Abstract
This paper presents a study on near cathode space charge region in BETA (Basic Experiments in Toroidal Assembly), a toroidal plasma device with purely toroidal magnetic field. A charge cylinder has been found to be embedded in the plasma center corresponding to the hot filament cathode location in poloidal cross section. This charge cylinder has been created by the primary electrons emitted from the filament surface, which in turn, leads to the formation of a potential well in the core plasma. We have proposed a model, which shows that a tiny fraction of injected energetic electrons is sufficient to sustain the observed potential well. We have examined the equilibrium of the charge cylinder in poloidal cross-section and found that it exhibits equilibrium configuration by forming circulation pattern of primary electrons. The circulation pattern is formed by vertical drift due to toroidal magnetic field and self-consistent poloidal E×B drift. We have concluded that the self-consistency is in adjusting the poloidal drift to the vertical drift of the trapped primary electrons.
Highlights
The near cathode space-charge behavior is of great interest in plasma discharges [1,2,3,4,5]
We have examined the equilibrium of the charge cylinder in poloidal cross-section and found that it exhibits equilibrium configuration by forming circulation pattern of primary electrons
The plasma in BETA device [9,10,11] is produced by a hot filament cathode discharge
Summary
The near cathode space-charge behavior is of great interest in plasma discharges [1,2,3,4,5]. These studies have attracted lot of interest due to their applications in plasma surface processing. Analytic and numerical models have been developed [2,3,4,5] to gain the understanding of the physical processes occurring in the cathode region of discharges. The knowledge of primary electron trajectories in this region helps in understanding and predicting various discharge characteristics. The influence of different magnetic fields on plasma discharge has been studied by tracing the primary electron trajectories near the filament
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