Conceptual Study of Electron Ripple Injection for Tokamak Transport Control
Authors: W. Choe, M. Ono, and C.S. Chang
A non-intrusive method for inducing radial electric field based on electron ripple injection is under development by the Princeton CDX-U group. The radial electric field is known to play an important role in the L-H and H-VH mode transition ac- cording to the recent theoretical and experimental research. It is therefore important to develop a non-intrusive tool to control the radial electric field profile in tokamak plasmas. The present technique utilizes externally-applied local magnetic ripple fields to trap electrons at the edge, allowing them to penetrate towards the plasma cen- ter via rB and curvature drifts, causing the ux surfaces to charge up negatively. Electron cyclotron resonance heating is utilized to increase the trapped population and the electron drift velocity by raising the perpendicular energy of trapped elec- trons. In order to quantify the effects of cyclotron resonance heating on electrons, the temperature anisotropy of resonant electrons in a tokamak plasma is calculated. For the calculation of anisotropic temperatures, energy moments of the bounce-averaged Fokker-Planck equation with a bi-Maxwellian distribution function for heated elec- trons are solved, assuming a moderate wave power and a constant quasilinear difu- sion coeffcient. Simulation using a guiding-center orbit model have been performed to understand the behavior of suprathermal electrons in the presence of ripple fields. Examples for CDX-U and ITER parameters are given.