Presentazioni e Articoli Tecnici

Radiofrequency discharges in Hydrogen have a range of applications. In particular, they are used as high-intensity sources of atoms for fundamental and material processing applications. In this study, we investigate a small volume, low pressure RF discharge used to produce Hydrogen atoms. A cylindrical quartz vessel is filled with Hydrogen at pressure in the range [10:20] Pa. The RF signal at 160 MHz is provided by a external flat spiral. A COMSOL axisymmetric model is built usign the Plasma, Magnetic Fields and Heavy Species Transport physics. (The PLASMA and AC/DC add-on modules are required). The Plasma physics is used to model the capacitive coupling of the RF source to the plasma. The plasma and neutral chemistry in the gas phase and at the vessel walls are included, together with secondary electron emission from electron and ion impact. The inductive coupling is modelled with the Magnetic Fields physics that computes the magnetic field produced by the current flowing in the coil. This is then coupled to the plasma via the Multiphysics couplings Plasma Conductivity and Electron Heat Source. A frequency-transient study solves the Magnetic Fields equations in the frequency domain and the Plasma equations in the time domain. The source terms for the dissociation/recombination kinetics are averaged over the RF period with auxiliary Domain and Boundary ODEs. These are then used in a Heavy Species Transport model to find the dissociation degree. Results are validated against PIC simulations [1]: plasma density, electron temperature, capacitive and inductive power absorption are compared for different working pressures. It is shown that COMSOL results reproduce accurately the PIC results at 20 Pa, but the differences increase at lower pressures when non-equilibrium effects arise in the sheaths and collisionless heating becomes important.