Low-temperature helium plasma generated at atmospheric pressures by non-standard field applicator

Due to their inherently high-energy, plasmas are typically sustained in high temperature or low-pressure conditions. As a consequence of the needed energy input to maintain the gas ionization and discharge, it is difficult to sustain a low-temperature plasma at atmospheric pressures. Over the last three decades, however, a few techniques have been developed to generate such plasmas.

New research hybridizes two of these plasma generation techniques to create a low-temperature helium plasma at atmospheric pressure using a radio frequency excitation waveform of 13.56 megahertz. Two 25-centimeter-long cylindrical electrodes are spaced only 2 millimeters apart to create oscillations in the electric field across the gas, which is enclosed in a glass tube, acting as a pair of separated dielectric barriers. This combination of electrodes and the glass tube allow a uniform plasma discharge to be sustained in the same modes as standard radio frequency plasmas. The high controllability of the gas flux, pressure and other process conditions also make this setup ideal for both basic and applied research.

The combination of parallel electrodes with the cylindrical geometry sustains the discharge with a cooling effect. This allows the plasma to operate in a closed system without gas flow, which is preferential, for instance, in lighting applications. This design also has practical applications in systems such as medical devices and materials processing, where lower temperature operation is imperative for safety or material protection.

Future work by the authors will examine other excitation waveforms and basic plasma parameters, such as electron temperature and electron density. Plasma processing applications that require the samples to be in direct contact with the plasma require slight modifications of the setup and are also under investigation.

Source: “Generation of a long uniform low-temperature RF discharge in helium up to atmospheric pressure,” by J.-S. Boisvert, F. Vidal, and J. Margot, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5025430 .