Magnetic Collimation of Relativistic Electron Beams through Resistivity Gradients

In the electron fast ignition (EFI) approach to inertial confinement fusion [1], a relativistic electron beam (REB) deposits energy into the core of relatively cold, isochoric fuel assembly, sparking ignition and burn. The electrons are accelerated from a thin foil by short-pulse lasers and must travel from the foil into the center of the compressed fuel (typically a few hundred microns). Short-pulse laser energy can be efficiently (~60%)[2] coupled into the REB. However, the resulting beam suffers from wide angular spread, making it difficult to focus the beam and deposit the energy into the small volume of the hotspot at a distance. One proposed solution is to collimate the beam through the use of self-generated magnetic fields. Such fields can be formed using resistivity gradients in the background material [3]. Previous simulations have demonstrated the potential of this scheme to improve electron collimation [4]. In this work we conduct particle-in-cell (PIC) simulations of fast electron transport through various guiding channels. We explore different system parameters (pulse duration, pulse intensity, and channel configuration) to inform design of future experiments at the Apollon and Titan laser facilities intended to demonstrate the resistivity gradient guiding scheme. We find that at the energies accessible with these laser systems, collimation performance can be sensitive to low-temperature resistivity of the background materials and must be considered in the design of initial experiments.

[1] Tabak, et al., Phys. Plasmas 1, 1626 (1994) ​

[2] Ping et al., PRL 109, 145006 (2012)​

[3] Robinson et al, Phys. Plasmas 14, 083105 (2007)

[4] Johzaki et al., Phys. Plasmas 29, 112707 (2022)​