Photon Beam Position Monitorsand Beam Stability
at the Swiss Light Source
Elsa van Garderen
March 12th, 2008
What are the XBPMs?• principle: 4 blades of Tungsten read the tails of the photon beam. Beam positiondeduced 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
xBPM2XBPM2
SPM1 SPM2
Source point
(front view)
(front view)
XBPM1
SPM1
SPM2
XBPM1
Beam profile
Beam profile
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 Module8201
Carrier board 8002
ADC 8401
VxWorksEPICS
Analogsignal
XBPM launcherSlow XBPM
feedback
Fast Orbit feedback
30 min archiverdata can be saved (for offline analysis)
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
without XBPM feedback (X09LA)
with XBPM feedback (X10SA)
DBPM before ID
DBPM after ID
DBPM before ID
DBPM after ID
x
y
x
y
XBPM DBPM
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
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
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0ho
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8.58.07.57.06.56.05.55.0UE24 gap size (mm)
IDFF off IDFF on
(a)
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XB
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8.07.57.06.56.05.55.0UE24 gap size (mm)
horizontal position vertical position
(b)
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)
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
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