Abstract
Two of the eight neutral-beam sources on the DIII-D tokamak were modified to allow injection below the midplane. To validate off-axis beam performance, the various beams are injected sequentially into low-power plasmas that are optimized for accurate neutron, neutral–particle, fast-ion D-alpha and fast-ion pressure measurements. As expected, the fast-ion profile is broader with off-axis injection than with on-axis injection. The driven toroidal rotation also broadens with off-axis injection and the central fast-ion density is several times smaller. The number of trapped ions in the core depends sensitively on the pitch of the magnetic field lines. Comparisons with classical predictions agree with the measurements for some diagnostics but are discrepant for others.
Highlights
Neutral-beam injection efficiently delivers power, current, torque and particles to a fusion plasma
The large uncertainties preclude definite conclusions, the analysis suggests that the difference in pressure is smaller than expected
The ratio of theory-to-experiment generally differs for the various sources. (To facilitate comparison, the co-going on-axis values have been adjusted to be nearly unity for all of the diagnostics.) In particular, it appears that the off-axis beams produce fewer fast ions than theoretically predicted
Summary
Neutral-beam injection efficiently delivers power, current, torque and particles to a fusion plasma. All early experiments [1] injected the neutrals towards the magnetic axis but, in recent years, many devices have explored off-axis neutral-beam injection [2] because it is widely believed that MHD-stable, steady-state operation requires broad current and pressure profiles [3, 4]. The DIII-D tokamak is equipped with eight neutral-beam sources housed in four beamlines. When the beamline is elevated, the pair of sources in this beamline inject neutrals through the midplane port and below the midplane in the plasma. The experiments reported here sought to confirm that the modified beamline successfully produces a population of off-axis neutral-beam ions and drives off-axis current
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