JAERI-Conf 95-021

PA15DEVELOPMENT OF AN RF ELECTRON BEAM IRRADIATION SYSTEM
T. Fujisawa, T. Hirasima, T. Katori, S. Wada, S. Kohmoto, andM. Odera

Denki kogyo Co. Ltd.
Nakatu-saku radai 4052-1, Aikawa-machi, Aikou-gun, Kanagawa 243-03
Abstract

Table 1. Specifications of ELECTRON SHOWER.

A prototype electron beam irradiation systemusing a re-entrant type RF accelerator(630 mm inlength, 500mm in diameter) has been studied to make acompact device for electron beam processing. Themaximum beam energy is planned to be 900 keV. Theresonator is powered by a self-excited oscillator.
This report gives brief explanations and the presentstatus.

1. Introduction
Recently many electron accelerators are used inindustries for electron beam(EB) processing. Most ofaccelerators whose beam energy is lower than 5 MeV are
DC type. The main reason why DC machines are used, isbecause the electric power sfficiency of DC machines
is much better than that of RF machines ,and principle
and technology of DC machine are well establishedcomparing to the RF machine. An RF accelerator,
however, is much smaller and less expensive than a DC
higher than 300 keV.
machine in the energy
Furthermore, RF accelerator is more reliable than DC
machine because any insulator is not used in the area
where strong electric field arises and no accessory is
required such as a reservoir of SF 6 gas for DC
machine. Then we studied a prototype RF electron
accelerator to develop an electron beam irradiation
system named 'ELECTRON SHOWER' for industrial use.
As shown in our previous report 1 >, the principleof the accelerator is established, that is, the selfexcited tetrode oscillator 2 > functions well andovercomes multipactoring i~ the resonator without useof the conventional DC bias s.cheme. The new beam
bunching system3 > gives a sufficiently good energy
resolution for EB processing. The maximum acceleration
voltage, however, was 300 kV and the averaged
intensity 0.1 mA. Then we have improved machine andincreased the acceleration voltage up to 800 kV andthe averaged beam intensity up to 10 mA. Now, a unit
for industrial application based on the study is underconstruction.

Beam energy:
Energy resolution:
Maximum beam intensity:scanning width:
Beam spot size:
Line power:
Space for accelerator:

300 -900 keV

±. 5X
10 mA
30 em
<20mm in diameter.
3 phase, 200V, 35 kVA max.
Floor:1500*900 mm,Height:1850mm

Figure 1 shows a cross-sectional view of ES. An
electron beam is extracted with DC grid voltage(5.5kV)
from a hot cathode(LaB6 , 4 mm in diameter) of anelectron gun mounted in the stem. Velocity of theextracted beam is modulated by the RF fields induced
at a bunch gap and electrons are concentrated in thenarrow phase of the main acceleration field at theacceleration gap. The beam accelerated at the gap istransported to a bending magnet and bent to thevertical direction. Then the, beam is scanned 30 em inwidth by a scanning magnet and extracted into the N2
gas or air through thin titanium foil. Samples placed
immediately below the foil are irradiated by electrons
electronturbo
molecular
fr==;f::=,f==.===::::::=::::::;;~
pump

circuitconveyor
NZ GasTiil

Fig.1 Cross-sectional view of ELECTRON SHOWER.
2. Specifications and Outline of ELECTRON SHOWER
Table 1 shows the specifications of 'ELECTRONSHOWER'(ES) that were already described in ourprevious report 1 >. These, however, are typicalvalues and should be changed according to requests bya small modification of the prototype without change
of fundamental construction.

3.

Radio-frequency system

The principle of radio-frequenc y system andstructures of ES were already described 1 ). Here, thebrief explanation for RF system and for its
modifications is given. Tl'le inner conductors( stem:
270mm in length, 80 mm in diameter) made of copper are-64-JAERI-Conf 95-021

mounted on the flat end of the outer cylindricalconductor and faced to each other at the center of theresonator. As shown in Fig. 2 the, stem in which theelectron gun is mounted, has slots to generate theelectric field for the velocity modulation of theextracted electron beam.
The resonant frequency of the fundamental mode is
182 MHz, and the measured Q-value is 15000. The shunt
impedance is estimated to be about 3 M ohm from the Qvalue of the resonator coupled to the oscillator. The
resonator is powered by self-oscillator system and theoscillation is switched by the control grid bias.
Table 2 shows the specifications of the oscillator.

cc=1beam

Fig.2 Cross sectional view of the stem with a beamfocusing. system and a buncher.
5. Present status

Table 2 Specifications of the oscillator.
Frequency
Maximum RF power
Duty
TypeTube

182 MHZ
130 kW pulse peak(when duty<1/10)
from 1/5 to 1/50
self-excited oscillation
4CW25000A

4. Increase of beam intensity
4-1. Electron gun
A triode type electron gun is operated in pulse
mode by switching the grid bias in synchronous to thepulse mode of the main RF ~ield. The cathode ofelectron gun made of LaB 6 ( 4mm in diameter) is heatedby pyrolytic-graphi tes set on both sides of thecathode. The grid made of molybdenum or tungsten meshis set in front of the cathode with gap of 1 mm. The
extraction electrode is one similar to a Pierce typeas shown in Fig. 2.

