Fig. 1. Laser-SR synchronization system. PZT Streak Camera 2 ns 40 ps SR Laser 2 ps 84.76 MHz 1 kHz PD Laser Oscillator 508.58 MHz RF Master Oscillator Undulator RF Cavity Electron Bunches Storage Ring Laser Amplifier Laser-SR Synchronization A synchronization system between an intense pulsed laser and synchrotron radiation ( SR ) pulses h a s b e e n d e v e l o p e d a t b e a m l i n e B L 2 9 X U L f o r laser + SR pump-probe experiments such as time- r e s o l v e d X - r a y d i f f r a c t i o n a n d a b s o r p t i o n measurements, and for studies on the mixing of X- r a y a n d o p t i c a l p h o t o n s . S i n c e t h e S P r i n g - 8 S R has a pulse duration of typically 40 ps ( FWHM ), the s y n c h r o n i z a t i o n t e c h n i q u e o f l a s e r p u l s e s w i t h a p r e c i s i o n o f l e s s t h a n a f e w t e n s p i c o s e c o n d s i s re qu ir ed to ac hi ev e a pe rf ec t ov er la p of bo th pu ls es . The synchronization scheme is shown in Fig. 1 . T h e o u t p u t t i m i n g o f a m o d e - l o c k e d T i : s a p p h i r e l a s e r ( o s c i l l a t o r ) i s s y n c h r o n i z e d w i t h t h e r a d i o f r e q u e n c y ( R F ) p r o v i d e d b y t h e m a s t e r o s c i l l a t o r which controls the RF cavity for acceleration of the electron bunches in the storage ring. The repetition ra te of th e la se r pu ls e is de te rm in ed by it s ca vi ty l e n g t h , w h i c h i s c o n t r o l l e d b y a p i e z o - e l e c t r i c t r a n s l a t o r w i t h a f e e d b a c k c i r c u i t . T h e i n t e n s e p i c o s e c o n d l a s e r p u l s e s w i t h a p u l s e e n e r g y o f ab ou t 1 mJ we re ob ta in ed by am pl if ic at io n of th e pu ls es pi ck ed up fr om a mo de -l oc ke d Ti :s ap ph ir e laser. The repetition rate of amplified laser pulses w a s c o n t r o l l e d t o b e 1 / n o f t h e R F , w h e r e n i s a multiple of the number of RF buckets in the ring, so that the laser pulses meet the SR pulses originated f r o m a p a r t i c u l a r e l e c t r o n b u n c h i n p a r t i a l f i l l i n g patterns [1]. A m o n i t o r i n g s y s t e m o f t h e t i m i n g f o r b o t h beams on a picosecond time scale should also be d e v e l o p e d , s i n c e c o n v e n t i o n a l m e t h o d s , s u c h a s o p t i c a l c r o s s c o r r e l a t i o n t e c h n i q u e , a r e s t i l l n o t av ai la bl e fo r th e ha rd X- ra y SR + la se r combination. We us ed a pi co se co nd X- ra y st re ak ca me ra as a t i m i n g m o n i t o r [ 2 ] . B o t h p u l s e s s i m u l t a n e o u s l y i r r a d i a t e d a p h o t o c a t h o d e o n t h e s t r e a k c a m e r a . This method ensures a precise measurement of the interval between both beams without being affected by the drift of the streak trigger timing. The laser a n d t h e X - r a y S R b e a m s w e r e i n t r o d u c e d t o t h e photocathode through a dichroic mirror made of a s u r f a c e - p o l i s h e d B e p l a t e i n s t a l l e d i n a v a c u u m c h a m b e r . F i g u r e 2 s h o w s t h e s t r e a k p r o f i l e s o b t a i n e d a t a f i n e a d j u s t m e n t o f t h e i n t e r v a l between the laser and SR pulses. Synchronization between the laser and the SR pulses was achieved with a precision of 2 ps. Application of this synchronization system to an i n v e s t i g a t i o n o f e l e c t r o n b u n c h d y n a m i c s i s a l s o d e s c r i b e d h e r e . S i n c e t h e l a s e r p u l s e s a r e p r e c i s e l y l o c k e d t o t h e p h a s e o f t h e R F i n t h e Timing Controller 88 Laser SR (b) ∆ ∆ t = 0 ps Time ( ps ) 300 100 200 0 Laser SR (a) ∆ ∆ t = 42.2 ps (a) Opened undulator gap RF Voltage for Acceleration Phase Difference (b) Closed undulator gap Storage Ring 14 Undulators ( (a) open/ (b) close ) Electron Bunch RF Cavity Undulator with fixed gap X-rays Streak Camera s t o r a g e r i n g , w e u s e d t h i s s y s t e m t o s h o w t h a t closing undulator gaps shifts the arrival time of the SR pulses, which is due to the electron energy loss produced by the undulator radiation. The graphs in F i g . 