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

Abstract

During the initial phase of microwave (MW) power launch inside a MW discharge ion source (MDIS), plasma and the electric fields are evolved with time together in the plasma volume. The spatio-temporal evolution pattern in the cavity of a MDIS is reported here, highlighting the role of these electric fields on power coupling processes. Evolution of electric field and so power coupling processes are calculated using Finite Element Method (FEM). Unlike PIC/MCC or hybrid fluid, here FEM model uses time dependent Poisson solver through drift-diffusion approach. In presence of plasma gradient in the volume ambipolar electric field is generated which interacts with the MW electric field to form resultant total electric field which continuously vary during this evolution period. It is observed that main power coupling mechanism is 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 upper-hybrid resonance (UHR) method, electro-static (ES) ion acoustic wave heating method are important heating mechanisms during the over-dense plasma condition, when density is above critical density for launched MW frequency, 2.45 GHz. An indirect verification of simulated temporal evolution of hot electron temperature and plasma density with the experiment, is done under similar configuration and operating environment. The experimental results are found to be agreed reasonably well with the simulation in the low power range.

Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase

“…It is observed that the plasma bulk electron temperature increases in the radial direction at the regions mainly in between the two pairs of the ring magnets with the increase in time from t = 280 ns to 730 ns [22]. Therefore, it can be summarized that with the increase in plasma density (or time, t = $45 ns to t = $280 ns) from underdense to overdense state, the plasma bulk temperature is increased by an amount of $80 eV mainly in the radial direction near the region, 24 mm < r < 40 mm, À25 mm < z < 25 mm.…”

“…The ridge dimensions are optimized from the analytical calculation as well as from the electromagnetic simulation. The mirror magnetic field is created by using two pairs of ring magnets that surround the microwave coupled reactor [22]. On the right side of the microwave coupled reactor (Figure 2), the ion beam extraction system is attached through a 5-mm hole on the wall of the reactor.…”

“…The similar computational domain is used in the experimental set-up (Section 5) to validate the simulated data. Here, the finite element method (FEM)-based COMSOL model is used [22].…”

Design of a Miniaturized Electron Cyclotron Resonance Ion Source for High-Voltage Proton Accelerator

“…Optimization is based on the simulation calculations with consideration that the microwave and plasma parameter evolution are decoupled to each other, because the time scale is at the ns level. Considering the time scale of microwaves, the motion of ions is assumed to be negligible in comparison to the motion of electrons, the model ignoring the plasma's specific evolution [20,21]. This means that we can only focus on whether the microwave is effectively absorbed and simplify the consideration of the type of ionized gas and the input power value.…”

“…The drift-diffusion equation can be used to determine the electron density and electron energy density if the electron density is constant in the resonance region, and the Debye length is much smaller than the microwave interaction length [21,23]:…”