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

Self-ampl ified

Emission

inaHybrid-FE L

Toshiyuki OZAKI
Accelerator Laboratory, High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba-shi, lbaraki-ken, 305, Japan

1.1 Setup for experiments

Abstract

Figure 1 shows the injector for prebunched beam
FEL experiments (refs. 4 and 5). The injector consist of aExperiments on self-amplified mic.rowave emission
were performed at the hybrid-PEL test-stand. The injector
has a two-cavity buncher which can be operated as aklystron. Without feeding external input RF power, the1.6-MeV induction linac and a prebuncher. The linac
consists of eight induction cells. The electron source is acarbon cathode.
The gap between the cathode and theanode is 10 em. The beam is emitted under a strong electricfield. The beam is transported by a solenoid magnetic field.

output from the cavities was measured. Also, the power
evolution in PEL without input was measured. These are· reported in section 1 and 2, respectively.

1.

Auto-bunchingbunching

The prebunching system consists of an input cavity andan idle cavity. These are analogous to those of a klystron.
InandEmission

inThere have been many experiments on the auto(self-excited bunching) of high-current beams

through cavities. At first, M. Friedman reported on theauto-bunching phenomenon (ref. 1). The frequency of thebunching was 500 MHz. The degree of amplitude
modulation exceeded 80%. The beam had a current of 15 kA

solenoid field near to the cavities was strengthen to 3 kG,
compared to 0.4 kG in the other region.
The beam profile can be expressed by an envelopeequation. A simulation with a normalized emittance of 0.1

and a voltage of 500 kV for 50 ns duration. I.A. Grishaev
also observed auto-bunching of 300 MHz (ref. 2). The autobunching occurred at a current in excess of 6 kA; theamplitude of the modulation increased with increasing thecurrent. However, K. Minami had not observed the autobunching because the beam was only on the order of 100 A
and the frequency, 2830 MHz, was too high (ref. 3).
A bunched beam generates coherent emission. Thecavity extracts power from the beam, and further bunches
the beam. Self-amplified emission is observed in cavities.

auto-bunchingexperiments, the beam had akinetic energy of 1.5 MeV with a 50 ns duration. The beam
current at the exit of the induction linac was 497 A, and thecurrent was 328A at the e.xit of the prebuncher. To pass thebeam through the narrow apertures of the cavities, theCavities

cm·rad. was consistent with the experiments. An emittancemeasurement performed at 700 keY was 0.41 cm·rad. (ref.
6). The improvement is considered to be due to the R&D
work on the geometrical shape of the cathode and anode.
The prebuncher is of conventional design, exceptthat the velocity (v) of 1.5 MeV beam is 0.967c , which is
close to the velocity of light (c). The cavities use the TM010
mode. The radius of the cavity is 26.77 mm, the radius of
the beam aperture is 15 mm, and the length is 10.2 mm. The
distance (drift space) between the cavities is 90 mm. Bothcavities have the port connected to the waveguide. The RF
source to excite the cavity is a 9.4 GHz magnetron. The
detected signal from the cavity is observed by aoscilloscope outside the shield.
Under the usual buncher operation, about 40 kW wasfed into the first cavity, the output power of 150 kW wasextracted from the second cavity. Therefore, the power
gain of this two-cavity klystron was 6.1. This agrees with
thevalue

whichisexpectedfrom

theusual

small-amplitude klystron theory.

Solenoid
1.2 Measurements of the Wake Field in the First Cavity

Induction Linac
Waveguide

Fig. 1 : FEL injector

Thefirstcavity

was connected with a longwaveguide and terminated at a calibrated diode. The wakefield from the beam was measured. Figure 2-a shows 20-326-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997
successive shots. The voltage on the diode changed at eachshot. The average voltage was 35 mV. The noise level was200

about 5 mV. The power corresponding to the voltage of 35:> 160-s

mV was 37 mW.
The power could be estimated as follows. Consider a120Q)

00
ttl

beam with radius b passing through a cylindrical cavity

80+Jwith inner radius a and length I. The current is assumed
to be linearly increasing. The induced field ( E, ) is given....-l040:>

0by the vector potential (A, );0E =-~iJt •412816ShotsIHere, A, is a solution of the following wave equation,

Fig.3:Amplifier experiment

which has a driving term:
M
1 iJlA,
1 . (vt-z), - c2 iJt2 =- e c2 Jo-whereT--r;-0( OSrSb) ,is the rise time of the _current, and j 0 is thecurrent density. Expanding the vector potential and thecurrent in terms of the normal mode series of the cavity,1.3 Measurements of the Power from the Second Cavity
The power from the second cavity was measured in
the absence of any input RF power to_ the first cavity.

and substituting them into the wave equation, we obtained
a solution of A, . The excited electric field on the axis of theFigure 2-b shows 20 successive shots. The average voltage

cavity is

was 77.5 mV, which corresponds to 114 mW. The power
gain to that from the first cavity was 3.1.
about

half

klystronthatoperationoftheusual

This gain was

high-power(as describedtwo-cavity
Thus, theseabove).

results demonstrate that self-amplified emission occurred
and the power isp=in the cavities.

