Langmuir probe measurements in a magnetized capacitive discharge around a tilted RF electrode
Résumé
Applications of magnetized Radio Frequency (RF) capacitive discharges for mirror cleaning applications of optical diagnostic systems in fusion devices and heat flux measurements have received a large interest in last years. Ensuring the homogeneity of heat flux to the RF electrode requires localized measurements of plasma parameters in front of the RF electrode. Langmuir probe measurements were taken in the ALINE plasma device around the active RF electrode with the help of a 3D manipulator equipped with RF compensation. Highly negative floating potential values were recorded inside the magnetic flux tube connected to the active RF electrode whilst plasma potential remained positive. Since the difference between the plasma potential and the floating potential is proportional to the average electron temperature, experimental investigations were carried out in capacitive coupling and direct coupling to study the change in floating potential as a function of input gas pressure, magnetic field strength (B), and the angle between the RF electrode and magnetic field. The observed change in floating potential is attributed to the stochastic heating leading to the generation of hot electrons, which was confirmed using the Langmuir probe current-voltage curves. The experimental results are found to agree with the analytical model based on the flux balance equations.