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

Developm ent of a ffigh-Cur rent Gun and Increase of the Electron Charge
in a Single-Bu nch Beam of the Electron linac at ISIR, Osaka University
Shuichi OKUDA, Tamotsu YAMAMOTO, Takahiro KOZAWA, Shoji SUEMINE*and Seiichi TAGAWA
The Institute of Scientific and Industrial Research, Osaka University
8-1 Mihogaoka, Ibaraki, Osaka 567, JAPAN*Unicon system

Abstract

2. Development of a New Electron Gun

A new electron gun using a YU-156 cathode-grid
assembly was developed for the L-band linac at The
Institute of Scientific and Industrial Research, Osaka
University. The configulation of the electrodes wereoptimized by using a simulation code and the bench testof the gun was performed. The maximum peak current of
an electron beam from the gun was 30.1 A for a pulse
width of 5 ns in FWHM at an anode voltage of 100 kV.
The characteristics of the electron beams generated fromthe linac equipped with the gun were much improved.
The maximum charge of electrons in a single-bunch beam
was 73 nC, which is higher than that obtained in theprevious accelerator system.

Previously, the electron gun ofModel-12 (ARCO) has
been used. For a new gun the cathode-grid assembly, YU156 (EIMAC), was adopted, in which the <:;onfiguration ofelectrodes should be designed.

1. Introduction

2.1 Design of the Electron Gun
When a single-bunch beam is generated from the linac
the pulsed beam at a width of 5 ns in FWHM is injectedfrom the electron gun. The width is determined from theoperational conditions of the SHPBs. The peak current of
the electron beam from the gun was 30 A for generating
the electron beams having the maximum charge of 67 nC.
In the design this value is made to be the goal of themaximum obtainable peak current at an anode voltage of
100 kV. The main properties of the gun are listed inTable 1. The diameter of the cathode is 20 mm, which issmaller than the previous one. Hence, the higher beam
quality is expected. The area of this cathode is 1.5 times
that of Y-796 (EIMAC) which is generally used forelectron linacs.
The configuration of the electrodes of the gun wasoptimized by calculation with an EGN2e simulation code.
The orbits of the electrons and the equi-potential planes
obtained by the calculation are shown in Fig. 1. In thisdesign the electron beams having a peak current below 30
A are stable in the gun. The current density of electrons at
the cathode is 10 Ncm 2 in maximum, which is 6.5 times

The 38 MeV L-band (1300 MHz) electron linac in
The Institute of Scientific and Industrial Research (ISIR),
Osaka University was constructed in 1978 for generating
the high-intensity single-bunch electron beams. After theimprovemen t of a subharmonic -prebuncher (SHPB)
system, a charge of electrons in the single-bunch beam of67 nC was obtained in maximum [1]. In the case thepeak current and a pulse width of the electron beam
injected from the gun were 30 A and 5 ns in FWHM,respectively. In order to achieve such a high current a
triode gun having a cathode of a large area 51 mm indiameter was used. The high-intensity electron beams
have been utilized to excite matters for analyzing ultrafast phenomena induced by the irradiation. Ma11y
Table 1 Characteristics of the previous and the new gunsother researches are also being made in various pulse
Model-12modes of the beams.
YU-156
Generation and application of the short electronStructure
Electron gun C-G assembly
bunches and the coherent light from the high-energy
Relative price
HighLowelectron beams are researches in the new fields.
Maximum peak current (A)30
30Brightness and pulse characterisitcs of electron beams
C diameter (mm ~ )51
20
are important factors, which depend strongly upon theC-G
distance(mm)2.5
0.15
features of the electron gun. In the present work a new~2000
electron gun was developed in order to improve the Maximum G drive voltage (V)700Pulse rise-up time (ns)characteristics of the electron beams of the L-band>1.5<1linac.
2~3 daysOut-gassing time
2~3 hours
C: Cathode, G: Grid-17-

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

that of the previous one, and hence, relatively strong
focussing is required for the configuration of theelectrodes.

Wehnelt
Anode60

2.2 Test Bench for the Electron Gun and the GridPulser System

The characteristics of the new electron gun was210
180
investigated with a test bench consisting of the gun,o150
a grid-pulser system, magnetic lenses, a beam current Cathodemonitor and a high-voltage power supply.
Fig. 1 Optimized configuration of the electrodes of the new
For the grid-pulser of the electron gun used inelectron gun and the calculated results for the electron
the present experiments four avalanche pulsers areorbits and equipotential planes at an anode voltage of 100
connected in parallel. In each pulser three transisters
kV for a peak current of the beam of30 A.
are connected in cascade. This was designed to havea relatively low impedance. The shape of the output
Anode Voltage (kV)pulse of the grid pulser is nearly rectangular and has a~~--------~----~----~~---100___,._9()
width of 5 ns. The peak output voltage of the pulser was---+---80
35-70
f'tx:ed and was estimated to be about 700 V during----a--60
operation. The injection current from the gun was~so-40
controlled by changing the bias voltage supplied to the'E: 25-30-20~·
cathode.

