The 9th Symposium on Accelerator &ience and Technology, Tsukuba, Japan 1993

Construction of a Heavy Ion Linac for Short-Lived Nuclei
S. Arai, M. Doi, Y. Hashimoto, T. Hattori', A. Imanishi, T. Katayama, H. Masuda,
K. Niki, Y. Takeda, N. Tokuda, M. Tomizawa, E. Tojyo, K. Yoshida, and M. Yoshizawa
Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188, Japan'Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152, Japan

Abstract
A heavy ion linac for short-lived nuclei isunder construction at INS. The linac is an accelerator complex composed of a 25.5-MHz split coaxial
RFQ (SCRFQ), a charge-stripper section, and a51-MHz interdigital-H (IH) linac. The SCRFQ withmodulated vanes accelerates ions with a chargeto-mass ratio (q/ A) greater than 1130 from 2 to170 keV /u. The stripper is a carbon foil. The IH
linac comprises four cavities and three magnetic
quadrupole triplets placed between cavities, accelerates
ions with q/ A E; 1/10, and varies the output energy
continuously in the range 0.17 "' 1.05 MeV /u.

Main parameters of the cavity are listed inTable 1. The cavity comprises four unit cavities,
whose structure is nearly same as that of the prototype as shown in Figure 1.
moss SI:CllON A-B-C-D-E-f~BackPlate\I rSpacing Rodv~\fJl.·-:-:·:·:-:-:-:.· :·:-:·:-:-:-: ·. ·:-:·:-:-:-:··.·
1<·1!;.;!

I. INTRODUCTION

1mbeam acceleration
A short-lived nuclear
facility, which is a prototype for the exotic nuclei
arena (E-Arena) of the Japanese Hadron Project(JHP), has been under construction at INS since
The short-lived nuclei, producedfiscal year 1992.
by bombarding a thick target with a 40-MeV 10-llA
proton beam from the existing SF cyclotron, areionized in an ion source, mass-analysed by meansof an isotope separator on-line (ISOL) and transported to a heavy ion linac through a 50-m long
beam line. The beam intensity extracted from theThe!SOL is expected to be 107 "' 1011 ions/s.
single-charge ions with a charge-to-mass ratio (q/ A)
greater than 1/30 are accelerated by a 25.5-MHz split
coaxial RFQ (SCRFQ) [1) from 2 to 170 keV/u,charge-stripped by a carbon foil, and transported
to a 51-MHz interdigital-H (Ill) linac [2) throughtwo magnetic-quadrupole doublets and a 25.5-MHz
The output beam energy fromrebuncher cavity.
the IH linac is variable between 0.17 and 1.05 MeV/u.
The duty factor of this linac complex depends on
ql A of the ions: nearly 100% at ql A ;;;; 1116, andgiven by 270 X (q/ A) 2 at 1/17 E; ql A ;;;; 1/30.

II. SPLIT COAXIAL RFQ
The 170-keV /u SCRFQ is an extended version
prototype model (25.5 MHz, q/ A ;;;; 1/30,of a
1 ..... 45.4 ke V /u, 0.9 m in diameter, 2.1 m in length).
This SCRFQ was designed on the basis of the result
obtained from the beam tests of the prototype (3).

A. Cavity Structure

Figure 1. Structure of the unit cavity.
We use the prototype cavity as the fourth unit cavity
and fabricate newly three unit cavities. The following improvements were taken in the new cavity:
in order to achieve a 30%-duty operation with amaximum peak power of 350 kW, we thickened thewater cooling pipes of the cavity, and removed thevane coupling rings installed in the prototype,because they caused appreciable shift of the resonant
frequency in high-power operations.
Table 1Main parameters of the SCRFQ cavity
Frequency (f)Cavity length
Cavity Inner diameter
Vane thickness
Stem width near vanes
Stem thickness
Outer diameter of spacing-rod
Total capacitance (C)Total inductance (L)Resonant resistance (Rp)
Intervane voltage (V) (for q/ A=1/30)
Power loss (P) (for q/A=1/30)

