Experimental Evidence for Self-Generated Magnetic Fields and Remote Energy Deposition in Laser-Irradiated Targets

Abstract

In a series of experiments using two-, four-, and eight-beam 10.6-jum-laser irradiation of a variety of target geometries, a significant amount of energy was found to be deposited in regions remote from the focal spots. The deposition patterns can be predicted with a self-generated magnetic field model. PACS numbers: 52.50.Jm Lateral transport of energy away from laser focal spots can play an important role in redistributing energy deposition in laser-fusion targets. Work has been reported investigating the qualitative 1 and quantitative 2 nature of this transport. Recently, using a plasma simulation, Forslund and Brackbill 3 have identified convective transport of electrons in self-generated magnetic fields as an important mechanism for surface transport in laser-irradiated foils. In one simulation with a laser intensity of 5 x 10 13 W/cm 2 in a 60-jum spot and a hot-electron temperature of 20 keV, peak fields of the order of 1 MG were calculated. The calculation has not been performed at higher intensities comparable to those used in experiment (~10 16 W/cm 2 ) because the code does not handle the relativistic effects of the high-energy electrons generated at these intensities. In general, the ratio of electron to magnetic field pressure is of order 1 in a magnetized sheath whose thickness is large compared with the electron gyroradius. This Letter presents experimental evidence for the nonuniformity of energy deposition predicted by the magnetic field model in a variety of target geometries progressing from flats to cylinders to spheres. From the simulations, a simple qualitative moddel has been developed. Briefly, the model describes lateral energy transport by electrons in magnetic fields generated at the periphery of the laser spot by lateral temperature gradients in the corona. These gradients are maintained by electrons confined and drifting in the magnetic field, resulting in the convective transport of energy from the beam spot to the edge of the magnetized region. The interaction of the magnetic field and electrons produces a thermal magnetic wave 4 which propagates across the surface until disrupted by fringing fields at the target edge or by destructive interference with the wave propagating from an adjacent beam.

A Simplified Implicit Maxwell Solver

“…Predictions of self-generated magnetic fields in laserplasma interactions [19] were confirmed by experiments in Ref. [62]. Implicit simulations of quasi-perpendicular [20] and quasi-parallel shocks [46] reveal complex behavior that is the subject of continuing study [52].…”

Review on Current Sheets in CME Development: Theories and Observations

“…Regarding the Biermann battery effect, interested readers are referred to a review by Widrow (2002) for more details. Yates et al (1982) realized the occurrence of magnetic reconnection when an unexpected X-ray emission was observed between two laser spots in a multi-beams experiment. As the two bubbles expanded laterally and encountered each other with oppositely directed magnetic fields, reconnection took place and the field lines were topologically rearranged in the diffusion region.…”

Proton radiography of dynamic electric and magnetic fields in laser-produced high-energy-density plasmas

“…High plasma densities (> 10 20 cm -3 ), high temperatures (~ 1 keV), intense B fields (~ 1 MG), and high ratios of thermal pressure to magnetic pressure (β >> 1) distinguish this novel regime from tenuous plasmas, of order 10 14 cm -3 or (usually much) lower, which are the more traditional venues of reconnection experiments 33,36,37 . Radiography experiments described in Ref.…”