Return To: Session EX5 - Transport, Internal Transport Barriers, H-Mode
Prev Page: (EX5/5) Behavior of Electron and Ion Transport in
Next Page: (EX5/D) Discussion of Session EX5

(EX5/6) A Comparative Study of Core and Edge Transport Barrier Dynamics of DIII-D and TFTR Tokamak Plasmas

   
E. J. Synakowski , M. Beer , R. E. Bell , K. H. Burrell ¹, B. Carreras ², P. H. Diamond ³, E. J. Doyle ⁴, D. Ernst , R. Fonck ⁵, P. Gohil ¹, C. M. Greenfield ¹, T. S. Hahm , G. W. Hammett , F. M. Levinton ⁶, E. Mazzucato , G. McKee ⁵, D. E. Newman ², H. Park , C. Rettig ⁴, G. Rewoldt , T. L. Rhodes ⁴, B. W. Rice ⁷, G. Taylor , M. C. Zarnstorff 
 
Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08543 USA
¹ General Atomics, P.O. Box 85608, San Diego, CA 92014 USA
² Oak Ridge National Laboratory, Oak Ridge, TN 37748 USA
³ University of California at San Diego, San Diego, CA 92093 USA
⁴ The University of California at Los Angeles, Los Angeles, CA 90024 USA
⁵ The University of Wisconsin at Madison, Madison WI 53706 USA
⁶ Fusion Physics and Technology, Torrance, CA 90503 USA
⁷ Lawrence Livermore National Laboratory, Livermore, CA 92075 USA

Abstract
Confinement bifurcations and subsequent plasma dynamics in the TFTR  core and the DIII-D  core and edge are compared in order to identify a common physics basis. Observations suggest a framework in which $\rm E \times B$ shear plays a dominant role in the barrier dynamics. In TFTR, bifurcations from the reverse shear (RS) into the enhanced reverse shear (ERS) regime with high power balanced neutral beam heating (above 25 MW at 4.8 T) resemble edge H mode transitions observed on DIII-D. In both, radial electric field ($\rm E_r$) excursions precede confinement changes and are manifest as localized changes in the impurity poloidal rotation. Reduced transport follows the excursions, and in both cases strong $\rm E_r$ shear is reinforced by the plasma pressure. These characteristics are contrasted with DIII-D negative central shear (NCS) barrier evolution with unidirectional beam injection. There, the improved confinement region can develop slowly, depending on the neutral beam input power and torque. Rapid expansion and deepening of this region follows an increase in the neutral beam heating power. The initial formation phase is modulated by confinement steps and interruptions. An analog for these steps is found in TFTR RS plasmas. Although these do not dominate the TFTR plasma evolution during low power (7 MW) heating, they can represent significant transport reductions when additional heating is applied. In both devices, no strong excursion in $\rm E_r$ precedes these latter confinement bifurcations . The triggering event of these steps may be related to current profile relaxation, but it is not always connected with simple integral or half-integer values of the minimum in the q profile. Finally, variations of $\rm E_r$ and the $\rm E \times B$ shear through the application of unidirectional injection on TFTR yielded plasmas with confinement characteristics and barrier dynamics similar to those of DIII-D NCS plasmas. The data underscore that the physics responsible for the enhanced confinement states is fundamentally the same in both devices.

     

Read the full paper in PDF format.