Laserwire Beam Size Monitor - TESLA Technology Collaboration

R&D towards a Laser Based Beam
Size Monitor for the FLC
T Kamps, BESSY FEL
TESLA Meeting, Hamburg
15 – 17 September 2003
WG-II APD Group Session
30/09/2003
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The next 20 minutes
• Motivation for the project
• History and status of Laserwire experiments
• Laserwire at PETRA
- Environment
- Detector simulations, design and calibration
- Laser scanning, transport and focusing
- Installation – status and plans
• Conclusions and Outlook
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Motivation
• Maximise Luminosity performance of Linear Collider
• Development of standard diagnostic tool for LC and LC
Test Facility (LCTF) Collider operation
• Control of transverse beam size and emittance in Beam
Delivery System (BDS) and at Interaction Point (IP)
• Features
- Full reconstruction with error smaller than 10%
- Fast (intra-train) scanning
- Non-destructive for electron beam
- Resistant to high power electron beam
Ø Optical scattering structures
2
L=
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nb N e f rep
4πσ σ
*
x
*
y
∗ HD
3
LC Layout and Parameter
CLIC
NLC
TESLA
BDS
σx/ µm
σy/ µm
3.4 to 15
0.35 to 2.6
7 to 50
1 to 5
20 to 150
1 to 25
IP
σx/ nm
nm
σy/ nm
nm
196
4.5
335
4.5
535
5
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Optical Scattering Structures
• Scanning of finely focused laser beam through electron beam
• Detection of Compton photons (or degraded electrons) as function
of relative laser beam position
• Challenges
- Produce scattering structure smaller than object to measure
- Fast scanning mechanism
- Efficient signal detection / background suppression
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SLC/SLD Laserwire
• Complex installation inside
SLD detector during shutdown
• Measured spotsizes as small
as 2.1 x 0.6 µm at IP
• Laser spotsize 380 nm with
Rayleigh range of 5 µm
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Laserwire Interest Group
• Elevate existing design ideas and experimental expertise
• Standard tool for LC and LCTF operation
• Collaborations
- SLAC/KEK Laserwire for ATF
- BESSY/CERN/DESY/RHUL on Laserwire for CTF and
PETRA
• Laserwire for µm spotsizes
• Advanced techniques for smaller spotsizes
- TE01 mode laserwire à sub- µm capabilities
• Next run of ATF Laserwire experiment
- Laser interferometer (Shintake) à tens of nm
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Laserwire for PETRA
• Positron Electron Tandem
Ring Accelerator
• Injector for HERA, upgrade to
synchrotron light source
• Free straight section
• Easy installation of hardware
due to existing access pipe
and hut outside tunnel area
• Q-switch Nd:YAG
• No clean longitudinal mode
• Laser from CERN LEP
polarimeter experiment
- Mode beating
- Measured with streak
camera
30/09/2003
PETRA parameter
Energy
Bunch Length
Charge/bunch
Hor. beam size
Ver. beam size
E/GeV
σz/ps
nC
σx/µm
σy/µm
4.5 to 12
~100
1 to 3
500 to 100
50 to 10
Laser parameter
Wavelength
Energy
Pulselength
Reprate
Beam size
Divergence
T Kamps, BESSY FEL
l/nm
E/mJ
dt/ns
frep/Hz
σx,y/mm
θ/mrad
1064/532
250/90
11/9
1 to 30
~7
0.7
8
Laserwire for PETRA
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Laser Focusing/Diagnostics
• Requirements
Ø RMS spotsize at interaction
smaller than electron beam
size, here 10 to 30 mm
Ø Rayleigh range larger than
horizontal beam, here 100 to
300 mm
Ø Resistant against high power
beam
Ø Beam stay clear distance
Ø Include diagnostics
• Solution commercially available
laser objective
• Imaging system for diagnosics
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Lab Measurements at RHUL
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Lab Measurements at RHUL
• Measurement of spot size at
focus and beam propagation
with knife edge technique
• Slicing of beam at several
longitudinal positions
• Piezo movement controlled by
interferometer
- high precision ~30 nm
• Tested with beam at CTF2
Laserwire experiment
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Fast Scanning
• Profile scans within one
bunch train of TESLA
requested
• Scan resolution to scan at
least a few points per
sigma
• Flexible scan pattern for
systematic studies
• Piezo driven platform with
high-reflective mirror
• Discrete and continuous
operation (up to 1 kHz)
• High damage threshold
• Tested in lab with homebuild driver unit and
amplifier
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Signal and Backgrounds
• Photon electron scattering
• Background sources
- Synchrotron radiation
- Cosmic rays
- Bremsstrahlung
• Simulation with Geant4/tool kits
Full simulation with realistic setup
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Detector Simulation
• Requirements for detector material
- short decay time (avoid pile up)
- short radiation length
- small Moliere radius
• Cuboid detector crystals made of PbWO4
• 3x3 matrix of 18x18x150 mm crystals
• Energy resolution
better than 5%
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Detector Calibration
• Detector studies with DESY II testbeam
• Beamline with electrons with energy
from 450 MeV to 6 GeV
• Ten detector crystals were calibrated
using a single PMT
• Combination of nine crystals in matrix
• Resolution
- High intrinsic resolution
- Full matrix less good
2
σ   p   p 
R 2 =  E  =  1  +  2  + p32
 E   E  E 
2
2
p1: stochastic contrib., p2: noise
p3: constant (inhomogeneity)
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Setup at PETRA
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Installation at PETRA
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Installation at PETRA
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Installation at PETRA
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Installation at PETRA
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First Photons 31.07.03
Laser on
Laser off
Calorimeter
Q-switch
Photodiode at IP
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Discussion (very prel.)
• Signal Rate
- Positron bunch: 7 GeV, 3 nC
- Laser beam: 9 MW
à Comptons photons total energy:
800 photons x 390 MeV mean energy = 312 GeV
• Positron Beam Size
- Laser beam at IP s L ~ 40 µm
- Signal disappered ± 200 µm à s m ~ 67 µm
- Positron beam s e = sqrt(s m2 - s l2 ) ~ 54 µm
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Conclusions & Outlook
• First Compton signal with Laserwire achieved
• Signal and background rate as expected
• Problems
- CCD cameras failures (heating, overvoltage)
- Laser beam diagnostics (power, trans. size)
- Dedicated beam time
• Outlook
- CCDs back from manufacturers workshop
- Laser beam diagnostics in place/in progress
- New laser trigger electronic
- No dedicated beam time
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People
G Blair, S Boogert, G Boorman, J Carter,
T Kamps, A Muir, F Poirier, I N Ross,
J Frisch, M Ross, Y Honda, H Sakai,
N Sasao, J Urukawa, K Balewski,
H C Lewin, P Schmüser, S Schreiber,
K Wittenburg, J Bosser, E Bravin,
S Hutchins, T Lefevre
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