Time evolution of ECRH power deposition in a tandem mirror plasma

Abstract

A mechanism has been developed by which a three-dimensional Electron Cyclotron Resonance Heating (ECRH) ray-tracing-absorption calculation may be coupled to a tandem mirror transport code. The radial temperature and density profiles of the transport code are expanded via flux conservation to provide the three-dimensional geometry required for the ray-tracing calculation. Absorption along the ray trajectory determines an equivalent radial ECRH power deposition profile for use by the transport code. This profile must be generated by using multiple ray-tracing calculations to simulate the spatial spread of the power launched from an antenna. A technique for artificially producing these multiple ray simulations is presented and compared with results where multiple ray-tracing calculations were performed. Examples of plasma build-up simulations for a tandem mirror using ECRH in the plug are provided. A positive feedback mechanism is identified which produces locally large electron temperatures. This occurs frequently near the plasma edge, shielding the electrons near the plasma axis from the incident ECRH power, and producing a hollow temperature profile. This may lead to the collapse of the plug plasma.

On stability of collisional coupling between relativistic electrons and ions in hot plasmas

“…to growth of T e /T i with decreasing P ei . Physically, this means that in plasmas where the ions are heated predominantly by the drag with hot electrons, positive feedback can appear [12][13][14][15][16] which leads to a rapid transition to a new steady state with T e ≫ T i defined by other factors. As a consequence, any further increasing of the electron heating power leads to further growth of T e (but not T i ) and might even provoke a thermal collapse of the ions.…”

“…As is well known from experiments in toroidal devices [12][13][14], electrons and ions exchange energy through collisions, but if the electrons are much hotter than the ions the two species "decouple". The collisional decoupling thus has not only the heating-power threshold, but also a threshold with respect to the ratio of electron and ion temperatures.…”