Electron avalanches and diffused γ-mode in radio-frequency capacitively coupled atmospheric-pressure microplasmas

Space-, time- and wavelength-resolved optical emission profiles suggest that the helium emission at 706 nm can be used to indicate the presence of high energy electrons and estimate the sheath in helium rf discharges containing small concentration of air impurities. Furthermore, the experimental data supports the theoretical predictions of energetic electron avalanches transiting across the discharge gap in rf microdischarges and the absence of an alpha-mode. Nonetheless, microdischarges sustained between bare metal electrodes and operating in the gamma-mode can produce diffuse glowlike discharges rather than the typical radially constricted plasmas observed in millimeter-size rf atmospheric-pressure gamma discharges.