0.01 0.1 10 1 0 50 100 150 200 before improvement after improvement Frequency (Hz) (b) Vibration of Beam Orbit Amplitude ( μ μ m / Hz) 0.0001 0.001 0.01 0.1 1 before improvement after improvement Frequency (Hz) (a) Vibration of Vacuum Chamber Amplitude ( μ μ m / Hz) 0 50 100 150 200 Fig. 1. Vertical vibration of the vacuum chamber and the stored electron beam before and after improvements. Fig. 1 ( ii ) To suppress the slow orbit drift, we increased the number of air-core type steering magnets with high resolution and low hysteresis from twelve to twenty-four in the summer shutdown of 2002. By this enhancement , we increase the degree of freedom for the correction phase. In principle, this is effective for reduc ing the slow drift. To avoid the mixing between the circumference change and orbit d i s t o r t i o n d u e t o t h e i n c r e m e n t i n t h e s t e e r i n g n u m b e r , w e a d o p t e d a n a l g o r i t h m t o s u b t r a c t t h e c o n t r i b u t i o n o f t h e e n e r g y s h i f t f r o m t h e m e a s u r e d o r b i t . B y u s i n g t h i s n e w p e r i o d i c c o r r e c t i o n system, horizontal and vertical orbit deviations were reduced down to about 5 μ m in rms for one-day operation. Orbit Stability A b eam orbit stabilization p roject was started in February 2001. Investigations in t he first y e a r f o c u s e d o n t h e s u r v e y o f f l u c t u a t i o n s o u r c e s , w h e r e a s i n t h e s e c o n d y e a r , 2 0 0 2 , f o c u s e d o n suppression of the vibration sources we had found. (i) By observing the correlation between the vacuum chamber vibration and the beam fluctuation, we found that the broad peak around 30 Hz in the vertical beam spectrum is caused by the vibration of an upstream chamber in a unit cell. The vacuum chamber vibrates in a quadrupole magnetic field, t hen eddy currents are induced on the vacuum chamber wall. The electromagnetic fields induced by the eddy currents shake the electron beam. W e have made some improvements o n the basis of these results. We reduced the vertical beam fluctuations around 30 Hz b y one order of amplitude , as shown in . This improvement is also effective for suppress ing the horizontal beam fluctuations from 50 to 100 Hz and the amplitude in this frequency range was reduced by factor three in amplitude. 126 Fig. 2 Top-up Operation Since 1999 we have been investigating the realization of “top-up operation” in the storage ring at SPring-8. In 2002, we set a target to introduce top-up operation to user time from the autumn of 2003. To meet this time schedule, we are rushing to upgrade a machine control, beam monitors, and a n i n t e r l o c k s y s t e m f o r r a d i a t i o n s a f e t y , a n d t o d e s i g n a n d m a n u f a c t u r e t h e n e w i n j e c t i o n b u m p magnets and their power supplies. There are two major problems, as follows. (i) Demagnetization of undulator magnets due to frequent beam injections: This phenomenon occurs d u e t o t h e l o s s o f i n j e c t e d b e a m s a t a n a r r o w v e r t i c a l a p e r t u r e o f t h e i n - v a c u u m u n d u l a t o r . C o n s i d e r i n g b o t h t h e e x p e r i m e n t a l a n d s i m u l a t i o n r e s u l t s , w e a r e o p t i m i z i n g t h e d e s i g n o f t h e collimator system to be installed in the beam transport-line from the booster synchrotron to the storage ring. ( ii ) Excitation of betatron oscillation of stored beam by beam injections: An off-axis beam injection i s t h u s a d o p t e d a s a n i n j e c t i o n s c h e m e f o r t h e s t o r a g e r i n g . T h e b u m p o r b i t f o r t h e i n j e c t i o n i s generated by four pulse bump magnets. The magnetic field pattern is a half-sine with a width of about 8 μ s. As this bump orbit is not completely closed, the stored beam suffers error kicks while passing t h r o u g h t h e f o u r b u m p m a g n e t s , a n d t h e n t h e b e t a t r o n o s c i l l a t i o n i s e x c i t e d . W e f o u n d t h a t t h e osc ill ati on exc ita tio n mai nly occ urr ed by two eff ect s: (a) One is the eff ect of non lin ear lit y due to sextupoles within the bump orbit. We are considering the correction of error kicks due to this effect by introducing