Transcript Document

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.
Laser-wire People
BESSY: T. Kamps
DESY : E. Elsen, H. C. Lewin, F. Poirier, S. Schreiber, K.
Wittenburg, K. Balewski
JAI@Oxford: B. Foster, N. Delerue, L. Corner, D. Howell, M.
Newman, A. Reichold, R. Senanayake, R. Walczak
JAI@RHUL: 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
Laser-wire Principle
PETRAII
• 2d scanning system
• DAQ development
• Crystal calorimeter
→ PETRA III
• Ultra-fast scanning
• Diagnostic tool
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
Skew Correction
y
x 
optimal  tan  
 y 
 68  88 at ILC
u
1

y
u
x
x
ILC LW Locations Eb = 250 GeV
Error on coupling term:
          
 xy   x y 4 u    x    y  
  u    x    y  


2
2
2
1
2
x(m
)
y
(m)
opt(°
)
u
(m)
39.9
2.83
86
3.99
17.0
1.66
84
2.34
17.0
2.83
81
3.95
39.2
1.69
88
2.39
7.90
3.14
68
4.13
Scan of an ILC Train of Bunches 2σL=2cτL
αtrainσe
2σe
2σscan
2αJσe
Ntrain bunches
2σe (1 + strain)
Not to scale!
Need for Intra-Train Scanning
L
N train N e2 f rep
4 x y
HD
1


1

1 
1
3
2
strain

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:
 e
 

 1.9 103  BPM  train
e
 100 nm 
For <0.5% effect, αtrain<2.6; otherwise, higher precision BPMs required
Machine Contributions to the Errors

e  
2
scan
  J  e    E 
2
2

1
2
Bunch Jitter
 e
2   J 
 5 10 

e
 0.5 
2
  BPM 


 100nm 
Dispersion
BPM resolution of 20 nm may be required
Assuming  can be measured to 0.1%,
then  must be kept < ~ 1mm
 e
2   

 2.3 / mm  

e
  
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  ey  1m, ex  10m
Beam jitter fixed at 0.25σ
BPM resolution fixed at 100 nm
√2σℓ
Laser Conventions
For TM00 laser mode:
laser beam
xR
σey
σex
σℓ
I  x, y, z  
I0
2 2
 y2  z2 
exp 2



f R x 
2

f
x
 R


1
 x 
f R  x   1   
 xR 
electron bunch
   M 2 f #
xR  4 M 2 f #2
2
Compton Statistics
N Detected  1212
2





1
y
exp  12   
  m  
2  m


Approximate – should
use full overlap integral
(as done below…)
Where :
Compton xsec factor
e-bunch occupancy
 det  P  N e    f   
 



m
10 
 0.05 10 MW  2 10  532 nm  0.2 
Laser peak power
Detector efficiency
(assume Cherenkov system)
Laser wavelength
TM00 Mode Overlap Integrals
 ey  1m,
 ex  10m
 ey  1m,
 ex  100m
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 
2
  
 e

 2.2  10 3 
e
 10 rad 
2
 


/ 10% 


 e  f #  2  M 2 
 M  2 
 
e  e 
 M 
TM01
Y. Honda et al
TM01 gives some advantage for larger spot-sizes
Estat
EM2
TM00
TM01
TM01
TM00
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 JAI@Oxford
based on fiber amplification. L. Corner et al
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
Error resulting from
5% M2 change
ATF2 LW; aiming initially
at f2; eventually f1?
Towards a 1 m LW
preliminary Resultant errors/10-3
Goals/assumptions
E
2.5
Epoint
2.2
Wavelength
266 nm
Ejitter
5.0
Mode Quality
1.3
Estat
4.5
Peak Power
20 MW
EM2
2.8
FF f-number
1.5
Total Error 8.0
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
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.
ATF Ext
S. Boogert, L. Deacon
ATF/ATF2 Laser-wire
•
•
At ATF2, we will aim to measure micron-scale electron spotsizes with green (532 nm) light.
Two locations identified for first stage (more stages later)
1) 0.75m upstream of QD18X magnet
2) 1m downstream of QF19X magnet
Nominal ATF2 optics
ATF2 LW-test optics
P. Karataev
LW-IP (1)
LW-IP (2)
σx = 38.92 m
σx = 142.77 m
σy = 7.74 m
σy = 7.94 m
LW-IP (1)
LW-IP (2)
σx = 20.43 m
σx = 20 m
σy = 0.9 m
σy = 1.14 m
 Ideal testing ground for ILC BDS Laser-wire system
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
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.