Hot Electron Transport in Magnetized Directly Driven Inertial Confinement Fusion Implosions

Measured hard x-ray fluxes hν > 60 keV from exploding pusher experiments on OMEGA are shown to increase by ~1.7x with application of a 10 T magnetic field compared to unmagnetized reference implosions and the overall capsule charge is shown to be reduced substantially by magnetization from measurement of the D3He-proton and DD-proton spectra. To explain these results, a model for the magnetic field evolution in the corona is derived demonstrating externally applied axial-magnetic fields rapidly reorient in the ablating plasma flow of an ICF implosion to become normal to the ablation front such that the magnetic field in the coronal plasma aligns with the laser propagation axis over a time scale of ∼ 200 ps which is much shorter than the typical timescale for the implosion hydrodynamics of ∼ 1 ns. The capsule charging condition on the electron pitch-angle in unmagnetized implosions is demonstrated to be equivalent to a confinement condition on electrons in magnetized implosions and the change in capsule charging is attributed to suprathermal electron confinement in the magnetic field. Correspondingly, the increase in hard x-ray flux is attributed to refilling of the hot electron loss-cone in the corona at a rate comparable to the electron oscillation-frequency. OSIRIS simulations of the two-plasmon decay in a longitudinal magnetic field are presented to rule out modifications to the non-linear stage of the TPD instability and related electron acceleration due to the presence of the longitudinal magnetic field.