1. Photon Beam Position Monitors and Beam Stability at the Swiss Light Source Elsa van Garderen March 12 th, 2008

2. What are the XBPMs? principle: 4 blades of Tungsten read the tails of the photon beam. Beam position deduced by asymmetries. Design of K. Holldack (BESSY), produced by FMB (Berlin). Front end of ID beamlines (top view) Front end of Bending beamlines (top view) Source point x BPM2 XBPM2 SPM1SPM2 Source point (front view) XBPM1 SPM1 SPM2 XBPM1 Beam profile

3. LCAD: Low Current Asymmetry Detector triaxe cables; Bias voltage= -70 V; I/U converter ID beamlines => XBPMs have motors VME signal processing (Hytec). 3.5 cm Transition Module 8201 Carrier board 8002 ADC 8401 VxWorks EPICS Analog signal

4. XBPM launcher Slow XBPM feedback Fast Orbit feedback 30 min archiver data can be saved (for offline analysis)

5. XBPM Feedback Fast Orbit Feedback (100 Hz) corrects electron beam movements. Based on readings of DBPMs. Problem: reference of DBPMs is not static. Fluctuations (μm level) due to: Air temperature variation at location of DBPM electronics Temperature changes in SLS tunnel due to beam loss Solution: XBPM feedback (slow: 0.5 Hz) photon beam changes = angle variation of orbit at source point → changes the reference of DBPMs Update: fast XBPM feedback (implies hardware upgrade). M. Böge et al., User operation and upgrades of the fast orbit feedback at the SLS, proceedings PAC05, Knoxville, USA DBPM1 DBPM2 Electron beam Photon beam Source point XBPM1 XBPM2

6. without XBPM feedback (X09LA) with XBPM feedback (X10SA) DBPM before ID DBPM after ID DBPM before ID DBPM after ID x y x y XBPMDBPM

7. XBPMs and Feed forward Feed forward (IDFF) corrects a priori distortions due to ID gap changes. (currently for in-vacuum undulators) Acts on correctors upstream and downstream of the ID. → good efficiency to stabilise electron beam. → but internal ID steering effects cause displacement of photon beam. Therefore, XBPMs are included in IDFF determination procedure (high level: Java): IDFF tables implemented on low level EPICS based control system (10 Hz). Move gap Observe effect on electron orbit Deduce correction kicks on electron orbit Observe effect on photon beam position Apply correction Step 1 Step 2

8. J. Chrin at al., A feedforward procedure to counteract orbit distortion and photon beam displacements from insertion device operation at the SLS, proceedings ICALPECS07, Knoxville, USA XBPM aligned for gap = 8.5 mm and calibrated for each gap IDFF off: 150 μm excursion when gap closes from 8.5 to 5 mm IDFF on: excursion removed U19 gap size (mm)

9. Conclusion XBPMs at SLS: XBPMs feedback (slow) part of FOFB (bending+ in vacuum ID beamlines) XBPMs used to create feedforward tables (in-vacuum ID beamlines) Future: XBPM feedback becomes fast feedback feedback and feedforward for all ID beamlines XBPMs in the world: Developed at BESSY Recently bought for SOLEIL and DIAMOND Interest of ALS Deep involvement of SLS

10. I would like to thank PSI: J. Krempaský for daily support M. Böge for FOFB and XBPM feedback J. Chrin for feed forward tables Th. Schmidt for insertion devices issues The beamline scientists of their comments Q. Chen and R. Wullschläger for technical support BESSY: K. Holldack for useful discussions