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A grating spectrometer for Raman free electron laser diagnostics
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A 331 (1993) 667-669 North-Holland
Section A
A grating spectrometer for Raman free electron laser diagnostics * Mingchang Wang, Zaitong Lu, Lifen Zhang, Jizhong Chen, Bibo Feng and Zhijiang Wang Shanghai Institute of Optics and Fine Mechanics, P.O. Box 800211, Shanghai 201800, China
A grating spectrometer is developed for Raman free electron laser (FEL) diagnostics. Spectral measurements of a Raman FEL by a microwave grating spectrometer are presented. The aluminum reflection gratings with groove constant d = 7.5 ram, d = 3.5 mm and blaze angle O = 30° are used. The spectrometer provides spectral coverage in the ranges of 6-10 mm and 2.2-4.8 mm with a resolution of 0.3 mm for an angle of incidence c~= 60°. The experimental results show that the central frequency of the spectra decreases with increasing the wiggler magnetic field, which is in accordance with the theoretical predictions concerned with waveguide mode effects on the collective FEL. The spectrum of an FEL oscillator with distributed feedback cavity was measured. 3 mm FEL radiation using a small-period wiggler is obtained.
1. Design considerations
T h e condition for constructive i n t e r f e r e n c e for radiation of w a v e l e n g t h A is given by
T h e diagnostics of an F E L o u t p u t r e q u i r e s meas u r e m e n t s with high special resolution a n d good spectral coverage. A dispersive waveguide of 85 m [1], a n d a dispersive line [2] were u s e d to m e a s u r e t h e F E L spectrum. A grating s p e c t r o m e t e r can directly m e a s u r e spectral characterastics of F E L emission. A t e n - c h a n n e l grating s p e c t r o m e t e r was designed for e l e c t r o n cyclotron emission p l a s m a diagnostics [3]. A grating s p e c t r o m e t e r is d e v e l o p e d for spectral m e a s u r e m e n t s of R a m a n free e l e c t r o n laser ( F E L ) diagnostics [4]. This p a p e r describes t h e microwave grating s p e c t r o m e t e r . Two a l u m i n u m reflection gratings with groove c o n s t a n t s d = 7.5 a n d 3.5 mm, a n d b o t h with blaze angle O = 30 ° are presently u s e d to allow t h e s p e c t r o m e t e r to o p e r a t e for a R a m a n FEL. T h e e x p e r i m e n t a l a r r a n g e m e n t of a s p e c t r o m e t e r is illustrated in fig. 1. T h e r a d i a t i o n e n t e r s t h e spectrome t e r t h r o u g h an i n p u t h o r n with a cutoff waveguide a n d a spherical collimating lens M 1. M 1 collimates the i n p u t b e a m towards t h e e c h e l e t t e grating G. T h e collim a t e d r a d i a t i o n is t h e n diffracted by t h e grating a n d focused by t h e cylindrical m i r r o r M 2 o n t o t h e detector. I n this configuration, t h e d e t e c t e r r e m a i n s fixed a n d the grating is rotated. Thus, t h e i n c i d e n t ray is always h o r i z o n t a l a n d t h e reflected ray (which is d e t e c t e d ) always m a k e s t h e same angle with respect to t h e horizon.
K h / d = sin O i + sin Od
* Work supported by the National Natural Science Foundation of China.
= 2 s i n ( O i - 4~) cos ~b, w h e r e O i a n d Od are t h e angles of t h e i n c i d e n t a n d r e f r a c t e d rays, respectively, with respect to t h e n o r m a l of t h e grating, a n d 2 q ~ = O i - O a. d is t h e spacing b e t w e e n t h e grating grooves a n d k is t h e o r d e r of interference, w h i c h may b e a positive or negative integer. If the i n c i d e n t a n d diffracted rays are o n opposite sides of t h e grating normal, t h e n O i a n d Od are of opposite sign. Thus, if the i n c i d e n t a n d diffracted rays are o n opposite sides of t h e z e r o - o r d e r reflected ray, t h e n k is n e g a t i v e . T h e s p e c t r o m e t e r configuration was c h o s e n b a s e d o n t h e c o n s i d e r a t i o n of high a n d relatively c o n s t a n t grating efficiency over a b r o a d spectral range.
