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On MHD Stability, Ellipticity, Omnigenuity, Constants of Motions and a Modified Thermal Barrier of a Straight Field Line Mirror

O. Ågren, V. E. Moiseenko (21R02)

Fusion Science and Technology / Volume 51 / Number 2T / February 2007 / Pages 200-203

Technical Paper / Open Magnetic Systems for Plasma Confinement

The straight field line mirror field is the unique field which gives the lowest ellipticity of the flux tube of an MHD stable minimum B mirror field. In this particular vacuum field, each gyro center bounces back and forth on a single magnetic field line, and a pair of two new constants of motion is associated with this property. Using these invariants in the Vlasov equation, it can be shown that the radial gyro center magnetic drift is absent to first order in the plasma beta, and the equilibrium is omnigenous. The neoclassical increase of the radial transport may thus be avoided without an axisymmetrization of this single cell mirror.

A scheme to improve end confinement of ions, and simultaneously create an electric potential barrier for the electrons and a sloshing ion component, has been proposed. The end plugging transforms ions under way to escape into the loss cone into sloshing ions by ion cyclotron resonance heating. Numerical studies on sloshing ion production by RF heating demonstrate strong absorption of the RF field near the fundamental gyro frequency resonance of the minority deuterium ions as well as near the tritium second harmonic gyro frequency resonance. The scenario indicates a possibility to achieve a high energy gain factor in this kind of single cell mirror with the proposed modified thermal barrier.

 
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