Amplitude of X[40] and Y[18] ( mm ) 0 0.005 0.010 0.015 0.020 0 4 8 12 16 20 24 X[40] Y[18] Time (hours) Routine Beam Refilling @10:00 and @22:00 Filling: 1/12 + 10 * Single Bunches Beam Lifetime: 23 hr @100 mA 1 μ m Orbit Stability B eam orbit stability is crucial for the generation of brilliant and stable photon beams for s y n c h r o t r o n r a d i a t i o n s o u r c e s . I t s r e a l i z a t i o n i s t h u s v e r y i m p o r t a n t a m o n g v a r i o u s a c c e l e r a t o r pr ob le ms . Re ce nt ly th e be am or bi t st ab il it y ha s be en ma rk ed ly im pr ov ed an d co ns eq ue nt ly or bi t jumps by periodic orbit correction are being suppressed to a level where jumps are undetectable to users. This is due to the combination of the following four improvements . First is an improve ment in c u r r e n t s t a b i l i z i n g c i r c u i t s i n t h e q u a d r u p o l e m a g n e t p o w e r s u p p l i e s , w h i c h m a r k e d l y r e d u c e t h e current ripples and drifts. Second, with a reduction of horizontal orbit oscillation around 1Hz by the above improvement in current stabilizing circuits and an averaging of the beam position data on VME, t h e r e p r o d u c i b i l i t y o f B P M s h a v e b e e n i m p r o v e d f r o m s e v e r a l t o a b o u t o n e m i c r o n i n r . m . s . T h e r e f o r e , a n y s e t t i n g e r r o r s o f c o r r e c t i n g d i p o l e m a g n e t s a r e d r a s t i c a l l y r e d u c e d i n e a c h o r b i t corr ecti on proc edur e. Thir d, to redu ce erro rs due to the sett ing reso luti on of the corr ecti ng dipo le magn et, air- core -typ e corr ecti ng dipo le magn ets with both high reso luti on and low hyst eres is were installed. The twelve correcting dipole magnets in each plane are now used in routine user operation. Fourth, the correction algorithm was changed to utilize the good reproducibility of the BPMs, i.e. , the real orbit deviation is used as a correction target instead of the re-synthesized one making use of the Fourier harmonics of the orbit deviation. Figure 1 shows the amplitude changes of the betatron tune- ha rm on ic s (4 0 th fo r ho ri zo nt al an d 18 th fo r ve rt ic al ) of CO D fo r on e da y af te r th e ab ov e im pr ov em en ts . Th e am pl it ud e ch an ge s st ay wi th in 1 μ m an d th e or bi t ju mp is sm al l be fo re an d af te r th e be am re fi ll in g. Fig. 1. Typical one-day orbit stability in a several bunch operation. B e f o r e t h e s e i m p r o v e - m e n t s , t h e a m p l i t u d e c h a n g e s w e r e a b o u t 5 μ m f o r h o r i z o n t a l a n d about 3 μ m for vertical. To achieve further orbit st ab il it y of su b- mi cr on , a t a s k f o r c e f o r o r b i t s t a b i l i t y w a s o r g a n i z e d a n d h a s b e e n c o n d u c t - i n g o v e r a l l i m p r o v e - m e n t a c t i v i t i e s f r o m a m u l t i l a t e r a l p o i n t o f view. 104 Emittance Ratio ε ε / / ε ε o Measured by Interferometer Calculation Sum of ∆ ∆ U by IDs ( MeV /turn) 0 0.5 1.0 1.5 2.0 2.5 1.00 0.95 0.90 0.85 0.80 Electron Beam Emittance at User Operation The lattice structure of the SPring-8 storage ring is a typical DBA configuration. This kind of configuration has an advantage in reducing electron beam emittance by extra radiation from insertion d e v i c e s ( I D s ) , b e c a u s e t h e I D r a d i a t i o n e n h a n c e s t h e r a d i a t i o n d a m p i n g b u t s c a r c e l y e x c i t e s t h e betatron oscillation. At present, 22 IDs were installed in SPring-8 and are routinely employed in user operation s . Most of them are in-vacuum types , the peak field of which is rather higher than an out-of- vacuum type. The above facts suggest us the possibility that the emittance reduction due to the ID radiation is not negligible. By using the ID parameters, we calculated dependence of the emittance on the extra radiation loss by IDs. We also measured the horizontal beam size variation by a visible light F i g . 