Relativistic electron beam transport for fast ignition relevant scenarios

Abstract

A crucial issue surrounding the feasibility of fast ignition, an alternative inertial confinement fusion scheme, is the ability to efficiently couple energy from an incident short-pulse laser to a high-density, pre-compressed fuel core. Energy transfer will involve the generation and transport of a relativistic electron beam, which may be subject to a number of instabilities such as the two-stream, Weibel, and filamentary instabilities that act to inhibit energy transport. This research addressed these issues by investigating the three main phases of the electron transport process: hot electron generation in the cone and the extent of confinement along the cone surface, linear instability growth in the outer plasma corona, and the nonlinear saturated state in the inner plasma corona. Analytical and computational models were constructed to include relevant physics that had been excluded from previous models, such as kinetic and collisional effects, and included use of a sophisticated particle-in-cell code (LSP). During the initial phase of transport, our simulation results showed that contrary to experimental claims, hot electron surface confinement is only a minor effect and the cone target angle is a minimal concern for design considerations. The discrepancy was attributed to a phenomenon known as escaping electrons and the enhanced intensity of electrons measured along the surface was attributed to target geometry, rather than surface confinement.

(cont.) The second phase of transport was modeled analytically with the Vlasov-Krook-Maxwell formulation, which included collisional effects, various assumed theoretical distributions, and a data fit obtained from a simulation of the laser-plasma interaction. Our primary results indicated that collisions generally suppress growth but do tend to enhance filamentary instability growth at some wavelengths; however, due to the large temperature of the data fit, the overall growth rates are relatively small for fast ignition considerations. Analysis of the saturated regime, including particle orbits, revealed similar conclusions that instabilities can be safely neglected for fast ignition conditions, especially in the high density region of the fuel, which would otherwise need to resolved at great computational expense.