Speaker
Mr
Walter Guttenfelder
(USA)
Description
Nonlinear simulations have progressed for multiple NSTX discharge scenarios to (i) validate with experimental turbulence and transport data, (ii) help differentiate unique instability mechanisms, and (iii) improve confidence in predictive modeling for future low aspect ratio fusion devices. First nonlinear gyrokinetic simulations of microtearing turbulence in a high-beta NSTX H-mode discharge predict experimental levels of transport that are dominated by magnetic flutter and increase with collisionality. This dependence is roughly consistent with energy confinement times in dimensionless collisionality scaling experiments providing evidence for the importance of microtearing modes in high-beta NSTX plasmas. In lower beta H-mode plasmas from a second collisionality scaling experiment microtearing modes are predicted to be stable. Instead, nonlinear simulations predict that ETG turbulence provides a significant fraction of the experimental transport, although the predicted transport is insensitive to variation in collisionality. ETG transport has also been predicted to be important in RF heated L-mode plasmas that exhibit electron internal transport barriers (e-ITBs) with strong negative magnetic shear (s<-0.5). Non-local simulations verify that at the base of the e-ITB the predicted ETG flux reaches experimental levels, but turbulence cannot propagate inward due to a nonlinear stabilizing effect from negative magnetic shear that occurs in the absence of strong ExB shear. Small differences in many parameters influence the microstability properties, and likely confinement scalings, to varying degree. For example, additional linear and nonlinear simulations predict that microtearing growth rates and transport increase with beta, s/q (for positive shear, s>0), and possibly even Zeff. On the other hand, ETG turbulence is often weakly dependent or stabilized with beta, and tends to be stabilized by increasing s/q (for s>0) and Zeff. Furthermore, both ETG and microtearing can be stabilized with sufficiently strong density gradient, while microtearing alone can be strongly suppressed with experimental levels of flow shear. In an effort to move towards predictive capability, first tests of the TGLF model for NSTX discharges have begun. This work is supported by US DOE contracts DE-AC02-09CH11466, DE-FG03-95ER54309, DE-AC52-07NA27344 and DE-AC05-00OR22725.
Country or International Organization of Primary Author
USA
Primary author
Mr
Walter Guttenfelder
(USA)
