Characterization of electron propagation via coherent transition radiation in two different conductivity regimes
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High intensity lasers interacting with an overdense target can accelerate high energy (“hot”) electrons at currents that would far exceed the Alfven limit. Hot electron propagation can be inhibited when bulk electron motion is unable to provide a return current such that the total current is nearly zero. The ability of the material to generate a sufficient return current, and permit propagation of the non-collisional hot electrons, is strongly affected by the material conductivity. Here we present an experimental study of the interplay between the conductivity of strongly heated solid density matter and the propagation of laser accelerated hot electrons. We diagnose the hot electrons by imaging the coherent transition radiation(CTR) generated from the target’s back surface into vacuum. The 1ω and 2ω harmonics of the CTR were imaged using a 10x microscope objective to CCD cameras. The spatial profile and energy emitted at the rear surface were evaluated, showing marked differences between high and low conductive materials. The conductivity is changed through both target temperature and material selection. CTR images of electrons propagating in high conductive targets, aluminum, displayed a high degree of collimation and a spot size 2.5x smaller than the focal spot. In contrast, CTR images from low conductive target exhibited significant expansion of the electron beam. Electron propagation through the dielectric experienced 2x divergence on average compared to the aluminum. The 1ω and 2ω CTR images of heated aluminum were both 1.2 times greater area on average than the corresponding unheated spot sizes. Evaluating the energy contained in a 7.5 x 7.5 micron square, the unheated targets have 1.6x more energy for 1ω and 2x greater for 2ω on average. The reduction in material conductivity produces an electrostatic field opposing the hot electron beam leading to a reduction in energy and increased divergence of the electron beam.