5-1.Beam loading
The insufficient maximum acceleration voltage of
300 kV in our previous report, was caused by poor
adjustments of coupling between oscillator andresonator, and also of feed back elements. It is,however, difficult to optimize the coupling and feedon theback ratio, because these depend strongly
acceleration voltage, biases of the tetrode and beamloading. Particularly, it is difficult to adjust theparameters when the beam is accelerated, because theshunt impedance of the resonator including beamloading is, for example, one thirds of the impedance
without beam loading in a case of 300 keV 100 mA(pulsepeak) acceleration. Figure 3 shows a time structure of
the acceleration voltage when the beam is accelerated.
Even if the self-excited oscillator were adjusted tooperate under beam loading, the acceleration voltage
decreased considerably when the beam was injected.
Now, intensity of 100 mA(pulse peak) has been stably
accelerated at 300kV.

4-2. Focusing system
The duty factor for normal operation of ES is
1/10. Then the peak current of 100 mA is required toaccelerate the averaged current of 10 mA. Furthermore,
the intensity more.than 1A is concentrated in thenarrow phase of RF acceleration field by velocity
modulation. In such a case, very high tension as a
few tens kV is usually applied to an extraction
electrode to suppress the. space charge effect. The
low extraction voltage of 5.5 kV is ,however, suppliedin ES because a machine for industrial application
should be as compact and inexpensive as possible.
For effective transportation of the low energy beam tothe main acceleration gap, a beam focusing system
consisting of cylindrical permanent magnets is mounted
in the stem( see figure 2). The beam focusing system
consisting of cylindrical magnets is also mounted in
the other stem to transport the electron beam to thebending magnet.
4-3. Buncher coupled inductively to the main resonator
We developed a new type buncher system whose RF
voltage is generated by an inductor in the middle ofthe stem 4 >. Figure 2 shows a schematic view of thestem having the buncher. An electron gun and also abeam focusing system are installed in the stem. The RF
electric field of the buncher is anti-phase with themain accelerating field.

Fig.3 Typical beam loading effect.
5-2. Acceleration test
Figure 4 shows the typical energy spectrum of theThe RF
electron beam analyzed by a bending magnet.
modulation voltage proportional to the main
acceleration voltage is adjusted to obtain the best
250 keV.
energy resolution at the beam energy of
The typical result of acceleration test is summarizedin Table 3. The energy resolution of 5X at 300keV issufficient for EB processing.

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ar b"1trary

6. EB system for industrial use

11Figure 6 shows the cross sectional view ofelectron beam irradiation system designed based on
study for prototype machine. The reasons why theaccelerator are set vertically are as follow:1) The energy resolution obtained is sufficient forusual EB processing. Then the beam energy has not tobe analyzed with a magnet.
2) Most of materials are irradiated in horizontalplane.
3) The transmission efficiency of beam is naturally
better for a system without a bending magnet.

1050~0100-~

rew fe300200Beam energy(kev)Fig. 4 Typical energy spectrum.

electrongun

Table 3 Typical result of acceleration test.
Maximum acceleration voltage
Beam intensity(33~ keV, duty=l/10)
Energy resolution(300 keV)turbopump

820kV
100 mA
5%6-3. Irradiation test
it is required to irradiate sample uniformly inwide area. In the present case, as the machine
operates in pulse mode(width:lOO micro sec, period: lmsec), the beam was scanned by so low frequency(60Hz)
that a beam spot of a pulse overlaps with that of. thesuccessive pulse. The uniformity of ± lOX is obtained.
The depth dose curve measured by use of CTA films
is shown together with one obtained with DC machinein Fig. 6. The disagreement between the data is little
problem for EB processing.

.,10···-DC(JAERI,radiachromic film)

Gl0't:l~---;,,.....
"' 5011-----------------.. ~~-~--~--~Gl>......

75,, A~

GlRF(Present resultwith CTA film)e>:~,',,,,---------------~-R--·---M -·-~-

25

Fig.6 Cross sectional view of ES for industrial use .

',,7. Conclusion-.,,

0102030
Depth(mg/cm2)

4060

Fig.6 Depth dose curve obtained by ES

We usually estimate the effect of irradiation bydose(Gy). Most of data of electron beam irradiation,however, are obtained by DC machine. Then we carriedout preliminary experiments such as sterilization,
cross linking of lubber, polymerization, coloration ofglass, and etc. In most of the tests, we could not
find out difference between effects of RF andDCmachines, but found that one thirds of dose required
for DC machinegave same effects on one ofpolymerization experiments. This result shows thetypical example that irradiation effect depends on
dose rate.

The prototype electron irradiation system was
successfully studied and also preliminary experiments
have been carried out. Now the machine is installed
in a university and used for several experiments. The
system for industrial application based on theprototype is under construction.
We thanks Dr. Batygin for his help in calculationof electron trajectories and Mr. Chiba for providing
convenience for calculation of RF electric fields.
References
1) T. Fujisawa, s. Kohmoto, T. Katori, s. Wada, Y.
Taniguti, T. Hirashima, and M. Odera, Proc. 9thSymp.
on Accelerator Science andTechnology,(1993)178.
2) Patent pending, TOKU GAN HEI 6-46338.
3) Patent pending, TOKU KAI HEI 6-296799.
4) Patent pending, TOKU GAN HEI 7-85825.

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