Photon Beam Position Monitors and Beam Stability at
Photon Beam Position Monitors and Beam Stability at the Swiss Light Source Elsa van Garderen March 12 th, 2008
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 view) Source point XBPM 1 XBPM 2 Beam profile Front end of Bending beamlines (top view) XBPM 1 x. BPM 2 (front view) SPM 1 Source point SPM 1 SPM 2 Beam profile SPM 2
VME signal processing (Hytec). Analog signal Transition Module 8201 Carrier board 8002 Vx. Works EPICS ADC 8401 LCAD: Low Current Asymmetry Detector triaxe cables; Bias voltage= -70 V; I/U converter 3. 5 cm ID beamlines => XBPMs have motors
XBPM launcher Slow XBPM feedback Fast Orbit feedback 30 min archiver data can be saved (for offline analysis)
XBPM Feedback • Fast Orbit Feedback (100 Hz) corrects electron beam movements. Based on readings of DBPMs. XBPM 1 DBPM 2 Source point XBPM 2 Photon beam Electron beam • 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 PAC 05, Knoxville, USA
XBPM DBPM without XBPM feedback (X 09 LA) DBPM before ID x y DBPM after ID x y with XBPM feedback (X 10 SA) DBPM before ID DBPM after ID
XBPMs and Feed forward • Feed forward (IDFF) corrects a priori distortions due to ID gap changes. Move gap (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). Step 1 Step 2 Observe effect on electron orbit Observe effect on photon beam position Deduce correction kicks on electron orbit Apply correction
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 U 19 gap size (mm) J. Chrin at al. , A feedforward procedure to counteract orbit distortion and photon beam displacements from insertion device operation at the SLS, proceedings ICALPECS 07, Knoxville, USA
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 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
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
- Slides: 10