E~( ne0oia 2~:(2.405)),

1.4 Experiments as an Amplifier with Low 1nput Power
where I= trb 2 j 0

is the total current;/ 0 and / 1 are Bessel

As the input source, a Gunn diode was used. Afunctions.
The Q-value was 4300, and the external Q-value ( QE )microwave power of 9 mW was launched into the firstwas 550. A power of 32mV was obtained by assuming awas measured. The average voltage in 20 shots (Fig.3) was

build-up time of 70 ns. This nearlyagrees with thecavity via an isolator. The power from the second cavity
115.5 mV. It corresponds to 200 mW at the exit of the secondcavity. The power gain was 4.3.

experimental value of ·37 mW.

As easily can be seen, the variation in the Fig.3 issmaller than200:>

s160Q)

12000
ttl

+J
....-l0:>

thevariationin Fig.2-b.

Thestandard

deviation of Fig.3 ia 47 mV, and the ratio to the average(a)power is 61%. Also, those of Fig.3 are 30.4 mV and 27%,respectively. It can be understood that the external input

80... .

40

power stabilize the output power.

..

Average

2.

0
0

12

16

20

Self-Amplified

SpontaneousEmissioninFEL

Shots-:>s-

The Self-Amplified Spontaneous Emission (SASE) in200!

FEL has attracted much interest in recent years. PELs in'(b)150

the oscillator mode can not be operated in the soft X-ray
regionQ)

00
ttl

+J

100

....-l50:>

0

Averagebecausefabrication

withofahighdifficulty
normalconcerningreflectivityandmirror
highpower-endurance. The use of SASE eliminates the need to0use mirrors. On the other hand, PELs in the single-pass
01216

amplifier mode, like the KEK X-band FEL, require an input ·20Shotssource. Since the SASE-mode FEL does not need any input
sources, the frequency of the output can be varied byFig.2:Power from cavity

controlling the beam energy and the wiggelr field.

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The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997
In the SASE-mode FEL, the electron beam throughwiggler will be self-bunched due to interaction with its
own radiation field. The radiation starts from the beam

100noise. If the wiggler is made long enough, and the beam
intensity is large, the spontaneous emission starts to beamplified

byanditself,beam,thetheradiation

26

growdB/m

exponentially.
0.1 ':-"-'-'-':-'-:'~"'-'-~......__L...o....<.........L..L.............J
2.5
21.5
1
0.5
0

2.1 Setup for Experiments

Wiggler length (m)The electron beam is generated in the induction linac,
and the beam is guided through a IS-period planar wigglerwith a period length of 16 em. The field in each period can
independently controlled by varying the charged
voltage of the capacitors in the pulsed power supplies. Themaximum field is 2 kG. The electron beam undergoes aFig.5:Prebunched beam experiment

bewiggling motion

interacts withandoversized waveguide (110 mmXa RF wave

inanpower line to the entrance of the calibrated diode wasdB. The least-observable RF power was abou.t 5 kW.

55 mm). The input RF

The obvious signal on SASE had not been observed.

source is a magnetron. The magnetron was turned off in

Severalreasons

exist.

the SASE experiments.

observesignalsinfractionof2.2 SASE from 750 keV BeamIn 1989-92, FEL experiments using a 750 keY beam wereperformed. The beam source was an one-stage induction
linac with 4 cells. The output RF power was passed thoughaRF-window

andradiatedinan

receiving horn picked up the signal.

anechoicroom.

AThe output signals

88.6

theThe new RF monitor could not
low-power

thebeambeginstoregion.
hit

theFurther,walltheof thewaveguide above the 1.2 kG wigger field. The center of theFEL resonance seems to exist at about 1.4 kG. The observed
signal may be caused by such a chopped beam.
2.4

RF Evolution by 1.5 MeV Prebunched BeamAs described above, SASE starts from the noise field in

were not stable because of a fluctuation of the voltage on

the beam. Therefore, the emission field

the induction linac.

evolve from the self-field in the prebunched beam as well.
The prebunched beam was produced by cavities(described in section 1 ). A beam having current

SASE had been observed.

The maximum power was 15kW, and the average was 4 kW. Fig. (4) shows the evolution

is expected toamplified gain was 20.3 dB/m.

modulation of 45% was introduced into the wiggler. Thewiggler field was 1.25 kG. The current was 730 A at the23 SASE from 1.5 MeV DC Beam

entrance of the wiggler and 50() A at the exit. Fig.S shows
the power evolution (ref. (5)). The average gain was aboutof SASE. The average SASE gain was 33.6 dB/m while theIn 1.5 MeV beam FEL experiments, the RF system wasimproved. The output RF power was absorbed into a longdummy load. The known fractions of the power were

26 dB/m. The power exceeded 100 MW. The saturation
length was 1.1 m, which is short compared with that of 1.9
m in ilie case of DC beam.

extracted by a directional coupler, which was installed in
front of the dummy load. The attenuation from the high-Acknowledgments
The author would like to thank to S. Hiramastu, J. Kishiro,
K. Takayama, K. Ebihara for their useful advises. He also
thanks Doctor T. Monaka for his contributions on the 750
KeY FEL experiments and Doctor K. Saito for 1.5 MeVexperiments.

33.6
dB/m

References(1) M. Frieman : Phys. Rev. Lett. 32 (1974) 92

Noise Level(2) I.A. Grishaev : Sov. Phys. Tech. Phys. 19 (1975)1087(3) K. Minami et al. : Jpn. J. Appl. Phys. 22 (1983) 1449

Wiggler length (m)(4) K. Saito et al. : Proc. of the 9th Ac. Sci. and Tech., 459(5) K. Saito et al. : N.I.M. A-375 (1996) 237

Fig.4:Evolution of SASE( 6) H. Kurino et al. : Particle Accelerator ( 1989) 1297

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