(EX/P1-9) Physics Advances in the ITER Hybrid Scenario on DIII-D

P.A. Politzer1), C.C. Petty1), R.J. Jayakumar2), T.C. Luce1), M.R. Wade1), M.E. Austin3), D.P. Brennan4), T.A. Casper2), M.S. Chu1), J.C. DeBoo1), E.J. Doyle5), M.E. Fenstermacher2), J.R. Ferron1), P. Gohil1), C.M. Greenfield1), C.T. Holcomb2), A.W. Hyatt1), R.J. La Haye1), C. Kim6), G.R. McKee6), M.A. Makowski2), M. Murakami7), T.W. Petrie1), R. Prater1), T.L. Rhodes5), G. Wang5), A.S. Welander1)
1) General Atomics, San Diego, California, United States of America
2) Lawrence Livermore National Laboratory, Livermore, California, United States of America
3) University of Texas-Austin, Austin, Texas, United States of America
4) University of Tulsa, Tulsa, Oklahoma, United States of America
5) University of California-Los Angeles, Los Angeles, California, United States of America
6) University of Wisconsin-Madison, Madison, Wisconsin, United States of America
7) Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States of America

Abstract.  Experiments on the DIII-D tokamak have developed a long duration, high performance discharge that is an attractive operating scenario for ITER. This ``hybrid scenario” regime is inductively driven with bootstrap current fractions of 35%-50% and a fully penetrated current profile with q01. The remarkably good transport properties of the hybrid scenario are demonstrated by a DIII-D discharge with high normalized fusion performance, βNH89P /q952 = 0.7, that is sustained for 5 current relaxation times. The measured ion thermal diffusivity for this case is equal to the neoclassical value across the plasma cross-section. Electron heat conduction (although small in an absolute sense) dominates the energy loss process, which is consistent with nonlinear GYRO simulations that show the TEM and ETG mode cause the majority of transport. For discharges with q95 > 4, a ρ* scan with the other dimensionless parameters held fixed showed that the effective thermal diffusivity has a scaling close to gyroBohm-like in the core (although more Bohm-like near the edge). A radiative divertor has been successfully applied to the hybrid scenario using argon injection, with good core confinement and high impurity enrichment in the divertor. Hybrid scenario discharges on DIII-D can have either a dominant 3/2 NTM or a dominant 4/3 NTM, depending upon initial conditions, with the latter having typically 15% higher H-factors (maximum 30% higher). One explanation for the lower confinement in 3/2 NTM hybrid plasmas is the flattening of the pressure profile near the q = 1.5 surface; the resulting ``missing” bootstrap current has been explicitly observed for the first time using a direct analysis of the MSE signals. The 3/2 NTM has the beneficial effect in hybrid plasmas of broadening the current profile and maintaining q0≥1. There is evidence from the MSE signals that this may be due to poloidal flux pumping, associated with the ELM modulation of the 3/2 NTM. Another possible explanation is counter current drive near the axis by the 2/2 component of the NTM, which can mode convert to a KAW and damp on electrons. Also being examined is the radial transport of fast ions, which may reduce the NBCD near the axis.
* Work supported by US DOE under DE-FC02-04ER54698, W-7405-ENG-48, DE-FG03-97ER54415, DE-FG03-01ER54615, DE-FG03-96ER54373, and DE-AC05-00OR22725.

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