2 . C a l c u l a t e d e m i t t a n c e r e d u c t i o n r a t e a g a i n s t radiation loss by IDs as compared to measured value by interferometer, where ε : with IDs and ε 0 : without IDs. i n t e r f e r o m e t e r w h e n I D g a p s a r e c l o s e d t o t h e m i n i m u m v a l u e o n e b y o n e t o e s t i m a t e the emittance variation. Figure 2 shows the c a l c u l a t e d h o r i z o n t a l e m i t t a n c e r e d u c t i o n ratio against the radiation loss increment by I D s t o g e t h e r w i t h t h e m e a s u r e d o n e . H e r e t h e h o r i z o n t a l a x i s s t a n d s f o r i n c r e m e n t o f the radiation loss by closing IDs. B oth data a g r e e w e l l , a s c a n b e s e e n i n F i g . 2 . T h e horizontal emittance reduces as the radiation i n t h e h o r i z o n t a l p l a n e i n c r e a s e s a n d i t r e a c h e s ~ 5 . 3 n m r a d w h e n a l l I D g a p s a r e m i n i m u m . O n t h e o t h e r h a n d , t h e v e r t i c a l e m i t t a n c e i s g e n e r a t e d b y s i x k i n d s o f I D s wi th a ho ri zo nt al ma gn et ic fi el d su ch as an elliptical multi-pole wiggler. Accelerator Diagnostics Beamline The accelerator diagnostics beamline #1 has a bending magnet light source, and wide band spectral availability including visible/ UV light, and soft and hard X-rays is expected. The beamline consists of a front end in the accelerator tunnel, an optics hutch in the experiment hall, a visible light t r a n s p o r t l i n e t r a n s p o r t i n g v i s i b l e / U V l i g h t f r o m t h e o p t i c s h u t c h t o a d a r k r o o m l o c a t e d i n t h e experiment hall, and an X-ray transport line in the optics hutch. The visible light transport line was completed in 2000. Single bunch impurity has been measured by a gated photon counting method, which utilizes fast Pockels cells for switching light pulses, and the bunch length has been measured by a st re ak ca me ra . Th e X- ra y tr an sp or t li ne ( Fi g. 3 ) wa s in st al le d in 20 01 . It ha s a do ub le cr ys ta l mo no ch ro ma to r, wh ic h co ve rs th e en er gy ra ng e of 4 to 14 ke V by Si (1 11 ) Br ag g re fl ec ti on . Th e monochromator crystals and their mechanisms can be moved off the beam axis in the monochromator vacuum chamber when use is made of white X-ray s including both soft and hard X-rays . The X-ray 105 t r a n s p o r t l i n e a s w e l l a s t h e f r o n t e n d h a s n o B e w i n d o w , w h i c h w o u l d o b s t r u c t s o f t X - r a y a n d visible/ UV light and potentially distort the wavefront and degrade the qualit y of beam diagnostics such as the imaging resolution. The precise measurement of the small vertical size of an electron beam is one of the most c h a l l e n g i n g s u b j e c t s o f t h e a c c e l e r a t o r b e a m d i a g n o s t i c s o f l o w e m i t t a n c e s y n c h r o t r o n r a d i a t i o n sources. The resolution of electron beam imaging is significantly improved by utilizing synchrotron ra di at io n in sh or te r wa ve le ng th re gi on s. X- ra y im ag in g ob se rv at io n of th e el ec tr on be am us in g a s i n g l e p h a s e z o n e p l a t e i s i n p r e p a r a t i o n a t t h e X - r a y t r a n s p o r t l i n e . A m o n o c h r o m a t i c X - r a y i s selected by the double crystal monochromator. The magnification factor of the zone plate is about 0.3, and an X-ray zooming tube will be used as a detector to compensate for demagnification. Fig. 3. X-ray transport line of accelerator beam