-10 0 10 20 30 40 50 60 -0.5 0 0.5 1 1.5 2 2.5 3 0 50 100 150 Path Length (m) Betatron Function (m) Dispersion Function (m) LSS Normal CG Cells Normal CG Cells β β x β β y η η x 0 10 20 30 40 50 60 -10 -0.5 0 1 1.5 2 2.5 3 0.5 0 50 100 150 Path Length (m) Betatron Function (m) Dispersion Function (m) β β x β β y η η x Fig. 1. Lattice structure and typical optical functions for Phase-I and Phase-II optics. Installation of Long Magnet-free Straight Sections The initial lattice structure (Phase-I optics) of the SPring-8 storage ring was composed of 48 unit cells ( DBA lattice). The ring has four long straight cells with missing bending magnets at intervals of eleven cells. In the summer of 2000, the long straight cells were changed into the long magnet-free s t r a i g h t s e c t i o n o f a p p r o x i m a t e l y 2 9 m l o n g b y r e - a r r a n g e m e n t o f t h e q u a d r u p o l e a n d s e x t u p o l e magnets. The new lattice structure and its optics ( Phase-II optics) are shown in Fig. 1 together with the Phase-I lattice. Matching Section Fig. 2. Optical system of the two-dimensional interferometer. The beam commissioning of this Phase-II optics was started on August 28 th , 2000 and within t h e s a m e d a y , t h e f i r s t b e a m s t o r a g e o f 0 . 2 2 m A w a s a c h i e v e d b y o n - a x i s - l i k e i n j e c t i o n . T h e c o m m i s s i o n i n g w a s s u c c e s s f u l l y c o m p l e t e d i n a b o u t t h r e e w e e k s , a n d u s e r s e r v i c e o p e r a t i o n commenced in the beginning of October. The beam performance achieved in the Phase-II optics is similar to that of the Phase-I except for the beam lifetime. The over-all beam performances ( Table 1 ) demonstrates that the beam lifetime is still improving, resulting from the beam self-cleaning effects of the photon absorbers installed in the new vacuum chambers. In December 2000, the beam lifetime achieved more than 100 hours for a total beam current of 100 mA in multi-bunch mode. Other Research and Development The following research and developments were performed: - Improvement of the orbit stability. Or bi t os ci ll at io n du e to c ur re nt f lu ct ua ti on s (1 Hz ) in t he m ai n qu ad ru po le m ag ne ts p ow er s up pl ie s, and synchrotron oscillation (2 kHz), resulting from RF noise in the synthesizer, were eliminated. - Analysis of the error field in the storage ring and the fine tuning of optical functions. - Analysis of beam instability, such as fast ion trapping. - Beam loss analysis in the injection process to realize a top-up operation. - Emittance measurement. The beam size of the SPring-8 storage ring has been measured utilizing a newly developed, two dimensional interferometer, employing the visible part of synchrotron radiation emanating from a b e n d i n g m a g n e t . T h e o p t i c a l s y s t e m i s s h o w n i n F i g . 2 a n d t h e t y p i c a l t w o - d i m e n s i o n a l i n t e r f e r e n c e pattern in Fig. 3 . The measured horizontal and vertical beam size is typically 150 μ m and less than 20 μ m, respectively. source point quad square slits 1st lens observation screen 2nd lens t w o - d i m e n s i o n a l i n t e r f e r e n c e p a t e r n b a c k f o c a l p o i n t o f 1 s t l e n s x θ y θ x o y H y b r i d / H H L V / 3 0 m - L S S Hybrid HHLV New Optic s Energy 8 GeV 8 GeV 8 GeV 8 GeV Circumference 1436 m - - - Number of bucket 2436 - - - Revolution time 4.79 ms - - - Symmetry 24 / 48 / 4 24 48 4 ( β x / β y ) at ID section (24 / 10), (1 / 8) 25 / 4 ( 2 4 . 