Controllability of Ion Out Flow Driven by Ambipolar Electric Field from Magnetic Mirror

A laminar flow is an ordered form of energy containers in plasma systems, hence tractable energy form to control or to convert to the other forms in plasmas. For this reason, plasma flows are utilized for various purposes, such as propulsion of spacecrafts, magnetohydrodynamic generators, and confinement of high-β fusion plasmas. In this study, we investigate properties of out flow of ions from a magnetic mirror in a laboratory experiment with a detailed measurement of axial profiles of plasma parameters for varied mirror ratios of the magnetic mirror. The experimental results reveal that the speed of the ion out flow is increased with the mirror ratio. This is considered to be attributed to enhancement of an ambipolar axial electric field accompanied by the increased mirror ratio. The relationship between the ambipolar potential height and the mirror ratio follows the relationship indicating a constant effective mirror ratio, which is an indication of an elimination of the mirror confinement effect by the enhanced ambipolar axial electric field. It is confirmed that the ion out flow satisfies the Bernoulli’s theorem and the continuity equation consistently. Accordingly, the ion out flow speed can be increased up to the speed corresponding to unity Mach number in the downstream of the magnetic mirror. Our work is the first detailed experimental confirmation of the Bernoulli’s theorem in a plasma flow system, which incorporates electrostatic potential variation in space. Another remarkable finding is that in the case of the mirror ratio lower than unity, the direction of the axial flow is reversed.