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Graduate Student Seminar - 9/27/2013

"Role of Trapped Particles in Tokamak Zonal Flows"

by Wrick Sengupta

Friday, September 27, 2013 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisor: Professor Adil Hassam

In a tokamak, flux tubes wind toroidally at a small angle. Particles follow field lines and stream parallel to the tubes while the tubes move perpendicularly due to drifts like ExB. The nonuniform magnetic field can, however, trap particles, thus severely restricting their poloidal motion. As a result of this, in the absence of collisions, the trapped particles (TPs) would slide only toroidally at a speed much higher than the ExB speed. We shall illustrate this point with the help of a simple toy model. The TP precession and the associated kinetic energy are not reproduced correctly in standard neoclassical calculations. In fact, these calculations predict a nonzero poloidal speed for TPs, which is not possible. We shall resolve this paradox by a careful examination of the trapped particle phase space.

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