Operating characteristics of a Transparent Cathode Discharge

A Transparent Cathode Discharge has been studied, with the aim of identifying operating regimes in which ions and neutrals are reactive species. The device consists of a cylindrical, powered cathode grid, that has generally been operated within a grounded anode grid, held concentric with the cathode. The principal mode structure evident across the operating pressure range is found to represent discharges sustained respectively by the activity of electrons at higher pressures, and ionic and neutral species at lower pressures. This is ascertained by analysis of the optical emission from the plasma, showing processes of excitation to be caused by these species. The discharge occurring at higher pressures, of tens to hundreds mTorr, has been characterised as an electron-driven 'cathode-confined' mode, that is shown to be sustained in part by the so-called 'hollow cathode effect'. The discharge occurring at lower pressures, of units to tens mTorr, is characterised as an 'ion beam' mode, composed of radially-convergent ion beams, from which Doppler-shifted light provides direct evidence for the presence of energetic ions and neutrals. Quasi-spherical space charge objects are observed to appear within the cathode, during operation at moderately low pressures of units to tens mTorr, and cathode currents of tens of mA. Larger space charge objects, termed 'fireballs', are observed to appear at a narrower range of conditions, and to show some analogous characteristics to these. Analysis of distributions of current and optical emission indicate the fireball objects to have a more complex internal structure. The physical dependences shown by the different operating modes upon the geometry of the electrode arrangement have been identified, so that the type and stability of plasma produced may be engineered by informed design. Discharge apparatus configurations have been identified that might be favourable for the creation of additionally-stable ion- and neutral-driven plasmas, and also for higher-power operation in convergent geometries