vertical emittance minimization at the sls through systematic and random-walk optimization

CLIC meeting, CERN, 20.04.2012

Vertical emittance minimization at the SLS through systematic and

random-walk optimizationCLIC meeting, CERN

20.04.2012

M. Aiba, M. Böge, N. Milas, A. Streun, PSI


Introduction

• Vertical emittance minimization is motivated by:– increase of brightness and transverse coherence– operational margin for small gap insertion device

(possibly even smaller undulator gap)– TIARA WP6 SVET (SLS Vertical Emittance Tuning)

• Collaboration: CERN-INFN-PSI+Maxlab• Establish VET means at SLS, for CLIC DR and SuperB

→ Fine corrections of betatron coupling and y, and maintaining small emittance during operation

• Beam size monitor R&D → NM’s talk• Intra Bunch Scattering studies


Swiss Light Source

Swiss Light Source – 3rd generation light source– 18(+2) beam lines– 2.4 GeV, 400 mA (top-up)– C~288 m

SLS


SLS vertical emittance• What was expected and what is achieved

– Achieved much better than expected, thanks to • 30 more skew quads installed (6 skew quads initially)• Better alignment on girder than expected• Girder realignment in 2011• Elaborated model based corrections• Random-walk optimization

Emittance ratio ≡ v/h

h~ 5 nm

(Insertion devices off)

Application of these methods achieved 0.9 pm!

1.8 pm in March 2011


Key component 1

– BBGA initially planned(see Backup slide)

– Switch to BAGA (later slides), more robust approach

• Magnet girder


Key component 2


Girder realignmentMotivation and approach

• BAGA (Beam Assisted Girder Alignment)– Remotely align girders based on survey data– Confirm the result online with beam and fast orbit feedback running

Girder discontinuity estimation from “corrector pattern”


Girder realignment result

BAGA resulted in:

-Gaussian like corrector kick distribution

-About half corrector kick

-About half dispersive skew correction

-Similar non-dispersive skew correction (Qaud rotation dominant?)

Result for Sector 1

Result for all sectors


Model based correction 1


Model based correction 2

ORM

* Contribution of BPM tilts subtracted

*


Random walk optimization

• Limitations in model based corrections…– Beam measurement errors– Model deficiencies

• Apply multi-variable optimization– Random-walk would be the best algorithm

• Model independent correction• The curse of dimensionality is avoided (#Knobs=12/24/36)• The optimum solution is within easy “walking-distance” after systematic

correction• Minimal effort to implement• Potential of online optimization, i.e. keeping small emittance during the

operation• NB: the optimization needs a target function, which is the measured vertical

beam size in our case

RAndom Coupling Correction (RACC) ~ Luck (in Japanese pronunciation…)


How it worksTypical successful step(Figure from first test)

Configure the parameters of RWO

Measure target function, f0

Generate random corrections (RC)

Add RC to the present correction variables

Measure target function, f

Update f0=f Remove RC

f<f0 f≥f0

Reconfigure RWO if necessary

Flowchart


MD on 6th Dec. 2011

Corrected vertical dispersion

• First dedicated MD after BAGA– v~1.3 mm rms with model based correction!– v~1.2 pm at the end of model based correction– v~0.9±0.4 pm with RWO in addition!

(Only ND skew quads optimized)– Better coupling correction with RWO was confirmed with ORM before and

after

1000 2000 3000 4000-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

I (

A)

Time (sec)

ND skew Q currents during RWO (plotted 4 out of 24)

Beam size measurement during MD


Model deficiencies?• What limits the model based correction?

– Step size of RWO was determined from the continuation of model based correction after it converged → Fluctuation of correction due to measurement error

KL~10-5 m-1 rms → Step size chosen in a range of KL=0.5~2×10-5 m-1 – However, RWO arrived at KL=7×10-5 m-1 rms – BPM and corrector tilts may be the sources

Simulation showed residual tilts of a few mrad already explain this

0 1 2 3 40

5

10

15

Re

sid

ual s

kew

qu

ad. K

L, r

ms

(10

-5 m

-1)

Residual corrector tilt, rms (mrad)

0 1 2 3 40

5

10

15

Res

idua

l ske

w q

uad.

KL,

rm

s (1

0-5 m

-1)

Residual BPM tilt, rms (mrad)


Summary

• Ultra low vertical emittance of 0.9 pm is achieved at the SLS !– BAGA + Model based corrections + RWO

• RWO– Successfully demonstrated, a good performance

booster– Potential applications:

• Beam lifetime of the SLS, SwissFEL undulator BBA etc.• Online optimization

Maintain small emittance during operation in CLIC DR and SuperB


Backup slides


Beam Based Girder Alignment


BAGA seen by the HLS in Sector 2

vertical emittance minimization at the sls through systematic and random-walk optimization

Documents

beam size measurement

measured vertical beam

small emittance

step size of rwo

approachbaga beam

beam lines2

rms step size

nd skew quads optimizedbetter