JAERI-Conf 95-021

PA16
COLD MODEL MEASUREMENT OF BIPERIODIC L-SUPPORT DAW
Y. Iwashita, A. Noda, H. Okamoto, T. Shirai, and M. Inoue
Accelerator Laboratory, Nuclear Science Research Facility,Institute for Chemical Research, Kyoto University,Gokanosho, Uji, Kyoto 611, JAPANAbstract

2. Biperiodic L-support DAW

Cold model test of a biperiodic L-support Disk-and-Washer
linac structure is performed. The structure is a variant of thebiperiodic 4-T support DAW. Each washer is supported by two
L-shaped supports 180° apart azimuthally. The results of thecold model tests are described.

The DAW structure has outstanding features in highstability, good vacuum properties, high shunt impedance, andease of fabricationS). It was found that the mode overlapping
problem can be overcome by the biperiodic supportconfiguration with the careful choice of the tank diameter (SeeFig. 1). There is variety of options for DAW linac structurewith such washer support. For example, in the configuration
with a large tank-diameter, the operating frequency drops
between two split TM11Hike) mode passbands, and the shunt
impedance is higher. When the tank diameter is small, both
passbands are above the operating frequency, and the modedensity is smaller. The basic configuration described here is theextension of the PIGMI6) geometries, except for the thicker
washers and the reduced tank diameter by 20%. This geometry
has fewer undesirable modes and a shorter filling timecompared with the large diameter 4-T support DAW. The
washer thickness is increased for the cooling water channels
inside the washers. Because the L-support configuration has
only two supports on a washer, there are only one inlet and oneoutlet for the cooling water, and the fabrication of the coolant
path is straightforward, although the design of the path is notstrait forward. This may simplify the fabrication problem
compared with the 4-T support geometry whose washer has
two inlets and two outlets on it7 ,8). The multiple inlets andoutlets may cause the fabrication problem when the paths havesplits and merges. A typical design specification based on
SUPERFISH calculation is listed in Table 1. The notations forthe DAW dimensions are shown in Fig.2.

1. Introduction
An electron linac 1) has been installed at the Accelerator
Laboratory, Institute for Chemical Research, Kyoto University.
It is mainly intended to be used as the injector for the electron
storag~ ring KSR 2,3), which is being assembled. The discloaded wave-guides are installed as the accelerator tubes, which
are operated at 2857MHz. Because of the limited space in thebuilding, only three accelerator tubes of three meter in length
can be installed. The available RF power from a klystron is up
to 20 MW for each tube, and then the output electron energy isexpected to be about 100 MeV at the peak current of 100 rnA.
In order to have a shorter damping time in the storage ring, thehigher injection energy is desirable. A new accelerating tube
with a higher shunt impedance is thus required to achieve thehigher accelerating gradient with the same RF power.
A cold model made of Aluminum4) is fabricated to study
the possibility of a DAW structure with biperiodic washer
supports. The coupling-mode frequency shift by the supports,
and biperiodic disturbance on the field distribution aremeasured and compensated in cold model tests. The mode
spectra is also measured.

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Fig.1 DAW with Biperiodic support-67-The positions of thewasher supports are
determined so that theireffect on the accelerating
mode is minimized. Then,the coupling mode
frequency fc is inevitably
disturbed by the existenceof the supports, and its
frequency is pushed up
from its calculated value.
Because fc should coincidewith the accelerating-mode
frequency fa, fc should becompensated. Besides thiseffect, the biperiodicity ofthe supports breaks theuniformity of the electricfield distribution on theaxis. Because the supports
reduce the electric fieldaround them, the coupling
coefficients between theJAERI-Conf 95-021

cells are not uniform, yielding the biperiodic modulation on thefield distribution. In order to improve the coefficient unbalance,
the disk radii Rdn and Rds (see Fig. 2) are changed
biperiodically; namely, the disks with the washer supports havea larger radius than that without supports, which enlarges thedisk-washer opening. Thus, the coupling coefficients areenhanced hi periodically, and the coupling frequency iscorrected by adjusting the average of Rdn and Rds. Finally theaccelerating frequency will be tuned by modifying the washer
radiusRw.
The tunings are performed· with a six-washer geometry.
The coupling mode frequency is measured in the geometry withthe half washer endplates, which has three disks with supports
and three disks without support (See Fig. 3-a). Although the.
simple biperiodicity in the whole system is broken, thisgeometry will give the correct coupling frequency4). There isanother option of the support direction; namely, the quadperiodic geometry where the support direction changes
alternatively (See Fig. 3-b). Photos 1 and 2 show the typicalparts for the model cavity, and the close view ofthe disksupport-washer assembly.
0.585

