Abstract
The initial experiments on off-axis neutral beam injection into high noninductive current fraction (fNI), high normalized pressure (βN) discharges in DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] have demonstrated changes in the plasma profiles that increase the limits to plasma pressure from ideal low-n instabilities. The current profile is broadened and the minimum value of the safety factor (qmin) can be maintained above 2 where the profile of the thermal component of the plasma pressure is found to be broader. The off-axis neutral beam injection results in a broadening of the fast-ion pressure profile. Confinement of the thermal component of the plasma is consistent with the IPB98(y,2) scaling, but global confinement with qmin>2 is below the ITER-89P scaling, apparently as a result of enhanced transport of fast ions. A 0-D model is used to examine the parameter space for fNI=1 operation and project the requirements for high performance steady-state discharges. Fully noninductive solutions are found with 4<βN<5 and bootstrap current fraction near 0.5 for a weak shear safety factor profile. A 1-D model is used to show that a fNI=1 discharge at the top of this range of βN that is predicted stable to n=1, 2, and 3 ideal MHD instabilities is accessible through further broadening of the current and pressure profiles with off-axis neutral beam injection and electron cyclotron current drive.
Highlights
In order to maintain the plasma in steady-state, the plasma current must be driven fully noninductively, a constraint that is best satisfied with a large bootstrap current fraction, fBS 1⁄4 IBS=Ip, in order to minimize the power required for externally driven current
This report describes the use of off-axis neutral beam injection and off-axis electron cyclotron current drive (ECCD) in DIII-D5 to modify the current density and pressure profiles toward a regime which is expected to be stable to low- n ideal MHD modes at bN % 5 and which is compatible with fully noninductive current drive
A one-dimensional (1-D) model has been used to demonstrate that a fully noninductive discharge that is predicted stable at bN % 5 is accessible in DIII-D using off-axis neutral beam and electron cyclotron (EC) heating and current drive to produce broad current and pressure profiles
Summary
This report describes the use of off-axis neutral beam injection and off-axis electron cyclotron current drive (ECCD) in DIII-D5 to modify the current density and pressure profiles toward a regime which is expected to be stable to low- n ideal MHD modes at bN % 5 and which is compatible with fully noninductive current drive. With guidance from the experimental results, modeling has been used to explore how increased capability for off-axis current drive in DIII-D would be used to produce discharges with, simultaneously, fNI 1⁄4 1 and bN % 5. The fNI 1⁄4 1 solutions described here for the weak shear q profile have bN approaching 5 as is desirable for a high Q reactor This contrasts with previous studies of equilibria with the larger pressure gradients and the correspondingly high JBS of an ITB-type discharge where the fNI 1⁄4 1 solution is at relatively low bN.. A one-dimensional (1-D) model has been used to demonstrate that a fully noninductive discharge that is predicted stable at bN % 5 is accessible in DIII-D using off-axis neutral beam and electron cyclotron (EC) heating and current drive to produce broad current and pressure profiles.
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