Studies on the dynamics of limited filaments

Abstract

A study on the dynamics of filaments in the presence of a diagnostic, conductive limiter is presented. Plasma filaments are coherent structures present in many fusion devices and transport a significant amount of particles and energy to vacuum chamber walls. The theories of filament propagation are based on a circuit model of current closure loops within the filament and are described herein. Two experimental configurations based on different modes of filament generation are utilized in the Versatile Toroidal Facility (VTF) to test the model. Along with the experimental observations of filament propagation, measurements of limiter sheath resistance are made and shown to depend upon environmental conditions and filament parameters. One experimental configuration creates long filaments (-6 meters) using electron cyclotron resonance heating (ECRH) and a toroidally symmetric solenoid. The other utilizes a newly constructed Argon plasma gun breaking down injected gas for creating short (~1 meter) plasma filaments. Arrays of Langmuir probes track the path of the plasma particles while the limiters are constructed to generate a uniform vertical electric field along its conductive side and measure the parallel current collected. Agreement is found with the model in a region of strongly negative vertical electric fields. Current collection data extends agreement to larger regions with eventual breakdown at strongly positive electric fields due to complex current collection and escape paths.