Haga, K

Kamiya, Yu

Katoh, M

Katsura, T

Mitsuhashi, T

Nakamura, N

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Particle Accelerators, 1990, Vol. 33, pp. 105-109Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaGLOBAL ORBIT FEEDBACK SYSlEM AT THE PHOTON FACTORYSTORAGE RINGKAIICIll HAGA, YUKIHIDE KAMlYA, MASAIllRO KATOH,TOMOTARO KATSURA, TOSIllYUKI MlTSUHASIllAND NORIO NAKAMURAPhoton Factory, National Laboratory for High Energy Physics(KEK), 1-1 Oho, Tsukuba, Ibaraki 305, JapanAbstract Daily drifting motion of the synchrotron radiation(SR) from the Photon Factory (PF) storage ring under lowemittance operation became a serious problem for mostusers. Horizontal and vertical positions of the SR have beenstabilized by a global orbit feedback system.INTRODUCTIONThe synchrotron radiation from a storage ring must be suppliedto experimenters with its beam position highly stabilized. Sincethe low emittance operation of the PF storage ring started,various stored-beam movements, especially daily drifting motion,became very large. As a result, the photon beam also moved witha large amplitude and then its motion became a serious problemfor users. The daily drifting motion of the stored beam is closelyrelated to the atmospheric environment around the storage ringbuilding. A study on the causes of the beam movements is nowin progress. 1In order to suppress the daily drifting motion of the photonbeam, a global orbit feedback system has been developed. Thissystem treats the horizontal and vertical motions separately. Ithas corrected the horizontal motion by compensating the changein circumference of the ring. The compensation is made byvarying the RF frequency. The vertical position of photon beam[1795]1105106/[1796] K. HAGA ET AL.has been stabilized by keeping the stored-beam orbit unchangedfrom a standard orbit chosen at the begin-ning of operation. Thestored-beam orbit is in turn stabilized by vertical steeringmagnets installed for this purpose.HORIZONTAL ORBIT FEEDBACKA schematical diagram of the global feedback system ofhorizontal orbit is shown in Figure 1. Horizontal closed-orbitdistortion (C.O.D.) is measured every twenty minutes with 45beam position monitors located around the ring. This systemthen corrects the difference (~xi) between the horizontal C.O.D.and its reference orbit by varying the RF frequency," ~x. 'fl.£..J I'" 1i ( i =1, 2,..., 45)where "i is the dispersion function at the i-th position monitorRFSYSTEM IRFCAVITYI~I~ /~llfRF~1 ~I~xAI- ~ ~~i '1l1~= 0( i 2fRF ~ "IiFIGURE 1 A diagram of the global feedback system ofhorizontal orbit.GLOBAL ORBIT FEEDBACK SYSTEM [1797]/107300.6EoS 0.4S 0.2~to-~>C 0.0S0-0.2;.:.>C-0.40 10 20PM No.-0-- Without F.B.--+- With F.B.40FIGURE 2 Differences of two horizontal C.O.D.'s with andwithout the global orbit feedback.and a is the momentum compaction factor.Figure 2 shows the differences between two horizontal C.O.D.'smeasured at 7 o'clock and 17 o'clock in both cases with andwithout the feedback. Data in both cases are those measuredwhen the daily drifting motion was relatively large. As shown inthe figure, the feedback system has reduced the change of thehorizontal C.O.D. by a factor of five. In Figure 3, the change in RF........E500.098 -0.4 E~20 Jul. 1989 CD500.097 -0.2 (,)c........ CDN-J: CD:E 500.096 0.0 'I-~ Ee- ::J(,)CD 500.095 0.2 --8ULLLL Ca: 500.094 0.4CDen500.093 0.6 Ca10 6 1 2 18 24 .c0Time (hr)FIGURE 3 The change in RF frequency for one day underthe operation of a global orbit feedback system and thecorresponding change in circumference of the ring.108/[1798] K. HAGA ET AL.frequency for one day under the operation of this feedbacksystem is shown with the corresponding change in circumferenceof the ring, which was estimated by a relation that ~ C / C = -~fRF/fRF. The RF frequency change has by no means induced anybeam instability.VERTICAL ORBIT FEEDBACKThe global feedback system of vertical orbit consists of verticalphoton-beam position monitors, computer control system andsteering magnet system. The vertical position of photon beam atBeamline 21 is measured by a wire monitor that detects photo-emission electrons from two tungsten wires. The controlcomputer changes the setting of the currents of twenty steeringmagnets so as to keep the vertical position at Beamline 21unchanged. The vertical feedback system is being improved sinceit was constructed in 1987. Details of this system are describedin Ref. 2.At some experimental beamlines, we have installed several. ..........,a~~••..............o BL-4a BL-6A BL.12• BL·2136 48 607/26 I 7/2724I127/25AAMA.M ~24 36I 5/17 1125/1 6WITHOUT FB ,pP'00oodJ°00oo°0°10000E 8000~ 6000c0 4000;:.;0 2000Q.c; 0u;:-2000"-CD>-4000-60000Time (hr)FIGURE 4 The vertical position of the photon beam atseveral beamlines with and without the global orbitfeedback. The vertical position in the figure is in a relativescale.GLOBAL ORBIT FEEDBACK SYSTEM [1799]/109photon-beam position monitors to measure the vertical beampositions. The variations of the vertical positions at thesebeamlines with and without the feedback are shown in Figure 4.For example, the vertical position of the photon beam at Beamline4 drifted about 5 mm between 15 o'clock May 16 and 7 o'clockMay 17 when the feedback system was turned off. With thefeedback system active, the change of the vertical position wassuppressed by a factor of about three.SUMMARYDaily drifting motions of SR at the PF storage ring have beenreduced by the global orbit feedback system for both horizontaland vertical directions to supply stable SR to the experimenters.We are now planning to directly measure the horizontal positionof the photon beam and thereby to make the horizontal photonbeam more stable. In order to suppress the fast varying verticalmotion of photon beam at each beamline, a feedback system thatmakes use of a local orbit bump is being tested at severalbeamlines. This local feedback system together with the globalfeedback system is expected to stabilize the vertical photon beamposition more sufficiently at the beamlines where highly stablephoton beams are required.REFERENCES1. N. Nakamura et. aI, in this conference.2. T. Katsura, Rev.Sci.lnstrum, 60, (1989), to be published.

Global Orbit Feedback System At The Photon Factory Storage ring

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