Dynamic regimes of cyclotron instability in the afterglow mode of minimum-Belectron cyclotron resonance ion source plasma

Abstract

Laulainen, J. (2016). Dynamic regimes of cyclotron instability in the afterglow mode of minimum-B electron cyclotron resonance ion source plasma. Plasma Physics and Controlled Fusion, 58 (4) AbstractThe paper is concerned with the dynamic regimes of cyclotron instabilities in nonequilibrium plasma of a minimum-B electron cyclotron resonance ion source operated in pulsed mode. The instability appears in decaying ion source plasma shortly (1 -10 ms) after switching off the microwave radiation of the klystron, and manifests itself in the form of powerful pulses of electromagnetic emission associated with precipitation of high energy electrons along the magnetic field lines. Recently it was shown that such plasma instability causes perturbations of the extracted ion current, which limit the performance of the ion source and generates strong bursts of bremsstrahlung emission. In this article we present time-resolved diagnostics of electromagnetic emission bursts related to cyclotron instability in the decaying plasma. The temporal resolution is sufficient to study the fine structure of the dynamic spectra of the electromagnetic emission at different operating regimes of the ion source. It was found that at different values of magnetic field and heating power the dynamic spectra demonstrate common features: decreasing frequency from burst to burst and always falling tone during a single burst of instability. The analysis has shown that the instability is driven by the resonant interaction of hot electrons, distributed between the ECR zone and the trap center, with slow extraordinary wave propagating quasi-parallel with respect to the external magnetic field. IntroductionMicrowave plasma discharges are extensively used in various branches of plasma technology, such as thin-film deposition, plasma etching, surface ion treatment and sputtering, sources of positive and negative ion beams, etc. A significant part of such discharges operate at low gas pressure with the plasma being heated through electron cyclotron resonance (ECR) and confined magnetically, e.g. in toroidal devices [1] and gasdynamic traps [2]. Electromagnetic emission from ECR-heated ITER-like high temperature plasmas, is used for imaging system with spatial resolution [3], for example.

Cyclotron instability in the afterglow mode of minimum-B ECRIS

“…These electrons are eventually expelled into the loss cone. The comprehensive study of cyclotron instability in the afterglow of ECR discharge in argon plasma 10 has shown that instability is driven by the resonant interaction of hot electrons with slow extraordinary wave, which propagates quasi-parallel to the magnetic field.…”

Measurements of the energy distribution of electrons lost from the minimum B-field—The effect of instabilities and two-frequency heating

“…The hump at higher energy is presumably linked to the B min value as discussed in the previous section. The hump at lower energy could be of similar origin, as a rich electromagnetic spectrum interacting with the electrons is generated during the instability event [6,15,16].…”

Pulse-Periodic Regimes of Kinetic Instabilities in the Non-Equilibrium Plasma of an Electron Cyclotron Resonance Discharge Maintained by Continuous-Wave Radiation of a 24 GHz Gyrotron

“…In the first case, slow extraordinary waves are excited most efficiently, which propagate at low angles to the magnetic field and interact with hot electrons distributed between the ECR heating zone and the center of the mirror trap. A similar mechanism of cyclotron instability, which explains the main properties of the dynamic spectra of pulses in the inherent electromagnetic radiation of the plasma in a magnetohydrodynamically stable trap of the "minimum-B" hexapole type, was proposed in [11]. A distinctive feature of excitation of a quasi-longitudinal slow extraordinary wave is the fact that the group velocity of the wave decreases as the plasma density grows, which results in an increase in the total instability growth rate even at fairly low densities of hot particles.…”

“…The use of high-power microwave radiation of gyrotrons allows one to increase the density and energy of the particles trapped in the mirror significantly (up to relativistic values), which allows one to study various regimes of wave excitation by energetic particles. (5), diagnostic camera (6), probe (7), CC-10 vacuum meter (8), microwave antenna (9), oscilloscope (10), turbomolecular pump (11), and forevacuum pump (12).…”