Transverse Coherent Transition Radiation (TCTR) Experiment First Ideas for a Measurement Setup
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Transverse Coherent Transition Radiation (TCTR) ExperimentFirst Ideas for a Measurement Setup
Max-Planck-Institute for PhysicsMunich
Olaf Reimann, Scott MandryGeneva, October 19, 2012
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Outline
• Short introduction▫ Why TCTR in frequency domain?
• Principle of the measurement
• First results
• Probes and Probe configuration
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What we are looking for?
• We are interested in the proton-beam modulation:▫Modulation frequency▫Modulation depth
• Modulation frequency:▫ 250 GHz for a 7 1014 cm-3 plasma
• Bunch-to-bunch changes?▫ Single-shot measurement
Electrooptic sampling
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A Problem!
• The protons are only pushed out of axis in the plasma cell. They are not disappearing.
The E-field outside the proton-beam is not modulated
We need a “converter”
Transverse coherent transition radiation is a good candidate!
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• Coherent Transition Radiation emitted radial around a charged beam along the surface of a (metallic) screen
• Normal (to the screen) electric field component• Dipole-like radiation pattern• Can be modulated by beam density
What is TCTR
Picture taken from A. Pukhov paper
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• Electric fields with amplitudes up to hundredths of kV at a distance of 10mm
• Signal is to the first order proportional to thebeam density
• High frequencies (several hundredth GHz) Make use of electrooptic sampling (EOS)
• But: No simple frequency response curve
TCTR Characteristics
Typical E-field for TCTR atdifferent radial distances
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• “Normal” time-domain single shot EOS-systems are measuring within a window of 10-20ps Too short for our expected frequency range
(250GHz) to achieve high resolution frequency information
▫ Additional problem: too complicated to use it at different probing positions
• Better: Time-Lensing EOS▫ But: has to be optimized for a “design“ frequency Not for the first experimental phase, but maybe
later
Measurement in the frequency domain
Why Frequency Domain?
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TCTR in Frequency Domain
• -Field of a charge distribution exiting a metallic screen:
with
• In frequency domain:
with retarded time
results in
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TCTR with Constant Beam Radius
• Beam density: for
for
• -field of a beam with constant radius:
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Const. Beam Radius and Density Modulation
• Modulation:
with
• Resultant E-field amplitude:
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Constant Radius vs. Constant Current
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m=0.66 m=0.33
1. Harmonic of detected TCTR-fieldMean beamradius: 600µm, radial distance: 40mm
|EZ| (
V/m
)
Modulation Frequency (GHz)
Constant radius
Constant current
Scott Mandry is looking todifferent configurations:- Probe placement- Foil with and without hole- …
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TCTR-Measurement using EO-Techniques
OSA
Optica lSpectrumAnalyzerM odulator
Proton-Bunch
XC W -Light
Phase modulation:Optical signal (electrical field): Modulation function:
t
NEW FREQUENCIES! Amplitudes for different frequencies:
2)(~)( fafI Measured intensity
Maximum phaseshift (<0.5)
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Some (very old) Simulations
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Frequency (THz)
20cm bunch, 150µm micro-bunch length,600µm spacing100GHz sine-wave, 1ns window
Some simulations (nonlinear field simulations):• 1ns optical pulse (“window”)• 100µm ZnTe probe• External E-field EZ=5MV/m
Base frequency 193THz (1.55µm) 1. Harmonic (signal) 2. Harmonic
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First Results
• Fourier spectrumMeasurement of a 6GHz signal with 100ps window
0.0 1.0n 2.0n 3.0n 4.0n 5.0n-1.1m
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plit
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First Results
• Fourier spectrum to show the resolution▫ Artificial (nonlinear) phase modulated spectrum▫ Comparison with 4-path grating spectrometer
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Frequency (THz)
EO phase modulated spectrum with 8 GHz line separation
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Advantages of the System
• Semiconductor laser based▫ Simple setup
• Fiber based signal transport
• Sampling-signal can be splitted und transported to many different probing positions
• Make use of the same EOS system for many probing positions
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Probe Configuration
GRIN-Lens with prism (GRINTECH)
Probe setup with a “closed” optical path using GRIN-Lenses and prisms:
Possible length of probe in longitudinal (beam) direction: 5mm
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Wishlist!!!
• Probing directly before (without foil) and after (with foil) the plasma cell
• At least four (maybe eight) probes at each probing position around the beam in the beam line
Picture stolenfrom anothertalk
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What we need in the Beam Line
Probing section• 20 cm per section (Length), • Metallic foil in the beam line
(maybe with a hole for the beam?)• 4 or 8 motorized stages around the beam line• Radial movable probes ( 1-2cm from beam axis?)• Probe diameter: 5mm • Access with two optical fibers (SMF28?) per probe
• Measurement system can be far away (10m, 100m, …)
• Connected by two fibers pro probe
• No Radiation ???
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Future Work
• Simulations of different probing configurations
• Increase resolution and sensitivity
• Studying nonlinearities of the system
• Building and testing probes
• Building a TCTR probe section and test it