Beam-ion confinement for different injection geometries

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/offaxis. 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 D α (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.

Energetic ion transport by microturbulence is insignificant in tokamaks

“…The broadened beam ion profile is consistent with D EI ¼ 0:5 m 2 =s, implying a considerable transport mechanism. Off-axis NBCD at DIII-D produced an example 44,45 of a similar process, with the transport enhancement increasing with P NB (since the beam energies are fixed at E b % 80 keV, this corresponds to a decreasing E b =T e ). A value of D EI ðrÞ ¼ 0:5v i ðrÞ, where v i is the power-balance computed ion thermal diffusivity, did not reduce the energetic ion density enough to agree with the experimental profile, and further increasing D EI produced greater discrepancy between the measured and simulated profile shapes.…”

Modeling the response of a fast ion loss detector using orbit tracing techniques in a neutral beam prompt-loss study on the DIII-D tokamak

“…1 of Ref. 9. The vertical extent of the neutral beams is most recently calculated and experimentally verified in Ref.…”

Calculation of the Non-Inductive Current Profile in High-Performance NSTX Plasmas

“…Before examining those calculations directly, it is useful to compare measurements and modeling of the neutron evolution before and after a step change in the neutral beam power. Previous experiments have studied the behavior of "dilute" populations of fast ions [32,49,50,74], including studies in NSTX L--mode plasmas [74]. The latter study showed that the prompt loss and slowing down of fast ions in those plasmas were consistent with classical behavior.…”

“…Alfven modes have also been shown to modify the current profile [63,64]. Recently, evidence from DIII--D [49,50] and ASDEX--Upgrade [60] has indicated that the electrostatic turbulence responsible for thermal plasma transport may also lead to radial diffusion of the injected fast ions.…”