The noninductive part of the measured current profile has been determined for DIII-D plasmas. A technique for determining the flux surface average of the quantity K~B and a model for the resistivity separates the current profile into inductive and noninductive portions. Analysis (1) where the cylindrical coordinate system (R, Z, @) is used, P = -fo BzR'dR' is the total poloidal flux per radian inside a major radius R, and F(p) = RB~. Furthermore, it is assumed that toroidally symmetric, nested fiux surfaces exist and can be labeled by either i/I or the en- gives a measurement at a fixed point.In a general toroidal geometry, the equation relating the current density to the electric field is (i B) = n '(E B) + (iNi B), (2) where (A) is the flux surface average of A, 71 is the parallel resistivity, and jNi represents any sources of noninductive current drive (including both bootstrap current and auxiliary driven currents). The flux surface average of the quantity (E B) can be shown to be [6] (E B) =
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A noninductive current drive concept, based on internal pressure-driven currents in a low-aspect-ratio toroidal geometry, has been demonstrated on the Current Drive Experiment Upgrade (CDX-U) [Forest et al., Phys. Rev. Lett. 68, 3559 (1992)] and further tested on DIII-D [in Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. For both experiments, electron cyclotron power provided the necessary heating to breakdown and maintain a plasma with high-βp and low collisionality (εβp∼1, ν*≤1). A poloidal vacuum field similar to a simple magnetic mirror is superimposed on a much stronger toroidal field to provide the initial confinement for a hot, trapped electron species. With application of electron cyclotron heating (ECH), toroidal currents spontaneously flow within the plasma and increase with applied ECH power. The direction of the generated current is independent of the toroidal field direction and depends only on the direction of the poloidal field, scaling inversely with magnitude of the later. On both CDX-U and DIII-D, these currents were large enough that stationary closed flux surfaces were observed to form with no additional Ohmic heating. The existence of such equilibria provides further evidence for the existence of some type of bootstrap current. Equilibrium reconstructions show the resulting plasma exhibits properties similar to more conventional tokamaks, including a peaked current density profile which implies some form of current on axis or nonclassical current transport.
Plasma discharges with negative central magnetic shear (NCS) and access to the ballooning second stability regime have been produced on DIII-D with edge plasma conditions similar to those in the standard L mode confinement regime. Confinement enhancement factors up to H identical to tau E/ tau ITER-89P~2.5 are obtained while maintaining the L mode edge. Compared with discharges with monotonic q profiles, highly peaked toroidal rotation (fphi (0) approximately=30-70 kHz), ion temperature (Ti(0) approximately=15-22 keV) and density (ne(0)/ne approximately=2.2) profiles are observed. This regime has yielded the highest DD neutron rates observed to date on DIII-D. Steep pressure gradients drive a large bootstrap current, with up to ~75% total non-inductive current drive. These features make this an attractive advanced tokamak operating regime for further study
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