Proceedings of the 25th Linear Accelerator Meeting in Japan (July 12-14, 2000, Himeji, Japan)[13P-29]

PULSE SHAPE EVOLUTION OF FELI FEL DUE TO CAVITY DETUNING
M. Yasumoto*), T. TomimasuA), Y. Kanazawa A), A. Zako A), M. Tanaka A), S. Kuma A) and N.
UmesakiOsaka National Research Institute (ONRI) 1-8-31 Midorigaoka Ikeda Osaka 563-8577 JapanA)
Free Electron Laser Research Institute (FELI) Hirakata Osaka 573-0128 Japan
Abstract
A 1.0% wavelength jitter (side band oscillation) of 11µm FEL at FELI was observed due to cavity detuning.
Although an ordinary wavelength jitter of the FELI FEL is controlled within 0.05%, the spectral behavior was appeared
during a cavity desynchronization, where the FEL output is given at about maximum power. Moreover, amplitude(pulse shape) of the macropulse is also observed to be modulated with some oscillation period at the same time. Thespectral side-band and the power modulation can be explained as the phenomena when the FEL is oscillated as thespiking mode operation in a long-pulse FEL case.

1.

LANL [4,5].

Introduction
We have been studying optical property of Free

Electron Laser (FEL) at Free Electron Laser Research
30

reported that the side-band is oscillated in spectrum due

25

to the cavity detuning and that the spectral modulation

20

Intensity (mW)

Especially in ref. [3] we

Institute (FELI)[1,2,3].

is affected from the electron energy perturbation.
Further experiment reveled that the side-band oscillationin the spectrum is associated with the macropulse shapeevolution during the cavity detuning.

4
λ =11µ m

25315

11050

On the contrary,-35

the side-band oscillation and the macropulse shape-30-25-20 -15 -10Distance (µ m)-505modulation are observed in the CLIO, the LANL andthe Stanford [4,5,6].

Fig. 1

They explained the phenomenausing the spiking mode theory.

Detuning curve of the 11µm FEL.

In this paper, wetherefore try to discuss the amplitude (pulse shape) and

Fig. 1 shows the detuning curve of the 11µm FEL at

frequency evolution of the macropulse caused from thethe FELI, when the length of optical cavity (δL) arecavity detuning of mid-infrared (MIR) FEL.

changed.

2.

3.

Cavity detuningAmplitude modulation

The MIR FEL facility of the FELI has a 165MeV

Fig.2 shows the amplitude shapes of the FEL

S-band linac, a 6.72m-length optical cavity and amacropulse for various δL values (1~5) indicated inundulator (N=58, λu=34mm).

Fig.1.

The slippage length

The time structures are measured with an MCT(Ls) and the electron bunch length are approximately

detector with 100ns rise time.

600µm and 3mm, respectively.

Therefore a slippage

structure can not be seen in these macropulses.

parameter (µc) is about 0.2 and the FELI FEL is

Although we unfortunately have not measured theclassified in a long pulse FEL like the CLIO and theduration of the FEL micropulse with an auto-correlation*)

M.Yasumoto, 0727-51-9537, yasumoto@onri.go.jp−364−

The micropulse

system, the CLIO result suggests that the time structureδL is the detuning length (δL=0 when the FEL power isof the FELI FEL micropulse is also modulated in amaximum.) and Lc is the cavity length.

theoretical slippage period (~2.1ps).

our experimental results and the calculated oscillationFig.3 indicatesperiod.

61

Measured data
Caluculated data

Oscillation period (µ s)52432
10-20 -18 -16 -14 -12 -10 -8 -6
Deyuning length (µ m)

3Fig. 3-4-2

Oscillation periods of the FEL macropulse of

the side-band as a function of cavity detuning length.

44.

Conclusion
We have discussed the shape evolution of the FELIFEL macropulse due to the cavity detuning.

5We foundboth the frequency modulation and the amplitude
modulation are strongly dependent with the laser cavitydetuning.

The oscillation period of the amplitude

modulation is good agreement with the calculated data.
Fig. 2 Pulse evolution of the FEL macropulse due toFurther measurement of the miropulse duration willcavity detuing.

more clearly explains the phenomena.

Moreover, the modulations of the macropulse have

References

double structure of 5 s and much shorter period. The[1] A.Kobayashi et al., Nucl. Instr. and Meth. A375 (1996)

5 s period modulation is observed in all time structure

317.

corresponding to the cavity detuning.

The modulationis unknown but is independent with the cavity detuning.
The shorter period modulation, which is not clearer than[2] Y. Kanazawa, et al., Proc. of the 24th Linear Accelarator
Meeting in Japan, Sapporo, July 7-9, 1999, pp274.
[3] M.Yasumoto, et al., Proc. of the 24th Linear Accelarator

the former modulation, is depended on the detuning

Meeting in Japan, Sapporo, July 7-9, 1999, pp281.

length described in Fig. 3.

[4] R.Prazer et al., Nucl. Instr. and Meth. A358 (1995) 212.

The LANL group reported that side-band oscillation
which is decided in the following equation.
N ⋅ λ 2 Lc
τ=⋅2 ⋅ δL cHere τ is oscillation period of the side-band, N is thenumber of undulator period, λ is the FEL wavelength,[5] R.W.Warren et al., Nucl. Instr. and Meth. A250 (1986) 19.
R.W.Warren et al., Nucl. Instr. and Meth. A285 (1989) 1.
[6] A. Marziali et al., Nucl. Instr. and Meth. A358 (1995) 252.
W.P. Leemans et al., Nucl. Instr. and Meth. A358 (1995) 208.