Measurement of the plasma potential in a magnetron discharge and the prediction of the electron drift speeds

Using an emissive probe the distribution of plasma potential V_p in the bulk plasma of a dc Magnetron discharge has been determined for a range of argon pressures (0.26, 0.53 and 0.78 Pa) and cathode voltages (between -236 and -338 V). The results reveal a large axial variation in the space potential in the confined plasma, with ΔV_p~25 V over a distance of 5 cm, from plasma to sheath-edge. By combining the derived electric field with the modelled magnetic field, the distribution of single-particle drifts have been found, namely the electron E∧B, ∇B and curvature drift speeds. The predicted E∧B drift speeds (with values up to about 1.5×10⁵ m s^-1) are typically two to three times higher than the ∇B and curvature drifts. The Hall current channel is a broad region extending from above the `racetrack' down to a position close to the axis, 6 cm from the cathode. The calculated total Hall current is approximately five times the discharge current. Using a simple model of the discharge, in which there is no spatial variation in electron current density J_e, the gyrofrequency to collision frequency ratio averaged over the plasma bulk is found to be ω/ν≈7.7±4.2. In an extension to the model, a possible distribution of electron current throughout the plasma is considered, which allows the determination of ω/ν locally in the bulk. Using this method, the maximum value of ω/ν is found to be about 25, however both models indicate that cross-field electron transport occurs more rapidly than from a classical prediction.