(EX/P3-1) Transport Improvement Near Low Order Rational q Surfaces in DIII-D

M.E. Austin1), K.H. Burrell2), R.E. Waltz2), K.W. Gentle1), P. Gohil2), C.M. Greenfield2), R.J. Groebner2), W.W. Heidbrink3), Y. Luo3), J.E. Kinsey4), M.A. Makowski5), G.R. McKee6), R. Nazikian7), C.C. Petty2), R. Prater2), T.L. Rhodes8), M.W. Shafer6), M.A. VanZeeland9)
1) University of Texas-Austin, Austin, Texas, United States of America
2) General Atomics, San Diego, California 92186-5608, United States of America
3) University of California-Irvine, Irvine, California 92697, United States of America
4) Lehigh University, Bethlehem, Pennsylvania, United States of America
5) Lawrence Livermore National Laboratory, Livermore, California, United States of America
6) University of Wisconsin-Madison, Madison, Wisconsin 53706, United States of America
7) Princeton Plasma Physics Laboratory, Princeton, New Jersey, United States of America
8) University of California-Los Angeles, Los Angeles, California, United States of America
9) Oak Ridge Institute for Science Education, Oak Ridge, Tennessee, United States of America

Abstract.  In DIII-D and other tokamaks, changes in transport have been linked to low-order rational q surfaces, especially integer q surfaces. Recent experiments on DIII-D have probed the confinement changes near integer q in reverse shear discharges with detailed, time-resolved measurements of electron and ion temperature, impurity rotation, radial electric field, turbulent fluctuation levels, and q profile evolution. In discharges marginal for transport barrier formation, it is found that transport either transiently improves or a transition to an sustaining core barrier occurs in the vicinity of integer values of the minimum in q, qmin, and that the change in confinement precedes the actual crossing of integer qmin by a short time. The timing of the transport changes plus measurements from magnetic probes and electron cyclotron emission indicate that magnetic reconnection is not acting as a trigger for the enhanced confinement state. The measured electron temperature gradient behavior near the radius of qmin matches predictions from nonlinear GYRO code simulations of time-averaged zonal flow structures near rational q values. These turbulence driven zonal flow structures occur due to the gap in rational surfaces near integer q surfaces and explain the observed temperature changes just before and just after qmin =integer is crossed. Measurements of turbulent density fluctuations near the integer qmin times show decreases in low and intermediate-k fluctuation amplitude coincident with the transport improvement. In addition, the BES diagnostic has detected localized increases in poloidal velocity at the qmin radius around the start of the barrier formation. The fluctuation level reductions and poloidal flow increases are consistent with the expected nature of the zonal flow structures. In cases of discharges with E×B shear close to a threshold for turbulence decorrelation the zonal-flow-induced transient can trigger changes in the equilibrium profiles which lead to formation of a sustained core transport barrier.
* Work supported by US DOE under DE-FG03-97ER54415, DE-FC02-04ER54698, SC-G903402, DE-FG02-92ER54141, W-7405-ENG-48, DE-FG03-96ER54373, DE-AC02-76CH03073, DE-FG03-01ER54615, and DE-AC05-76OR00033.

Full paper available (PDF)