BEAM CHARACTERISTICS OF PF LINAC INJECTION SYSTEM
A. Asami, S. Ohsawa and J. Tanaka
Photon Factory,
National Laboratory for High Energy Physics

Abstract
The injector components of PF linac are briefly described. The capture
efficiency and bunch width of the bunching system were measured to give a good
agreement with the design expectations.
]. Accelerator Components and Systems A ' 2

The injector is a small linear accelerator consisting of the following
accelerator components; (1) an electron gun operated at a voltage of 100 kV,(2) a prebuncher and a buncher accelerating electrons up to about 5 MeV, (3)two of 2m long accelerator guides with which electrons gain an energy of
35 MeV.
A development was made in theelectron gun which has an assemblyof a barium-oxide coated cathodeand grid, currently used for aplanner triode. The gun has
advantages that the amplitude of
the grid pulse required is muchsmaller than is usually used, andthat a high current beam caneasily be obtainable with a small
diameter (Fig. 1).
The prebuncher is of trav8
eling wave type, and consists of 6cavities. The buncher consists of
two sections, one is a buncher
section with 9 cavities whose(1) grid lead (2) heater (3) insulator
phase velocity ranging from 0.75
(4) focusing (5) cathode (6) grid
to 0.98, and the other a regular(7) anode (8) current monitor
section with 12 cavities. The
Fig.l Electron gun assembly.
prebuncher operates with several
kW rf, accelerating electrons a little, while the buncher powered with an rf
power of about 7 MW. With this bunching system about 70% of the injected
electrons are designed to be captured and have a bunch width of less than 4°.
The microwave power for the prebuncher, buncher and two accelerator
guides comes from a standard klystron. The waveguide system for the prebuncher and the buncher is different from that for the accelerator guides inthat it has an attenuator and a phase shifter in the high power waveguides
pressurized by SF6 gas. In the beam transport system magnetic lenses andsolenoid coils are used for the low energy part of the injector, and quadrupoles for the high energy part, together with steering coils distributed along
the injector.
2.

Beam Characteristics 3 , 4

Test operation of the injector was made with an rf power of about 20 MW.
Beam currents along the injector were measured with four current monitors-125-located behind the gun (labeledas 01), in front of prebuncher(02), the 1st accelerator guide(03), the 2nd guide (04), andtwo in the first sector (10,11). At an injection current
the bunching and transportelements were adjusted toproduce the maximum accelerated
currents. The results are shownin Fig. 2. At low currents
about 70% of the beam arecaptured in a good agreementwith the calculation. When theinjected current increases, thecapture efficiency decreases alittle, but still remains at 60
to 55%.

100:z0en
en:::E~<C50·~·-~.

CM 01 CURRENT·~~·-·-•'=~=--·-·""'·.:::::::.·-·

7Z5 mA

=
1-457 mA

64 mmAA
Z15319 mA:::E<C.....

=
oL-~-L~~L-L-~~---01 oz 03 04 10 11 1Z
The width of a beam bunch
CORE MONITOR
was estimated from the beamenergy spread. To do thisFig.2 Beam transmission.
rrecisely, the energy spreadarising from other reasonsshould be much smaller than that<, >from the actual bunch width. In
5' o·
10'
20'
15'
order to satisfy this condition
approximately, the spectrum wasmeasured using an energy analyzing system isntalled at the1.5
end of the first accelerator
sector. There are 8 klystrons
in the first sector, and their>uphases were first carefullyil!!
adjusted for the electrons to be> 0.5
on the top of rf wave. The theinjector rf phase was deliberately varied from those of the_.?-·-·-,/
first sector in such a way thatO.OI L.,.-133::;:0=-3~40::--3~50::----:3:!-ID::---:3::70:---;;3!;10:---;;3!;;10:--;40:;;-'0
the center of bunch deviate fromIMA&(A)the rf peak by 5°, 10°, and 15°
over the sector to produce
Fig.3 Energy spectrum vs phase.
appreciable energy spread. At
the end of the electrons thus
accelerated had the energy spread which was mainly caused by the finite bunch
width. An example of measurements in shown in Fig. 3. The width changes
with the variation of the injector conuition, but it was usually less than 4°
and likely to be 2° to 3° when well adjusted, and showed the design expectation was satisfied.

e\;\Reference
1) A. Asami et al: Proc. 6th Meeting on Llnac in Japan, July 1981,SAPPORO, JAPAN (1981) p59.
2) J. Tanaka et al: Proc. 1981 Linac Conf., Santa Fe, Oct. 1981,LA-9234-C (1981) 360.
.
3) A. Asami et al: Proc. 7th Meeting on Linac in Japan, August 1982, KEK,JAPAN (1982) to be published.
4) S. Ohsawa et al: ibid-126-