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Transverse Coherent Transition Radiation (TCTR) Experiment First Ideas for a Measurement SetupPowerPoint Presentation

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 Mandry

Geneva, 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

- Single-shot measurement

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

- 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

- 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?

- “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

- -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

Constant radius

Constant current

Scott Mandry is looking todifferent configurations:- Probe placement

- Foil with and without hole

- …

TCTR-Measurement using EO-Techniques

Phase modulation:

Modulation function:

Optical signal (electrical field):

NEW FREQUENCIES!

Amplitudes for different frequencies:

Maximum phaseshift (<0.5)

Measured intensity

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

- 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

- 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 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)

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

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