Photon Beam Position Monitors and Beam Stability at
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 INTRODUCTION ABSTRACT principle: 4 blades of Tungsten read the tails of the photon beam. Beam position deduced by asymmetries. Photon Beam Monitors (PBPMs), in a 3 rd 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. Design of K. Holldack (BESSY), produced by FMB (Berlin). 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. CALIBRATION and ALIGNMENT Vertical asymmetrical bumps XBPM 1 DBPM 1 vert. bump (μm) Calibration using machine bumps [1]: XBPM 2 Photon beam DBPM 2 BPM after source point BPM before source point Electron beam time (s) Source point Response of the blades (well aligned monitor) blade signal (V) VME signal processing (Hytec). Analog signal 2 BPM before source point Carrier board 8002 Transition Module 8201 3 EPICS ADC 8401 1 4 LCAD: Low Current Asymmetry Detector triaxe cables; Bias voltage= -70 V; I/U converter time (s) Response of the blades (badly aligned monitor) blade signal (V) 2 3 3. 5 cm 1 4 ID beamlines => XBPMs have motors time (s) Calibration using machine bumps is preferred to calibration using motors as it is a tool to detect alignments. BEAM STABILITY XBPM FEEDBACK FEED FORWARD Fast Orbit Feedback (FOFB) corrects electron beam movements. Based on readings of DBPMs [2]. Feed forward (IDFF) corrects a priori distortions due to ID gap changes. Acts on correctors upstream and downstream of the ID [3]. 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 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): Solution: XBPM feedback (slow: 0. 5 Hz): photon beam changes = angle variation of orbit at source point → changes the reference of DBPMs Implemented on in-vacuum undulator beamlines. IDFF determination procedure (for each gap) Move gap Step 1 Step 2 Observe effect on electron orbit Observe effect on photon beam position Deduce correction kicks on electron orbit Implemented on bending beamlines and in-vacuum undulator beamlines. Apply correction Results: XBPM DBPM before ID x y 30 μm Without XBPM feedback (X 09 LA) 5 μm preliminary calibration DBPM after ID XBPM aligned at gap = 8. 5 mm x y • Without Feed Forward: Gap closed to 5 mm → 150 μm excursion DBPM before ID With XBPM feedback (X 10 SA) μm stability! • With Feed Forward: → no excursion [1] DBPM after ID [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) U 19 gap size (mm)
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