Transverse Coherent Transition Radiation (TCTR) ExperimentFirst Ideas for a Measurement Setup

Max-Planck-Institute for PhysicsMunich

Olaf Reimann, Scott MandryGeneva, October 19, 2012

Outline

• Short introduction▫ Why TCTR in frequency domain?

• Principle of the measurement

• First results

• Probes and Probe configuration

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

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!

• 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

• 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

• “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?

TCTR in Frequency Domain

•      -Field of a charge distribution exiting a metallic screen:

                                    with                    

• In frequency domain:

                                                    

with retarded time                            

results in

                                                                               

TCTR with Constant Beam Radius

• Beam density:                           for            

                           for         

                                                                                        

•      -field of a beam with constant radius:

                                                                  

Const. Beam Radius and Density Modulation

• Modulation:                                  

with                 

• Resultant E-field amplitude:

Constant Radius vs. Constant Current

0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400.00

10k

20k

30k

40k

50k

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70k

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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- …

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)

Some (very old) Simulations

190 191 192 193 194 195 1960

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ld S

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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

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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-950.0µ

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-750.0µ

Am

plit

ud

e (

V)

Time (s)

0 GHz

First Results

• Fourier spectrum to show the resolution▫ Artificial (nonlinear) phase modulated spectrum▫ Comparison with 4-path grating spectrometer

3 4 5 6 7

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plit

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Time (ns)

192.90 192.95 193.00 193.05 193.10

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plit

ud

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Frequency (THz)

EO phase modulated spectrum with 8 GHz line separation

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

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

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

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 ???

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