STUDIES ON ELECTRON INJECTOR OF LINAC
FOR PICOSECOND SINGLE BEAM
Hitoshi Kobayashi, Toru Ueda, Toshiaki Kobayashi,
and Yoneho Tabata
Nuclear Engineering Research Lab., Faculty of Engineering
Univ. of Tokyo, Shirakata Shirane 2-22, Tokai-mura
Ibaraki, 319-ll, JapanAbstract
A 35 MeV S-band linac with capability of producing picosecond
single electron beams (pulse width < 18 ps) has been used as irradiation sources of pulse radiolysis-systems for research of radiation chemistry with a time response of 18 ps. Generally speaking,the intense irradiation sources are desirable for research of radiSo the program to increase the intensity of picoation chemistry.
second single beam has been continued. The pulse width of emission
from an electron gun must be less than 0.8 ns to produce a picosecond single beam without satellite pulses. To increase the intensity of picosecond single beam, the electron injector which canemit and transmit the intense and short electron beam is required.
The development of electron guns and improvements of the focusing
system will be discussed.
Electron GunElectron gun which contain a grid-cathode structure taken froma planar triode was developed.< 1 > Short pulse response of the electron gun was measured using the grid modulator which has been usedin the present linac system. The output current of ~4 amps withpulse width of ~0.8 ns (F.W.H.M.) is obtained. The output of thegun is shown in Fig-1. The perveance of the electron gun is ~0.18
xl0- 6 • The profile of the output beam is affected by the anode.
To reduce the anode hole effect (as a concave lens), the anode hole
The electron beam profile
with a mesh made~of molybdenum is used.
with and without the anode mesh are shown in Fig.2. The electron
But thegun has good high frequency response and is unexpensive.
oxide cathode is used and has comparatively short life time at
So the electron gun that contain a barium
high current output.
impregnated cathode is now being investigated.
Beam Bunching and Capture Efficiency
Electron injector of the present system is shown in Fig-3.
Electron beam emitted from the electron gun is bunched by the subharmonic pre-buncher (S.H.B) operated at 1/6 frequency of accelerating microwave and is further bunched by the prebuncher (traveling wave type) and also the buncher section of the accelerating
When the emission with current of 5 amps andwaveguide I (Acc-I).
with pulse width of 0.8 ns is injected, the capture efficiency of
90 % is obtainable on calculation using one dimensional disk spaceIn the present linac system, the capture efficiency
charge model.
So the improvement of a focusing systemof only ~38 % is obtained.
is also important for increasing the intensity of output beam andis now being investigated.
-137-Beam Focusing System
As described, the transmissio n efficiency of the present injection system in not so good.
The electron beam orbits, focusedby various kinds of focusing systems, are measured at a test bench.
In the present injector the beam diameter oscillate with considerably large amplitude, judging from the results obtained by using
the test bench.
The beam diameter affect the space charge force
on Z-axis (acceleratin g direction). The larger the beam diameter
becomes the weaker the space charge force on Z-axis becomes. Thenear parallel flow electron gun will be disirable to transmit thebeam with constant diameter.
The parallel flow electron gun has
been developed by adjusting the shape of the gun electrodes. The
beam profile of the near parallel flow gun is shown in Fig-2.
The Brillouin flow which keeps the beam diameter 10 mm (same asthe diameter of the cathode) in now being investigate d.
Referencel)

H. Kobayashi et al. J. Fac. Engng Univ. Tokyo 1981, 36B, 85

CURRENT: 4A

L--------·-----·-----·----- --------=-"' c:ic(a:~:::;c(wm0---~~WITH ANODE MESHANODE-~ PARALLEL FLOW GUN' - - - - - - WITHOUT ANODE MESH

Q23456(cm)

DISTANCE FROM ANODE

Fig.l The output current ofthe gun.lA/div, 500
psjdiv

Fig.2 The beam profile of the gun

PREBUNCHER

PUMP-OUTTO KLYSTRONBEAM CURRENTMONITOR

Fig.3 The block diagram of the injector-138-