Observations on core turbulence transitions in ASDEX Upgrade using Doppler reflectometry

In Doppler reflectometry the antenna tilt angle ?t induces a Doppler frequency shift fD = u? 2 sin ?t/?o in the measured spectrum which is directly proportional to the rotation velocity u? = vE ? B + vph of the turbulence moving in the plasma. Measurements in ohmic ASDEX Upgrade core plasmas show u? of the order of 1?2?km?s?1 and reversing direction with increasing collisionality. Numerical simulations of the turbulence phase velocity vph using the GS2 linear gyrokinetic code reveal a change in the dominant core turbulence from ion temperature gradient (ITG) to trapped electron mode (TEM) with decreasing collisionality. The transition coincides with the u? reversal. Extracting the E ? B velocity from the measured u? and the simulation vph indicates that the core radial electric field reverses sign with the turbulence. Using the radial force balance equation vE ? B = v? ? v* and measured diamagnetic velocities v* gives a perpendicular fluid velocity v? i reversing from ~ 5 to 7.5?km?s?1 (ion direction) at low collisionality to ?0.7?km?s?1 (electron direction) at high collisionality. Neoclassical predictions using the NEOART and NCLASS codes give poloidal Deuterium fluid velocities too small by factor of ten. Toroidal ion fluid velocities would need to be significant (> 30?km?s?1) at low ?* to account for the difference. A clear ITG to TEM transition in the core turbulence has also been demonstrated using on-axis electron cyclotron heating to perturb the collisionality.