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

Accelerator Structure of PNC High Power Quasi-CW Electron Linac
YuanlinWANG, MasahiroNOMURA, Shin'ichiTOYAMA, TakashiEMOTO and IsamuSATO*
Oarai Engineering Center (OEC)Power Reactor and Nuclear Fuel Development Corporation (PNC)
4002, Narita-cho,Oarai-machi, Ibaraki-ken,311-13, JAPAN*,Atomic Energy Research Institute, Nihon University
Shurugadai 1-8, Chiyodaku, Tokyo 101, JAPAN1. Introduction

Abstract

The injector test ofPNC high power electron linac (1]had been done in the last year. Now all accelerator sectionshave already been set in the accelerator room. The beam
test will be started in the beginning of the next year. The
scheme and main specifications of PNC high powerelectron linac[2] are shown in Fig.l and Table 1
respectively. To design such kind of high powerquasi-CW
linac, a short accelerator section with a traveling wave
resonant ring was adopted, a constant gradient structure
under lOOmA beam loading was used for each accelerator
section, and a progressive stop-band technique was adopted
to avoid the cumulative type of beam breakup as much as

For PNC high power quasi-CW electron linac, one of thekey points is how to enhance the threshold current ofBBU.
A short L-band accelerator section with a traveling wave
resonant ring was adopted. Each accelerator section is aconstant gradient structure under 1OOmA beam loading. And
the progressive stop-band technique was adopted for allsections. In this paper, we put emphasis on the accelerator
structure and measurement. A nonresonant bead
perturbation method was used to measure the electric fields
in the accelerator structure. The resonance frequencies of
TMlt-like mode were measured. The results of themeasurement were in good agreement with the designvalues.

QM

AC2possible~

AC3A04,Y- Attenuator,d"- Ptwe Shifter

AY'- Matcher

C - Chopper Cavity
B- BWlCher
S - Signal Generator

CS - Chopper Slit
SC - Solenoids Coils
A - Amplifier

PB - Prebuncher
AC - Accelerator
PD ... Phase Detector

AC3

G- GWlST - Steering CoilQM- Q Magnets

Fig.l Scheme of PNC high power electron linac-233-AC1AC6v-ValveBM- Beam Monitor
K - KlystronThe lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Table 1. Specification of PNC high power linac
EnergyCurrent
Pulse duration
Pulse repetition
Duty factor
Frequency
Energy spectrum
Bunch widthEmittance
Efficiency
Length of accelerator sections
Number of accelerator sections
Klystron power
Number of klystrons
Gun voltage
Gun beam current

lOMeV
IOOmA
4ms
50Hz
20%
1249.135 MHz
<0.5%

ACCZ
ACC6
ACC5
ACC4

ACC3
ACC2

<50I01t mmmrad
70%
1.2m

ACC12a (mm)8l.OMW
2200KV6462585452

50

Fig.2, Ranges of iris diametersof all accelerator structure

300mA

2. Accelerator Structure

Eo=In the usual case, an accelerator structure is designed at
the constant gradient structure under zero current. It canenhance the threshold current of BBU, and keep dP/dz
constant under zero current, but under beam loading dP/dz
cannot be kept constant. For PNC high power linac, we
designed the accelerator structure at the constant gradientstructure under 1OOmA beam current. It means that eachcavity in one section has greater difference, and can keepdP/dz constant under 1OOmA beam current. The former canenhance the threshold current of BBU, the latter canmaintain thetemperature constant over the entire
accelerator structure easily.
The progressive stop-band technique is that for eachaccelerator section, the oscillation frequency of beaminteraction with TMu-like mode is located at a stop bandof TMu-like mode of all subsequent accelerator sections.
This technique requires that the first iris diameters in thesection be smaller than those in any preceding section butlarger than those in subsequently located section.
According to above mention, the ranges of iris diametersof all accelerator sections are shown in Fig.2. One
accelerator section has 15 cavities. The difference of theiris diameters between the neighboring cavities is 0.3 mm,
which brings about the con$tant gradient structure underlOOmA beam current. And the difference of the first iris
diameters between the neighboring sections is 1.8 mm,
which is required by the progressive stop-band technique.

