JAERI-Conf 95-021

PB42
X-band Prebunched FEL Amplifier
Kazuyoshi Saito 1, Ken Takayama2 , Toshiyuki Ozaki2,Junichi Kishiro2, Kiyokazu Ebihara2, Shigenori Hiramatsu2
1 Hitachi Research Laboratory, Hitachi, Ltd.
7-2-1 Omika, Hitachi, lbaraki, 319-12, Japan
2 National Laboratory for High Energy Physics (KEK)
1-1 Oho, Tsukuba, Ibaraki, 305, Japan

Abstract
We have designed and constructed a prebunched free~lectron laser (FEL) amplifier consisting of a prebuncher

and a standard FEL. Microwave power saturation with ashort wiggler length was realized by the prebunched beam.
We also examined the microwave phase evolution. It wasfound the adjustable range of the output microwave phaseby changing the input microwave phase was restrictedwithin a narrow band due to spontaneous emission radiatedby the prebunched beam.

1. Introduction
We developed the X-band FEL [1-3], which is acandidate for possible high power microwave sources(-IGW) for future linear colliders and other applications.
We tried a new sort ofFEL experiment in which the FELis driven by a prebunched beam [4-7]. The experimental
configuration is that of a standard FEL accompanied by aprebuncher. Hereafter we call this a prebunched FEL. In
this paper we describe the prebunched FEL experiment
which has two important purposes. The first is to realize
a compact I efficient FEL with a large gain per wiggler
length. By attaining the power saturation with a short
wiggler length, we can enhance the power through theremaining length by tapering the wiggler field. The
second purpose is to experimentally control the output
microwave phase by changing the phase of the seed
microwave to the wiggler. Achieving the latter purpose isa crucial issue in FEL physics in a multi-stage FEL which
is driven by a bunched beam in the FEL-TBA linear
collider [8].

with a fraction of the power introduced from the external
magnetron, leading to modulation, in the beam-velocity tosome degree. Velocity-modulation gradually becomescurrent-modulation. The gain cavity, placed 9cm
downstream the input cavity, is excited by induced currentmodulation and can give the driving beam further velocitymodulation. To get the current-modulation of 30% at thewiggler entrance, the cavity gap voltage of 170kV isrequired according to a theoretical estimation. In the twocavity configuration, the input cavity requires a microwave
power of 15kW to achieve the gap voltage of 170kV in thegain cavity. The maximum surface electric field can bereduced to 13MV/m in the gain cavity when beam-loaded,
which should be sufficiently low enough to avoid surface
breakdown. The same sized pill-box cavities without nose
cones are employed for the input and gain cavities.

GunIDUFig.l Configuration of the prebunched FEL amplifier.

2. Prebuncher
A schematic view of the prebunched FEL amplifier isshown in Fig. I. The prebuncher is placed between thedownstream induction unit {IDU) and the wiggler. An
electron beam of 1.5MeV/850A generated by the induction
gun is prebunched at the same frequency as that of the FEL(9.4GHz) before entering the wiggler. An input microwave
signal is amplified through the wiggler due to the FEL
interaction (stimulated operation regime ). Even if nomicrowave signal is inputted, the strong spontaneousemission radiated by the prebunched beam should be
amplified in the same way (superradiant operation regime).
Considering the limited magnitude of the available
microwave power, we have adopted a prebunching section
with a two-cavity configuration as illustrated in Fig.2. It
is similar to the bunching section of a conventional
klystron. The input cavity is excited in the TMOlO mode

Fig.2 Prebunching cavities.

3. Prebunching Experiment
A magnitude of current-modulation attained at theentrance of the wiggler was measured in the frrst step of
the prebunched FEL experiment to optimize and evaluate
the prebuncher system. A bunch-monitoring cavity-368-JAERI-Conf 95-021

immersed in an axial magnetic field was installed at thelocation corresponding to the wiggler entrance. The
magnitude of current-modulation was determined from themicrowave power extracted from this cavity. Its extemalQ (Qext) is designed to be much smaller than the unloadedQ (Qu ) and the beam-Q. Under this approximation, theAC beam current lac is estimated from the extracted power
P out,mon by the following relation'Q=--u-;P.:::-o-ut-,m-on2
Iac == TTF(O) Rsh Qext
where Rsh is the shunt impedance and TTF(O) is thetransit time factor for a particle passing through the cavity
center. This method is not so precise, but it is sufficientto estimate the fundamental component of modulation.
Figure 3 shows the experimental results in which themaximum current-modulation of 45% was achieved withan input power of 35kW and the dependence of the currentmodulation on the input power seemed to be fairly
consistent with the design calculation. Current-modulation
beyond 45% was limited in practice by pulse-shortening
phenomena at the gain cavity.

0.5,

-

1oo ·············r············~r·~~~-·

c:ti :.;tzt
•r:f

1~

oo ••

l~c/Idc j45%·9/···-]-··············i············*mutat~on·······:,/ . . . . .J..............l.....!?.....IhcLidc..lfi%. .. .

0.01 •:!
0.5:~Ji ----- s'mulatlon
11.5Z (m)22.5

Fig.4 Microwave power evolutions in the case ofsuperradiant operation.

0.4~-~

00.6CJ

However, an enhanced output power has not been observed
yet, probably due to poor beam transport in the wiggler.
The beam loss was rather severe around the resonant field(-1.4kG), where the beam should be well-bunched andtrapped in the pondermotive potential ideally. Although
the saturation power was a maximum for the wiggler fieldof 1.25kG, at which the beam loss was mitigated, thebunch structure in the pondermotive potential becomes
rather elongated due to mismatching in the longitudinal
phase space, resulting in poor efficiency of the wiggler
field taperings.

