α , γ, and normal, abnormal glow discharge modes in radio-frequency capacitively coupled discharges at atmospheric pressure

Abstract

Discharge modes, α and γ, of a radio-frequency helium capacitively coupled discharge at atmospheric pressure were investigated with the discharge gap distance between electrodes varied from 1 to 5mm. As similarly observed in other experiments, the α and γ mode and the α–γ mode transition were observed with large drops in the voltage (310–179V) and the phase angle between the voltage and current (54°–18°), and a contraction of the plasma volume (8.5–0.17cm3, at 3mm gap distance). The discharge voltage where the α–γ mode transition occurred versus the gap distance showed a similar behavior with the Paschen curve for a gas breakdown. Depending on the gap distance, normal and abnormal glow regimes were observed in the αmode. At 1 and 2mm, the α mode remained in the abnormal glow discharge until the α–γ mode transition occurred as the discharge current increases. At 3mm, however, the α mode was excited as a normal glow discharge with a constant current density (17mA∕cm2) but it became an abnormal glow discharge as the current increased. At 4mm, the α mode was sustained as a normal glow discharge, then the transition to the γ mode occurred. Using a simple resistor-capacitor circuit model and a α sheath breakdown model, the discharge modes and the mode transition properties were studied.

Mode transition in radio-frequency atmospheric argon discharges with and without dielectric barriers

“…2͑b͒, where one bright thin line is seen near each of the two electrodes suggesting the existence of the negative glow and a strong sheath region. [11][12][13][14][15][16] Combining with a highly localized optical emission, these suggest that the rf DBD is in the ␥ mode at point E.…”

“…Then at point C, it expands abruptly to fill up the entire interelectrode space. The resulting homogeneous discharge is most likely to be an ␣ mode, [12][13][14][15][16] judging by the lack of a clear sheath structure in its image ͑not shown͒ and by its positive differential conductivity shown in Fig. 1͑a͒ 13,14,16 With further reduction of the input power, the rf APGD remains homogenous until point D after which it is extinguished.…”

“…11 Its characters are therefore that of the ␥ mode. [12][13][14][15][16] From the standpoint of the underpinning science and practical use, this constricted ␥ mode is different from the large-volume ␥ mode observed in rf DBD ͑Refs. 10 and 11͒ and microgap rf APGD.…”

Atmospheric glow discharges from the high-frequency to very high-frequency bands

“…It is known that with sufficient input power the sheath region could undergo an abrupt collapse thus causing the discharge to enter the less stable ␥ mode. [5][6][7][8][9] In the 20 MHz case, point a marks the ␣ − ␥ mode transition beyond which increasing input rf power was found to constrict the discharge into a bright plasma column typically less than 1 mm in diameter. In the 40 and 60 MHz cases, points b and c indicate plasma conditions under which the discharge became radially inhomogeneous, consisting of an optically intense channel surrounded by a weak but expansive and diffuse region.…”

“…Compared to low-pressure discharges, homogeneous APGDs are much more susceptible to rapid transition into a constricted ␥ mode when excited in the common high-frequency ͑HF͒ band of 3-30 MHz. [5][6][7][8][9] By reducing the input rf power, it is possible to operate in the more stable ␣ mode but at the expense of low electron densities thus compromising their application efficiency. 10 One technique to achieve high plasma density without plasma constriction is to add a dielectric barrier to the electrodes so as to control the growth of the discharge current.…”