Radial drift invariant in long-thin mirrors |
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Authors: | O Ågren V E Moiseenko K Noack A Hagnestål |
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Institution: | 1.?ngstr?m laboratory,Uppsala University,Uppsala,Sweden;2.Institute of Plasma Physics, National Science Center “Kharkov Institute of Physics and Technology”,Kharkiv,Ukraine |
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Abstract: | In omnigenous systems, guiding centers are constrained to move on magnetic surfaces.
Since a magnetic surface is determined by a constant radial Clebsch coordinate,
omnigeneity implies that the guiding center radial coordinate (the Clebsch coordinate) is
a constant of motion. Near omnigeneity is probably a requirement for high quality
confinement and in such systems only small oscillatory radial banana guiding center
excursions from the average drift surface occur. The guiding center radial coordinate is
then the leading term for a more precise radial drift invariant
I
r
, corrected by oscillatory “banana
ripple” terms. An analytical expression for the radial invariant is derived for long-thin
quadrupolar mirror equilibria. The formula for the invariant is then used in a Vlasov
distribution function. Comparisons are first made with Vlasov equilibria using the
adiabatic parallel invariant. To model radial density profiles, it is necessary to use the
radial invariant (the parallel invariant is insufficient for this). The results are also
compared with a fluid approach. In several aspects, the fluid and Vlasov system with the
radial invariant give analogous predictions. One difference is that the parallel current
associated with finite banana widths could be derived from the radial invariant. |
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Keywords: | |
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