3 5 / 5 . 7 7 ) / ( 2 3 . 4 2 / 1 4 . 4 4 ) Current : single bunch 5 mA 16 mA 16 mA - multi bunch 100 mA 100 mA 100 mA 100 mA Bunch length ( FWHM ) 36 ps 36 ps \ 1 32 ps \ 2 - Emittance 6 . 9 9 / 6 . 3 / 6 . 6 n m ∑ r a d 6 . 8 0 . 5 n m ∑ r a d \ 3 6 n m ∑ r a d \ 3 5 . 9 n m ∑ r a d \ 3 Tunes ( ν x / ν y ) 51.16 / 16.36 43.16 / 21.36 40.15 / 18.35 C h r o m a t i c i t i e s ( ξ x / ξ y ) : n a t u r a l ( - 1 1 5 . 9 / - 4 0 . 0 ) / ( - 1 0 5 . 9 / - 5 1 . 2 ) / ( - 9 0 . 3 / - 4 0 . 7 ) operation 3.2 / 3.9 7.0 / 4.0 7.0 / 6.0 Momentum acceptance ~ 2% \ 1 1.3% \ 1 1.9% \ 1 (2.8% \ 2 ) 2.0% \ 2 Energy spread ( ∆ E/E) 0.0011 0.0012 0.0011 0.0011 Coupling less than 10% ≤ 0.06% \ 4 ≤ 0.04% \ 4 ~ 0.06% \ 4 Lifetime : 100 mA (multi bunch) 24 hr ~ 70 hr \ 5 ~ 140 hr \ 6 ~ 110 hr \ 6 1 mA (single bunch) - ~ 5 hr \ 1 ~ 11 hr \ 1 (~25 hr \ 2 ) ~ 22 hr \ 2 COD : horizontal ( rms ) - < 0.1 mm < 0.1 mm < 0.1 mm vertical ( rms ) - < 0.1 mm < 0.1 mm < 0.1 mm Beam size at ID section : horizontal ( rms ) - 400 μ m / 86 μ m 390 μ m 380 μ m vertical ( rms ) - 6.7 μ m / 6 μ m 3 μ m 4.5 μ m Residual dispersion at non-dispersive section : h o r i z o n t a l ( r m s ) 0 9.8 mm 7.0 mm 4.4 mm vertical ( rms ) 0 2.7 mm 4.5 mm (1.1 mm \ 7 ) 1.3 mm \ 7 Orbit stability (tune harmonics) horizontal ( s t a n d a r d d e v i a t i o n ) - 1.1 μ m 0.7 μ m 1.3 μ m vertical ( s t a n d a r d d e v i a t i o n ) - 0.7 μ m 0.35 μ m 0.35 μ m \ 1 Vrf =12 MV \ 2 Vrf =16 MV \ 3 calculated by using designed β -function \ 4 single resonace approximation \ 5 2/3-filling, Vrf =12 MV \ 6 24/29-filling, Vrf =16 MV \ 7 with correction by 24 skew Q’s Table 1: Beam Performance Fig. 3. Two-dimensional interference pattern. Installation of ECS and Single Pass BPM An energy compressor system ( ECS ) was installed at the end of the linac to reduce the beam en er gy sp re ad re su lt in g fr om be am lo ad in g an d th e en er gy fl uc tu at io n. As a pr el im in ar y re su lt , a reduction rate of 50% was achieved in the energy spread. To improve the orbit stability, 12 single pass BPM’s with a position resolution of a few tenth micron (2 σ ) were installed in the Linac. Operation of this new BPM system will begin in the autumn of 2001. Development of RF-gun To realize high quality electron beam of 1 nC, 1 π mm • mrad with bunch length of sub-psec, a development of an RF-gun was carried out in the test bench and high-power RF, up to 27 MW, was fed into the cavity. The maximum electric-field gradient of 140 MV /m is reached on the cathode and th e el ec tr on be am of 2 nC pe r bu nc h wa s ac ce le ra te d up to 3. 2 Me V. Th e mi ni mu m no rm al iz ed emittance of 17 π mm • mrad was obtained at 90 MV /m and 0.8 nC /bunch. Also, a three-dimensional beam simulation code has been developed to understand the formation of beam emittance in the RF- gun. 100 0 200 300 400 500 600 700 Intensity ( a.u. ) x ( mm ) y ( mm ) -2.0 -1.5 -1.0 2.0 -0.5 0.0 0.5 1.0 1.5 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Noritaka Kumagai SPring-8 / JASRI
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