Among third-generation light sources, SPring-8 is distinguished by the presence of four long str aig ht sec tio ns all owi ng the ach iev eme nt of sup er bri lli ant syn chr otr on rad iat ion . Dur ing sum mer sh ut do wn of 20 00 , a la tt ic e of th e st or ag e ri ng wa s mo di fi ed to cr ea te ma gn et fr ee 30 m st ra ig ht se ct io ns [1 ] . At th e sa me ti me , th e fi rs t lo ng in se rt io n de vi ce wa s in st al le d at th e X- ra y be am li ne BL19LXU. The first device is an in-vacuum undulator with a 32 mm magnetic period similar to the SPring-8 standard type in-vacuum undulator. Main parameters of the undulator are detailed in Table 1 . The net length of the undulator magnet is 25 m and all magnet blocks are arranged continuously. This is one of the advantages of in-vacuum devices. In the construction of long insertion devices, the ma g n e ts o f th e o u t o f va cu u m i n se rt i o n d e vi ce a re se p a ra te d b y fl a n g e s a t th e va cu u m ch a mb e r co nn ec ti on po in ts as sh ow n in Fi g. 1 (b ) . On th e ot he r ha nd , fo r in -v ac uu m de vi ce s, th e va cu um chambers are connected externally using bellows, therefore, there is no gap between magnet arrays ( Fig. 1 (a) ). As a result, all radiations emanating from each magnet array are superposed in phase; therefore, no complicated phase matching techniques are necessary. The 27 m undulator consists of 5 segments ( Fig. 2 ), each possessing a similar mechanical st ru ct ur e to a 4. 5 m lo ng in -v ac uu m un du la to r, wh ic h ha s be en su cc es sf ul ly op er at ed fo r ye ar s at SPring-8 [2] . Each segment has an individual undulator gap driving system, controlled simultaneously by one signal since magnet arrays are mechanically connected between segments. The segment is also an unit to carry out magnetic field measurements [3] and transportation. The five segments were transported independently to the storage ring, then assembled together inside the ring tunnel. Figure 3 is a photograph of the installed undulator in the SPring-8 storage ring. Type P u r e p e r m a n e n t m a g n e t ( halbach ) Periodic length 32 mm Number of periods 780 Gap range 12 ~ 50 mm Maximum K 1.76 at 12 mm gap E 1st 7.4 ~ 18 keV Table 1. Parameters of the 27 m long in-vacuum X-ray undulator for BL19LXU Fig. 2. Schematic of the 27 m undulator consisting of 5 segments. Fig. 1. Schematics of the magnet array connections utilized for the long undulator construction, (a) in-vacuum device, (b) out of vacuum device. Flanges Magnet array Vacuum chamber Bellows Magnet array Vacuum chamber (b) (a) Segment 1 Segment 2 Segment 3 Segment 4 Segment 5 27 m 25 m Fig. 4. First observation of the undulator radiation at BL19LXU. Fig. 3. 27 m in-vacuum undulator installed in the SPring-8 storage ring. The beamline was commissioned in October 2000, and the first undulator radiation was observed on October 25th ( Fig. 4 ). The brilliance of the 27 m undulator was increased approximately 4~5 times from that of 4.5 m undulators. The measured spectrum at 12 mm undulator gap utilizing a front-end (FE) slit aperture of 0.2 mm × 0.2 mm [4] demonstrated that 50% of the calculated flux was obtained under these conditions ( Fig. 5 ). The undulator radiation Bose degeneracy surpasses one during high peak current operation mode of the ring. Experiments using high photon flux and spatial coherency of the undulator radiation will dominate the use of the beamline. The success of the 27 m undulator will be the basis of the upcoming self-amplified spontaneous emission ( SASE ) based FEL project, which requires the precise control of the electron beam inside the undulator. For example due to its long length, we have observed the effect of a weak u n i f o r m f i e l d o f 0 . 1 G , s u c h a s t h e e a r t h f i e l d , o n t h e u n d u l a t o r s p e c t r u m . T o g e t h e r w i t h t h e t e c h n i q u e s o f t h e f i e l d a l i g n m e n t a n d c o n s t r u c t i o n , t h e e x p e r i e n c e i n u s i n g t h e 2 7 m i n - v a c u u m undulator forms the next step in pursuit of innovative light sources. Fig. 5. Undulator spectrum measured at 12 mm gap with a 0.2 mm × 0.2 mm FE slit. References [1] N. Kumagai, SPring-8 Information, No.1, 6 (2001) 5. [2] H. Kitamura, J. Synchrotron Rad. 7 (2000) 121. [3] T. Tanaka et al. , SRI2000, to be published in Nucl. Instrum. Meth. [4] T. Ishikawa et al. , to be published in J. Jpn. Soc. for Synchrotron Rad. Res. 100 × 10 12 80 60 40 20 0 25 20 15 10 Photon energy ( keV ) Flux (photon/sec in 0.1% b.w. ) Toru Hara and Hideo Kitamura SPring-8 / RIKEN • JASRI
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