2. Measurements T h e pulse line a c c e l e r a t o r is u s e d to p r o d u c e kiloa m p e r e s of electrons with energies a p p r o a c h i n g 0.4 M e V . Fig. 2 shows a s c h e m a t i c d i a g r a m of a R a m a n F E L in S I O F M ; t h e energy is first stored in t h e capacitor w h i c h is c h a r g e d to - V. W h e n t h e primary switch is triggered, t h e capacitor discharges t h r o u g h t h e transf o r m e r to t h e transmission line. W h e n sufficient voltage has built u p o n t h e transmission line, the secondary switch b r e a k s down a n d t h e s t o r e d energy flows t h r o u g h t h e diode. T h e m a g n e t i c field lines cross the v a c u u m system walls a n d f o r m a b e a m d u m p in t h e region just b e y o n d t h e wiggler. T h e applied axial m a g n e t i c field is typically
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
XI. OPTICS
668
M. Wang et al. / A grating spectrometer for diagnostics
;\l CYLINDRIIALMIRROR Fig. 1. A grating spectrometer configuration for a Raman FEE 9.35 kG. The diode and b e a m drift tube regions are p u m p e d down to a pressure of 5 × 10 -5 Tort. A typical electron beam of 800 A is accelerated across a diode potential of 400 kV and then guided through a wiggler section by an axial magnetic field. Peak microwave powers of 17 M W have been observed in the circular TM01 m o d e at a frequency of 42 GHz. The radiation spectrum of the F E L with distributed feedback cavity has been measured as shown in ref. [5]. Fig. 3 illustrates the relation between output power and frequency spectrum. The experimental results have shown that the central frequency of the spectra dicreased with increasing the wiggler field, which is in accordance with the theoretical predictions concerned with waveguide m o d e effects on the Raman collective free electron laser. Stimulated emission is observed at a wavelength near 3 m m from the SIOFM F E L with new wiggler recently. A helical bifilar wiggler with a period of 10 m m and a length of 600 m m is used. The 15 ns pulse at an energy of 9 mJ is measured. The experimental parameters of the F E L are listed in table 1. The tuning ability of the F E L is shown as in fig. 4 by adjusting the guided field. The maximum power appears at B 0 = 7500 G. The conclusion is that the spectrometer is developed to measure the spectrum of an F E E 3 m m radiation of an F E L with a small period wiggler is obtained. The work of improving the F E L efficiency is being undertaken.
10, i
F a
I
L 6
d
2~ I
0' 20
70
1.2 I J
Eb=0.3MeV Ib=400A B~=t000G
1.0 J
0.8 /
/ Jx
x x
/ i /
• 0.6
"~0.4
/ ×
0.2
/ F
0.0
'
0
II
I
2
4
6 Bo
8
10
12
14
(kG)
Fig. 4. The radiation power of an FEL with a small period wiggler vs the guided field B o.
,
'
I
WIGGLER
60
Fig. 3. The relation between output power and frequency spectrum: B o = 9.35 kG; (a) B w = 1.26 kG; (b) B w = 0.98 kG.
,
DIODE
40 50 f (GHz)
30
GUIDEDFIELD
)~r
Eb=0.4MeV Ib=800A Bo=g.35kG
I
8~
DRIFT
'
TL~3E
Fig. 2. Schematic diagram of the SIOFM Raman FEE
M. Wang et aL / A grating spectrometer for diagnostics Table 1 Experimental parameters of the S I O F M F E L at 3 m m wavelength Accelerator B e a m voltage V B e a m current I B e a m radius r e Pulse duration ~Wiggler Wiggler period Aw Wiggler length L Wiggler field B w Internal radius r i External radius r o Radiation wave Wavelength As Output power Po Efficiency ~
300 400 3 50
kV A mm ns
10 m m 600 m m 1000 G 8 mm 15 m m
669
References [1] R.A. Kehs, M.C. W a n g et al., I E E E Trans. Plasma Sci. PS-13 (6) (1985) 559. [2] J. Fajans, G. Bekefi et al., Phys. Rev. Lett. 53 (1984) 246. [3] J. Fisher, D.A. Boyd et al., Rev. Sci. Instr. 54 (1983) 1085. [4] Z.T. Lu, L.F. Zhang, M.C. W a n g and Z.J. Wang, Chin. J. Lasers 18 (12) (1991) 881. [5] M.C. Wang, Z.J. W a n g et al., Nucl. Instr. and Meth. A304 (1991) 116.
