JAERI-Conf 95-021

PB43
Development of Infrared Free Electron Laser System
Using Compact Linac
Tsuyoshi SHINZATO, Katsuji EMURA, Tsuyoshi HAGA and Hiroshi TAKADA
Harima Research Laboratories, Sumitomo Electric Industries Ltd.
1431-12, Harima Science Garden City, Kamigori, Hyogo, 678-12 Japan
Abstract
The present status of infrared free-electron laser (FEL)system using compact linac in Sumitomo Electric Industries
Ltd. (SEI) is reported. Characteristics of the beam transportsystem was studied through numerical simulations. For
precise simulating, initial condition of the beam at the endof the linac was examined in details. As a result, beamtransport condition to realize well-focused beam in undulator
was obtained. Spontaneous emission of peak power of
0.4m W was observed in recent experiment.

l. Introduction
We have developed a compact lOOMc V linac inHarima Research Laboratories. This linac not only provides
electron beam for compact superconducting SR ring NIJI-III,
but is utilized for infrared FEL experiments.

2. Compact linac lll
Main design parameters of the compact linac arc listedin Table I. The linac produces electron beam with pulse

Table I

Main design parameters of the linac.

Electron gun

CathodeVoltage
Emission current
Normalized emittance

EIMAC Y646B
200 kV - DC
1.5 A
7 1t mm mrad

SHPB

TypeFrequency
Input power

Standing wave
476 MHz

Prebuncher

TypeFrequency
Input power

Standing wave
2,856 MHz
5kW

Buncher

Jype
Frequency
Input power

Traveling wave
2,856 MHz
8MW

Accelerator

Jype
Mode
Frequency
Beam energy

Traveling wave
27r/3
2,856 MHz
100 MeV( short pulse)
76 MeV( long pulse )
IOOmA
0.5%
60 1r mm mrad
2 pps
22 MeV/mMacropulse current
Energy spread
Normalized emittance
Repetition rate
Accelerating gradient

4.6kW

width of 1j.Ls for injection into NIJI-III, lOj.Ls for FELexperiments. Accelerating rf power is supplied by klystron(Thomson-CSF TH2146). Pulse length depends on theconnections in pulse forming network (PFN) in the klystron
modulator.
Since the long-pulse operation mode has been designed
to marking FEL oscillation in target, which includes someof the essential elements to obtain high quality electron
beam suitable for FEL.
First of all, emittance of the beam should be reduced
for FEL experiments. We have acomplishcd normalized
emittance of 7 n mm mrad in the electron gun, where smallsize thermionic dispenser cathode (EIMAC Y646B) isassembled and high DC voltage of 200kV is applied toanode-cathode gap. The gun container is filled with SF 6
insulation gas, and the structure around the gun isdetermined by numerical simulation to avoid electric fieldconcentration.
Secondary, energy spread of the electron beam fromlinac should be narrow. Fluctuation of resonant frequency of
the accelerating tube causes energy shift of the beam. To
avoid that, temperature of accelerating tubes are precisely
controlled within ±0.02'C by water cooling system. Beamenergy spread is also strongly influenced by the stability ofpulses generated by klystron modulator. To obtain flatness
and stability of output pulses, PFN is composed of twoparallel 21 stages, each using the capacitors with low self
inductance less than 0.1J.LH. We achieved the pulse voltage
deviation from flatness less than 0.3%, and the long term
voltage drift less than 0.5%/Hr.
Next, subharmonic prebunchcr system (SHPB) isinstalled. Accelerating rf frequency is selected to be
476MHz, that is one-sixth of the dominant frequency of2,856MHz. Power supply system for SHPB is constructedby full solid-state circuits for its maintainability. Also innersurface of the stainless-steel cavity is partially coated with
OFHC to optimize the Q-value, intending to reduce filling
time and wake-field effect.

3. FEL experiment system
Figure 1 shows the schematic view of our FEL
experiment system. The FEL beam transport system (FELBTS) contains two bending magnets and five quadrupoles,
showing S-shape with bending angle of 25 degrees. Beamslit installed at down-stream of BM1 is expected to cut thetail of energy distribution. Equipments for beam diagnostics;
two current monitors and five position monitors, also
installed to observe focusing property in the undulator. To
realize efficient and precise adjustment of beam trajectory,
1mm-thick aluminum OTR position monitors in theundulator section have 3mm-radius pin-hole in the center,-371-

JAERI-Conf 95-021

Fig. 1. FEL experimental system.