where r and ro are the reflection with bead and withoutbead. It should be possible to analyze them and todecompose Eo into forward and backward wave
components. After data processing by computer, theforward wave components of electric fields gave us valuesof the accelerating fields shown in Fig.3.

c201.8o--o--0-o--o--o-·..a--o--CI-0.o-.o-..a--0o--o--o-·CIo--0·
6..a--a--o-·.o-.o-..a--0·~-o--o-·0
o-.o--D·-o--o-·CJ-·u-.o~.o-.o-..Q--0-o--o--o-·0
o-.o--D A..a-..o--o--o.o-.o·<P-D-..a-..a·- - - -0~:.a- .o-.o3a- .o- ..Q--0~ .o--0- -o- ·CI- -CJ- -o--0~ o- -CJ- -0
o-.o-.o·-D 2o- .o- .o-..a--o--o--o--~o-.o-.o-.o- - -Acc N0. 7·j 1.9::::: 1.7~ 1.6~ 1.5:!2 1.4~ 1.3-s 1.2jw

1.1
1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Z Axis Cavity Numberi2~ 1.9~ 1.8.._. 1.7

e 1.6:;;:IE 1.5:; 1.4~ 1.3

3. Measurement-s 1.2

A nonresonant bead perturbation method was used tomeasure the electric field in the accelerator structure bymeasuring the reflection produced at the input port by aperturbation bead. In the accelerator along the z axis, there
are no magnetic fields, one can get electric field zcomponent Eo as following:-234-~ 1.1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Axis Cavity Numberz

Fig.3, Electric fields in accelerator structures

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

The upper figure shows design data. The electric fields
increase with z, because of no beam loading. The lower
figure shows measurement data. They are in. goodagreement with design data.
The backward wave components of electric fields in eachsection showed us the information about the reflection ofaccelerator structure from where. This method was used tocheck the accelerator structure finally.
We used two loop couplers into the accelemtor input andoutput couplers to measure frequency chamcteristics one
section by one section by means of a network analyser.
Fig.4 shows the frequency characteristic of one accelerator
section. The left pass band is TMot accelerating mode andright one is TMn-like mode.

If(MHz)I
lAce No.7

1900
1800

1700
1600
1500
1400
1300

Fig.5, Brillouin ( m-!3) diagramsof all accelemtor sections by measurement
Fig.4, Frequency chamcteristic of one section
We can detect the frequency of each mode (from 0 to 7t),then draw the Brillouin (m-j3) diagrams of all accelerator
sections which are shown on Fig.5. The results ofmeasurements satisfied us. The upper part is TMn-likemode dispersion chamcteristics' curves which intersect
vp=c line at At,A2, ...A1 . They correspond to the request of
the progressive stop-band technique. For the first accelerator
section, the probable oscillation frequency of the beaminteraction with TMn-like mode is near the intersection Atpoint which is located at the lowest point and below theTMn-like mode pass bands of all subsequently locatedaccelerator sections. The following sections are the samesituations. It is to avoid accumulation of the beam transverse
TMn-like mode oscillations one section by one section.
The lower part is TMot accelerating mode dispersion
chamcteristics' curves, where only shows three curves(No.1, No.3 and No.7 section), and all of them intersect at
2x/3 and fundamental frequency (1249,135MHz) point
which is operation point
This kind accelerator structure which we designed canopemte at low gmdient, high beam current lOOmA and longpulse 4ms. During the last year injector test, we accelerated
a lOOmA beam with 3ms dumtion pulse to 3MeV
successfully .[1]References[l],Y.L. Wang, et al."Injector Adjustment of PNC HighPower Linac" Proceedings of the 21st Linac Accelerator
Meeting in Tokyo, Japan, Sep. 1996. P 305.
[2], Y.L. Wang,etal."DesignofHighPowerElecttonLinac
at PNC". Journal of Nuclear Science and Technology,
30[12], Dec.l993. pp1261-1274.

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