(EX/1-2) Progress Toward High Performance Steady-State Operation in DIII-D

C.M. Greenfield1), M. Murakami2), A.M. Garofalo3), J.R. Ferron1), T.C. Luce1), M.R. Wade1), E.J. Doyle4), T.A. Casper5), R.J. Jayakumar5), C.E. Kessel6), J.E. Kinsey7), R.J. La Haye1), J. Lohr1), M.A. Mahdavi1), M.A. Makowski5), M. Okabayashi6), C.C. Petty1), T.W. Petrie1), R.I. Pinsker1), R. Prater1), P.A. Politzer1), H. Reimerdes3), J.T. Scoville1), H.E. St. John1), E.J. Strait1), T.S. Taylor1)
1) General Atomics, San Diego, California, United States of America
2) Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States of America
3) Columbia University, New York, New York, United States of America
4) University of California, Los Angeles, California, United States of America
5) Lawrence Livermore National Laboratory, Livermore, California, United States of America
6) Princeton Plasma Physics Laboratory, Princeton, New Jersey, United States of America
7) Lehigh University, Bethlehem, Pennsylvania, United States of America

Abstract.  Advanced Tokamak (AT) research in DIII-D works toward development of a scientific basis for steady-state high performance scenarios. These scenarios are a high level goal for ITER, in recognition of their potential for high duty cycle and reduced cyclic fatigue in a power plant. Bootstrap supplies most of the current ( fBS≈ 60%-75%), with the remainder supplied by neutral beam (NBCD) and electron cyclotron (ECCD) current drive. In recent experiments, βN≈4≈6li has been maintained for 2 s. These plasmas operate well above the no-wall stability limit (∼4li), enabled by active error field and resistive wall mode control. This has been achieved only during toroidal magnetic field ramps, which appear to broaden the current profile to improve coupling with the wall and control coils. These plasmas have internal transport barriers, contrasting previous experience where βN is limited below 2. In other experiments, fully noninductive conditions fNI≈ 100% have been sustained for several confinement times or about half a current relaxation time (τR). Similar discharges, with fNI≈ 90%-95%, are stationary for the entire 2 s ( ∼1τR) ECCD pulse. These plasmas have βN≈3.5, qmin≥1.5 and weakly reversed magnetic shear in the core. Recent efforts in this scenario focus on mapping the operational space. fNI is found to increase with both q95 and βN. Fusion performance decreases with q95, so these experiments suggest continued emphasis on increasing beta at moderate q95. At the same time, low density is observed to be advantageous for noninductive operation, primarily through its impact on the effectiveness of external current profile control tools. Improvements now underway on DIII-D include additional long-pulse ECCD and fast wave (FWCD) and density control for high triangularity double-null configurations. Theory-based simulations including these new capabilities predict in-principle steady-state conditions with high βN maintained for several τR. The same models, applied to ITER, extrapolate current DIII-D results to steady-state scenarios with Q≥5.
* Work supported by US DOE under DE-FC02-04ER54698, DE-AC05-00OR22725, DE-FG03-98ER53297, DE-FG03-01ER54615, W-7405-ENG-48, DE-AC02-76CH03073, and DE-FG02-92ER54141.

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