Spatially resolved measurements of plasma parameters in a broad-beam ion source

Abstract

We measured spatially resolved plasma parameters inside the discharge chamber of a broad-beam ion source using the Langmuir-probe technique. Radial profiles of non-uniform charge-carrier densities between 1010 and 1011 cm−3 are detected which depend on discharge current and voltage. The plasma potential is determined as being equal to the anode potential. Two maxima characterize the electron-energy distribution, a low-energy one produced by thermalized electrons and a second maximum of primary electrons accelerated in the filament sheath. The strength of the multipole magnetic field of the ion source strongly affects the spatial charge-carrier distribution. At a small magnetic-field strength the radial carrier-density distribution is more uniform than at a strong one. These different radial charge-carrier densities in the discharge chamber are also reflected in the extracted ion-beam profiles and ion-beam currents.