Abstract
The DIII-D tokamak is equipped with neutral beam sources that inject in four different directions; in addition, the plasma can be moved up or down to compare off-axis with on-axis injection. Fast-ion data for eight different conditions have been obtained: co/counter, near-tangential/near-perpendicular and on-axis/off-axis. Neutron measurements during short beam pulses assess prompt and delayed losses under low-power conditions. As expected, co-injection has fewer losses than counter, tangential fewer than perpendicular and on-axis fewer than off-axis; the differences are greater at low current than at higher current. The helicity of the magnetic field has a weak effect on the overall confinement. Fast-ion Dα (FIDA) and neutron measurements diagnose the confinement at higher power. The basic trends are the same as in low-power plasmas but, even in plasmas without long wavelength Alfvén modes or other MHD, discrepancies with theory are observed, especially in higher temperature plasmas. At modest temperature, two-dimensional images of the FIDA light are in good agreement with the simulations for both on-axis and off-axis injection. Discrepancies with theory are more pronounced at low fast-ion energy and at high plasma temperature, suggesting that fast-ion transport by microturbulence is responsible for the anomalies.
Highlights
Neutral beam injection (NBI) is a standard heating method in tokamak experiments
This paper presents a careful quantitative study of beam-ion confinement based on neutron and fast-ion D-alpha (FIDA) data
For a discharge where the density was nearly constant at the transition from off-axis to on-axis injection, the neutron data imply that, during the off-axis phase, the number of confined beam ions is ∼67% of the number in the on-axis phase
Summary
Neutral beam injection (NBI) is a standard heating method in tokamak experiments. In many devices (including the DIII-D tokamak), it is the primary source of auxiliary heating. The observed spatial transport is consistent with the low levels of diffusion predicted by neoclassical theory. Recent studies of neutral beam current drive (NBCD) on ASDEX Upgrade suggest that larger fast-ion diffusion may occur when the injection energy is closer to the plasma temperature [25]. Reference [30] predicts that, in the high-energy limit, the diffusivity of passing fast ions DB is proportional to (E/T )−3/2, while [31] predicts (E/T )−1 scaling for electrostatic turbulence and no reduction for electromagnetic microturbulence Both gyromotion and drift motion are important but phase-averaging could be valid for the gyromotion without applying for the drift motion [32, 33].
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