25.5 MHz
8.6 m
0.9 m
3 em
15 em
3 em
3.8 em1548 pF
25.2 nH
22 kQ108.6 kV
280 kW

In the cavity, the rf current flows mainly on theinner surface of the cavity cylinders, the stems andthe back-plates of the vanes. The cavity is cooledby eleven water channels running in parallel. Three-226-

channels for cooling the cavity cylinders have a flowrate of 10 1/min per channel. Four channels forthe stem flanges, the spacing rods, the stems andthe diamond-shaped back plates have a flow rateof 44 1/min per channel. Four channels for the stem
flanges, the spacing rods, the stems and the triangular back plates have a flow rate of 22 1/min.
The flow rate of each channel was determined soas that the temperature increase of the water might
be less than 1.4 o C under a 30% duty operation witha peak power of 350 kW. For cooling the stems
and the back plates, copper pipes, 12.7 mm in outer
diameter and 0.8 rnrn in thickness, are soldered along
the sterns and around the back plates.

By using these values, we estimated a resonant
frequency of the 8.6-m long cavity (12 modules) toIf thebe 25.33 MHz as shown in Figure 3.
experimental value is different from 25.33 MHz andhigher than 25.45 MHz, we will roughly tune theresonant frequency to 25.45 MHz by attaching thecapacitive tuners on the sterns. If the experimental
value is lower than 25.45 MHz, the tuning will bedone by adjusting the area of the stem-flange
The fine tuning to 25.5 MHz will bewindows.
performed by means of the piston tuners.

B. Vane Structure
The improvements on the vane structure areas follows: the vanes in the first unit cavity aremachined by means of a three-dimensional cutting
technique, and for the other vanes a two-dimensional
cutting technique is adopted. For the both vane-tip
geometries, we made a correction on the apertureparameter a and modulation m (A10 correction) tobring the actual field close to an ideal one. A
detailed description on the vane is elsewhere (1).

III. INTERDIGITAL-H LINAC
Figure 2. Equivalent circuit of the unit cavity.
When the all coupling rings are removed from a unit
cavity, the electrostatic capacitance between vanes
decreases from 453.2 to 386.7 pF. Without changing
the dimensions of the unit cavity, we intend toadjust the resonant frequency of the 8.6-rn longcavity to 25.5 MHz by using the stern inductance.
The unit cavity is a three module cavity as shownin Figure 1. The equivalent circuit for the unit
cavity is illustrated in Figure 2. The total inductance of the unit cavity is given by a tank inductance(L0) and a stern inductance (Ls) as follows:
LLo(Lo + 3Ls)
3Lo + Ls

In the design of the IH linac, the following
principles were adopted to accelerate ions with aq/A ;;:; 1/10 from 0.17 to 1.05 MeV/u, to vary theoutput energy continuously between 0.17 and1.05 MeV/u, and to obtain the high shunt impedance:
1) accelerating periodic structure is a 7r-7r mode; 2)
synchronous phase is not 0° but -25° to assure
the stable longitudinal motion; 3) no transverse
focusing element is installed in the drift tubes, and4) cavity is divided into four cavities to set thetransverse focusing elements locally, and to varythe output energy easily. The cavity structure isillustrated in Figure 4.
A(1)

Flux Inducer""'~"""':?""4!

From the rf measurements of the prototype, the tankinductance and the stern inductance were obtainedto be Lo = 201 nH and Ls = 29.3 nH.
28i!

127j_\--

26~ 25 0\\0~1mQuadrupole Trip letFigure 4. Cavity structure of the IH linac.

r--

15105
Number of Modules

A. Beam Dynamics
20Figure 3. Resonant frequency as a functionof the number of modules.

Beam from the SCRFQ has a transverse
emittance (normalized) of 0.67r rnrn ·rnrad at maxilongitudinal emittance of 757r
mum and akeV/u·deg. The subject we have put emphasis on-227-

is to make the acceptances of the IH lina.c as large
a.s possible. The both acceptances greatly dependon each cavity length a.nd drift-space length betweencavities. A quadrupole triplet is best as the transverse focusing element placed in the drift space.
Each optimized cavity length is given in Table 2.
The drift-space length wa.s determined to be 47.5 em,by considering the technical feasibility of the quadrupole triplet. As a. result, the transverse acceptanceof 2.47r mm ·mra.d a.nd the longitudinal acceptanceof 2007r keV/u·deg were obtained. Through a. beamsimulation, it wa.s verified that the output energy
wa.s continuously varied by changing the rf powera.nd the phase in a. last cavity of the working ones.
Table 2
Design parameters of the IH cavities
IIIII