3 a r e o b t a i n e d u n d e r t h e c o n d i t i o n s t h a t t h e g a p s o f 1 4 u n d u l a t o r s a r e f u l l y o p e n e d ( a ) a n d closed (b) while the BL29ID gap is fixed to monitor the timing. The shift of the SR pulses between (a) an d (b ) ha s a goo d ag re em en t wi th th e expectations for the increased power loss [3]. Some picosecond time-resolved X-ray diffraction e x p e r i m e n t s w e r e p e r f o r m e d u s i n g t h e l a s e r - S R s y n c h r o n i z a t i o n s y s t e m . F i g u r e 4 ( a ) s h o w s t h e t i m e - r e s o l v e d r o c k i n g c u r v e s o f a G a A s c r y s t a l , ob ta in ed by va ry in g th e de la y be tw ee n th e pu mp laser and the probe X-ray pulses. The Bragg peak is shifted by the lattice expansion with a response time of a few hundred picoseconds. The diffracted Fig. 2. Overlap of laser pulses with SR pulses. Intensity ( arb. units) (a) ∆ ∆ t = 105 ps (b) ∆ ∆ t = 76 ps (e) ∆ ∆ t = 0 ps (d) ∆ ∆ t = 22 ps (c) ∆ ∆ t = 50 ps SR Laser ∆ ∆ t 100 150 50 0 Time ( ps ) Fig. 3. Temporal drift of SR pulses due to the undulator power. 89 150 100 50 0 Laser Laser SR Time ( ns ) Time ( ns ) Time ( ns ) I n t e n s i t y ( a r b . u n i t s ) 4 0 2 2 1 0 — 200 200 0 — 200 200 0 — 200 200 0 X - r a y i n t e n s i t y a t a c e r t a i n o f f s e t a n g l e i s drastically changed according to the Bragg peak shift as shown in Fig. 4(b) . It is to be noted that the lattice recovered from the expansion within 1 ms corresponding to a laser pulse repetition rate o f 1 k H z . W e a l s o i n v e s t i g a t e d t h e o p t i c a l swi tch ing met hod of the X-r ays usi ng the lat tic e e x p a n s i o n , a s s h o w n i n F i g . 5 . A s i n g l e p u l s e w a s e x t r a c t e d f r o m t h e s y n c h r o t r o n r a d i a t i o n pulse train using a double crystal arrangement of GaAs, in which the two crystals were irradiated by w a y o f t w o s u c c e s s i v e l a s e r p u l s e s w i t h a n ap pr op ri at e ti me de la y [4 ]. Th is te ch ni qu e ma y e n a b l e i n d i v i d u a l b e a m l i n e - u s e r s t o e m p l o y S R p u l s a t i o n w i t h a p a t t e r n r e q u i r e d f o r t h e i r e x p e r i m e n t s , w h i c h i s u s u a l l y s u p p l i e d w i t h a filli ng patte rn of elect ron bunch es in the stora ge r i n g . F u r t h e r d e v e l o p m e n t o f a f a s t e r X - r a y sw it ch wi ll al lo w fo r an ul tr as ho rt X- ra y pu ls e to be shaped from a single SR pulse. References [ 1 ] Y . T a n a k a e t a l . , N u c l . I n s t r u m . M e t h . A 4 6 7 - 468 (2001) 1451. [ 2 ] T . H a r a e t a l . , R e v . S c i . I n s t r u m . 7 1 ( 2 0 0 0 ) 3624; Nucl. Instrum. Meth. A 467-468 (2001) 1125. [3] Y. Tanaka et al. , Rev. Sci. Instrum. 71 (2000) 1268. [4] Y. Tanaka, T. Hara, H. Yamazaki, H. Kitamura, T. Ishikawa, to be published in J. Synchrotron Rad. (2002). Fig. 4. (a) Time-resolved rocking curves and (b) change in diffracted X-ray intensity at an offset angle of — 7 arcsec. Yoshihito Tanaka SPring-8 / RIKEN E-mail: yotanaka @ postman.riken.go.jp Fig. 5. Extr acti on of a sing le X-ra y puls e from the SR pulse train by optical-switching using laser-induced lattice expansion. Intensity (arb.untis) Angle (arcsec) 1000 500 0 -500 Intensity (arb.untis) - 200 0 200 400 600 Time (ps) 1.0 0.8 0.6 0.4 (a) (b) Time (ps) - 20 0 10 20 90
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