Momentum Transfer in Plasma Flows at High Alfvén Mach Numbers

Abstract

Momentum coupling between two interpenetrating plasmas in the presence of a weak magnetic field has been studied experimentally. A coaxial plasma gun ionizes and accelerates aluminum (0.4 mg to 10 cm/μsec) into a low-pressure background gas which has been preionized by ultraviolet radiation from the gun. A steady magnetic field is applied parallel to the aluminum flow with maximum amplitude such that (1) the ratio of dynamic (aluminum) pressure to magnetic pressure, is of order 103, (2) the ion gyroradius is much larger than over-all flow scale, and (3) the Alfvén Mach number is greater than 10. Under these conditions, the field should have no direct influence on the deceleration rate of the aluminum plasma. Streak camera photographs show little momentum transfer between the two plasmas when no field is applied (verifying collisionless conditions) and almost complete transfer when field is applied. Magnetic field probes show that the applied field has been pushed aside by the aluminum plasma creating a field free bubble region. The scale length of the electron density gradients has been measured with a holographic interferometer indicating momentum transfer lengths less than 1 cm and thus two orders of magnitude less than the ion gyroradius.