diagnostics beamline #1. The R&D of accelerator components and new types of light sources are other major research subject s . In the X-ray transport line, there are two dummy vacuum pipes of approximately 2 m length, which will be replaced by an apparatus for the specific purposes of R&D. For example, the study of the effects of synchrotron radiation on cooling water in vacuum components such as absorbers is in progress. Production of γ -ray photons with energ y of the order of 10 MeV is in preparation, which utilizes the backward Compton scattering of the far infrared laser photons injected to the storage ring. Other Research and Developments Activities The following research and development activities were performed: - Analysis of beam instability and bunch-by-bunch feedback test. - Beam loss analysis in the injection process and installation of new injection septum magnets to realize a top-up operation. - Test of low energy operation at the booster synchrotron and the storage ring. 30m 35m 40m Abs orbe r 4-J aw S l it Fi lt er Fl uore s ce nt S cre en 4-J aw S l it Fl uore s cen t S cr een Wi re S canne r Fl uore s cent Sc ree n 4-J aw S li t X-ra y Zoom ing T ube Doubl e C rys ta l M onoch rom at or Pha s e Zone Pl at e M i rror Se par at ing UV/ Vi s ibl e Li ght from X-r ay B eam Dum my V ac uum P ipe s 25m Distance from Source Point 106 Accelerator Stabilization An energy compression system ( ECS ), which was completed in 2000, achieved remarkable beam performances improvements as follows: The energy spread of the 40 ns beam was compressed from 3.5% to 1.4% at the beam current of 35 0 mA . Co ns eq ue nt ly , th e in je ct io n cu rr en t in to th e sy nc hr ot ro n wa s in cr ea se d a bo ut fi ve ti me s without decrease the injection efficiency. The energy fluctuation of the 1 ns beam at a beam charge of 1.9 nC was reduced from 0.06% rms to 0.02% rms as illustrated in Fig. 4 . The injection rate into the New SUBARU storage ring – 1.5 GeV synchrotron radiation source for VUV region – reached more than 90% and maintained this during one operation cycle of three- or four-weeks. Time (min) 0.2 0.1 0 – 0.1 – 0.2 0.2 0.1 0 – 0.1 – 0.2 2 4 6 0 8 10 σ = 0.06% (a) ECS OFF (b) ECS ON σ = 0.02% Fig. 4. ECS reduced the energy fluctuations of 1 ns beams at 1.9 nC. Uniform Bunch Current at Several Bunch Operations In the several bunch operation s of the storage ring, each bunch current is equalized at the injection to the storage ring by adjusting manually the current of the linac gun. The current can be c h a n g e d b y m o d i f y i n g t h e v o l t a g e o f t h e g r i d p u l s e r , o r b y i n s e r t i n g a n i r i s i n f r o n t o f t h e g u n . Though a a change in the accelerating charge results in a change of the beam loading which leads to a change in the beam energy, an ECS works to stabilize the beam energy extracted from the linac. The stored bunch current of the storage ring is measured by the monitoring signal amplitude from a button pickup using an oscilloscope which monitors the trigger delays. The typical deviation of the bunch current to the mean value is less than 3%. 107 Development of RF-gun We introduced a new 0.3 TW laser s y s t e m f o r t h e R F - g u n i n o r d e r t o stabilize the laser power and make the l a s e r p u l s e w i d t h v a r i a b l e . I t h a s a p o w e r s t a b i l i t y o f a b o u t 3 % a n d t h e pulse width can be select ed from 1 to 19 ps. The vacuum system was also r e i n f o r c e d , w i t h t h e r e s u l t t h a t t h e d a r k c u r r e n t f r o m t h e c a t h o d e p l a n e w a s r e d u c e d t o 1 / 1 0 o f i t s p r e v i o u s value. A p r e l i m i n a r y e x p e r i m e n t p r e s e n t e d t h e m i n i m u m n o r m a l i z e d b e a m emittance of 6 π mm mrad at a beam c h a r g e o f 0 . 