Measurements of the cross-phase angle between density and electron temperature fluctuations and comparison with gyrokinetic simulations

This paper presents new measurements of the cross-phase angle, alpha(neTe), between long-wavelength (k(theta)rho(s)< 0.5) density, (n) over tilde (e), and electron temperature, (T) over tilde (e), fluctuations in the core of DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] tokamak plasmas. The coherency and cross-phase angle between (n) over tilde (e) and (T) over tilde (e) are measured using coupled reflectometer and correlation electron cyclotron emission diagnostics that view the same plasma volume. In addition to the experimental results, two sets of local, nonlinear gyrokinetic turbulence simulations that are performed with the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] are described. One set, called the pre-experiment simulations, was performed prior to the experiment in order to predict a change in alpha(neTe) given experimentally realizable increases in the electron temperature, T-e. In the experiment the cross-phase angle was measured at three radial locations (rho=0.55, 0.65, and 0.75) in both a "Base" case and a "High T-e" case. The measured cross-phase angle is in good qualitative agreement with the pre-experiment simulations, which predicted that (n) over tilde (e) and (T) over tilde (e) would be out of phase. The pre-experiment simulations also predicted a decrease in cross-phase angle as T-e is increased. Experimentally, this trend is observed at the inner two radial locations only. The second set of simulations, the postexperiment simulations, is carried out using local parameters taken from measured experimental profiles as input to GYRO. These postexperiment simulation results are in good quantitative agreement with the measured cross-phase angle, despite disagreements with transport fluxes. Directions for future modeling and experimental work are discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3323084]