SELENE FEL

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

Nuclear Instruments and Methods in Physics Research A 341 (1994) ABS 94 North-Holland

Sec honA

Three dimensional simulations of the Novosibirsk/SELENE FEL

J . Blau, R.K. Wong, D .D. Quick, W.B. Colson Physics Department, Nacal Postgraduate School, Monterey, California 93943, USA

Three dimensional simulations of the proposed Novosibirsk/SELENE FEL are used to predict the steady state power in the klystron oscillator, and to look at optical guiding and power evolution in the radiator . The SELENE proposal [11 is a two-stage FEL based on the Novosibirsk racetrack microtron [2]. The parameters for this paper are chosen for the current experiment in Novosibirsk, which will be modified to reduce the wavelength and increase the power for the SELENE project. An FEL klystron oscillator consisting of three identical undulators separated by two dispersive magnets is used to bunch the electron beam . Only the bunched electrons are delivered to an external radiator, which creates high average power over a single pass [3]. Fig. 1 shows a three dimensional simulation in (x, y, t) of the FEL klystron oscillator . The dimensionless current density i = 2500 leads to high gain over a single pass, so the oscillator quickly saturates [4]. The two klystron magnets, located at dimensionless time ,r = 1/3 and -r = 2/3, enhance electron bunching and reduce the saturated optical power. However, it may be necessary to operate the resonator at a low Q < 10 to prevent the power from growing excessively over many passes . The purpose of the oscillator is only to bunch the SELENE OSCILLATOR WAVEFRONTS 41a43.0I'Ifa(-Y)I

27 .61

3=2500

I

x =0 0 v =1 0 =0 .5

a =0 .14 e a =1 0 1 0=0 .4 D=0 .66

aG 3

N=120

r =1 .9

r =1 .2

rh0.0

q=10

edg1os=0 .1

i 11 IO

n

10-/2

i

3n/2 0

lal

n

Fig. 1 . Multi-pass evolution of the oscillator .

' amx l

10

SELENE RADIATOR WAVEFRONTS 3 1 .(x,1)1

I [la (x,Y)1 i

I~

31 3 a(x,1)

7=6000

I l a(x,Yr)

1

a 0 .14 e~ ao=2

v~ 1

III

==0 .375

D~0 .25

aG4

N=160

0

la(x,1)I

0

la(x,Y)I a( .") a(x,y)

lo

L0

loo") IIP (1) ln

0

1

I~~

w

1 -n/2 ,

Ç

3rz/2 0

122 1221 720' 7201

1 5x10 3

I

I

..=0 .3

D=0 .5

1+G11))

6 .69

1

1

Fig. 2 Single-pass evolution of the oscillator and radiator electrons, and the phase space plot in the lower center of Fig. 1 shows evidence of bunching . The achromatic bending magnet used to extract the electrons from the oscillator [5] would add more dispersion and further improve the bunching . Fig. 2 shows a single pass through the oscillator and the radiator . The dimensionless parameters have been resealed to include the additional undulator. When the electron beam enters the radiator at T = 3/4, the dimensionless optical field a is reset to zero . Light is created in the radiator by self-amplification of spontaneous emission from the bunched electron beam . Although there is substantial gain in the radiator, the optical power does not saturate . This may indicate the need for a longer undulator, or increasing the strength of the dispersive magnets. [1] HE . Bennett et al ., these Proceedings (15th Int. Free Electron Laser Conf., The Hague, The Netherlands, 1993) Nucl . Instr. and Meth . A 341 (1994) 124, [2] N.G . Gavrilov et al .. Nucl . Instr. and Meth A 304 (1991) 228. [3] D.D . Quick et al ., ref. [1], p. ABS 92 . [4] W.B . Colson, in : Free Electron Laser Handbook, eds . W.B Colson, C. Pellegrim and A . Remeri (Elsevier, Amsterdam, 1990) chap 5 . [5] N.G . Gavrilov et al , Nucl Instr. and Meth . A 304 (1991) 63 .

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