Analysis of plasma dynamics of a negative ion source based on probe measurements

Abstract

Measurements and analysis of the plasma flow in an ion source made for negative ion extraction are reported in this article. The plasma flow has been measured using a Mach probe having two orthogonal probe heads. The plasma flow along the axis is driven by the electron pressure gradient, dragging along the ions via a measured ambipolar electric field against the collisional drag on the background gas. The force on the ions created by the electric field is mainly balanced by the collisional drag force. The collision between the ions and the background gas creates a pressure gradient along the flow direction. The one-dimensional plasma dynamic analysis supports the consistency of the experimental observations. The presence of a transverse magnetic filter reduces the plasma flow velocity, which could affect the negative ion production on the cesiated grid surface. A simple analysis shows that a strong plasma flow could enhance the surface production of negative ions.