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E. J. Synakowski, M. Beer, R. E. Bell,
K. H. Burrell1, B. Carreras2,
P. H. Diamond3, E. J. Doyle4, D. Ernst,
R. Fonck5, P. Gohil1, C. M. Greenfield1,
T. S. Hahm, G. W. Hammett, F. M. Levinton6,
E. Mazzucato, G. McKee5, D. E. Newman2,
H. Park, C. Rettig4, G. Rewoldt,
T. L. Rhodes4, B. W. Rice7, 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
E×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
(
Er) 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
Er 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
Er 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
Er and the
E×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.
IAEA 2001
