Abstract Details
Characterization of Energetic Electrons Accelerated by Intense Laser-Solid Interactions by Measurement of Coherent Transition Radiation
Author: Gilliss M Dyer
Requested Type: Oral Only
Submitted: 2009-04-21 16:36:32
Co-authors: B. I. Cho, J. Osterholz, A. C. Bernstein, T. Ditmire
Contact Info:
University of Texas at Austin
2511 Speedway
Austin, tx 78712
USA
Abstract Text:
Laser absorption and energetic electron transport in intense laser-solid interaction are applicable to fast ignition in inertial confinement fusion and the development of ultrafast x-ray sources for radiography. Recently, it has been shown that laser-accelerated hot electrons can exhibit a characteristic micro-bunching, as evidenced by coherent transition radiation (CTR) from electrons crossing the plasma-vacuum interface. Here we present an investigation of the temperature, population, and temporal and spatial characteristics of energetic electrons produced by resonance absorption and j × B heating.
Experiments were performed at the THOR laser at the University of Texas at Austin. THOR is a Ti:Sapphire chirped pulse amplification system delivering up to 600 mJ of 800 nm light on target in a 40 fs pulse. The laser was focused using an off-axis parabola with f/# 2.8 to a 7 μm (FWHM) spot, producing intensities over 10^19 W/cm^2, on a flat, 10-20 μm aluminum foil target held at 45° with respect to the beam. Optical emission from the target rear surface was imaged with 20x magnification onto both a CCD and the entrance slit of a spectrometer. Signals were filtered with BG39 glass to avoid exposure from infrared light. In addition, bremsstrahlung x-ray spectra from 100 keV to 1 MeV were obtained using NaI scintillating detectors with various cutoff filters.
In images of the CTR emission, two clearly distinct, bright spots were observed with FWHM of 5-6 μm, comparable to the size of the laser focus. Further, these spot sizes remained approximately the same for targets of thickness 10 μm and 20 μm, implying the beams were collimated. One beam travelled in the target normal direction, the other travelled along the laser direction. By observing the distinct scaling of CTR brightness in the two spots and comparing to simulation, we were able to show that the signal from target-normal directed electrons were consistent with resonance absorption, with a single electron micro-bunch per laser cycle, whereas the signal from laser directed electrons had a peak at 400 nm, consistent with j × B scaling with two bunches per cycle. We thus conclude that we have simultaneously observed signals from two distinct streams of electron micro-pulses generated by resonance absorption and j × B heating. We will discuss details of these measurements and analysis, as well as further observations of spatial characteristics of electron propagation, and x-ray measurements, from a series of target thicknesses.
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