$ao
5 20

2.3 Results for the Experiments in the Bench Testu.::.!.
IV~-101510In Fig. 2 the peak current of the pulsed electron
beams from the gun, measured with the beam-current
monitor, is plotted versus the bias voltage and the anode200300400
100500600
voltage . At a certain anode voltage, with decreasing theBias Voltage (V)bias voltage the peak current increases, and then saturates,
Fig. 2 Dependence of the peak current of an electron
as shown in this figure. The cause of the saturation was
beam from the gun on the grid bias voltage andestimated to be the space-charge effects working in thethe anode voltage: pulse drive voltage for theelectron beams between the grid and the anode [2]. At agrid is estimated to be 700 V.
point indicated with "a" in this figure the measured pulse
shape of the electron beam from the gun is shown in Fig.
3. The response of the gun to the pulsed voltage fromthe grid pulser was improved compared with the previous
one. The pulse width is about 5 ns which satisfies the:::5.
condition for generating the single~bunch beam. In Fig. 'ECD
4 the maximum peak current obtained is plotted versus ...
:; -10the anode voltage. At a voltage of 100 kV the maximum 0current is 30.1 A. This value well agrees with that c:eobtained in the design. The present results are also listed 0 -20
in Table 1. Most characteristics of the gun was much .!!1w
improved.

3. The Characteristics of the Beam from the Linac
Equipped with a New Gun
The characteristics of the single~bunch beam
generated from the linac equipped with a new gun wereinvestigated. By using the new gun which has a cathodesmaller than the previous one, beam quality and theefficienty of the beam transportation are expected to be.improved.
3.1 Accelerator System and Operational Conditions-30 L---------1------L----_J__----.J__--____JL_____I-10-5
15051020Time (ns)Fig. 3 Pulse shape of the electron beam from the gun at ananode voltage of 90 kV for the point indicated by"a" in Fig. 2.

The accelerator system of the ISIR linac isschematically shown in Fig. 5. The linac is equippedwith an electron gun, three SHPBs (two at the twelfth-18-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

100 MHz 108 MHz

216 MHz

1.3GHz1.3GHz...... 30~....c:~:::luElectronGun20c:

12thSHPB

12thSHPB

6thSHPB

PreBuncberbuncher

Accelerating Waveguide....eFig. 5 Schematic diagram of the ISIR linac.

ru 10

electrons in a bunch was 73 nC, which is higher than thatof the previous value .

&l.::.<"'Q)c..

Acknowledgemento.-~~L-~~--~-L~--J-~--L-~~

020

40
60
80
Anode Voltage (kV)

100120Fig. 4 Dependence of the maximum peak current of theelectron beam from the gun on the anode voltage.

The authors would like to thank the staff of thetechnical expert workshop at ISIR for producing theelectrodes of the electron gun.
Referencessubharmonic of the fundamental and one at the sixth),
fundamental bunchers and an accelerating waveguide 3 mlong. The maximum beam energy is 3 8 MeV for no
beam-loading. The details are described in Ref. 3.
3.2 Measurements of the Beam Characteristics

Pulsed electron beams were injected from the gun at apulse width of 5 ns in FWHM, a peak current of 24 A andan energy of 104 keV. The peak powers of microwave
supplied to the three SHPBs were 4, 12, 13 kW,
respectively. The energy for acceleration was 27 MeV.
The charge of electrons in a single-bunch beam wasmeasured with an aluminum block placed at an output
window for the electron beam of the linac. The maximum
charge of electrons measured is 73 nC/bunch. This yalue
is higher than that for previous one. In the previous' casethe peak current of the electron beam injected from the gun
was 30 A. The difference between the beam currents forinjection from the guns is attributed to the improvementof the efficiency of the transportation of the beam in thepresent case, according to the higher quality of theelectron beam. The energy spread of the electron beam isabout 2% in FWHM. The charge of the single-bunch
beam is expected to be increased for higher injection
current from the gun. The other characteristics of theelectron beams are being measured.

[1] S. Takeda, K. Tsumori, N. Kimura, T. Yamamoto,T. Hori, T. Sawai, J. Ohkuma, S. Takamuku, T.
Okada, K. Hayashi and M. Kawanishi, IEEE Trans.
Nucl. Sci. NS-32 (1985) 3219.
[2] S. Okuda, S. Suemine, T. Yamamoto, T. Kozawa andS. Tagawa, Proc. 22th Linear Accelerator Meeting,(1997) (in press).
[3] S. Okuda, Y. Honda, N. Kimura, J. Ohkuma, T.
Yamamoto, S. Suemine, T. Okada, S. Takeda, K.
Tsumori and T. Hori, Nucl. Instr. and Meth. A358
(1995) 248.

4. Conclusion
For the purpose of improving the characteristics of theelectron beam generated with the L-band linac at ISIR, a
new electron gun using a YU-156 cathode-grid assembly
was developed. In the bench test of the gun the electron
beam having a pulse width of 5 ns in FWHM and themaximum peak current of 30.1 A was obtained at an
anode voltage of 100 kV. The single-bunch beam wasgenerated from the L-band linac equipped with the newgun at an energy of 27 MeV. The maximum charge of-19-