Theory Weekly Highlights for May 2015

May 29, 2015

EFIT equilibrium reconstructions performed under OMFIT for a DIII-D discharge with significant fast-ion pressure and large core MHD modes show that magnetic measurements can be used to discriminate the level of anomalous fast-ion transport necessary for TRANSP simulations to match measured neutron rates. The pressure and current profiles are limited in their accuracy by the assumptions made in models used in the transport codes. It was found that the magnetics χ2 can be used to discriminate the accuracy of the pressure model. A study of the effect of adding an anomalous diffusion coefficient of increasing strength to a kinetic equilibrium reconstruction constrained by the results of the TRANSP simulations found that the minimum in the magnetics χ2 occurs for a fast-ion diffusion coefficient of about 0.6 m2/s. This is consistent with the level of fast ion diffusion that is necessary for the TRANSP simulation to match the measured neutron rates, providing an independent and reliable estimate of the level of anomalous fast-ion transport.

May 22, 2015

A new mode of operation was added to TGYRO to enable dynamic evolution of the Helium ash profile. The approach balances the turbulent (TGLF) and neoclassical (NEO) ash fluxes with the effective ash source from self-consistent thermonuclear alpha production. To our knowledge this is the first time a self-consistent ash calculation is available in a reactor transport model.

Most of the GA Theory staff attended the DoE Planning Workshop video conference on Integrated Simulations in Magnetic Fusion Energy Sciences.

May 15, 2015

The effects of plasma rotation on the toroidal spreading of impurities during massive gas injection have been studied in massive gas injection (MGI) simulations using NIMROD. In an initial simulation, a rotation level comparable to a post-MGI DIII-D plasma (~1kHz) was imposed. Without rotation, it was confirmed that the impurities preferentially spread in one direction (see highlight 29 Aug 2014), and the imposed rotation profile was chosen to have the opposite sign of the preferential spreading. In the simulations the toroidal rotation was strongly modified locally by the MGI and in fact reversed sign, and the overall effects on the impurity transport were small. The primary effect of the toroidal rotation was to shift the peak of the toroidal radiated power distribution in the direction of the rotation, which is associated with rotation of the n=1 mode away from the initial phase determined by the gas jet location. Simulations with higher levels of imposed rotation (comparable to pre-MGI levels) and rotation of the opposite sign will be carried out.

May 08, 2015

Local nonlinear GYRO simulations of a DIII-D discharge, intended to verify the critical gradient model of Alfvén eigenmode (AE) transport, confirm that the onset of stiff transport obeys a simple linear threshold condition. Stiff transport of energetic particles (EPs), indicated here by transport runaway, forces the driving EP density gradient to stay at or below a critical value. When the leading AE growth rate meets or exceeds the ITG growth rate at the same toroidal n number, the transport runs away. This simple condition arises from the fact that microturbulence-driven zonal flows can suppress AE transport for sufficiently weak drive. Below the threshold, this ITG-AE interaction keeps AE enhancement of EP transport low (previously called the “soft” transport enhancement regime). An important detail revealed by the present result is that the simple linear stiffness condition applies only as long as all driving gradients (thermal species and EPs) are retained in extrapolating the intersection point of AE and ITG growth rates; these gradients push the stiff transport critical gradient lower than previously estimated in some parts of the domain, possibly explaining the slight over-prediction of the stored energy under the existing transport model.

May 01, 2015

Phil Snyder, Gary Staebler, Chris Holland, Orso Meneghini, and Eric Bass attended the 2015 US/EU Transport Task Force Workshop in Salem, MA April 28 - May 1. Phil Snyder presented an oral plenary talk; “The EPED Pedestal Model: Super H-mode, Statistical Tests and Integrated Modeling.” Posters were presented by C. Holland and E. Bass on “Testing the Skill of Core Transport Models in Accurately Capturing Rotation and Heating Effects in I and H-mode Plasmas,” and “L-mode Edge Instabilities Simulated in BOUT++,” respectively. Gary Staebler chaired a plenary session on “Understanding H-and I-mode Pedestals”. Presentations were also made at the breakout sessions on a variety of topics: “Transport Modeling of Dithering H-mode Transitions,” and “Discussion of Methods and Validation Metrics for Predictive Experimental Design,” (Staebler), “Quantifying the Onset of Stiff Energetic Particle Transport by Alfvén Eigenmodes Using Nonlinear Gyrokinetics,” (Bass), and “Initial Physics Applications of the Advanced Tokamak Modeling (AToM) Project.” (Meneghini).

These highlights are reports of research work in progress and are accordingly subject to change or modification