1. Laser wakefield accelerated electrons
Production of MeV Gamma Rays through Inverse Compton Scattering
Keegan Behm1, J.Cole2, E. Gerstmayr2, S.P.D. Mangles2, J.C. Wood2, C. Baird3, C. Murphy3, K. Krushelnick1, A.G.R. Thomas1
1Center for Ultrafast Optical Science, University of Michigan, Ann Arbor
2Plasmas Group, Imperial College of London, London, UK
3The University of York, York, UK
Abstract
Experimental Setup
Research for high energy photon sources has been continuing since the discovery of X-rays in 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 a0 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.
Probe beam
f/40 drive wakefield driver
f/2 scattering
beam
15 mm gas jet
Spatial and temporal alignment tools
Gamma Spectrum
Actual Signal
Calculated Signal
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.
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.
Objectives
Results and Discussion
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.
Inverse Compton Scattering
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 a0 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 (a0 = 20 and 1 GeV electrons), the critical energy of the produced gamma rays would be over 300 MeV.
Conclusions
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.
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.
Counter-propagating pulse
Laser wakefield accelerated electrons – – – – – – – – – – – – – –
References
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. – –
γ-rays
[1] Sarri, G., et al. "Ultrahigh brilliance multi-MeV γ-ray beams from nonlinear relativistic thomson scattering." Physical review letters  (2014):
[2] Di Piazza, A., K. Z. Hatsagortsyan, and Christoph H. Keitel. "Quantum radiation reaction effects in multiphoton Compton scattering." Physical review letters  (2010):
[3] Corde, Sébastien, et al. "Femtosecond x rays from laser-plasma accelerators." Reviews of Modern Physics 85.1 (2013): 1.