Observations on core turbulence transitions in ASDEX Upgrade using Doppler reflectometry

摘要

In Doppler reflectometry the antenna tilt angle theta(t) induces a Doppler frequency shift f(D) = u(perpendicular to)2 sin theta(t)/lambda(o) in the measured spectrum which is directly proportional to the rotation velocity u(perpendicular to) = v(ExB) + v(ph) of the turbulence moving in the plasma. Measurements in ohmic ASDEX Upgrade core plasmas show u(perpendicular to) 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(perpendicular to) reversal. Extracting the E x B velocity from the measured u(perpendicular to) and the simulation vph indicates that the core radial electric field reverses sign with the turbulence. Using the radial force balance equation v(ExB) = v(perpendicular to) - v* and measured diamagnetic velocities v* gives a perpendicular fluid velocity v(perpendicular to i) reversing from similar to 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 v* 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.