Turbulent flow, velocity shear, and neutral beam injection on the MAST-U spherical tokamak

The interconnected effects of plasma flow, flow shear, and neutral beam injection with turbulence and transport in the MAST-U tokamak are examined using an eight channel Doppler backscattering system (DBS). MAST-U currently has two neutral beams (NBI, 2.5 MW each) that are injected along toroidal tangents. One beam is injected off axis with respect to the vacuum vessel major axis while the second beam is on-axis. DBS is a widely used active radar like technique that measures the local density turbulence level and its flow velocity. It is found that the turbulence velocity (often dominated by the local ExB velocity) responds quickly to NBI initiation (≤ 5ms) consistent with the generation of internal radial currents from the larger gyroradii of the locally ionized beam particles. In contrast, the local velocity decay times, in response to the NBI turn off, vary from 30 to 60 ms depending on the radial location of the measurement (psi_n=0.9 to 0.6 in this case). Note that these decay times are significantly longer than the NBI fast particle decay time (~5-10 ms as measured by the neutron production decay rates). It is hypothesized that these longer decay times and their radial variation are due to different physical processes dominating at the different radial locations, eg momentum source vs transport variations, beam deposition, etc. These different possibilities, as well as the effect of the neutral beam injection geometry (ie on- vs off-tangential axis) are examined in this report.