Theory Weekly Highlights for September 2018

September 28, 2018

Gerrit Kramer visited DIII-D this week from PPPL and collaborated with GA and UCSD theorists on understanding the observed radial phase variation in TAE and RSAE modes. This variation is precluded by ideal MHD but does appear in non-ideal simulations. Progress was made during the visit in understanding a possible link between the phase variation and a net Poynting flux that arises from it.

September 21, 2018

Gary Staebler, Chris Holland, and Federico Halpern participated in the 2018 Joint EU-US TTF Conference held in Seville, Spain. Holland presented transport simulations from the ATOM project and Staebler showed interesting new results on a rigorous approach to closure of gyrofluid equations with Onsager symmetry preserved, a prelude for a new version of TGLF. Halpern delivered a review talk focussed on recent theoretical and experimental developments in understanding Scrape-off Layer (SOL) profiles. The talk succinctly explained why a 1/B_pol scaling is expected from essentially any model; the scaling results from balanced quasineutral plasma flows within a finite-length flux tube. Recent specific developments included heuristic explanations of why SOL profiles flatten or develop a density shoulder when the density Greenwald fraction exceeds 1/2, a result reported experimentally by J. Boedo and D. Rudakov in DIII-D in 2001-5. Finally, the state of theory-experiment validation was reviewed. A number of approaches ranging from 0-D reduced models to 5-D gyrokinetic simulations have been employed to explain SOL phenomena. Surprisingly, there has been a lot of success in explaining profile lengths, and a rather optimistic outlook on approaching the missing parts of the problem is developing. In particular, one frontier area that has been identified is integration of the SOL with a robust pedestal. Untangling this issue appears to be a top priority according to discussions within the TTF.

September 14, 2018

Calculations of ITER Alfvén eigenmode (AE) stability with the TGLF companion code TGLFEP show a steep rise in AE instability below magnetic shear s=0.1. Most potential ITER current configurations have low shear over a wide radial range and this is where most AE-driven transport of alpha particles and neutral beam injection (NBI) ions is predicted by the Alpha-TGLFEP 1D critical-gradient model. Surprisingly, the AE critical gradient, corresponding to the linear stability threshold, seems to scale very weakly with the magnetic safety factor q at these low shear values. As a consequence, reducing the plasma current from 15MA down to 7.5MA, consistent with ITER steady-state scenarios (with a corresponding reduction of q across the domain) shows only a moderate increase in AE instability and transport of alpha-particle and NBI ions. The Alpha-TGLFEP transport model predicts that core-contained AEs broaden the alpha-particle and NBI ion profiles, but predicts no edge or near-edge AE-driven flux for both high and low current configurations.

Lang Lao visited PPPL to participate in a Fusion Whole Device Model Exascale Computing Advisory Meeting.

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