compensation pulse magnets in the ring. (b) The other is caused by the existence of two types of bump magnet. These two have different eddy current patterns on the end plates of bump magnets, which change the shape of the field overshoot. We are now investigating cures for these oscillation excitations, and designing new magnets that have end plates made of insulating material to reduce eddy current effects. New Optics There exist several methods to reduce natural emittance. We attempted to reduce the emittance by breaking the achromatic condition imposed on Chasman-Green cells. This method is effective for the case where undulators with a moderate field are used as main insertion devices ( IDs ). The SPring-8 storage ring just meets this condition , and c alculation s sho w that an approximately 20% e x t r a r e d u c t i o n i s a l s o o b t a i n e d b y c l o s i n g a l l I D s g a p s t o t h e m i n i m u m , e v e n a f t e r b r e a k i n g t h e achromatic condition. In the summer shutdown, to realize the new optics we modified cabling of the quadru poles in the disper sive arc to change the streng ths of the quadru poles while maintai ning the p h a s e m a t c h i n g c o n d i t i o n o v e r e a c h l o n g s t r a i g h t s e c t i o n . S i n c e S e p t e m b e r 2 0 0 2 w e h a v e b e e n machine - tuning this new optics ( ) with the distributed dispersion. We plan to release this new optics to user operation from November 2002. The expected value of emittance is about 2.8 nm ∑ rad with all ID gaps closed. 127 Fig. 3 Accelerator Diagnostics Beamlines The accelerator diagnostics beamline #1 ( BL38B2 ), which has a bending magnet light source, is in operation. The visible synchrotron light is used for longitudinal diagnostics of the stored electron beam, such as bunch length and single bunch impurity. Single - bunch impurity is measured by a gated pho ton cou nti ng met hod tha t uti liz es fas t poc kel s cel ls for swi tch ing lig ht pul ses . To imp rov e the extinction ratio or isolation of the light shutter, the optical system was improved so that two pockels cells are arranged in tandem. For the main bunch, we have achieved a sensitivity to satellite bunches in the order of 10 -10 . A beam profile monitor based on a phase zone plate has been installed ( ). Synchrotron radiation from a dipole magnet source is imaged by a single-phase zone plate. The m onochromatic X- ray is selected by a double crystal monochromator, which covers the energy range of 4 to 14 keV by Si (111) reflection , while a n X-ray zooming tube observes the X-ray image of the electron beam. Results f r o m p r e l i m i n a r y e x p e r i m e n t s s h o w t h a t t h e o b s e r v e d p r o f i l e o f t h e b e a m i s a f f e c t e d b y a s m a l l deformation in the monochromator Si crystals, which was predicted from extensive measurements of the rocking curves of the monochromator. The experiments will resume after improvement in the crystal holders of the monochromator. The accelerator diagnostics beamline #2 ( BL05SS ) has a straight section of the storage ring, where IDs for light sources can be installed. Synchrotron radiation from the edges of two bending magnets adjacent to the straight section can also be observed. In 2002, the components of the front end wer e des ign ed and man ufa ctu red , and the des ign of rad iat ion shi eld ing hut che s is in pro gre ss. I n s t a l l a t i o n o f t h e f r o n t e n d i n t h e a c c e l e r a t o r t u n n e l a n d t h e c o n s t r u c t i o n o f t h e h u t c h e s w i l l b e completed in early 2003. Fig. 2. Optical function for new optics. –10 0 10 20 30 40 50 60 – 0.2 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 50 100 150 Dispersion Function η x (m) β Path Length from Injection Points (m) Betatron Function β x , y (m) LSS β x β y η x N o r m a l C G C e l l s M a t c h i n g S e c t i o n N o r m a l C G C e l l s 128 F i g . 