Photon Beam Position Monitors and Beam Stability at the Swiss Light Source E. van Garderen, J. Krempaský, M. Böge, J. Chrin, T. Schmidt Paul Scherrer Institute, Villigen, Switzerland ABSTRACT Photon Beam Monitors (PBPMs), in a 3rd generation light source, are inevitable diagnostics instruments for both the machine and the beam lines. They are used to determine the photon beam position and are ultimately utilized in feedback loops for position stabilization. At the Swiss Light Source (SLS) in operation since mid-2001, PBPMs have been installed at the bending and insertion device beamlines. In the introduction the operating principle of the PBPM is explained. Then, a calibration method utilizing local bumps in the electron orbit is presented, and it is demonstrated how this method can be used to detect misalignments. Finally, the role of PBPMs in achieving sub-micron beam stability by means of feed-forwards and PBPM feedbacks at the SLS is highlighted. BEAM STABILITY INTRODUCTION XBPM aligned at gap = 8.5 mm Without Feed Forward: Gap closed to 5 mm → 150 μm excursion With Feed Forward: → no excursion [1] FEED FORWARD U19 gap size (mm) ID beamlines => XBPMs have motors VME signal processing (Hytec). 3.5 cm Transition Module 8201 Carrier board 8002 ADC 8401 EPICS Analog signal CALIBRATION and ALIGNMENT Without XBPM feedback (X09LA) With XBPM feedback (X10SA) μm stability! DBPM before ID DBPM after ID DBPM before ID DBPM after ID x y x y XBPMDBPM Calibration using machine bumps [1]: Calibration using machine bumps is preferred to calibration using motors as it is a tool to detect alignments. BPM before source point BPM after source point Vertical asymmetrical bumps Response of the blades (well aligned monitor) time (s) Response of the blades (badly aligned monitor) time (s) blade signal (V) vert. bump (μm) XBPM FEEDBACK Fast Orbit Feedback (FOFB) corrects electron beam movements. Based on readings of DBPMs [2]. 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 Implemented on bending beamlines and in-vacuum undulator beamlines. DBPM1 DBPM2 Electron beam Photon beam Source point XBPM1 XBPM2 Feed forward (IDFF) corrects a priori distortions due to ID gap changes. Acts on correctors upstream and downstream of the ID [3]. Problem: IDFF has a good efficiency to stabilise electron beam but internal ID steering effects cause displacement of photon beam. Solution: XBPMs are included in IDFF determination procedure as shown (note: XBPMs need to be calibrated for each gap): Implemented on in-vacuum undulator beamlines. 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 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). Results: IDFF determination procedure (for each gap) LCAD: Low Current Asymmetry Detector triaxe cables; Bias voltage= -70 V; I/U converter BPM before source point time (s) 30 μm 5 μm preliminary calibration [1] E. van Garderen et al., Characterisation of the systematic effects of the insertion devices with Photon Beam Position Monitors, proceedings DIPAC 2007, Venice, Italy [2] M. Böge et al., User operation and upgrades of the fast orbit feedback at the SLS, proceedings PAC 2005, Knoxville, USA [3] J. Chrin at al., Local correction schemes to counteract insertion devices effects, Nuclear Instruments and Methods in Physics Research A (2008)