Electron cyclotron plasma startup in the GDT experiment

Abstract

The paper reports on a new plasma startup scenario in the Gas Dynamic Trap (GDT) magnetic mirror device. The primary 5 MW neutral beam injection (NBI) plasma heating system fires into a sufficiently dense plasma target ("seed plasma"), which is commonly supplied by an arc plasma generator. In the reported experiments, a different approach to seed plasma generation is explored. One of the channels of the electron cyclotron resonance (ECR) heating system is used to ionize the neutral gas and build up the density of plasma to a level suitable for NBI capture. After a short transition (about 1 ms) the discharge becomes essentially similar to a standard one initiated by the plasma gun. The paper presents the discharge scenario and experimental data on the seed plasma evolution during ECR heating, along with the dependencies on incident microwave power, magnetic configuration and pressure of a neutral gas. The characteristics of consequent high-power NBI discharge are studied and differences to the conventional scenario are discussed. A theoretical model describing the ECR breakdown and the seed plasma accumulation in a large scale mirror trap is developed on the basis of the GDT experiment.

First results of ECE measurements at the GDT mirror trap

“…The latter agrees with theoretical limit of maximum energy due the resonance escape outside the microwave beam for the relativistic electrons. More accurate analysis confirms that the energy spectrum of suprathermal electrons lays in the range 5 -100 keV what is in good agreement with our study of the electron kinetics during the ECR breakdown [8,9]. The "gap" for the X mode emission is most likely related to a topological bifurcation of the ECR surface.…”

Electron cyclotron heating and diagnostics of plasma at the second harmonic in the GDT device

“…Besides, measured by this diagnostics non-thermal ECE have unambiguously confirmed the existence of suprathermal electrons in GDT with energies about 10 keV. These electrons are generated by the intense microwave field during the ECR discharge start-up and heating of the main plasma [11]. Presence of suprathermal electrons should be taken into account in the analysis ECE at the second harmonic.…”

Electron cyclotron emission at the fundamental harmonic in GDT magnetic mirror

“…27, where a possibility for further acceleration of a minor fraction of electrons up to relativistic energies is not excluded.…”