Electron heating mode transition induced by ultra-high frequency in atmospheric microplasmas for biomedical applications

Abstract

The electron heating mode transition induced by ultra-high frequency in atmospheric-pressure microplasmas was investigated using particle-in-cell simulation with a Monte Carlo collision. Interestingly, this discharge mode transition is accompanied by non-monotonic evolution of electron kinetics such as effective electron temperature, plasma density, and electron energy on the electrode. In this study, the highest flux of energetic electrons (ɛ > 4 eV) usable for tailoring the surface chemistry in atmospheric microplasmas is obtained at the specific frequency (400 MHz), where an optimal trade-off is established between the amplitude of sheath oscillations and the power coupled to electrons for sub-millimeter dimensions (200 µm).

Abnormal electron-heating mode and formation of secondary-energetic electrons in pulsed microwave-frequency atmospheric microplasmas

“…Moreover, an electron-heating mode and its transition, which are induced by changes in the input power and frequency, have been reported as in the cases of low-pressure discharges. 9,[13][14][15] However, the previous studies of the electron-heating mode were based on fluid models; 11,12 the effects of heating on the electron kinetics has not yet been clarified. In particular, we are interested in the electron energy distribution function (EEDF) on the electrodes in atmospheric discharges for biomedical applications.…”

“…1-8 For atmospheric radio-frequency (rf) capacitive discharges, a variety of microplasmas operating over a wide range of frequencies and sub-millimeter dimensions have been reported. 1,4,[9][10][11][12] We previously studied single-frequency (SF) 9 and dual-frequency (DF) 10 atmospheric-pressure helium microplasmas, which can be sustained over a wide range of microwave frequencies from 400 MHz to 2.45 GHz. These microwave-induced microplasmas are important because they may be used to develop safe, portable, and long-lifetime devices capable of operating at atmospheric pressure.…”

Modeling of microplasmas from GHz to THz

“…2 At 0.5 GHz, the plasma behavior is similar to that of a typical capacitive coupled RF discharge. 1,[29][30][31] The central plasma density remains static in time while the plasma boundaries are strongly timemodulated by the applied electrode potential. Since the ion plasma frequency is much smaller than the excitation frequency, the ion density is immobile and responds only to the time-averaged electric field.…”

Electron confinement and heating in microwave-sustained argon microplasmas

“…The reduction in voltage reduces the coupling of power to ions being accelerated through the sheath. 7,9,10 In helium below a few hundred MHz, this attenuation of ion power loss increases the electron density. 11 Once most power is coupled to electrons, however, the electron density falls as continued reductions in sheath voltage reduce the population of high-energy electrons.…”

“…11 Once most power is coupled to electrons, however, the electron density falls as continued reductions in sheath voltage reduce the population of high-energy electrons. 7,9,11 In other gases, only limited frequency ranges have been modeled: electron densities that rise with the excitation frequency have been predicted for helium-oxygen mixtures 12 up to 0.1 GHz and for argon 6 at 0.9 and 1.8 GHz.…”