Role of Electric Field on the Power Coupling Mechanisms During Evolution of Plasma in an Off-Resonance Microwave Discharge

Abstract

The role of electric field on the different power coupling processes is reported during evolution of over-dense plasma inside a microwave (MW) discharge ion source (MDIS). This power coupling through different parametrically induced waves can yield very high density plasma and self-focused ion beam. Unlike PIC/MCC or hybrid fluid, here the FEM based fluid model uses time dependent Poisson solver through drift-diffusion approach. Results demonstrate that the pumping wave (2.45GHz) energy decays through electron cyclotron resonance (ECR), scattered upper hybrid (UH)/electron Bernstein wave (EBW), polarization reversal and ion type waves(i.e., Ion Bernstein wave (IBW),Ion Acoustic Wave (IAW), Electrostatic Ion cyclotron wave(EICW)) respectively with the advancement of time of MW launching. ECR based power absorption takes place up to 5 $\mu \mathrm{s}$ , till it reaches the critical density. The energy channeling through different waves is determined by the plasma density gradient, the pump wave $\tilde{E}$ -field strength and its distribution within the cavity. The MW power absorption through UH/EBW starts occurring mainly from time t = ~ $40 \mu \mathrm{s}$ onwards since the initiation of plasma breakdown time. The change in guided-wave mode (m = +1 to -1, polarization reversal) frequency from low to high, comes into play from time $\mathrm{t}=\sim 25\mu \mathrm{s}$ at the polarization reversal point which is affected by E- field, magnetic field gradient, and the radial boundary condition. The theoretically obtained $\tilde{\mathrm{E}}$ -field was compared with the experimentally obtained EICWs intensity lines.