Overview of results from supported mode operation of the Levitated Dipole Experiment
Author: Jennifer L. Ellsworth
Requested Type: Consider for Invited
Submitted: 2006-12-18 15:53:57
Co-authors: A.C. Boxer, D.T. Garnier, I. Karim, J. Kesner, M.S. Kim, M.E. Mauel, E.E. Ortiz
NW17-221, 175 Albany St. 175
Cambridge, MA 02139
The levitated dipole experiment is designed to study the behavior of fusion relevant plasmas confined by a dipole magnetic field. We report on new results from experiments carried out during this past year that use spectroscopy and multi-channel interferometry to better understand the behavior of bulk thermal plasma confined by a strong dipole magnet. The LDX diagnostic set includes pickup coils and flux loops for equilibrium measurement, Mirnov coils to measure magnetic fluctuations, electrostatic probes, visible imaging diagnostics, and an x-ray camera. A four-channel interferometer and visible spectrometer have also been installed. The three super-conducting magnets (floating coil, levitation coil, and charging coil) have been tested and recent experiments utilized a similar magnetic geometry to the levitated field in which the plasma is limited by a magnetic separatrix produced by operating the levitation coil while the floating coil is suspended from three thin supports. Hot electron plasmas are produced using multi-frequency electron cyclotron heating and neutral gas fueling.
The equilibrium pressure profiles are reconstructed from magnetic measurements using an anisotropic pressure model. Peak beta values of greater than 20% have been observed. The pressure gradients routinely exceed the MHD stability limit for interchange modes (although the plasma remains stable to these modes). We measure centrally peaked density profiles with gradients that are steeper than predicted by an adiabatic mixing process. In plasmas produced in the supported mode, as these are, most of the particle and energy loss occurs along the field lines to the supports. Later this year the floating coil will be levitated and we expect that the plasmas produced will have higher densities, higher temperatures, and higher degrees of thermalization. We further expect that the density profiles will change significantly as field-aligned losses are eliminated. Spectroscopic measurements reveal a helium impurity content and that the plasma discharges have a bright blue color because of oxygen impurities. We often observe a low frequency (1-10 kHz) quasi-coherent mode with a large radial extent and toroidal mode number, m = 1. The mode appears to be related to the density gradient and can be suppressed by transient neutral gas fueling. When the neutral pressure is properly adjusted, we are able to simultaneously suppress the high frequency hot electron interchange mode. During these periods, we find fluctuations to be undetectable, even at high beta, in the LDX device.