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Transverse Coherent Transition Radiation (TCTR) Experiment First Ideas for a Measurement Setup. Max-Planck-Institute for Physics Munich Olaf Reimann , Scott Mandry Geneva, October 19, 2012. Outline. Short introduction Why TCTR in frequency domain? Principle of the measurement First results

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Transverse coherent transition radiation tctr experiment first ideas for a measurement setup

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

Max-Planck-Institute for PhysicsMunich

Olaf Reimann, Scott Mandry

Geneva, October 19, 2012


  • Short introduction

    • Why TCTR in frequency domain?

  • Principle of the measurement

  • First results

  • Probes and Probe configuration

What we are looking for
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
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!

What is tctr
What is TCTR

  • 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

Picture taken from A. Pukhov paper

Tctr characteristics
TCTR Characteristics

  • 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

Typical E-field for TCTR atdifferent radial distances

Why frequency domain
Why Frequency Domain?

  • “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

Tctr in frequency domain
TCTR in Frequency Domain

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


  • In frequency domain:

    with retarded time

    results in

Tctr with constant beam radius
TCTR with Constant Beam Radius

  • Beam density:                           for                                        for         

  •      -field of a beam with constant radius:

Const beam radius and density modulation
Const. Beam Radius and Density Modulation

  • Modulation:                                   with

  • Resultant E-field amplitude:

Constant radius vs constant current
Constant Radius vs. Constant Current

Constant radius

Constant current

Scott Mandry is looking todifferent configurations:- Probe placement

- Foil with and without hole

- …

Tctr measurement using eo techniques
TCTR-Measurement using EO-Techniques

Phase modulation:

Modulation function:

Optical signal (electrical field):


Amplitudes for different frequencies:

Maximum phaseshift (<0.5)

Measured intensity

Some very old simulations
Some (very old) Simulations

  • Some simulations (nonlinear field simulations):

  • 1ns optical pulse (“window”)

  • 100µm ZnTe probe

  • External E-field EZ=5MV/m

20cm bunch, 150µm micro-bunch length,

600µm spacing

100GHz sine-wave, 1ns window

Base frequency 193THz (1.55µm)

1. Harmonic (signal)

2. Harmonic

First results
First Results

  • Fourier spectrumMeasurement of a 6GHz signal with 100ps window

0 GHz

First results1
First Results

  • Fourier spectrum to show the resolution

    • Artificial (nonlinear) phase modulated spectrum

    • Comparison with 4-path grating spectrometer

EO phase modulated spectrum with 8 GHz line separation

Advantages of the system
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
Probe Configuration

Probe setup with a “closed” optical path using GRIN-Lenses and prisms:

Possible length of probe in longitudinal (beam) direction:  5mm

GRIN-Lens with prism (GRINTECH)


  • 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
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
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