Modeling of air plasma generation by electron beams and high-voltage pulses
Many of both known and emerging applications of weakly ionized gases, or plasmas, require that plasma be produced with minimum power. Minimization of the power budget can be achieved if the energy of ionizing electrons is high, from tens to thousands of electronvolts. Low-energy (1-10 eV) electrons in conventional nonequilibrium electric discharges dissipate most of the discharge power in inelastic collisions with molecules, primarily exciting molecular vibrational states. In contrast, high-energy electrons would ionize the gas quite efficiently, greatly reducing the energy cost of newly produced electrons. In this sense, high-energy electron beams injected into the gas represent the most efficient way of nonequilibrium ionization. High-energy electrons can also be produced in the gas if a pulse of very high voltage is applied to it. Calculations show that increased electric field in the pulse would decrease the energy cost of newly produced electrons. In order not to waste power by overshooting electron density, the pulse length must be very short. To maintain the mean electron density at a prescribed level, pulse repetition rate should match the recombination time. Analytical calculations show that the power budget in the high-voltage, repetitive pulse mode can be significantly lower than in the DC regime, but still much higher than in the case of electron beam ionization. For each pulse length, there exists an optimum electric field that minimizes the power budget. A detailed modeling of spatio-temporal dynamics of pulsed discharges reveal that voltage displacement into the cathode sheath plays a critical role. Fully coupled modeling of non-local kinetics of high and low-energy electrons, ionization processes, charge particle transport, and electrodynamics was performed for the first time for pressures of 1 Torr and higher. The kinetic modeling shows formation of a large group of high-energy electrons in high-voltage nanosecond pulse and dramatic increase in ionization rate. The ionization rate across the discharge gap has maxima both near the cathode and near the anode. The high-energy electrons produced during the pulse can generate additional ionization cascades while moving and losing their energy after the pulse, resulting in substantial ionization and excitation of atomic and molecular states between the pulses. 2000 by S.O. Macheret, M.N. Shneider, and R.B. Miles.