3.2 m m 600 kW 0.5%
XI. OPTICS
Nuclear Instruments and Methods in Physics Research A 331 (1993) 667-669 North-Holland
Section A
A grating spectrometer for Raman free electron laser diagnostics * Mingchang Wang, Zaitong Lu, Lifen Zhang, Jizhong Chen, Bibo Feng and Zhijiang Wang Shanghai Institute of Optics and Fine Mechanics, P.O. Box 800211, Shanghai 201800, China
A grating spectrometer is developed for Raman free electron laser (FEL) diagnostics. Spectral measurements of a Raman FEL by a microwave grating spectrometer are presented. The aluminum reflection gratings with groove constant d = 7.5 ram, d = 3.5 mm and blaze angle O = 30° are used. The spectrometer provides spectral coverage in the ranges of 6-10 mm and 2.2-4.8 mm with a resolution of 0.3 mm for an angle of incidence c~= 60°. The experimental results show that the central frequency of the spectra decreases with increasing the wiggler magnetic field, which is in accordance with the theoretical predictions concerned with waveguide mode effects on the collective FEL. The spectrum of an FEL oscillator with distributed feedback cavity was measured. 3 mm FEL radiation using a small-period wiggler is obtained.
1. Design considerations
T h e condition for constructive i n t e r f e r e n c e for radiation of w a v e l e n g t h A is given by
T h e diagnostics of an F E L o u t p u t r e q u i r e s meas u r e m e n t s with high special resolution a n d good spectral coverage. A dispersive waveguide of 85 m [1], a n d a dispersive line [2] were u s e d to m e a s u r e t h e F E L spectrum. A grating s p e c t r o m e t e r can directly m e a s u r e spectral characterastics of F E L emission. A t e n - c h a n n e l grating s p e c t r o m e t e r was designed for e l e c t r o n cyclotron emission p l a s m a diagnostics [3]. A grating s p e c t r o m e t e r is d e v e l o p e d for spectral m e a s u r e m e n t s of R a m a n free e l e c t r o n laser ( F E L ) diagnostics [4]. This p a p e r describes t h e microwave grating s p e c t r o m e t e r . Two a l u m i n u m reflection gratings with groove c o n s t a n t s d = 7.5 a n d 3.5 mm, a n d b o t h with blaze angle O = 30 ° are presently u s e d to allow t h e s p e c t r o m e t e r to o p e r a t e for a R a m a n FEL. T h e e x p e r i m e n t a l a r r a n g e m e n t of a s p e c t r o m e t e r is illustrated in fig. 1. T h e r a d i a t i o n e n t e r s t h e spectrome t e r t h r o u g h an i n p u t h o r n with a cutoff waveguide a n d a spherical collimating lens M 1. M 1 collimates the i n p u t b e a m towards t h e e c h e l e t t e grating G. T h e collim a t e d r a d i a t i o n is t h e n diffracted by t h e grating a n d focused by t h e cylindrical m i r r o r M 2 o n t o t h e detector. I n this configuration, t h e d e t e c t e r r e m a i n s fixed a n d the grating is rotated. Thus, t h e i n c i d e n t ray is always h o r i z o n t a l a n d t h e reflected ray (which is d e t e c t e d ) always m a k e s t h e same angle with respect to t h e horizon.
K h / d = sin O i + sin Od
* Work supported by the National Natural Science Foundation of China.
= 2 s i n ( O i - 4~) cos ~b, w h e r e O i a n d Od are t h e angles of t h e i n c i d e n t a n d r e f r a c t e d rays, respectively, with respect to t h e n o r m a l of t h e grating, a n d 2 q ~ = O i - O a. d is t h e spacing b e t w e e n t h e grating grooves a n d k is t h e o r d e r of interference, w h i c h may b e a positive or negative integer. If the i n c i d e n t a n d diffracted rays are o n opposite sides of t h e grating normal, t h e n O i a n d Od are of opposite sign. Thus, if the i n c i d e n t a n d diffracted rays are o n opposite sides of t h e z e r o - o r d e r reflected ray, t h e n k is n e g a t i v e . T h e s p e c t r o m e t e r configuration was c h o s e n b a s e d o n t h e c o n s i d e r a t i o n of high a n d relatively c o n s t a n t grating efficiency over a b r o a d spectral range.
