Spatio-temporal evolution of electric field inside a microwave discharge plasma during initial phase of ignition and its effect on power coupling

摘要

During the initial phase of microwave (MW) power launch inside a MW discharge ion source (MDIS), plasma and the total electric field (MW and ambipolar) are evolved with time together in the volume. In the presence of a plasma gradient, an ambipolar electric field is generated which interacts with the MW electric field forming a resultant total electric field which continuously varies during this evolution period. The spatio-temporal evolution pattern inside the MDIS volume is reported here, highlighting the role of the total electric field in power coupling processes. The Finite Element Method (FEM) is used to calculate the evolutions of the electric field and power coupling processes. Unlike a Particle-in-cell/Monte Carlo collision or a hybrid fluid, here, the FEM model uses time dependent Poisson solver through the drift-diffusion approach. It is observed that the main power coupling mechanism is the electron cyclotron resonance (ECR) method; however with the evolution of plasma, the mode shifts from ECR to off-ECR type heating with time. Off-ECR heating in the form of the upper-hybrid resonance method and the electrostatic ion acoustic wave heating method are two important heating mechanisms during the over-dense plasma condition, when the plasma density is above the critical density for a launched MW frequency, 2.45 GHz. In addition, the shifting of heating mechanisms from ECR to off-ECR methods is discussed based on the 3D maps of spatio-temporal evolution of plasma density and hot electron temperature. Furthermore, simulated temporal hot electron temperature and plasma density values are validated with the experiment under a similar configuration and an operating environment. Published under license by AIP Publishing.