3 n C / b u n c h . F i g u r e 5 s h o w s a p h o t o g r a p h o f t h e R F - g u n experimental setup. Fig. 5. RF-gun experimental setup on an optical bench. Improvement s of the Timing System of the Booster Synchrotron The timing system of the synchrotron receives a 508.58 MHz RF reference signal and a 1- cycle signal at a rate of 1 Hz from the RF station of the storage ring and regenerates many timing signals such as a gun trigger, pulse magnet triggers and ramping patterns. We improved the timing system of the synchrotron to give it better stability and flexibility. In the SPring-8 RF timing system, a phase-locked-loop ( PLL ) feedback, using the signal returned in the same optic fiber reflected by the mirror located at the end point was applied for phase stabilization. PLL feedback was already adopted most of the entire signal-transmission line between the RF station of the storage ring and the RF low p o w e r s y s t e m o f t h e s y n c h r o t r o n . A l s o , t h e p h a s e c o n t r o l p a r t i n t h e R F l o w p o w e r s y s t e m w a s s ta b il iz ed b y P LL s . In 2 0 0 1 , a P LL fe ed b ac k w as in tr o d u ce d in th e re ma in in g p ar t o f th e s ig n al - transmission line. As a result , the fluctuation in the RF phase between the synchrotron and the storage ring is less than 0.3 degree. The fluctuation in the RF phase between the linac and the synchrotron has been remarkably improved. The measured time jitter of a gun trigger to the RF signal is 18 ps in r.m.s. This improvement results in benefits to other advanced operations of the synchrotron, for example, storing an electron beam for longer than 1 second, changing the injection cycle from 1 Hz to a slower fre que ncy to inc rea se the RF kno ck- out ope rat ion per iod and eje cti ng the low ene rgy bea m dur ing ramping up. 108 Development of the New RF Synchronization System between the Linac and Circular Accelerators A new synchronization system for two different RFs was introduced. A 508.58 MHz RF is used in both the booster synchrotron and the storage ring, and the linac uses a 2856 MHz independent RF. The phase between the 508.58 and 2856 MHz RFs was not locked. In the new synchronization m e t h o d , t h e p r e - t r i g g e r s i g n a l t r i g g e r s a 2 8 5 6 M H z R F g e n e r a t o r , w h i c h c o n s i s t s o f a n a r b i t r a r y F i g . 6 . B l o c k d i a g r a m o f t h e n e w s y n c h r o n i z a t i o n method between the 508.58 MHz and 2856 MHz RFs. Haruo Ohkuma and Noritaka Kumagai SPring-8 / JASRI w a v e f o r m g e n e r a t o r a n d a f r e q u e n c y multiplier. The time width generating the 2856 MHz RF is about 290 μ s. The RF for a linac is generated by the RF of a circular accelerator. The uniqueness of this method is that an RF for a linac i s n o t c o n t i n u o u s l y g e n e r a t e d b u t p u l s i n g . T h e R F g e n e r a t o r a p p a r a t u s for a linac is simple and can be applied to any combination of two RFs. With thi s new met hod , bea m int ens ity fro m the linac was kept almost constant even with higher peak current, and the shift o f t h e b e a m e n e r g y c e n t e r b e c a m e smaller than that when an independent synthesizer is used. The block diagram is shown in Fig. 6 . The energy stability was not onl y alm ost con sta nt but als o f e l l t o 0 . 0 1 5 % a n d b e a m q u a l i t y w a s remarkably improved. ( × × 32 ) Frequency multiplier Bandpass filter Bandpass filter Arbitrary waveform generator ( SONY-Tektronix : AWG2041 ) 2855.981281 MHz Mechanical phase shifter 508 MHz SUC Gun trigger signal Bandpass filter 508.58 MHz RF Bandpass filter 109
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