The II th Symposium on Accelerator Science and Technology, Harima Science Garden City, I997

A Cold Model Test of the CW-DTL for the Neutron Science Project at JAERI
Etsuji ClllSlllRO, Hiroshi INO*, Nobuo OUCill, Zensaburo KABEYA* and Motoharu MIZUMOTO
Proton Accelerator Laboratory, Japan Atomic Energy Research Institute,Tokai-mura, Naka-gun, Ibaraki-ken, 319-11, Japan*Mitsubishi Heavy Industries, LTD. Nagoya Aerospace Systems,Oye-cho, Minato-ku, Nagoya, 455, Japan
Abstract

2. Experimental equipment

The RF characteristics of the DTL which accelerates
proton beam from 2 to 10 MeV has been studied using acold model. The cold model is fabricated of 60-cell made ofaluminum and is assembled with post couplers to adjust andstabilize the accelerating field. In this study, the dependence· of the RF characteristics on number of installed post
couplers in the cold model has been investigated. Installing
the post coupler every 1-cell, we can not obtain the uniform
field distribution because some modes, which are neither
cavity modes nor post modes, strongly perturb the field
distribution. Decreasing the number of the post couplers,
these modes disappear and the uniform distribution isobtained. The stabilization effects such as the field
distribution, the detuning sensitivity and the mode spacing
between TMOlO and TMOll become weak with decreasing
number of the post couplers.

2.1 Cold model of CW-DTL injector
We have fabricated the cold model (2-10MeV, 60-cell)of which the geometrical size is reduced at factor of 113 for
the CW-DTL. The designed parameters of the cold model islisted in Table 1 and the cross sectional view is illustrated in
Fig.1. The cold model has been assembled with
manufacturing tolerance of +/-0.3 mm. Post couplers arealternately inserted from side to side. A post length CLp)
which is a distance between the post coupler and the drift
tube is adjusted with setting accuracy of +1-0.4 mm to tune
the post coupler frequency for the stabilization. Each post
coupler has an extension tab. In this study, the tab is kept toalign in vertical. To investigate the dependence on numberof post couplers, an installation interval of the post couplers
for the cells is changed from every 1-cell to 5-cell. In eachcase, total number of post couplers corresponds to 59, 29,
19, 14 and 11 couplers.

1. Introduction

Table 1
Designed parameters of the cold model

JAERI has planned the Neutron Science Project (NSP)
for exploring basic researches and nuclear wastetransmutation technology1>. The . NSP requires a highintensity proton linac which consists of radio frequency
quadrupole (RFQ) linac, drift tube linac (DTL) andsuperconducting linac. In this linac the DTL accelerates
proton from 2 to 100 MeV.
The DTL has utilized post couplers to stabilize andadjust the accelerating field. In general, since the stabilized
accelerating field has non-zero group velocity because twopass band (post band and cavity band) are confluent at theaccelerating mode, the field distribution is almost
insensitive to perturbations such as effects of stem, wall loss,

Frequency
Tank Radius
Tank Length
Number of Cells
Ace. Energy
Cell Length
Drift Tube Radius
Bore Radius
Stem Radius
Post Radius
Material

detuning, and beam loading2l.

However, the stabilization using the post couplers is not
simple at the low energy section of the DTL. This is because
the cell length is short and varies rapidly along a tank3>. The
short cell length causes a strong coupling among post
couplers that is dependent on the distance between the postcouplers. The change of the cell length causes thedifferential resonance frequency of each post coupler that isdependent on the length of the drift tube.
To assist the design of the CW-DTL, the RF
characteristics·has been investigated using a cold model. In
this paper, we describe the dependence of the stabilization
effects on number of the post coupler in the cold model.

600 MHz153.5 mm3096.9mm60-cell2-10MeV
32.9-72.7 mm
33.3 mm
3.3mm
6.0mm
5.0mm
Aluminum

2.2 Measurement
The axial field (E(z)) is measured by the bead
perturbation method4>. A radius of the bead made ofAluminum is 2 mm. Averaged field across i-th cell (Ei) andtotal averaged field <E> across the tank are given byfollowing equations.

-215-Ei =_!_f E(z)dz ,4 i-cell{E)= - 1- f E(z)dz .
Lwt Tank(1)(2)

The 11th Symposium on Accelerator Science mid Technology, Harima Science Garden City, 1997

We had attempted the stabilitzation of the cold modelwith a post coupler installed every 1-cell. Here we express
this installation interval with PIC (Post/Cell)=l/1. Tuning
the post couplers, a minimum Dx was obtained at 4 = 26mm, but undesirable modes we~re excited around TMOlO
mode (Plotted by "*" in Fig.2). These modes were neither

5Fig. 1 Cross sectional view of the cold model.
where L,. and Lr.ot are i-th cell length and tank length,
respectively. To evaluate filed distribution, distortion
parameter (Dx) is defined ash60

Dx =!tiE; -(E~/(E} .

(3)

Also a network analyzer is used to measure the resonantmode frequency.

