Probe beam - PowerPoint Presentation

Slide1

Probe beam

f/40 drive wakefield driver

f/2 scattering

beam

15 mm gas jet

Spatial and temporal alignment tools

Inverse Compton Scattering

Objectives

Abstract

Measure a radiation reaction in the electrons by colliding them with a strong counter-propagating beam.

Produce MeV-level gamma rays through inverse Compton scattering of the electron beam.

Develop a gamma ray spectrometer by measuring the penetration depth of photons in the crystal array.

Results and Discussion

Conclusions

References

Production of MeV Gamma Rays through Inverse Compton Scattering

Experimental Setup

Keegan Behm

1

,

J.Cole

2

, E. Gerstmayr

2

, S.P.D. Mangles

2

, J.C. Wood

2

, C. Baird

3

, C. Murphy

3

, K. Krushelnick

1

, A.G.R. Thomas

1

1

Center for Ultrafast Optical Science, University of Michigan, Ann Arbor2Plasmas Group, Imperial College of London, London, UK3The University of York, York, UK

47 x 33

CsI

crystal array used as primary gamma ray diagnostic.

CsI

crystals are 5 x 5 x 50 mm.

Beam incident on side of spectrometer to measure penetration depth.

[1] Sarri, G., et al. "Ultrahigh brilliance multi-MeV γ-ray beams from nonlinear relativistic thomson scattering." 

Physical review letters

 113.22 (2014): 224801.

[2] Di Piazza, A., K. Z. Hatsagortsyan, and Christoph H. Keitel. "Quantum radiation reaction effects in multiphoton Compton scattering." 

Physical review letters

 105.22 (2010): 220403.

[3]

Corde

,

Sébastien

, et al. "Femtosecond x rays from laser-plasma accelerators." Reviews of Modern Physics 85.1 (2013): 1.

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This is a method of high energy photon production (on the MeV) level by scattering an electron beam with a counter-propagating laser. In this experiment, we accelerated electrons from a 15 mm gas jet to 800 MeV and collided them with a counter-propagating laser with an a

0

of 20. The intense electric field of the scattering laser causes the electrons to wiggle in the field, thus releasing very high energy photons. The critical energy of the photons is proportional to both the energy of the electrons (

γ

in the equation below) and the intensity of the laser. For the ideal scenario in this experiment (a

0

= 20 and 1 GeV electrons), the critical energy of the produced gamma rays would be over 300 MeV.

γ

-rays

Laser

wakefield

accelerated electrons

Counter-propagating pulse

The raw data obtained from the

CsI

crystal is shown on the left. The image is 1024 x 1024 pixels with the dark spots due to the Al face plate blocking the light. The data within each crystal was averaged together into a single data point to make it possible to analyze with an iterative algorithm and MCNP simulations.

In an attempt to maximize the signal on the

CsI

scintillator, a raster scan was performed to try and improve the overlap between the electron beam and the counter-propagating laser. On the left shows an area of highest signal was found in the middle. To turn the figures above into a spectrum, several MCNP simulations were performed of

monoenergetic

photon beams entering a simulated

CsI

block, results shown on the right.

Research for high energy photon sources has been continuing since the discovery of X-rays in 1895. Here we present data showing the production of gamma rays as high as 100 MeV through inverse Compton scattering of a laser

wakefield

accelerated (LWFA) electron beam. One of the reasons for studying high energy photon sources on an all-optical device is because they have a high degree of

tunability

and it is possible to eliminate timing jitter between various arms of the experiment. At the Astra-Gemini laser system at Rutherford Appleton Labs (RAL), we collided an 800 MeV electron beam with a counter-propagating ultra-short pulse with a maximum a

0

of 20 [1]. The goal for this experiment was to measure a radiation reaction due to the immense energy radiated away by the electron beam [2]. A

CsI

crystal array positioned parallel to the photon beam was used to detect the high energy gamma rays and provide information about the penetration depth of the gammas and the vertical divergence. Figure 1 shows an example of the data obtained from the fluorescing

CsI

crystals within the detector array. With this detector we can analyze correlations between vertical divergence of photon flux and characteristics of the electron beam such as charge or maximum electron energy.

Successfully beam overlap between the electrons and counter-propagating f/2 heater beam.

There was no evidence of a radiation reaction in the electron beam on the electron spectrometer.

Produced gamma rays of 100 MeV or greater through inverse Compton scattering.

The simulation curves struggle to match up with the data in the low energy regime due to the sharp rise in signal at the start of the

CsI

array.

The iterative algorithm is currently not producing a very accurate spectrum. It is likely that the light yield from the

CsI

scintillation is not linearly proportional to energy deposited.

An iterative algorithm was used to create a sample spectrum and calculate what the resulting

CsI

signal would look like.

Starting with a flat spectrum, small perturbations were made to gradually form an input spectrum that can match the signal data.

The calculated signal was then checked against the actual experimental data and perturbations to the spectrum were kept if the matching was improved and thrown away if it was not improved.

The simulations cannot match the first few bricks of the signal very well resulting in an overestimated energy in the gamma ray spectrum.

A cause for this is likely a nonlinear relationship between energy deposited and light yield in the

CsI

crystals.

Calculations of perfect laser beam overlap with the electrons suggest that scattering could produce up to 300 MeV gamma rays.

Actual Signal

Calculated Signal

Gamma Spectrum