2. Measurements T h e pulse line a c c e l e r a t o r is u s e d to p r o d u c e kiloa m p e r e s of electrons with energies a p p r o a c h i n g 0.4 M e V . Fig. 2 shows a s c h e m a t i c d i a g r a m of a R a m a n F E L in S I O F M ; t h e energy is first stored in t h e capacitor w h i c h is c h a r g e d to - V. W h e n t h e primary switch is triggered, t h e capacitor discharges t h r o u g h t h e transf o r m e r to t h e transmission line. W h e n sufficient voltage has built u p o n t h e transmission line, the secondary switch b r e a k s down a n d t h e s t o r e d energy flows t h r o u g h t h e diode. T h e m a g n e t i c field lines cross the v a c u u m system walls a n d f o r m a b e a m d u m p in t h e region just b e y o n d t h e wiggler. T h e applied axial m a g n e t i c field is typically
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
XI. OPTICS
668
M. Wang et al. / A grating spectrometer for diagnostics
;\l CYLINDRIIALMIRROR Fig. 1. A grating spectrometer configuration for a Raman FEE 9.35 kG. The diode and b e a m drift tube regions are p u m p e d down to a pressure of 5 × 10 -5 Tort. A typical electron beam of 800 A is accelerated across a diode potential of 400 kV and then guided through a wiggler section by an axial magnetic field. Peak microwave powers of 17 M W have been observed in the circular TM01 m o d e at a frequency of 42 GHz. The radiation spectrum of the F E L with distributed feedback cavity has been measured as shown in ref. [5]. Fig. 3 illustrates the relation between output power and frequency spectrum. The experimental results have shown that the central frequency of the spectra dicreased with increasing the wiggler field, which is in accordance with the theoretical predictions concerned with waveguide m o d e effects on the Raman collective free electron laser. Stimulated emission is observed at a wavelength near 3 m m from the SIOFM F E L with new wiggler recently. A helical bifilar wiggler with a period of 10 m m and a length of 600 m m is used. The 15 ns pulse at an energy of 9 mJ is measured. The experimental parameters of the F E L are listed in table 1. The tuning ability of the F E L is shown as in fig. 4 by adjusting the guided field. The maximum power appears at B 0 = 7500 G. The conclusion is that the spectrometer is developed to measure the spectrum of an F E E 3 m m radiation of an F E L with a small period wiggler is obtained. The work of improving the F E L efficiency is being undertaken.
10, i
F a
I
L 6
d
2~ I
0' 20
70
1.2 I J
Eb=0.3MeV Ib=400A B~=t000G
1.0 J
0.8 /
/ Jx
x x
/ i /
• 0.6
"~0.4
/ ×
0.2
/ F
0.0
'
0
II
I
2
4
6 Bo
8
10
12
14
(kG)
Fig. 4. The radiation power of an FEL with a small period wiggler vs the guided field B o.
,
'
I
WIGGLER
60
Fig. 3. The relation between output power and frequency spectrum: B o = 9.35 kG; (a) B w = 1.26 kG; (b) B w = 0.98 kG.
,
DIODE
40 50 f (GHz)
30
GUIDEDFIELD
)~r
Eb=0.4MeV Ib=800A Bo=g.35kG
I
8~
DRIFT
'
TL~3E
Fig. 2. Schematic diagram of the SIOFM Raman FEE
M. Wang et aL / A grating spectrometer for diagnostics Table 1 Experimental parameters of the S I O F M F E L at 3 m m wavelength Accelerator B e a m voltage V B e a m current I B e a m radius r e Pulse duration ~Wiggler Wiggler period Aw Wiggler length L Wiggler field B w Internal radius r i External radius r o Radiation wave Wavelength As Output power Po Efficiency ~
300 400 3 50
kV A mm ns
10 m m 600 m m 1000 G 8 mm 15 m m
669
References [1] R.A. Kehs, M.C. W a n g et al., I E E E Trans. Plasma Sci. PS-13 (6) (1985) 559. [2] J. Fajans, G. Bekefi et al., Phys. Rev. Lett. 53 (1984) 246. [3] J. Fisher, D.A. Boyd et al., Rev. Sci. Instr. 54 (1983) 1085. [4] Z.T. Lu, L.F. Zhang, M.C. W a n g and Z.J. Wang, Chin. J. Lasers 18 (12) (1991) 881. [5] M.C. Wang, Z.J. W a n g et al., Nucl. Instr. and Meth. A304 (1991) 116.
3.2 m m 600 kW 0.5%
XI. OPTICS