Abstract
Modification of the two existing DIII-D neutral beamlines is planned to allow vertical steering to provide off-axis neutral beam current drive (NBCD) peaked as far off-axis as half the plasma minor radius. New calculations for a downward-steered beam indicate strong current drive with good localization off-axis so long as the toroidal magnetic field, BT, and the plasma current, Ip, point in the same direction. This is due to good alignment of neutral beam injection (NBI) with the local pitch of the magnetic field lines. This model has been tested experimentally on DIII-D by injecting equatorially mounted NBs into reduced size plasmas that are vertically displaced with respect to the vessel midplane. The existence of off-axis NBCD is evident in the changes seen in sawtooth behaviour in the internal inductance. By shifting the plasma upwards or downwards, or by changing the sign of the toroidal field, off-axis NBCD profiles measured with motional Stark effect data and internal loop voltage show a difference in amplitude (40–45%) consistent with differences predicted by the changed NBI alignment with respect to the helicity of the magnetic field lines. The effects of NBI direction relative to field line helicity can be large even in ITER: off-axis NBCD can be increased by more than 30% if the BT direction is reversed. Modification of the DIII-D NB system will strongly support scenario development for ITER and future tokamaks as well as provide flexible scientific tools for understanding transport, energetic particles and heating and current drive.
Highlights
Advanced tokamak (AT) research [1, 2] on DIII-D seeks to provide the scientific basis for steady-state, high-performance operation for ITER and future tokamak reactors
The main focus for steady-state scenario development in DIII-D is the demonstration of fully noninductive current sustainment for more than twice the current relaxation time at progressively higher pressures to meet the requirements of ITER and future tokamak reactors
The prospect for off-axis neutral beam current drive (NBCD) in DIII-D to supply a substantial amount of off-axis current drive needed for development of steady-state, advanced tokamak scenarios has been studied
Summary
Advanced tokamak (AT) research [1, 2] on DIII-D seeks to provide the scientific basis for steady-state, high-performance operation for ITER and future tokamak reactors. The most expedient solution to get substantial off-axis CD in DIII-D is to modify two of the four NB lines to allow vertical steering to drive current peaking as far off-axis as half the plasma radius This capability should greatly increase the parameter space available for AT scenario development. The effects of the alignment of the neutral beam injection (NBI) relative to the magnetic field pitch can be large: for DIII-D the magnitude is 40% higher in the case with BT and Ip in the same direction, and for ITER [10] it is up to 20% This prediction has been tested successfully by off-axis NBCD experiments utilizing small cross-section plasmas that are vertically shifted, which places the peak deposition of the neutral beams near the mid-radius of the plasma. By shifting the plasma upwards or downwards, or by changing the sign of BT, predicted differences in the off-axis NBCD profiles from the difference in magnetic alignment have been successfully validated against the measurements
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