Escaping Electrons from Intense Laser-Solid Interactions as a Function of Laser Spot Size

Dean Rusby,David Neely,Paul Mckenna,Nick Butler,Bernhard Zielbauer,Ross Gray,Rachel Dance,Graeme Scott,Vincent Bagnoud

doi:10.1051/epjconf/201816702001

Abstract

The interaction of a high-intensity laser with a solid target produces an energetic distribution of electrons that pass into the target. These electrons reach the rear surface of the target creating strong electric potentials that act to restrict the further escape of additional electrons. The measurement of the angle, flux and spectra of the electrons that do escape gives insights to the initial interaction. Here, the escaping electrons have been measured using a differentially filtered image plate stack, from interactions with intensities from mid 1020-1017 W/cm2, where the intensity has been reduced by defocussing to increase the size of the focal spot. An increase in electron flux is initially observed as the intensity is reduced from 4x1020 to 6x1018 W/cm2. The temperature of the electron distribution is also measured and found to be relatively constant. 2D particle-in-cell modelling is used to demonstrate the importance of pre-plasma conditions in understanding these observations.

Highlights

When a high intensity laser (IIλλ# > 10'( W/cm2 μm2) interacts with a solid target, multi-MeV electrons are accelerated and travel into the target
Understanding the internal electron distribution present during laser solid target interactions is important to enable a greater degree of control and optimisation of secondary sources such as Bremsstrahlung x-rays, ions and neutrons
The entire angular distribution of the forward escaping electrons in the 3-20 MeV energy region has been observed as a function of defocus distance from copper foil targets

Summary

Introduction

When a high intensity laser (IIλλ# > 10'( W/cm μm2) interacts with a solid target, multi-MeV electrons are accelerated and travel into the target. At intensities > 1016 W/cm, resonance absorption usually plays a dominant role This mechanism accelerates the electrons perpendicular to the target or critical surface. Many investigations have been conducted to show how the laser energy and pulse duration affects the emitted protons[1] and x-rays[2] It has been demonstrated in the past that the size of the focal spot changes the dynamics of the laser interaction with the target. This reduction was smaller for a given intensity than that demonstrated when the target was kept at best focus and the laser energy reduced This measurement of the maximum proton energy is consistent with the data found by Brenner et al [4,5] for shorter pulse laser conditions. No studies have examined in detail the escaping electron population as a function of focal spot size

Methods

Results

Conclusion