Interpretation of plasma impurity deposition probes. Analytic approximation

Abstract

Insertion of a probe into the plasma induces a high speed flow of the hydrogenic plasma to the probe which, by friction, accelerates the impurity ions to velocities approaching the hydrogenic ion acoustic speed, i.e., higher than the impurity ion thermal speed. A simple analytic theory based on this effect provides a relation between impurity fluxes to the probe Γ_imp and the undisturbed impurity ion density n_imp, with the hydrogenic temperature and density as input parameters. Probe size also influences the collection process and large probes are found to attract a higher flux density than small probes in the same plasma. The quantity actually measured, c_imp, the impurity atom surface density (m⁻²) net‐deposited on the probe, is related to Γ_imp and thus to n_imp by taking into account the partial removal of deposited material caused by sputtering and the redeposition process.