# Presentation: 297138

## Photon Beam Position Monitorsand Beam Stability at the Swiss Light Source

**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 end of **Bending** beamlines *(top view)*

- Source point

- xBPM2

- XBPM2

- 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

**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
- Module
- 8201

- Carrier board 8002

- ADC 8401

- VxWorks
- EPICS

- Analog
- signal

## XBPM launcher

**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.
_**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

## XBPM

**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*

##      XBPM aligned for gap = 8.5 mm and calibrated for each gap

*from insertion device operation at the SLS*, proceedings ICALPECS07, Knoxville, USA

-      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