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

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Summary

INTRODUCTION

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.

DISCHARGE PRODUCTION AND ANALYSIS
MODIFICATION OF THE PLASMA PROFILES
CONFINEMENT
SCALING OF FULLY NONINDUCTIVE SOLUTIONS
A bN55 SOLUTION FROM A 1-D MODEL
CONCLUSION
Findings
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