|eeo_icc_60213.pdf||2006-02-27 16:58:34||Eugenio Ortiz|
Effects of the Hot Electron Interchange Instability on Plasma Confined in a Dipolar Magnetic Field
Author: Eugenio E Ortiz
Submitted: 2005-12-21 22:07:27
Co-authors: A.Boxer, J.Ellsworth, D.Garnier, A.Hansen, I.Karim, J.Kesner, and M.Mauel
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The Levitated Dipole Experiment (LDX) explores the confinement and stability of plasma created within the dipole field of a strong superconducting magnet. In this presentation, the results from initial experiments are reported that describe the formation of long-pulse, quasi-steady state microwave discharges lasting more than 10 seconds and having peak beta more than 20%. In these initial experiments, the high-field superconducting floating coil was supported by three thin supports. Detailed measurements have been made of discharge evolution, plasma dynamics and instability, and the roles of gas fueling, microwave power deposition profiles, and plasma boundary shape. The plasma is created by multi-frequency electron cyclotron resonance heating at 2.45 and 6.4 GHz, and a population of energetic electrons, with mean energies above 50 keV, dominates the plasma pressure. Creation of high-pressure, high-beta plasma is possible only when intense hot electron interchange (HEI) instabilities are stabilized by sufficiently high background plasma density. The instabilities resonate with the magnetic drift motion of the energetic electrons and can cause rapid radial transport. A dramatic transition from a low-density, low-beta regime to a more quiescent, high-beta regime is observed when the plasma-fueling rate and confinement time are sufficiently large. We understand this behavior to result from the significant improvement in plasma confinement as the HEI mode is stabilized. A nonlinear, self-consistent numerical simulation of the growth and saturation of the HEI instability reproduces many observations from the experiment including the global mode structure of the HEI [Levitt, et al., POP, (2002)]. Measurements of the electrostatic and magnetic fluctuations of the HEI instability are described along with observations and explanations of the observed beta-limiting processes. In particular, we describe how measurements of the magnetic fluctuations induced by the HEI in the high-beta plasma of LDX can be used to interpret the evolution of the perturbed pressure profile.