Summary of Transient Transport Working Group Meeting November 5,1998 in New Orleans

Jon Kinsey (GA/Oak Ridge Associated Universities) was selected Working Group representative to the TTF executive committee.

Jim DeBoo (GA) described modulated ECH experiments on DIII-D as a test of transport theories.  The target plasma was heated with 4 MW of NBI and limited on the inside to obtain clear L-mode and negative central shear to avoid sawteeth.  The discharges were ideal for seeing small variations in electron temperature.  Densities were low  (electron-ion equilibration 200 ms), but the beam heating kept electron and ion temperatures close.  The modulation was a square wave of 40 ms period.  Several positions of the ECH resonance off axis were used, but the results were similar.  The temperature modulation at the resonance position was ~200 eV, about half that predicted by the theories.  The unique feature of these experiments is that the variations in both electron and ion temperatures were measured. Both amplitude and phase of the temperature variations from the resonance position to the axis were obtained with reasonable accuracy.  Changes in turbulence levels were also measured by BES, but they could not be done for conditions with good ECE temperature measurements.  For fluctuations at rho = 0.55 and the resonance at rho = 0.7, the density fluctuation level varied 15% in phase with the modulation and the toroidal velocity varied 10% out of phase, rather large effects.

Jon Kinsey discussed the interpretation of these ECH experiments as a preview of his invited talk the following day.  A number of models have now been tested, but none gave good fits to the data.  There were even problems fitting the equilibrium profiles.  If both amplitude and phase of both electron and ion temperatures were considered, no theory fits.  Some did better with one species, but those failed on the other species.  GL23 with ExB shear, one of the most sophisticated models, gave reasonable fits to the equilibrium profiles and the modulated ion temperatures, but the modulated electron temperatures differed from the observations.  A full discussion will appear shortly in the proceedings issue of Physics of Plasmas.

Phil Efthmion (PPPL) presented the most recent analysis of tritium transport in Enhanced Reversed Shear discharges on TFTR.  For these discharges, the minimum in q exceeded two and occurred near rho = 0.4, within which particle diffusion was greatly reduced.  The D for both tritium and helium is roughly neoclassical, as is the scaling with magnetic field.  Carbon peaks on axis in these discharges, but with a pinch velocity ~ 1.5 m/sec, which is less than the neoclassical value of 4 m/sec.  The pinch observed for tritium also seems less than neoclassical.

Max Austin (U. Texas, GA) showed evidence of complex structure in the electron temperature profiles during the evolution of the q profile in discharges with negative central shear in DIII-D.  The temperatures tend to increase in a step-wise manner rather than smoothly as q(0) gradually decreases to 1.3.  The strongest effects occur when q(min) crosses an integer value.  Structure is also seen in the evolution of the central ion temperature.

 We look forward to seeing everyone at the US-EU TTF meeting in April.  The announcement will be out soon.

 Ken Gentle