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 V_{θ,E(r) × B} velocity, which is in turn due to a rapidly changing radial electric field E_r, 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 E_r 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.