Ion accumulation in an electron plasma confined on magnetic surfaces

Accumulation of ions can alter and may destabilize the equilibrium of an electron plasma confined on magnetic surfaces. An analysis of ion sources and ion content in the Columbia Non-neutral Torus (CNT) [T.S. Pedersen, J.P. Kremer, R.G. Lefrancois, Q. Marksteiner, N. Pomphrey, W. Reiersen, F. Dahlgreen, and X. Sarasola, Fusion Sci. Technol. 50, 372 (2006)] is presented. In CNT ions are created preferentially at locations of high electron temperature, near the outer magnetic surfaces. A volumetric integral of ne iz gives an ion creation rate of 2.8× 1011 ionss. This rate of accumulation would cause neutralization of a plasma with 1011 electrons in about half a second. This is not observed experimentally, however, because currently in CNT ions are lost through recombination on insulated rods. From a steady-state balance between the calculated ion creation and loss rates, the equilibrium ion density in a 2× 10-8 Torr neutral pressure, 7.5× 1011 m-3 electron density plasma in CNT is calculated to be ni =6.2× 109 m-3, or 0.8%. The ion density is experimentally measured through the measurement of the ion saturation current on a large area probe to be about 6.0× 109 m-3 for these plasmas, which is in good agreement with the predicted value.