Langmuir probe analysis in negative ion beams

Negative ion beams are used as precursors of neutral beams for additional heating of fusion plasmas. By ionizing the background gas, positive ions are confined and cumulated in the negative potential well of the ion beam, until the space charge is neutralized and the negative ion beam can propagate in the absence of defocusing electric fields. Experimental measurements of the negative ion beam plasma parameters can provide essential data for the beam neutralization in gas and plasma neutralizers. This paper presents a method to analyze Langmuir probe data obtained in a negative ion beam, in which the beam ion current dominates on the current given by the secondary ion and electron plasma. The sheath model is discussed for different compensation degrees of the beam space charge, also including the secondary electron emission form the surface. The analytical curves are fitted to experimental data by adjusting the plasma parameters. Plasma potentials along a profile transverse to the beam axis are deduced from the probe data.