Simulation and modeling of radio-frequency atmospheric pressure plasmas in the non-neutral regime

Radio-frequency-driven atmospheric pressure plasma jets (RF APPJs) play an essential role in many technological applications. This work studies the characteristics of these discharges in the so-called non-neutral regime where the conventional structure of a quasi-neutral bulk and an electron depleted sheath does not develop, and the electrons are instead organized in a drift-soliton-like structure that never reaches quasi-neutrality. A hybrid particle-in-cell/Monte Carlo collisions (PIC/MCC) simulation is set up, which combines a fully kinetic electron model via the PIC/MCC algorithm with a drift-diffusion model for the ions. In addition, an analytical model for the electron dynamics is formulated. The formation of the soliton-like structure and the connection between the soliton and the electron dynamics are investigated. The location of the electron group follows a drift equation, while the spatial shape can be described by Poisson–Boltzmann equilibrium in a co-moving frame. A stability analysis is conducted using the Lyapunov method and a linear stability analysis. A comparison of the numerical simulation with the analytical models yields a good agreement

Evolution of the bulk electric field in capacitively coupled argon plasmas at intermediate pressures

The physical characteristics of an argon discharge excited by a single-frequency harmonic waveform in the low-intermediate pressure regime (5–250 Pa) are investigated using particle-in-cell/Monte Carlo collisions simulations. It is found that, when the pressure is increased, a non-negligible bulk electric field develops due to the presence of a "passive bulk", where a plateau of constant electron density forms. As the pressure is increased, the ionization in the bulk region decreases (due to the shrinking of the energy relaxation length of electrons accelerated within the sheaths and at the sheath edges), while the excitation rate increases (due to the increase of the bulk electric field). Using the Fourier spectrum of the discharge current, the phase shift between the current and the driving voltage waveform is calculated, which shows that the plasma gets more resistive in this regime. The phase shift and the (wavelength-integrated) intensity of the optical emission from the plasma are also obtained experimentally. The good qualitative agreement of these data with the computed characteristics verifies the simulation model. Using the Boltzmann term analysis method, we find that the bulk electric field is an Ohmic field and that the peculiar shape of the plasma density profile is partially a consequence of the spatio-temporal distribution of the ambipolar electric field

Frequency coupling in low-pressure dual-frequency capacitively coupled plasmas revisited based on the Boltzmann term analysis

Electron power absorption dynamics is investigated in radio-frequency (RF) argon capacitively coupled plasmas (CCPs) at low pressure (4–70 Pa) excited by a dual-frequency waveform with frequencies of 27.12 MHz and 1.937 MHz. Based on the spatio-temporal dynamics of the ambipolar electric field a novel interpretation of the mechanism of frequency coupling is given, which is not based on the hard wall model, as in previous explanations. Within this framework, frequency coupling arises due to the decreased size of the ambipolar region outside the sheath when the low-frequency sheath is close to its full expansion, which leads to decreased ionization in this region. It is shown, under the circumstances considered here, ohmic power absorption is dominant. The spatio-temporally averaged ambipolar power absorption shows nonmonotonic behaviour as a function of pressure, first increasing, then, after reaching a local maximum, decreasing as the pressure is increased. It is shown, that the reason for this nonmonotonic behaviour is ultimately connected to the frequency coupling mechanism