0.3

4.2 Stimulated Operation Regime

0.2(a) Power evolution

CJ~

0.1
10203040

soPin, input (kW)Fig.3 Current-modulation attained at the wiggler entrance.

4. FEL Experiment
4.1 Superradiant Operation Regime

A seed power of 60kW was introduced into the wiggler
through a microwave input coupler with beam-passing
metal-mesh which reduced the transmitted beam-current toDC500A. Figure 5 shows the power evolution for
current-modulation of 40%. Its initial growth was very
sensitive to the phase relation between the seed microwave
and the current-modulation. When both were in-phase, thefield evolution was quite similar to that in the superradiantoperation regime. When both were in anti-phase, a rapid
damping in power was observed at the initial stage of thewiggler and beyond that the power grew quickly. The
current-modulation being larger, the field recovery became
quicker and the power-evolution curve in the case of theanti-phase approached that of the in-phase after the quick
recovery. This is quite clear from the fact that themicrowave power PRF at the initial stage of the wiggler
may be approximated as

In this regime, a prebunched beam of 1.5MeV/DC750A
was introduced into the wiggler and the transmission
efficiency through the wiggler was 70%. Figure 4 shows
the power evolution measured at the wiggler field of
1.25kG for two cases of current-modulations of 45% and10%. Both results indicated stagnation in power-growth at
the wiggler length of 0.4m. They had different saturationlevels and saturation distances; 120MW/l.lm and
150MW/1.5m, respectively. These features could be wellreproduced by simulations. The field growth stagnation
where e is the phase difference between the seed microwave
was caused by debunching. The prebunched beam isfield A R,IN and the prebunched superradiant emission
debunched once and again bunched due to space-charge
AR,PB· The prebunched superradiant emission AR,PB
oscillation and partially trapped in the pondermotive
grows rapidly and surpasses the seed microwave fieldpotential. Without the space-charge effect, a significant
AR IN after a few wiggler periods for our beam parameterspart of the prebunched beam would be trapped in thein the (high-gain) Raman regime. If cose is negative, thepondermotive potential without any serious debunching.
microwave power PRF drops rapidly from the seed power
To increase the output power beyond the saturation
due to the last term and soon increases again due to thelevel, we carried out tapered FEL experiments.
term. These features can be well-reproduced bysecondSimulations told us that an output power over 350MW
and are quite different from those ofsimulations
fields.
wigglertaperedappropriate
can be achieved with-369-JAERI-Conf 95-021

prebunched FELs operating in the low-gain regime [5][7].
In the case of low-gain prebunched FELs including an
optical klystron, the second term is negligible, because
AR,PB<<AR,IN· If cose is negative, the power gain isnegative namely PRp<PRF,IN·( lac/Ide = 0.40 )100changed the input microwave phase from o· to 360. every
30'. We can say that the current-modulation being higher,
the output microwave phase was determined by thesuperradiant emission of the prebunched beam. The
adjustable range of the output microwave phase became
restricted within a narrow band. When the currentmodulation became higher than 30%, the adjustable range
was limited to less than 40' for the input microwave
power of only 60kW. These phenomena were expectedbased on the simulations.
~I! 300 \ ........... l..............l.... . ~. .J~P.~E~.~~-~.L..

-=...

~
c\\~ 200~ 0.01

~

01

0.5

1.5

2::c.s{/) 100

2.5=Z (m):e'<Fig.5 Microwave power evolutions in the case ofstimulated operation.
Microwave phase was measured with a double balanced
mixer using a magic tee. The phases of input and output
microwaves of the wiggler were measured with reference tothe magnetron's output microwave phase. Figure 6 showsthe experimental results of phase evolution for currentmodulation of 15%. Initial phase variations are caused by
a complicated mechanism, because intense FEL interaction
starts in the matching section where a non-resonant uptapering field is present for beam-orbit matching.
However, we note that the initial phase difference of 180'
decreased to 80' due to FEL interaction with theprebunched beam.
( lac/Ide = 0.15 )18 0 I'""""'...,....,"'T"............,..............T"""......'O"T".......-.......,"tS90~II)

~.c

0~

~~

\,f. .;__t~~l :jo.~~~~~~~--~~~--~-~--~~
0.5
0.4
0.3
0.2
0.1
0Fig.7 Adjustable range of the output microwave phaseby changing the input microwave phase.

5. Conclusion
The saturation power of 120MW was attained at thewiggler length of l.lm by a 1.5MeV prebunched beamwith a 45%-modulated DC750A current. However, FEL
performances were deteriorated by beam losses in thewiggler.
The controllability of the output microwave phase wasexamined by changing the phase of the input seed
microwave to the wiggler. When the current-modulationof the injection beam (1.5MeV-DC500A) was higher than30%, the adjustable range was limited to less than 40• by
the input microwave power of only 60kW.

References~~i -------simulationiii

Current Modulation ( lac/Ide )(b) Phase evolution-ii-90-180 ....-..........................,_...............r.................~...o..o.........
0.5
0
1.5
1
2.5
2Z (m)
Fig.6 Microwave phase evolutions for different initial
phases of the input microwave. The zero phase iswhen the beam bunch-center is located at themaximum decelerating field of the input microwave.
The response of the output microwave phase wasexamined for the change of the input microwave phase ofthe wiggler. Figure 7 plots the experimental results of theadjustable range of tlile output microwave phase when we[1) K.Takayama, J.Kishiro, K.Ebihara, T.Ozaki,S.Hiramatsu and H.Katoh,Nucl. Instr. and Meth. A358(1995)122.
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