Session UF2: Diffuse Discharges

Operating modes of a low pressure, high current, magnetized rare-gas plasma

We have investigated high current operation of a magnetically-enhanced rare-gas plasma for use as a high-voltage, high-power gas switch in grid-scale electric power conversion. It is desirable to operate at a low voltage to minimize incident ion energy and reduce the cathode erosion rate by sputtering. We have determined the key characteristics of low-voltage mode operation by correlating the electrical pulse and optical emission spectroscopy data with observations from high-speed movies. We have identified four key modes of operation, namely magnetron mode (200-300 V), a constricted, rotating mode (70-120 V), an edge-enhanced constricted mode (40-60 V), and metal/arc mode (20-40 V), along with the cathode geometry and the operating conditions required to select the desirable modes. It is essential to minimize operation time in either magnetron mode or metal mode due to the high erosion rate of cathode material in each case. The edge-enhanced constricted mode has a favorably low voltage but is characterized by the plasma attaching to sharp/rough surface features. The constricted, rotating mode is the preferred operating regime due to low voltage losses and the absence of cathode material observed in optical emission spectra.   

Ionization instabilities of plasma column at high electromagnetic field frequencies

The stability problem of spatially limited high frequency plasma discharge is analyzed. This instability breaks the spatial plasma homogeneity and leads to forming of complex spatial structures. In traditional works its development is associated with the kinetic processes in plasma [1] and the total electric current is conserved. Here the exact solution of Maxwell's equations in the form of a series is found. The solution takes into account both potential and vortex perturbation. A separate solution contains the field, associated with conversation law, as separate term. It is shown that the traditional approach can only be used far from electrodynamic resonances of the plasma. For plasma without a magnetic field the resonances are associated with the surface waves. Instability increments for different ratios of the collision frequency to the frequency of the field are found. The phenomenological model, that describes nonlinear stage of instability and new stationary state, is proposed. Satisfactory agreements between theory and experiment [2] are obtained. We demonstrate that the ionization instability, leading to the appearance of plasma inhomogeneity, can be observed in any discharge system, when several types of electromagnetic waves can propagate simultaneously.\\[4pt] [1] D. Mackey, L. Planti\'e, M.M. Turner, Appl. Math. Lett., 18, 865 (2005).\\[0pt] [2] S. Dvinin et al., Sov. Phys.: Fizika Plazmy, 9, 1297 (1983).   

DC Non-thermal Atmospheric-pressure Plasma Jet Generated by Syringe Needle Electrode

In this paper, non-thermal plasma jet operated in the atmospheric-pressure environment is presented. Plasma jet is generated by applying dc source voltage between 1.2 mm of inner diameter syringe needle anode with flowing Argon gas and planar or hollow copper cathode. Two operating discharge modes, which are self-pulsing discharge and continuous discharge mode, are mainly controlled by the limitation of current flowing in the discharge circuit. Rated current flowing in the circuit and ballast resistor (800 kiloohm and 1 Megaohm ballast resistors are chosen in this research) are important factors affecting on the limitation of operating discharge mode. Gas breakdown are initially generated in the self-pulsing discharge mode at the source voltage of 1.2 kV, which is slightly higher than the breakdown voltage, at the experimental condition of Argon gas flow rate of 1 LPM and electrode gap distance of 1 mm. The self-pulsing discharge currents are up to 15-20 amperes with self-pulsing frequency in the range of 10-20 kHz. The continuous discharge mode could be observed at the higher source voltage, compared with those of self-pulsing discharge mode, with the continuous discharge current in the range of a few milliamperes.