UCLA

Magnetized plasma expansions from explosive phenomena often have characteristically large ratios of kinetic ram pressure to the ambient magnetic field pressure, β≫1. In the presence of a tenuous, ambient plasma, collisions are incapable of transferring much energy due to the high relative velocities. These expansions, however, generate large magnetic field variations, ΔB/B∼1, in the form of a diamagnetic cavity as well as potentially large electric fields.

A high-β expansion was created using a laser-produced plasma that expanded, v_exp=1.28?〖10〗^7 cm/s, β∼〖10〗^6, into a uniform, magnetized background plasma. The processes that are capable of transferring energy and momentum from the high-β expansion to the ambient plasma without collisions were explored. The combination of a magnetic probe and a novel emissive probe yielded measurements of the total electromagnetic field in three-dimensions. These constituted the first measurements of the total electric field in such an environment. The electrostatic field structure comprised the predicted inward field of a diamagnetic cavity as well as previously unobserved features including an outward field associated with a magnetic field compression and an intense electrostatic pulse preceding the LPP. All these components were stronger than the largest observed induced electric field from Faraday’s law. Direct measurements of argon ion velocities moving through these fields were made with a planar, laser-induced fluorescence diagnostic which showed ions being pulled inward against the expansion direction. Orbit solvers show that the characteristic velocity observed, v_r=-3?〖10〗^5 cm/s, is consistent with the measured fields. The inward electrostatic field exhibited a linear variation with the magnetic field while the outward field and pulse exhibited at most a weak dependence. No significant differences in the fields were observed between helium and argon background plasmas. A qualitative model to describe the evolution of the high-β expansion in the context of weak coupling was developed. The model and the experimental field structure yielded important scaling relations for similar expansions and a qualitative extrapolation to the strong-coupling case.