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 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 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 the beam 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 at different 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 Constant radius Constant current Scott Mandry is looking to different configurations: - Probe placement - Foil with and without hole - …

TCTR-Measurement using EO-Techniques Phase modulation: Optical signal (electrical field): Modulation function: NEW FREQUENCIES! Amplitudes for different frequencies: Measured intensity Maximum phaseshift (<0.5)

Some (very old) Simulations 20cm bunch, 150µm micro-bunch length, 600µm spacing 100GHz sine-wave, 1ns window Some simulations (nonlinear field simulations): 1ns optical pulse (“window”) 100µm ZnTe probe External E-field E Z =5MV/m Base frequency 193THz (1.55µm)1. Harmonic (signal)2. Harmonic

First Results Fourier spectrum Measurement of a 6GHz signal with 100ps window 0 GHz

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 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 stolen from another talk

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