Speaker
Dr
Walter Guttenfelder
(Princeton Plasma Physics Laboratory)
Description
The inward momentum convection or “pinch” observed in many tokamaks can be explained by the Coriolis drift mechanism, with relatively good quantitative agreement found with gyrokinetic predictions of the ion temperature gradient (ITG) instability. Here we attempt to validate this model over a broader range of beta and aspect ratio by extending into the spherical tokamak (ST) plasma regime using data from NSTX and MAST. Previous perturbative measurements in NSTX H-modes have indicated the existence of an inward momentum pinch with a magnitude similar to that observed in conventional aspect ratio tokamaks. However, linear gyrokinetic simulations run for these cases predict the microtearing mode, which only transports electron energy, is the dominant micro-instability in the region of interest due to the relatively large plasma beta. Although weaker, there is also evidence of a variety of unstable electrostatic and electromagnetic ballooning modes. Quasi-linear calculations for all of these ballooning modes, assuming they contribute substantially to the momentum transport, predict a pinch that is small or directed outward, in contradiction to the experimental results. Additional scans show that the weak pinch is a consequence of how both electromagnetic effects (at relatively large beta) and low aspect ratio influence symmetry-breaking of the instabilities. To minimize electromagnetic effects, similar experiments were performed in MAST L-mode plasmas at relatively low beta using the time-dependent rotation response after the removal of a short n=3 applied magnetic field perturbation. The inferred inward pinch is similar in magnitude to conventional tokamaks and the NSTX H-modes. However, linear gyrokinetic simulations indicate that even for low beta L-modes the predicted momentum pinch is relatively small and cannot reproduce the large experimentally inferred pinch. Based on the above observations and simulations, the Coriolis pinch mechanism predicted from local, linear gyrokinetic theory does not appear to explain perturbative momentum transport at low aspect ratio. Other mechanisms neglected thus far are being investigated as possible solutions to the apparent discrepancy, including nonlinear effects, perpendicular ExB shear driven transport, centrifugal effects and profile shearing.
| Country or International Organization | USA |
|---|---|
| Paper Number | TH/P3-14 |
Primary author
Dr
Walter Guttenfelder
(Princeton Plasma Physics Laboratory)
Co-authors
Dr
Anthony Field
(CCFE)
Mr
Ivan Lupelli
(CCFE)
Dr
Jeff Candy
(General Atomics)
Dr
Jong-Kyu Park
(Princeton Plasma Physics Laboratory)
Mr
Michael Peters
(Indiana University, Bloomington)
Dr
Stanley Kaye
(Princeton Plasma Physics Laboratory, Princeton University, Princeton NJ, 08543 USA)
Dr
Tuomas Tala
(VTT, Association Euratom-Tekes)
Dr
Wayne M. Solomon
(Princeton Plasma Physics Laboratory)
Dr
Yang Ren
(Princeton Plasma Physics Laboratory)
