Observations of fluctuations and sudden confinement jumps of non-neutral plasmas in the CNT stellarator.
Author: Thomas Pedersen
Submitted: 2005-12-21 09:57:03
Co-authors: H. Himura, J. P. Kremer, J. Berkery, R. G. Lefrancois, Q. Marksteiner
500 W 120th Street, S.W. Mudd
New York, NY 10027
The Columbia Non-neutral Torus (CNT) is a relatively small, ultracompact stellarator which has been in operation at Columbia University since November 2004. It is devoted to the study of non-neutral and electron-positron plasmas confined on magnetic surfaces.
As reported at this conference by J. P. Kremer et al., macroscopically stable pure electron plasma equilibria have been created and studied in some detail, using internal filaments as electron sources and probes. These plasmas have low fluctuation levels. Here, we will report on large fluctuations and sudden confinement changes that have been observed under certain other operating conditions in CNT.
When operating with an ion density of ~14% of the electron density, repeated bursts of growing sinusoidal oscillations were seen. The frequencies of these oscillations are on the order of tens of kHz, consistent with ExB rotation frequencies as well as ion orbital frequencies. This may be a toroidal manifestation of the ion resonance instability [R. H. Levy, J. D. Daugherty, O. Buneman, Phys. Fluids 12 p 2616, (1969)].
Even though ions appear to destabilize the plasma, oscillations have been seen even when the ion fraction is very small, <1%. In general, it appears that a combination of larger electron density and lower magnetic field strength triggers the fluctuations, but this is not consistently the case.
When the filament bias voltage is scanned at very weak magnetic field strengths, the general trend is for the filament emission current to increase faster than linearly but smoothly as the filament is biased more strongly negative. However, sudden jumps in emission current are seen at certain values of the bias voltage, implying changes in the confinement time of more than a factor of two. These jumps can go in either direction, ie. confinement may suddenly improve or degrade, but appear to be reproducible. These may be related to the inertial density limit of a pure electron plasma confined in a stellarator [A. H. Boozer, Phys. Plasmas 12 034502 (2005)].