4 A bunch-by-bunch feedback system for suppression of transverse beam instabilities is being d e v e l o p e d f o r t h e s t o r a g e r i n g a n d w i l l b e i n s t a l l e d i n t h e s u m m e r s h u t d o w n o f 2 0 0 3 . S e v e r a l i n s t a b i l i t i e s a r e o b s e r v e d i n S P r i n g - 8 : a m u l t i - b u n c h i n s t a b i l i t y d r i v e n b y v e r t i c a l r e s i s t i v e - w a l l im pe da nc e of sm al l ga p in -v ac uu m un du la to rs , ho ri zo nt al mu lt i- bu nc h in st ab il it y dr iv en by hi gh er order mode impedance of RF cavities , of which horizontal betatron function is i ncreased by one order o f m a g n i t u d e f r o m i t s d e s i g n v a l u e , a n d v e r t i c a l a n d h o r i z o n t a l s i n g l e - b u n c h m o d e - c o u p l i n g instabilities driven by broad-band impedance of discontinuities of the beam-pipe wall of the vacuum chamber. Currently, those are suppressed by high chromaticity, 8 in the horizontal direction and 8 in the vertical direction . However, such high chromaticities reduc e momentum acceptance of the ring and increases i njected beam l oss because of its longitudinal phase space mismatch . T his beam loss is one of the most serious problems for the top-up operation. The bunch-by-bunch feedback s y s t e m u s e s a t l e a s t f o u r p a r a l l e l m odules , and each module is composed o f t w o A D C s , o n e F P G A a n d t w o D A C s , a s s h o w n i n . E a c h m o d u l e is driven by an 85 - MHz c lock which is o n e s i x t h o f t h e S P r i n g - 8 R F a c c e l e r a t i o n fr eq ue nc y. FP GA is qu it e fa st er th an D S P s a n d c a n h a n d l e t w o 9 - t a p F I R f i l t e r a l g o r i t h ms s i m u l t a n e o u s l y w i t h this frequency. Using this module, we s u c c e s s f u l l y c u r e d a h e a d - t a i l s i n g l e - bunch instability that was intentionally d e s t a b i l i z e d b y s e t t i n g c h r o m a t i c i t y n e g a t i v e a n d o b t a i n i n g d a m p i n g t i m e o n e - o r d e r o f m a g n i t u d e s h o r t e r t h a n radiation damping. Fig. 3. Optical system of the X-ray imaging of the electron beam. Double Crystal Monochromator Phase Zone Plate Wire Scanner 4-Jaw Slit X-ray Zooming Tube Source Point e - Bunch-by-bunch Feedback Fig. 4. Block diagram of a digital signal-processing unit with four modules on one PCI board. ADC ADC ADC ADC ADC ADC ADC ADC FPGA FPGA FPGA FPGA DAC DAC DAC DAC DAC DAC DAC DAC Multiplexer Multiplexer Divider Divider 4 2 . 4 M H z - 2 5 5 M H z f r o m B P M 1 0 k H z - 4 2 . 4 M H z f r o m B P M 4 2 . 4 M H z - 2 5 5 M H z t o H F K i c k e r 1 0 k H z - 4 2 . 4 M H z t o L F K i c k e r 85 MS/s on ONE PCI board 129 F i g . 5 Low Energy Operation In general, the emittance of stored beam is proportional to the square of its energy , and can be reduced by lowering the beam energy. The bunch length is also reduced when we lower the energy. T h i s r e d u c t i o n o f e m i t t a n c e a n d b u n c h l e n g t h w i l l o p e n u p n e w o p p o r t u n i t i e s f o r u s i n g b r i g h t e r synchrotron radiation with shorter pulse lengths at SPring-8. For this aim, we r amp ed down the beam energy from a design value of 8 GeV and we performed beam injection at 4 GeV. I n the ramping- down experiments, we first stored a low-current 5 mA b eam in a multi-bunch mode t hen lowered the energy, step by step, t o 4 GeV. At each step of beam energy we measured beam parameters, such as b e a m s i z e , b u n c h l e n g t h , s y n c h r o t r o n f r e q u e n c y , e t c . , a n d c o m p a r e d t h e m w i t h e x p e c t e d v a l u e s o b t a i n e d f r o m a s i n g l e - p a r t i c l e p i c t u r e , a s s h o w n i n . W e f o u n d n o s i g n i f i c a n t d i f f e r e n c e between measured and expected values. Beam instabilities were not observed in the above-mentioned c u r r e n t n o r i n a f i l l i n g m o d e . W e t h e n p e r f o r m e d a b e a m i n j e c t i o n a t 4 G e V . T o i m p r o v e t h e efficiency of beam injection and hence increase the stored current, further studies are planned , such as optimization of the strength of harmonic sextupole magnets. 40 60 80 100 120 140 160 0 10 20 30 40 50 60 3 4 5 6 7 8 Energy ( GeV ) Bunch Length σ σ { ( ps ) σ x ( μ m ) = 16.327 × E ( GeV ) + 25.286 Beam size ( μ m) Measured Calculated value Calculated value + jitter Fig. 5. Horizontal beam size and bunch length as a function of beam energy. Beam size σ σ x ( μ μ m ) 130
home
- JP
- EN