Rc/A.
B
Frequency
L=BA./4
Rc (cavity radius)
Rd (disk radius)
Td (half disk thickness)
Rw (washer radius)
Tw (half washer thickness)e (nose angle)
Rn (nose radius)Rb _{bore radius)
G (gap)Rt (supporting point)
Rr (support curvature)

1.0
2.856
26.24

Verticalplane(a)Horizontal
plane

Verticalplane(b)Horizontal
plane

Fig. 3 · Geometry for the measurement of the coupling mode
frequency. (a) biperiodic (b) quad-periodic.

GHzmm

61.40
49.6
12.53
42.
2.530mmmm
mm
mm
degree1.25.13
14.84
32.3
9.

mm
mmmmmmmmmm

Photo 1 The typical parts for the DAW cold model

Table 1 DAW cavity dimensions

TwPhoto 2 Close view of the disk-support-washer assemblyR

4. Measurements and results

Fig. 2 Notations for DAW dimensions

Figure 4 shows the geometry for the mode spectra
measurement. Although it is intended to be the "hi-periodicconfiguration", the hi-periodicity is broken by the terminations.
It does not have a mirror symmetry even. Figure 5 shows thepreliminary result of the mode spectra measurement for the "hiperiodic configuration". Because of the broken hi-periodicity
and the fewer number of cells, the understanding of the mode
spectra is somewhat complicated. Because the stem modes
have very low Q-values, and could not be observed easily, they-68-JAERI-Conf 95-021

are not shown in the figure. The TEn mode passband isdegenerated in this support configuration, and only one
passband is shown. Because the symmetry of the supportconfiguration breaks the degeneracy, TE21 mode has two
passbands; namely TE21E// (electric field parallel to thesupport) and TE21E..L. (electric field perpendicular to thesupport). The TE21FJ/ couple strongly to the TMQ1 modes, andthey are mixed together particularly around the 1t mode. It
makes the mode identification process difficult.
The mode overlapping on the acceleration mode is avoided
in this configuration. The TMu(-like) passband splits into two
narrow passbands and both passbands sit 86MHz above theoperating frequency. The slope of the ™02 passband aroundthe 1t mode shows the high coupling and is the same as thedesigned one, which indicates that the confluence condition
comes close sufficiently. Because of the boundary condition of
the termination, the coupling mode cannot be observed in thesame configuration as the one that shows the acceleratingmode. This situation makes the confirmation of the confluencedifficult. The measurement with more cells is going on.

Vertical
plane

MHz

4000

3500

3000

2500

Horizontal
plane

2000

Fig. 4 Geometry for the accelerating mode

5. Acknowledgment
The authors thank Mr. Tonguu for his help in themeasurement. The present work is supported by Grant-in Aidfor Scientific Research from Ministry of Education Science andCulture of Japan.

1500

6. References
1) T. Shirai, et al., "A 100 MeV Injector for the Electron
Storage Ring at Kyoto University", Proc. of the 1995 Particle
Accelerator Conference, in print.
2) A. Noda, et al., "Design of an Electron Storage Ringfor Synchrotron Radiation", Proc. of the 4th European Particle
Accelerator Conference, London (1994) p.645.
3) A. Noda, et al., "Electron Storage Ring, KSR for
Light Source with Synchrotron Radiation", Proc. of 1995
Particle Accelerator Conference, in print.
4) Y. !washita, "Disk-and-washer structure withbiperiodic support", Nucl. lnst. and Meth. in Phys. Res. A348
(1994) 15-33
5) R.K.Cooper, et al. "Radio-Frequency Structure
Development for the Los Alamos/NBS Racetrack Microtron",
LosAlamos National Laboratory document LAUR-83-95 (Jan.
1983)
6) L.D.Hansbrough, Compiler, "PIGMI: A Design
Report for a Pion Generator for Medical Irradiation", LANL
report LA-8880(1980).
7) Y. !washita, "Biperiodic L-Support DAW For Electron
Acceleration", Bull. Inst. Chern. Res., Kyoto University, Vol.
73 No.1 (1995), p.97.
8) Y. I washita, et al., "Cold Model Test ofBiperiodic Lsupport Disk-and-Washer Linac Structure", Proc. of the 1995
Particle Accelerator Conference, in print.

1000500-69-001t

021t

01t

241t

051t

01t

Phase ShiftFig. 5 Mode spectra for hi-periodic configuration.