IV

51
0.59
1.34
947
8
3.6
2.0
351
0.84
1.34
1047
8
4.4
2.4
351
1.15
1.34
11
47
8
5.2
2.8
351
1.53
1.34
1247
8
6.0
3.2
3751
200
5510
250
11345
315
22244
370
40

Cavity number
Frequency (MHz)Cavity length (m)Cavity inner diameter (m)N urn ber of cells
Ridge height (em)Ridge width (em)Drift-tube diameter (em)Bore diameter (em)Stem diameter (em)
Effective shunt
impedance (MfVm)
Ma.x. ga.p voltage (kV)
Ma.x. peak power (k W)

B. Cavity and Quadrupole Triplet
The design parameters of the cavities a.re listed
in Table 2. The inner diameter of four cavities isconstant, while the a.xia.l length is different fromeach other. Each ga.p length between drift tubes isconstant over a. cavity a.nd equal to one half ofthe first cell length. We estimated a. resonant frequency a.t each cell by using a. capacitance obtained
approximately by SUPERFISH a.nd a.n inductancecalculated a.na.lytica.lly in a.n assumption that themagnetic flux density might be constant in thecavity. Both end structures of the cavity, i.e. themagnetic flux inducer and the gaps between theend wall and ridge, are determined experimentallyso as that the longitudinal field distribution becomesflat over a. cavity.
We designed the quadrupole triplet compactly
to make the drift space as short as possible.
Therefore, the quadrupole triplet, 2 em in bore
radius, 19 em in yoke radius and 9, 14 and 9 emin pole length, requires high field gradient such as
55 Tim a.t maximum. As a. coil, a hollow conductor,

4X6 mm 2 in outer size and 2X4 mm 2 in inner size,
is wound 34 turns on a. pole. Power supplies, 300 A
and 35 V at maximum, are used for these magnets.

IV. CHARGE STRIPPER SECTION
We designed a. compact stripper section [ 4]by using a. 12C+ ion beam, because 12C+ ion haslarger energy-deposit and energy-straggling per
nucleon in the stripper than other ions. The energyloss is about 6 keV/u and the energy-straggling isabout ±1 keV/u when the beam with an energy of
170 keV/u passes through a. carbon foil, 10 j.lg/cm2in thickness. Since the transverse emittance-growthin the stripper is proportional to the beam size, thestripper is placed just behind the SCRFQ, where
the horizontal and vertical beam radii are 7 and4 mm. The emittance-growth rate of the beam isabout 1.6 in the horizontal plane and about 1.2 inthe vertical one. The longitudinal matching of thebeam is done by means of a rebuncher cavity. It
is convenient for matching that the position of therebuncher is near to the SCRFQ, because thedebunching due to the drift space becomes small.
The distances between the SCRFQ and rebuncher,and between the rebuncher and IH lina.c were
determined to be 2 and 0.9 m, so as that it might
be possible to set a. quadrupole doublet behind therebuncher.

V. STATUS
The construction of the 170-keV/u SCRFQ will
be completed this fiscal year. A 350-kW rf amplifier
for the RFQ has been already constructed. TheIH linac will be constructed from this year on thebasis of the studies performed by means of half scale
The basic design of the charge
model cavities.
As thestripper section was almost completed.
rebuncher, a. 25.5-MHz spiral loaded cavity is being
developed.

VI. ACKNOWLEDGMENTS
tothanks
theirexpress
authors
TheT. Nomura. and M. Kiha.ra for their encouragement.
This work is supported by Accelerator Research
Division, High Energy Physics Division, and Nuclear
Physics Division of INS. The computer works weredone on FACOM M780 in the INS Computer Room.

VII. REFERENCES[1) S. Ara.i et al., Proc. IEEE PAC, 1993.
[2) M. Tomizawa. et al., Proc. IEEE PAC, 1993.
[3) N. Tokuda. et al., INS-Rep.-897, 1991.
(4) K. Niki et al., Proc. IEEE PAC, 1993.

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