Observations of Low Frequency Modes in LDX
Author: Jay Kesner
Requested Type: Poster Only
Submitted: 2006-12-18 16:39:58
Co-authors: A.C. Boxer, J. L. Ellsworth, D.T. Garnier, I. Karim, M.E. Mauel, E.E. Ortiz
Contact Info:
MIT PSFC
175 Albany St
Cambridge, Ma 02139
USA
Abstract Text:
Confinement in a closed field line system such as a levitated dipole imposes particular restrictions on collective effects; notably the plasma compressibility will play an important stabilizing role. In recent experiments in LDX (in the supported mode of operation), the levitation coil was operational which creates a magnetic separatrix that limits the plasma extent and changes the outer boundary conditions. The plasma was heated by multiple frequency ECRH, which produces plasma that contains a thermal and a hot electron species.
Plasma that is heated by ECRH can be subject to instability that feeds on the free energy of either a hot or a thermal plasma component. The thermal species contains most of the plasma density while the energetic electron species contains most of the plasma stored energy. In addition to high frequency fluctuations reported elsewhere [1] we observe a low frequency semi-coherent mode (f<10 kHz) that is presumed to be are driven by the thermal species. The mode exists in a window of neutral pressure and can eliminated by gas puffing. The toroidal mode number is measured to be m=1 and exhibits a broad radial structure. Theoretical considerations of thermal plasma driven instability indicate the possibility of MHD-like behavior of the background plasma (including rotating convective cells), drift frequency (entropy mode) modes and rotationally driven modes.
1. Garnier et al, Physics of Plasmas 13 (2005), 056111
Characterization: A2,E1
Comments:
Place with LDX posters of J. Ellsworth, D. Garnier & A. Boxer