2

0.5 L.-...L-........__,__ _....._....__.._........_ , _ _.........._.____.
24262830
3234L (mm)p

Dependence of Dx on Lp. PIC= lin indicatesa installed post coupler every n-cells

Fig. 4

3. Experimental results

3.1 Field distribution with a post coupler
installed every 1-cell
610

N'4=26mm:::r:~

P/C=l/1

605~ 600!

i' 5951.1..

590____01Mode Number~5'--~~~~--_.._-2580

600

620Frequency (MHz)Fig.2-1....._~~~

2Fig. 5 Dispersion curves of TM and post modes

Resonant modes of the cold model
Post couplers are installed every 1-cell•

6~....v

1.4

PIC

<E>

Dx

0 -1/1 ------

1.48
1.40

3.45
5.42

...

2530..
..·..·:::::!

~!:!:!,-1.2

0

0.8
0

10

20

30

40

50

5101520
Number of Posts

60

Fig.6 Group velocity (vg) and mode spacingvs. number of post couplers

Cell Number

Fig. 3 Field distribution with post coupler every 1-cell(PIC= 11) and without post coupler (P/C=O)-216-N.. c 6 J:... 'The 11th Symposium on Accelerator Science and Technology, Harimll Scie}'ICe Garden City, 1997

the cavity (TM) modes nor the post modes, and arose when
the post band closed the ·cavity modes. The field
distributions with P/C=111 and without post coupler{P/C=O) are shown in Fig.3. At the P/C=O, the field level
gradually varies. On the other hand, the distribution at thePIC= 111 periodically oscillates. Nevertheless the cold model
was tuned by the post couplers, the Dx is larger than that of· PIC=O. It seems that the distribution is perturbed by theundesirable modes, but the excitation mechanism of themodes has not been clarified.

3.2 Dependence on number of post couplers
Expanding the installation interval of the coupler, we
reduced the total number of the post couplers to eliminate

86

1/3

•
2o~~--_.--~~--~--~~--~

o.o5

0.1 oo.15

o.2oo.25

Detuning Frequency (MHz)the contribution of the undesirable modes. At the interval of
P/C=1/2, these modes were not excited around the TM010
mode and stabilized field was obtained. Fig.4 shows thedependence of Dx on Lp at conditions of P/C=1/2,1/3,1/4
and 115. When a miniJ:num value (Dxmm) of Dx wasachieved at a Lp, we regard that condition as the cold modelwith the Lp was stabilized. Narrowing the inteival, that is tosay, inaeasing number of the post couplers, smaller Dxmm
was produced. The smallest Dxmm was measured at· theP/C=l/2. The group velocity (va) and the mode spacing
between TM010 and TMOll were calculated from·dispersion curves (Fig.5) of the stabilized cold model. Fig. 6shows the v1 and the mode spacing as a function of total
number of the post couplers. Both v1 and mode spacing
became largest for all conditions at the P/C=1/2.
_In the stabilized cold model, the detuning ·sensitivity
was measured by shifting a resonant frequency of TM010
mode, which was accomplished by pulling out a half-drift
tube of the low energy end. Fig. 7 shows the dependence ofDxmm on the detuning frequency. As shown in the figure,Dxmm is linearly increasing at all conditions and the tilt. isgradual at the narrow interval condition. At the P/C=1/2
detuning peiturbation slightly mfluenced the field
distribution. We defined a - detuning senSitivity as
perturbation growth against detuning frequency. In this _evaluation it indicates an enhancement factor of Dxmm perdetuning frequency of 1 MHz. Fig. 8 plots the sensitivity as
a function of total number of posts. The detuning sensitivity
exponentially decreases with increasing number of post
couplers.
4. Conclusion

Fig. 7 Dependence of Dm~r~ on detuning frequency

The stabilization effects become large with inaeasing
number of post couplers. However, too many post couplers,or too closing post couplers causes the excitatioo of theundesirable modes. These modes seriously affect the .field
distribution.

f" 50 ........No Post
N
J:-~

•.

..,,::::!

.

~c:0..

•• ••20

Reference

P/C=1/S·.....

1~ ' ' .......•m 1o

1/3 •••••a·-c~c[1] M. Mizumoto, et al. "Development of High httensity

...,
'....

1~~50

510

15

20

2530

Number of Posts
Fig. 8 Detuning sensitivity_vs. number of post couplers

Proton Accelerator for Neutron Science ResearchPlan", Proc. of The 21st Linear Accelerator Meeting inJapan, (1996), 86.
[2] P. M. Lapostolle ed. -"Linear Accelerator" JOHN
wn.EY & SONS,INC. New York, (1970).
[3] F. Naito, et. al. "A Tuning Method of Post Couplers
for a Low fl Drift Tube Linac", Proc. of the XIVInternational Coof. oo High Energy Accelerator inJapan, (1989).
[4] S. W. Kitchen, et. al. J. Appl. Phys. 26 (1955) 618.

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