Analysis of the high-energy electron population in surface-wave plasma columns in presence of collisionless resonant absorption

Abstract

In surface-wave plasmas, the energy can be transferred to the plasma electrons through both ohmic (collisional) and collisionless heating mechanisms. At very low pressure, when the electron-neutral collision frequency is much lower than the wave frequency (collisionless regime), a resonance is excited close to the tube walls where the electron plasma frequency in the radially-inhomogeneous plasma column reaches the wave frequency. In such conditions, the sharp rise of the component of the surface-wave electric field perpendicular to the tube axis can induce transit-time heating. At the resonant point, the long-wavelength electromagnetic surface wave can also be converted into short-wavelength electrostatic Langmuir waves that propagate down the density gradient. In this work, spatially-resolved trace-rare-gases optical emission spectroscopy combined with collisional-radiative modeling is used to analyze the electron energy distribution function (EEDF) and wave-particle interactions in low-pressure argon plasma columns sustained by an electromagnetic surface wave at 600 MHz (over-dense plasma). The EEDF is found to depart from a Maxwellian with the presence of a high-energy tail. The relative population of high-energy electrons increases with the axial distance towards the end of the plasma column where the electron density decreases and the resonance point becomes closer to the discharge axis. Over the range of experimental conditions examined, the high-energy tail increases with the characteristic length of the plasma density gradient at the resonance point; a feature that can be linked to collisionless electron heating by Landau damping of Langmuir waves.

Multi-scale investigation in the frequency domain of Ar/HMDSO dusty plasma with pulsed injection of HMDSO

“…3. Collisional-radiative modeling of the argon emission lines: consideration of the collisional quenching induced by HMDSO and its fragments OES measurements of argon 2p-to-1s transitions (in Paschen's notation) between 700 and 900 nm are compared to the predictions of a zero-dimensional argon collisional-radiative model [52]. This approach is used to determine the electron temperature (assuming a Maxwellian electron energy distribution function (EEDF) [53]), the electron density, as well as the population of argon metastable and resonant (or radiative) 1s states.…”

Time-Resolved Analysis of the Electron Temperature in RF Magnetron Discharges with a Pulsed Gas Injection

“…Once the theoretical emission line intensities are obtained for every (T e , n e ) pair, they are compared to the experimental (measured) ones by calculating a percentage standard deviation and the T e and n e values resulting in the best fit are assumed to correspond to the real plasma parameters. In order to run, the model requires input parameters such as the operating pressure and the neutral gas temperature (to calculate the number density of argon atoms in the ground state via the ideal gas law), as well as the absorption length along the line of sight of the optical emission spectroscopy measurement to account for optically thick Ar 2p-1s transitions [18,32,33].…”

“…Figure 3 shows the evolutions of the operating pressure (blue plot) For all conditions reported in this work, the measured line intensities are compared to those computed from a collisional-radiative model based on Donnelly's trace-rare-gases optical emission spectroscopy method [31] in order to determine time-resolved electron temperature in these pulsed injection conditions. As discussed previously [18,32,33], the model is however adapted with respect to Donnelly's work to account for radiation trapping and stepwise excitation through resonant 1s levels.…”

Uniformity enhancement of a microwave surface-wave plasma by a field agitation

“…1(a)] of the TM mn0 mode in microwave SWP. 8,13,14) For a plasma surface treatment of a 300 mm wafer, the chamber diameter is chosen as 450 mm. The numbers for the mode (m = 1, and n = 3) are determined from the operation frequency of a microwave generator source (2.458 GHz) and the diameter of the cylindrical cavity (395 mm).…”