(EX/P1-4) Feedback Control of the Safety Factor Profile in DIII-D Advanced Tokamak Discharges

J.R. Ferron1), V. Basiuk2), T.A. Casper3), E.J. Doyle4), Q. Gao5), P. Gohil1), C.M. Greenfield1), F. Imbeaux2), J. Lohr1), T.C. Luce1), M.A. Makowski3), D. Mazon2), M. Murakami6), Y. Ou7), J.-M. Park8), C.C. Petty1), P.A. Politzer1), T.L. Rhodes4), M. Schneider2), E. Schuster7), M.A. Van Zeeland9), M.R. Wade1), A. Wang5)
 
1) General Atomics, San Diego, California, United States of America
2) Association Euratom-CEA, CEA-Cadarache, France
3) Lawrence Livermore National Laboratory, Livermore, California, United States of America
4) University of California-Los Angeles, Los Angeles, California, United States of America
5) Southwest Institute of Physics, Chengdu, China.
6) Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States of America
7) Lehigh University, Bethlehem, Pennsylvania, United States of America
8) National Fusion Research Center, Dae Jeon, Korea
9) Oak Ridge Institute for Science Education, Oak Ridge, Tennessee, United States of America

Abstract.  Active feedback control for regulation of the safety factor (q) profile at the start of the high stored energy phase of an advanced tokamak discharge has been developed for the DIII-D tokamak. The time evolution of the on-axis and minimum values of q is controlled during and just following the period of ramp-up of the plasma current, using electron heating to modify the rate of relaxation of the inductive component of the plasma current profile. This concept of using changes in the σ profile to modify the current profile evolution is unique to DIII-D experiments and contrasts with maintenance of a constant current profile in steady-state, which utilizes sources of localized current drive and which has been the focus of experiments at other tokamaks. In L-mode and H-mode discharges, feedback control of q is effective with the appropriate choice of either off-axis ECH or neutral beam heating as the actuator. The q profile is calculated in real time from a complete equilibrium reconstruction fitted to external magnetic field and flux measurements and internal poloidal field measurements from the motional Stark effect diagnostic. Comparisons of experimental measurements and transport code predictions of the time evolution of the tokamak equilibrium are used to validate transport codes for use in testing of real-time feedback control algorithms. Improved real-time controllers are being developed by including the feedback algorithm into the transport code simulation.
* Work supported by US DOE under DE-FC02-04ER54698, W-7405-ENG-48, DE-AC05-00OR22725.

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