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

140

120

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0ho

rizo

nta

l dis

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cem

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t (m

)

8.58.07.57.06.56.05.55.0UE24 gap size (mm)

IDFF off IDFF on

(a)

2.0

1.6

1.2

0.8

XB

PM

lin

ea

r ca

libra

tion

fact

ors

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