(EX/1-5) Maintaining the Quasi-Steady State Central Current Density Profile in Hybrid Discharges

M.S. Chu1), D.P. Brennan2), V.S. Chan1), M. Choi1), R.J. Jayakumar3), L.L. Lao1), R. Nazikian4), P.A. Politzer1), H.E. St John1), A.D. Turnbull1), M.A. Van Zeeland5), R. White4)
1) General Atomics, San Diego, California, United States of America
2) University of Tulsa, Tulsa, Okalahoma 74104, United States of America
3) Lawrence Livermore National Laboratory, Livermore, California 94550, United States of America
4) Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, United States of America
5) Oak Ridge Institute for Science Education, Oak Ridge, Tennessee 37831, United States of America

Abstract.  The Hybrid scenario, with confinement and beta improved above the baseline ELMy H-mode, is an attractive operation scenario for ITER. One intriguing feature of these discharges in DIII-D as well as in JET is that the plasma evolves into a quasi-steady state without sawteeth. The central safety factor (q0) is kept close to 1 and correlates with a rotating 3/2 magnetic island. The central current density is found to be smaller than expected without taking into account the effect of the 3/2 island. In this work, we report on results from investigation on the causal relationship between the 3/2 island and the non-sawteething nature of the discharge. Two mechanisms were invoked; both rely on a large co-rotating 2/2 sideband excited by the rotating 3/2 island when the plasma central q drops towards 1. The first mechanism is the excitation of the kinetic Alfvén wave (KAW) by the 2/2 sideband in the central region, which (due to its short perpendicular wavelength) provides an efficient current driver in direction counter to the plasma current. In the second mechanism, the 3/2 and its 2/2 sideband are assumed to redistribute the energetic ions resulting from the injected neutral beam. Thus, reducing the beam-driven current in the plasma center. Equilibria reconstructed from discharges in DIII-D are analyzed using the MHD stability codes. The 3/2 island is found to develop a 2/2 sideband with increasing amplitude as q0 approaches 1, as expected from standard MHD theory. The 2/2 sideband is shown to become resonant in the plasma center and converts into KAWs. The ONETWO transport code, which takes into account the neutral beam current drive from energetic ions, has been utilized to simulate the development of the discharge. When there is additional transport for the energetic ions, the profile of neutral beam driven current becomes broader, indicating equivalent counter current drive inside the 3/2 surface. These two mechanisms have been found sufficient for driving the negative current necessary for maintaining the steady current profile.
* Work supported by U.S. DOE under DE-FG03-95ER54309, DE-FC02-04ER54698, W-7405-ENG-48, DE-FC02-04ER54698, and DE-AC05-76OR00033.

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