bore radius (O.-in /Clbea.)(0 em4)
Transmission(5 em4)(10 em4)

180

800
1.941
1.349
2.098
gr

0.015

-o.1oo-90- -30
1.83
180
200
0.485
0.337
2.0gr

1rem· mrad

kVdegememem%

73%%

94

Module length20049.4em

Radius of cavity

8020emAve. radius of inner conductor

205em

Electrode capaci lance

150150pF/m

Rosonant resistance per module
Unloaded Q value
rf power per module

209

209

kll

4850

4850

65

4.0

44260

16.2

Number of Modules

Total power

92

0.01681
1.68- 45
0.06

Kilpatrick factor

Schematic diagram of the split coaxial resonantcavity is shown in Fig. 1. This resonator is considered
to be a modification of the re-entrant cavity of TEM
mode. By spliting electrodes and by setting eachsplited electrode into each split, constant voltage is
produced between adjacent electrodes. The potential
difference between the electrode and the outer conductorof cavity is shown in Fig. 1.
The general merits of S.C. cavity are summarized asfollows: 1) The diameter of the tank becomes small. 2)
There is no deflection mode near the fundamental mode3) Voltage flatness along the beam axis is inherent
without tuning. However, the high capacitance between

1/4--Scale

12.5

Kinetic energy (T)

SPLIT COAXIAL RESONANT CAVITY

2381f>

kllk\1

The length and diameter of each module aredetermined by compromizing with following demands: toobtain the higher
resonant
resistance,Rp,fordecreasing the rf power loss, to make short the interval
between the supporting points of vanes for getting themechanical stability and to make small the radius ofcavity for decreasing construction cost. The relation
between the cavity radius, rb, and the module length, L,
is obtained by determining the electrode capacitance, C,
the radius of inner conductor, r 0 , and the stem width,

The injection energy from ion source into RFQ issuppressed as low as possible in order to make use of
the merit of RFQ linac sufficiently.
Output energy of 45 keV/u has been determined fromthe condition that the field gradient of focusing
quadrupole magnet which is used in the drift tube linacof next stage, is 10 kG/em at maximum. Vane length of 8
m is based on the tank design of the multi-module cavity
structure.
By fixing the output energy and the vane
length as above, vane parameters have been searched soas to increase the output beam current as much as
possible.
Two computer codes GENRFQ and PARMTEQ havebeen used to determine the vane parameters.
In the design of radial matching section, it has tobe considered that the potential on beam axis is notzero but one half of the potential difference between
electrodes. In order to obtain the required quadrupole
field, the geometrical arrangement of the four vanes inthe transversal cross section is asymmetric with respect
to the beam axis. The length of radial matching section
is chosen so that the acceleration of particles isnegligible on all phases.
The design parameters of the 12.5 MHz RFQ structurefor 238u4+ acceleration are compared with that of the 50
MHz RFQ structure in Table 1. The 50 MHz RFQ structure
can accelerate ions with the charge to mass ratio morethan 1/15. In the case of proton acceleration, rf powerof about 40 W is sufficient.

w.

r = 0.15 ma

C = 122 pF

C = l66 pF

MULTI-MODULE CAVITY STRUCTURE
A fundamental diagram of the multi-module cavity
structure is shown in Fig. 2. The cavity consists ofcavity modules divided by two stems. One module
Horizontal andcorresponds to one resonant cavity.
vertical vanes are suppoted by the verticalandhorizontal stems every one module periodically andalternately.
In the case of four-modulecavity,
horizontal and vertical vanes are supported at two orthree points, respectively.

III\100 1.0r-----------,_-'~----~__,

LOOP COUPLER

I.

1.0

2.0

1.5
L( m)

Fig. 3. The relations of cavity parameters.

CAVITY DIAMETER = 40 CM
FREQUENCY= 135.63 MHz

UNIT MODULE

Fig. 2. Illustration of multi-module cavity structure.
Since each module is coupled by the vanes and stems
with each other, the cavity is excited in a x-mode where
the phase of field changes every one module by x .
Inthis structure, diamond-shaped plate beams ( triangularplate beams in both end modules ) are used in order toimprove the voltage flatness along the beam axis and inorder to strengthen the support of vanes. Since theshape of the beam is simple, the manufacturing andassembling of electrodes are also easy.

Fig. 4. Cross section of the four vane cavity.

93

-.=L-1--

Fig. 5. 50MHz S.C. RFQ cavity structure.
copper. The resonant frequency of each moduleisadjusted with a loop tuner.
This 50 MHz RFQ structure is manufactured atmachine shop of INS. Vane disign has been carried out
withthe computer Ml80-II-AD of the INS computer
facility.

In order to examine the contribution oftheelectrode capacitance and stem width to the length anddiameter of a module, the relations are given in Fig. 3
assuming the capacitance, the width and the radius of
innerconductor
appropriately.
Theelectrodecapacitance has been obtained by using the computer code
SUPERFISH.
The capacitance is derivedfrom
theresonant frequency of a four vane cavity as shown inFig. 4 assuming that the density of magnetic flux isconstant.
Dimensions of the 12.5 MHz and 50 MHz
cavities are given in Table 1.
A multi-module cavity structure of 50 MHz is shownin Fig. 5.
All parts of the cavity are made from brasssince that is cheap and comfortable to machine. In thelow power test for examining the rf characteristics,
vanes from brass have not modulation. In the step ofacceleration
test,
modulated vanes are made fromREFERENCES

1.
2.
3.
4.
5.

94

R. W. Mueller, GSI-Report 79-7, May 1979.
R. W. Mueller et al., IEEE Trans. Nucl. Sci., Vol.
NS-28, No. 3, June 1981.
A. Moretti et al., Proc. 1981 Linear Ace. Conf.,
Santa Fe, NM, USA, LA-9234-C (1982).
S. Arai, GSI-Report 83-11, 1983.
K. R. Crandall et al., Proc. 1979 Linear Ace. Conf.,
Montauk, N.Y. , USA, P. 205.