1. Laser-wire Measurement Precision
Grahame Blair
Beijing- BILCW07,
6th February 2007
Introduction
Overview of errors
Ongoing technical work in this area
Plans for the future.

2. Laser-wire People BESSY: T. Kamps
DESY : E. Elsen, H. C. Lewin, F. Poirier, S. Schreiber, K. Wittenburg, K. Balewski
B. Foster, N. Delerue, L. Corner, D. Howell, M. Newman, A. Reichold, R. Senanayake, R. Walczak
G. Blair, S. Boogert, G. Boorman, A. Bosco, L. Deacon, P. Karataev, S. Malton , M. Price I. Agapov (now at CERN)
CCLRC: I. Ross
KEK: A. Aryshev, H. Hayano, K. Kubo, N. Terunuma, J. Urakawa
SLAC: A. Brachmann, J. Frisch, M. Woodley
FNAL: M. Ross

3. Laser-wire Principle PETRAII 2d scanning system DAQ development
Crystal calorimeter
→ PETRA III
Ultra-fast scanning
Diagnostic tool

4. The Goal: Beam Matrix Reconstruction
50% Reconstruction success
<5% error on σy
NOTE: Rapid improvement
with better σy resolution
Reconstructed emittance
of one train
using 1% error on σy
Conclude: Essential to measure
the spot-size at the few % level
or better
I. Agapov, M. Woodley

5. Skew Correction Error on coupling term: x y u
ILC LW Locations Eb = 250 GeV
x(m)
y (m)
opt(°)
u (m)
39.9
2.83 86
3.99
17.0
1.66 84
2.34 81
3.95
39.2
1.69 88
2.39
7.90
3.14 68
4.13
44.7
2.87
4.05
Error on coupling term:

6. Scan of an ILC Train of Bunches
2σL=2cτL
αtrainσe
2σe
2αJσe
2σscan
Ntrain bunches
2σe (1 + strain)
Not to scale!

7. Need for Intra-Train Scanning
For <0.5% effect, strain<0.12; otherwise, the effect must be subtracted
For 1m bunches, the error after subtracting for any systematic shift (assumed linear ±αtrainalong the train) is:
For <0.5% effect, αtrain<2.6; otherwise, higher precision BPMs required

8. Machine Contributions to the Errors
Bunch Jitter
Dispersion
BPM resolution of 20 nm may be required
Assuming  can be measured to 0.1%,
then  must be kept < ~ 1mm

9. Alternative Scan Mode
R&D currently investigating ultra-fast scanning (~100 kHz)
using Electro-optic techniques
Alternative: Keep laser beam fixed and use natural beam jitter
plus accurate BPM measurements bunch-by-bunch.
Needs the assumption that bunches are pure-gaussian
For one train, a statistical resolution of order 0.3% may be possible
Single-bunch fit errors for
Beam jitter fixed at 0.25σ
BPM resolution fixed at 100 nm

10. √2σℓ laser beam xR σey σex σℓ electron bunch Laser Conventions
For TM00 laser mode:

11. Compton Statistics Approximate – should use full overlap integral
(as done below…)
Where :
Compton xsec factor
e-bunch occupancy
Laser wavelength
Laser peak power
Detector efficiency
(assume Cherenkov system)

12. TM00 Mode Overlap Integrals
Rayleigh Effects obvious
Main Errors:
Statistical error from fit ~ -1/2
Normalisation error (instantaneous value of ) – assume ~1% for now.
Fluctuations of laser M2 – assume M2 known to ~1%
Laser pointing jitter 

13. TM01 gives some advantage for larger spot-sizes
Y. Honda et al
TM01 gives some advantage for larger spot-sizes
Estat
EM2
TM00
TM01
TM01
TM00

14. Laser Requirements Wavelength  532 nm Mode Quality  1.3 Peak Power
 20 MW
Average power
 0.6 W
Pulse length
 2 ps
Synchronisation
 0.3 ps
Pointing stability
 10  rad
ILC-spec laser is being developed at
based on fiber amplification. L. Corner et al

15. TM00 mode Statistical Error From 19-point scan
Optimal f-num1-1.5 for = 532nm
Then improve M2 determination
f-2 lens about to be installed at ATF
Relative Errors
ATF2 LW; aiming initially
at f2; eventually f1?
Error resulting from
5% M2 change

16. Towards a 1 m LW preliminary Resultant errors/10-3 E 2.5 Epoint 2.2
Ejitter
5.0
Estat
4.5
EM2
2.8
Total Error
8.0
Goals/assumptions
Wavelength
266 nm
Mode Quality
1.3
Peak Power
20 MW
FF f-number
1.5
Pointing stability
10 rad
M2 resolution 1%
Normalisation () 2%
Beam Jitter
0.25
BPM Resolution
20 nm
Energy spec. res
10-4
Final fit, including dispersion
Could be used for  measurement
→ E

17. Lens Design + Tests
f-2 lens has been built and is currently under test.
Installation at ATF planned for this year
M. Newman, D. Howell et al.
Designs for f-1 optics are currently being studied, including:
Aspheric doublet
Vacuum window
N. Delerue et al.

18. ATF Ext
S. Boogert, L. Deacon

19. ATF/ATF2 Laser-wire
At ATF2, we will aim to measure micron-scale electron spot-sizes with green (532 nm) light.
Two locations identified for first stage (more stages later)
0.75m upstream of QD18X magnet
1m downstream of QF19X magnet
Nominal ATF2 optics
LW-IP (1) LW-IP (2)
σx = m σx = m
σy = 7.74 m σy = 7.94 m
ATF2 LW-test optics
LW-IP (1) LW-IP (2)
σx = m σx = 20 m
σy = 0.9 m σy = 1.14 m
P. Karataev
 Ideal testing ground for ILC BDS Laser-wire system

20. ATF LW Plans March 07: Start upgrading ATF LW hardware
April 07: aim to install f2 lens system
May/Jun 07: aim to take first micron-scale scans
Longer term
Upgrade laser system to reduce spot-size further
Install additional LW systems, building towards emittance measurement system for ATF2.
Investigate running with UV light.
Implement ultra-fast scanning system (first to be tested at PETRA, funding permitting)
Build f-1/1.5 optical system

21. Summary Very active + international programme: Hardware Optics design
Advanced lasers
Emittance extraction techniques
Data taking + analysis
Simulation
All elements require R&D
Laser pointing
M2 monitoring
Low-f optics
Fast scanning
High precision BPMs
Look forward to LW studies at PETRA and ATF
ATF2 ideally suited to ILC-relevant LW studies.