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

E. van Garderen , J. Krempaský, M. Böge, J. Chrin, T. SchmidtPaul 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

140

120

100

80

60

40

20

0ho

rizo

nta

l dis

pla

cem

en

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

fa

cto

rs

8.07.57.06.56.05.55.0UE24 gap size (mm)

horizontal position vertical position

(b)

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 Module8201

Carrier board 8002

ADC 8401

EPICS

Analogsignal

CALIBRATION and ALIGNMENT

WithoutXBPM feedback (X09LA)

With

XBPM feedback (X10SA)μm stability!

DBPM before ID

DBPM after ID

DBPM before ID

DBPM after ID

x

y

x

y

XBPM DBPM

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 pointVertical asymmetrical bumps

Response of the blades(well aligned monitor)

1

2

3

4

1

2

3

4

time (s)

Response of the blades(badly aligned monitor)

time (s)

bla

de

sig

na

l (V

)b

lad

e s

ign

al (

V)

vert

. b

um

p (

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: Results:

IDFF determination procedure (for each gap)

LCAD: Low Current Asymmetry Detectortriaxe cables; Bias voltage= -70 V; I/U converter

BPM before source point

time (s)

30 μ

m5

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)