| |
 |
|
| |
Equilibrium and stability of rotating plasmas in a mirror geometry
| Author: | Aydemir A. Y. |
| Coauthor: | |
| Institution : | University of Texas |
| Abstract text: | Plasma flows of significant magnitude seem to be a ubiquitous feature of
confined plasmas. The flow by itself, or in combination with the shear in the
flow profile, is now recognized to have a stabilizing influence on large scale
instabilities such as the resistive wall and internal kink modes in tokamaks,
and microturbulence. Long before the beneficial effects of these incidental
flows were discovered, flows driven externally as in integral part of a
confinement scheme were also examined[1]. Recently some of these ideas have
been revived, as in the centrifugal confinement experiment MCT[2]
of Maryland and the magneto-Bernoulli experiment of FRC at Texas[3].
Both use a mirror field in conjunction with a radial electric field to drive
large cross-field flows. We have done a systematic study of the MHD equilibrium and stability of these configurations. Although we have easily found centrifugally confined detached states, where the bulk of the plasma mass is pulled away from the mirror ends towards the center of the device, our stability calculations have consistently yielded negative results. The expected shear stabilization of the interchange modes is observed for rotation frequencies comparable to the growth rate of the flute modes. However, centrifugal confinement becomes effective at much higher frequencies, where a plethora of ideal MHD modes are observed, both with k_parallel=0 (interchange modes), and with finite k_parallel (Parker modes). These rotationally-driven modes are observed even in a pure theta-pinch configuration without the unfavorable curvature of the mirror fields. Therefore, we conclude that accessibility of these detached states look highly questionable. Since they are ideally unstable, they also cannot be manifestations of ``relaxed states\'\' in a variational sense.
[1] B. Lehnert, Nucl. Fusion {\\bf 11}, 485 (1971).
[2] R. F. Ellis, A. B. Hassam, S. Messer, and R. B. Osborn, Phys. Plasmas {\\bf 8}, 2057 (2001).
[3] H. Quevedo, P. M. Valanju, and Roger D. Bengtson, Bull. Am. Phys. Soc. {\\bf 47}, 67 (2002).
|
|
|