....... 6

Thble II Parameters of the undulator.
'!Ype
Magnet material
Period length
Number of periods
Total length
Gap width
Magnetic field
K parameter

8EExHalbach
Nd-Fe-84.0cm

b2502.0m
16-32 mm0.62-0.18 T
2.4- 0.7010.......
aligned to designed orbit using He-Ne laser.
Thble II shows main parameters of our horizontal
polarized Halbach type undulator with Nd-Fe-8 permanent
magnet. Magnetic field was accurately measured with
8ELL-9900 Gauss meter. Dispersion of peak value ( dB I B)
was less than 1.0%, and second-order integral was less than5.4 x 10 5 Gauss mm 2.
Optical resonators consists of two concave Au coated
OFHC mirrors with the curvature of 4.42m and 4.14m.
Resonator length between up- and down-stream mirrors was
designed to be 7.5576m. Up-stream mirror has reflectance ofmore than 99.2%, and down-stream mirror has lmm-radius
pinhole to extract FEL.
Down-stream mirror can be adjustea manually with
precision of O.Olmm, and remote-controllable driver unit
with stepping motors and piezoelectric actuator enables fineposition adjustment of up-stream mirror. In particular,piezoelectric actuator gives the precision of lOnm to theadjustment along the z-axis.

EE........
>b__0

on n n n

Triplet 1Triple12 BM1QM1 OM2 QM3...".. .
.....
,-· .\ ..

. ..

,.'\BM2 QM4QM5

undulator

Fig. 2.
Calculated beam size in FEL-8TS.
Characters ax and a, stand for half-width of thebeam envelope in the x- and y-plane respectively;
result of parametric fitting to observed beam size(dashed line), and well-focused beam with initialcondition estimated at the exit of linac (solid line).

beam waist 1.84mmt

4. Beam transport experiments
Lattice structure of FEL-8TS was designed through thenumerical simulation with evaluation of (i) achromaticity,(ii) quasi-isochronism and (iii) focusing property of thebeam in the undulator section.
In FEL experiments the electron beam envelope should
be well-focused and finely matched to the optical beamenvelope in the undulator section to increase theirinteraction. To simulate characteristics of the beam in theFEL-8TS, particularly in the undulator section, we studied
initial condition in details; not only the size and divergence-372-O'y-------------------

undulator

Fig. 3. Electron beam and optical envelope in theundulator.

JAERI-Conf 95-021

but also focusing or defocusing character of the beam at theexit of the linac was taken into account.
As the first step, we injected the beam into actual FELBTS to collect the experimental parameters such as beamenergy and energy spread, beam size at the position
monitors and magnetic fields applied to Q-magnets. Settingthese parameters as input values, we simulated beamtransportation to search for initial condition that satisfies
characters of the beam observed in the experiment.
Calculated Twiss parameters a, {3, and of the beam at theexit of linac are listed in Table III. We can see theparameter a has negative value, which means the beam
initially has focusing character at the exit of the linac.

r

Table III
Twiss parameters of the beam at the exit of linac.
Negative values of a corresponds to the focusing
character of the beam.

x-planey-planeafJ

r

-22.3
-9.9

41.7
26.7

119.9
37.6

---5 (J1 s/div)

Fig. 4. Result of spontaneous emission experiment;(1) beam current at the entrance of undulator, (2)spontaneous emission.

In the next step calculation with the initial condition ofobtained above, we changed the combination ofbeam
thefields of Q-magnets to produce well-focused beam matchedto optical envelope, as shown in Fig.2 . Envelope of opticalbeam and electron beam in the undulator are shown in Fig.3,
where the optical envelope of the wavelength of 2.78J.L01 is
assumed to be Gaussian beam that has beam waist of
1.84mm at the center of the undulator. Through the study ofinitial condition of the beam at the end of the linac, we canprecisely predict the beam characteristics in the FEL-BTS
by the numerical simulation, and it is helpful to arrange theoperating parameters.

5. FEL experiments
Referring the result of transport experiments, we carriedout spontaneous emission experiment. Figure 4 showsobserved spontaneous emission under the conditions listed inTable IV. With the assumption the micropulse is lOps-long,the peak power is estimated to be 0.4mW.
Table IV Conditions of FEL experiment.
Beam energy
Peak current
K parameter
Magnetic field
FEL wavelengthGain

We also roughly estimated the small-signal gain to be
10% through the calculation method used in the FELIX

facility[2]. Though energy spread is known to eliminate
gain, it is hard to make it narrower only by adjusting
operational parameters of linac. Subharmonic prebuncher
system (SHPB) can increase the. micropulse current and it
will lead to increase of the gain. Recently we observed
desirable bunching effect throughout the macropulse.

6. Summary
Characteristic behavior of the beam in our FEL
experiment system can he well predicted through thenumerical simulation with the initial condition at the end of
the linac. The calculation result indicates the experimental
condition of the next stage that we can produce the beam
well-focused and matched to optical envelope in theundulator.
Spontaneous emission was observed as the primarystage of our FEL experiments using the compact linac.
Also we intend to achieve PEL oscillation through theincrease of micropulse current by SHPB system and fineadjustment of the optical resonator.

50 MeV
25A
0.84
0.23T
2.8 Jlm
10%References[1] T.Haga et al., on this proceedings.

[2] P.W. van Amersfoort et al., The FELIX projectstatus report April 1988, Oijnhuizen Report 88-176-373-