Manipulating the Reactivity-Stability Balance in Radio-Frequency Atmospheric Pressure Glow Discharges

Summary form only given. One of the most fundamental open questions in the science and technology of atmospheric pressure glow discharges (APGD) is how to enhance plasma reactivity (application efficacy) while maintaining plasma stability (process controllability). This is particularly challenging since the homogeneous state of glow discharges is prone to instabilities, particularly at high and atmospheric pressures. Such instabilities often evolve APGD into thermal arc plasma thus hampering their use for a wide range of applications. For radio-frequency (RF) APGD however, we have recently established that the glow-to-arc transition is preceded by APGD operation in a so-called gamma mode in which secondary electrons dominate gas ionization and the differential conductivity is negative. If APGD operation can be kept in the alpha mode of lower discharge current with the same electron density and energy (hence similar plasma reactivity), plasma stability can then be improved for the same without compromising plasma reactivity. This will offer a possible strategy to enhance plasma reactivity while sustaining plasma stability for APGD. In this contribution, we study RF APGD in helium and present strategies to sustain an alpha-mode operation of RF APGD (hence better plasma stability) whilst maximizing mean electron energy and electron density. This will be achieved using higher radio-frequencies than the mainstream 13.56 MHz. Our approach is a computational one, based on a hybrid APGD model that treats electrons kinetically and heavy particles hydrodynamically. Through a suite of numerical examples, it is shown that it is possible to manipulate the reactivity-stability balance and this can be used to tailor dynamic properties of RF APGD. To assess these numerical results, time-resolved emission spectroscopy and nanosecond imaging are also used. Comparison between simulation and experimental data confirms unequivocally the proposed technique for manipulating the reactivit- -stability balance