Ionization by bulk heating of electrons in capacitive radio frequency atmospheric pressure microplasmas

Abstract

Electron heating and ionization dynamics in capacitively coupled radio frequency (RF) atmospheric pressure microplasmas operated in helium are investigated by Particle in Cell simulations and semi-analytical modeling. A strong heating of electrons and ionization in the plasma bulk due to high bulk electric fields are observed at distinct times within the RF period. Based on the model the electric field is identified to be a drift field caused by a low electrical conductivity due to the high electron-neutral collision frequency at atmospheric pressure. Thus, the ionization is mainly caused by ohmic heating in this "Ω-mode". The phase of strongest bulk electric field and ionization is affected by the driving voltage amplitude. At high amplitudes, the plasma density is high, so that the sheath impedance is comparable to the bulk resistance. Thus, voltage and current are about 45 • out of phase and maximum ionization is observed during sheath expansion with local maxima at the sheath edges. At low driving voltages, the plasma density is low and the discharge becomes more resistive resulting in a smaller phase shift of about 4 • . Thus, maximum ionization occurs later within the RF period with a maximum in the discharge center. Significant analogies to electronegative low pressure macroscopic discharges operated in the Drift-Ambipolar mode are found, where similar mechanisms induced by a high electronegativity instead of a high collision frequency have been identified.

Particle-in-cell and global simulations ofαtoγtransition in atmospheric pressure Penning-dominated capacitive discharges

“…At low powers, the maximum bulk field is about 1/3 of the maximum field at the wall while at high powers, the bulk field is small due to the high density. This effect is captured by the previous global model [18] and has been discussed in [16]. The implication is that even at the lowest power in the base case, most of the metastable production corresponds spatially to the high-field sheath regions.…”

“…The second term on the right-hand side can be important at low currents and high frequencies, as will be seen in the next section; it essentially represents a phase shift between J and E [16]. However, in most cases, we have the simpler result n 2 e T e = m He 6 e 3 J 2 ,…”

Coupling effects of driving frequencies on the electron heating in electronegative capacitive dual-frequency plasmas

“…At high pressures, e.g. in atmospheric pressure microplasmas, the ionization may be dominated by ohmic heating in the bulk ( -mode) [14]. In single-frequency (SF) discharges with high electronegativity, the drift-ambipolar (DA) operation mode has recently been identified and its physical origin has been clarified [15].…”

Numerical study on discharge modes in atmospheric dielectric barrier discharges and their transition excited by the medium frequencies

“…比如, Boisvert等人 [21,22] 最近的实验 表明, 大气压氦气介质阻挡放电在中频域存在两种放 电模式, 即击穿电压以上的混合模式和击穿电压附近 的Ω模式. 所谓Ω模式是指在较高频率下, 电子密度最 大值及光发射最大值均出现在气隙中部的放电模式, 其电子能量主要来自气隙中部的欧姆加热 [26][27][28]…”