Refluxing of fast electrons in solid targets irradiated by intense, picosecond laser pulses

Abstract

The propagation of fast electrons produced in the interaction of relativistically intense, picosecond laser pulses with solid targets is experimentally investigated using K α emission as a diagnostic. The role of fast electron refluxing within the target, which occurs when the electrons are reflected by the sheath potentials formed at the front and rear surfaces, is elucidated. The targets consist of a Cu fluorescence layer of fixed thickness at the front surface backed with a propagation layer of CH, the thickness of which is varied to control the number of times the refluxing fast electron population transits the Cu fluorescence layer. Enhancements in the K α yield and source size are measured as the thickness of the CH layer is decreased. Comparison with analytical and numerical modelling confirms that significant refluxing occurs and highlights the importance of considering this phenomenon when deriving information on fast electron transport from laser-solid interaction experiments involving relatively thin targets.

Relativistic High-Current Electron-Beam Stopping-Power Characterization in Solids and Plasmas: Collisional Versus Resistive Effects

“…Also note that the rear side (i.e., the Sn, Cu, and PP layers) of the solid-sample targets is also irradiated by the LP beam in order to create rear-side K-shell tracer conditions similar to the case of the driven samples. More particularly, this generates a long PP plasma tail where the background electron density is greater than the REB density over an extra $100 m beyond the Cu layer: the fast electrons are confined far beyond the K-shell tracers [24], allowing us to quantify the REB energy losses on the samples from only one REB transit.…”

Investigation of relativistic intensity laser generated hot electron dynamics via copper Kα imaging and proton acceleration

“…This experiment did not use a buried layer configuration 16 or a thick plastic layer at the rear to prevent recirculation of the electrons. 37 The overall K a yield can be enhanced by the recirculating electrons, but it has been demonstrated, down to a thickness of 20 lm, that this has only limited effect on the size of the K a image. 37 A Thomson parabola ion spectrometer measured the proton and ion spectra in the rear-surface target normal direction.…”

“…29 This is related to the decrease in hot electron density with increasing distance through the target due to the natural electron beam divergence. [14][15][16] There is an intrinsic connection between the fast electron beam divergence and the rear side proton acceleration 4,28,36 and experimental studies from different thickness targets were recently discussed by both Quinn et al 37 and Coury et al 38 In this paper, we vary the thickness of a copper target to measure the correlation between the hot electron transport and electron density at the rear target surface, which determines the rear electric sheath field for proton acceleration. Experimental measurements of copper K a imaging were used as a diagnostic for the hot electron transport.…”

Fast electron beam with manageable spotsize from laser interaction with the tailored cone-nanolayer target

“…Before the FEB reaches the rear surface of the cone tip, the hottest laser generated electrons can escape from the cone, establishing around the latter a space-charge sheath potential that hinders further electron escape (Quinn et al, 2011). The induced sheath electric field is roughly E sheath ∼ T h /eλ D (Mora, 2003), where λ D = (T h ε 0 /n e e 2 ) 1/2 is the Debye length, n e and T h are the number density and temperature (in eV) of the hot electrons in the sheath region, and ε 0 is the vacuum permittivity.…”