Comparison of Doppler back-scattering and charge exchange measurements of E x B plasma rotation in the DIII-D tokamak under varying torque conditions

Measurements of the E X B toroidal angular velocity, omega(ExB) = E-r/RB theta (E-r is the radial electric field, B-theta is the poloidal magnetic field), are made using the Doppler back-scattering (DBS) and charge-exchange recombination (CER) spectroscopy diagnostics. DBS uses the Doppler shift of wavenumber-resolved density fluctuations while CER uses the Doppler shift of impurity emission lines to independently measure plasma parameters for calculating the local radial electric field. DBS and CER profiles of omega(ExB) as a function of normalized toroidal flux (rho) are compared at various levels of neutral beam applied torque on the plasma. Under standard neoclassical theory omega(ExB) is a flux surface quantity, making it appropriate to compare across diagnostics. DBS and CER generally show good agreement when comparing omega(ExB) profiles at different levels of neutral beam injection-applied torque. Furthermore, the DBS values have close to the same precision as CER values when averaged over a similar time-scale and effects, such as prompt-torque are considered. DBS is able to observe the rapid (<10 ms) modification of the E-r profile by the diagnostic neutral beam 'blips'. This modification is most pronounced when the blip applies a large relative change in torque on the plasma. Overall, these results could have implications on transport analysis and suggests using DBS and CER in conjunction to constrain values of the E x B-shear (sometimes called gamma(ExB)).