Prompt radial electric field response to neutral beam injection

A variety of low density discharges in the DIII-D tokamak exhibit a prompt response ( < 10 ms) to neutral beam injection as evidenced by changes in the Doppler shift of the density fluctuations. This variation in the Doppler shift, attributed to a changing Vθ,ErxB velocity, which is in turn due to a rapidly changing radial electric field Er, is observed from the deep core to the edge of these plasmas. The core changes are large and occur much more quickly than collisional beam equilibration times (≥ 120 ms for the core) indicating a different momentum transfer mechanism. Towards the edge the Doppler shift times approach the collisional transfer times. Theoretical predictions of changes in the core Er due to radial fast particle currents are not large enough to explain the observations. Although not yet understood this effect could lead to a tool for relatively fast feedback control of core electric fields in advanced confinement regimes. (author)