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Plasma Transport in a Converging Magnetic Field with Applications to Helicon Plasma Thrusters

Abstract

Plasma transport in the convergent magnetic field region of an RF plasma source with a movable rear gas injection wall is investigated using an RF compensated Langmuir probe to support the development of helicon plasma thrusters. Measurements of the radial plasma density profile show that the plasma structure consists of convective and diffusive regions. The width of the convective region is shown to scale according to the bounding magnetic flux surface that intersects the gas injection plane. The extent of the diffusive region is governed by a parameter, τ, which is the ratio of the convective to diffusive timescales. The plasma is observed to transition to a convective mode for τ < 0.1, which represents a new design criteria for helicon plasma thrusters.