Proceedings of the 20th Linear Accelerator Meeting in Japan (September 6—8,1995, Osaka, Japan)⑰ -22
A SINGLE PREBUNCHER CAVITY RESONANT AT
FUNDAMENTAL AND HARMONIC WITH HIGH CAPTURE EFFICIENT
Y.LWANG, T.EMOTO, M.NOMURA and LSATO*
Oarai Engineering Center (OEC)Power Reactor and Nuclear Fuel Development Corporation (PNC)
4002, Narita-cho,Oarai-machi，lbaraki-ken,311-13, JAPAN*National Laboratory for High Energy Physics (KEK)
1-1 Oho, Tsukuba-shi, lbaraki-ken，
305, JAPANABSTRACT
A new prebuncher system is mentioned. It has high capture efficient There is only one prebuncher
cavity which TM110 mode resonated a t the fundamental frequency fo and TM310 mode resonateda t second harmonic frequency 2fo.

1 . INTRODUCTION
For PNC high power CW linac, its beamenergy is lOMeV, beam current 100mA, energy
spread lower than 1.0% and bunch width lower
than 5°[1]. The injector consists o f a electron
gun, chopper, prebuncher, buncher and one
accelerator section. The beam from electron gun
is 300mA at 200kV. After chopper system, in one
RF period only 120° bunch length passes throughthe chopper slit. The 120° bunched beam enters
the prebuncher bunched to 20°. It means that ahuge amount beam power (200m A 200kV) willloss on the chopper slit. It w ill cause a big heating
dissipation problem.
In principle a normal prebuncher cavity canbunch beam from about 120° to 15°. It is shown
on F ig .l. Its capture efficient is about 33%. If
the field in the cavity is not a sinuous wave but asaw-wave, it w ill be a ideal prebuncher shown onFig.2 , one period 360° beam can be bunched in15°. Its capture efficient is 100%. But it is very
difficult to make such field.
Some laboratories use the fundamental andsecond harmonic to increase beam capture
efficiency[2,3]. But they used two cavities that
one is resonated at the fundamental frequency/oand another is at second harmonic frequency 2fo.
We had already developed single chopper cavity
resonated at the fundamental frequency/^ and at
second harmonic frequency 2/o[4,5].

F i g .1 A normal prebuncher cavity
characteristic.

So it w ill be realized that one prebuncher cavity
can be resonated at the fundamental frequency
fo and the second harmonic frequency 2fo. Its
capture efficient w ill be 70 %.

E中in
Fig.3 The fundamental electric field, thesecond harmonic electric field and theirsuperposition field.

(a) electric field(b) magnet field

Fig.4 TM no mode fieldin the prebuncher cavity.

Fig. 2 A ideal prebuncher cavity characteristic.

% PREBUNCHER CAVITY
Fig.3 shows the fundamental electric field Efo
and the second harmonic electric field Elfo andtheir superposition field E t. If one chose TM 110
mode and TMsio mode to bunch the beam, but
in one cavity the frequency ofT M sio mode is notequal to two times o f the frequency o f T M nomode. The special stub tuners are needed toadjust the frequency o f two modes to makeTM nomode resonate at the fundamental
frequency/^ and TM310 mode at the second
harmonic frequency 2fo. Fig.4 and Fig.5 show
TMllO mode fields and TM310 mode fields in(a) electric field
Fig.5(b) magnetic field

TM310 mode fields in the prebunchercavity.

the prebuncher cavity. Fig.6 shows the structureo f the prebuncher cavity. There are four mainstub tuners to adjust the frequency in the opposite
direction changing for TM 110 mode and TM310
mode respectively, and other four side stub tuners
to adjust the frequency in the same direction but
different changing ratio.
According to the MAFIA calculation, the cavity
size is a=110.0mm, d=129.55mm, h=60.0mm,
g=20.0m m , rb=l 0.0mm , rsm=8.0mm, andhsm =l 6.75mm.

Fig.7 Results o f the beam longitudinal dynamiccalculation.
STma|nie -©

xafter suitable drift space. Its capture efficient isabout 70%.
It is not so difficult to realize according to ourchopper system test.

References

Fig.6

Structure o f the prebuncher cavity

3. DYNAMIC CALCULATION
According to the beam longitudinal dynamic
calculation, using different ratio o f the amplitudeo f fundamental and second harmonic, and giving
some offset angle, the results are shown on Fig.7.
Vg=ASin(cot)+0.3ASin(2cot+7C-7c/8).
In this case, about 252° beam w ill bunch in 30°[1 ]Y.L. Wang,et al/'Design of High Power ElectronLinacat P N C ' Journal of Nuclear Science andTechnology, 30[12], Dec.1993, ppl261-1274.
[2] P.M. Lapostolle and A.L.Septier, "LinearAccelerators”,1970, P.506[3] A.Moretti,et.al. IEEE Trans. Nucl. Sci. Vol.NS-28,No.3, June 1981，P. 304.
[4] Y.L. Wang,et al. "A Novel Chopper System with
Very little Emittance Growth."，Proceedings of the 4th
European Particle Accelerator Conference, in London,Junel994.
[5] Y.L. Wang, et al."A Novel Chopper System forHigh Power CW Linac" Proceedings of the 1994
international Linac Conference, in Tsukuba, Japan, Aug.
1994, P 205.