Plasma is sustained with gaseous discharges. A high enough voltage across molecular gas has to be applied to transform it into the plasma state. Power supplies of voltages between about 500 and 1,500 V are often used for sustaining plasma in low pressure reactors, and between 2,000 and 20,000 V for atmospheric pressure discharges. Our team is specialized in low-pressure plasma of high dissociation fraction and rather low power density. To suppress the loss of radicals by heterogeneous surface reactions reactors have to be made from materials of low recombination coefficients . These materials are dielectrics, so the electrodes are placed outside the plasma chamber. Such configurations are called “electrodeless discharges”, which are only possible using the oscillating electric field. We often use the power supplies operating in the frequency rage between about 100 kHz and 100 MHz (radiofrequency sources) or at 2.45 GHz (microwave sources). Radiofrequency generators are coupled to plasma either in the capacitive or inductive mode. Our team is specialized in inductive mode. At low power and/or elevated pressure the discharge remains in the capacitive mode although it is powered with an inductor. This configuration is known as “inductively coupled plasma in the E-mode”. At higher powers and/or lower pressures the electron density is high enough to make plasma electrically conductive enough for the plasma to act as a low-voltage part of a transformer (the high-voltage part is the coil). When such conditions are met, the plasma is very bright – the configuration is called “inductively coupled plasma in the H-mode”. The transitions between the E- and H-modes are abrupt, and hysteresis is observed: higher power of the RF generator is needed to enable the transition from E- to H-mode than the transition in the opposite direction. A flexible matching network is needed to enable such transitions.

Microwave plasma is ignited at the point of high electromagnetic filed. Once ignited, the electrically conductive plasma does not allow for penetration of the field deep into the plasma due to the skin-effect. The electromagnetic field propagates along the sheath between conductive plasma where electrons gain kinetic energy by changing the direction of oscillation at collisions with gaseous molecules. The best coupling is obtained at conditions where the oscillation amplitude is similar to the electron mean free path.

Atmospheric-pressure plasma can be sustained in the continuous mode with RF of MW discharges, but large gas flow is needed to keep the kinetic temperature low. Namely, practically all discharge power is spent for heating of the gas at atmospheric pressure. Low-frequency discharges are preferred for sustaining gaseous plasma at atmospheric pressure. Our team often uses a noble gas with a small admixture of molecular gas to suppress the loss of low-energy electrons. Plasma sustained with low-frequency discharges (typically 10-30 kHz) is temporally not uniform, but rather in the form of numerous electron streamers of the duration of the order of a microsecond, what is also a typical life-time of plasma particles at atmospheric pressure. We usually use plasma jets of a diameter of a few millimeters in diameter and several centimeters in length. Such confined plasma is useful for rapid treatment of small segments on any product surface. Pulsed DC discharges are used for sustaining plasma streamers in a highly controlled manner.