Spatial Variations of Plasma Parameters in a Hollow Cathode Discharge

The transformations of the electron energy distribution (EEDF), their concentration, and plasma space potential along the discharge gap between the hollow rectangular cathode and the mesh anode were experimentally studied. The discharge was 3 cm long, with a cross-section of 10 cm2. A new version of measurements with several single probes with the formation of current-voltage characteristics (IVC) in the probe circuit was proposed with the simultaneous application of voltages in the form of a combination of periodic and noise signals. The proportions of the signals were varied for different sections of the current-voltage characteristics. Measurements along the central axis of the discharge were taken. The dynamic range of the EEDF was not less than 4 orders of magnitude at the electron concentrations of 2–13 × 1010 cm−3, which exceeds the best known achievements. Measurements for discharge in helium at reduced pressures of 1–1.2 mbar and currents of 150–400 mA showed that the EEDFs differ from Maxwell ones, with an excess of fast electrons in the region of 10–20 eV at medium energies 4–6 eV. The fraction of fast electrons decreased in regions closer to the anode, which is associated with the nonlocality of the mechanism of the spectrum formation of free electrons. EEDFs transformations led to the space dependence of electron drift velocities on the plasma area. The dependence of the voltage drop across the cathode on the gas pressure and discharge current was noted.