Theory Weekly Highlights for June 2017

June 30, 2017

The first set of stability calculations for the diverted H-mode discharge #154802, which reached q95 < 2 found, as in the L-mode case (see Highlights from July 15, 2016 at Theory Weekly Highlights for July 2017, and October 23, 2015 at Theory Weekly Highlights for October 2015), that the discharge equilibria are ideally stable but unstable to a resistive kink. The initial resistive stability calculations with the MARS code used a Spitzer profile enhanced near q = 2. There appear to be some significant differences compared to the previous L-mode results. Most noticeably, the growth rates are significantly lower and scale weakly with resistivity, suggesting the discharge is already in the ‘almost ideal’ regime, despite the low growth rate. The scaling also does not appear to follow the universal scaling found earlier in the L-mode discharges. Further calculations using the Sauter resistivity and an effective resistivity from Ohm’s Law are underway; in the previous L-mode case, the growth rates were significantly larger when these profiles were used.

June 23, 2017

The unexpected discovery of steep scrape-off layer plasma gradients in L-mode limiter discharges recently forced a redesign of the ITER inner-wall limiter panels to accommodate for an excessively projected large heat flux. Recent theoretical work asserted that the profile steepening stems from a complex interaction between plasma flows and turbulence at the edge of the device [Nucl. Fusion 57, 034001 (2017)]. An extensive validation of the theory was carried out using dedicated C-Mod and TCV discharges featuring state of the art diagnostic techniques. These measurements were replicated in non-linear turbulence simulations for a detailed one-to-one comparison. Langmuir probe measurements of the plasma profiles are in excellent agreement with the theoretical predictions, and, furthermore, non-linear simulations of these experiments quantitatively replicate many features of the plasma dynamics. The key features predicted by the plasma dynamics, such as scrape-off layer sheared flows, and current at the limiter plates, have all been confirmed. Altogether, it appears that the physical mechanism for profile steepening has been unraveled. The findings are reported in two papers to appear in the Physics of Plasmas [Phys. Plasmas 24, 062508 (2017), and Phys. Plasmas 24 (2017), in press].

June 16, 2017

Full sonic toroidal rotation effects have been implemented in CGYRO. These include centrifugal effects, which are second-order in the ion Mach number (Mi), in addition to the standard ExB rotation, Coriolis drift, and parallel velocity shear, which are first-order in Mi. Resolving these effects requires high resolution in poloidal and pitch angle. The code has been benchmarked against the GKW code for simple linear test cases. While the ITG mode is primarily affected by the stabilizing Coriolis drift for low ion Mach number, at large Mi, the dominant effect of sonic rotation is on the electron dynamics. Though the centrifugal force is small for electrons due to their small mass, enhancement of the mirror force leads to an increased effective trapped electron fraction. This has a strong destabilizing influence on trapped electron modes (TEMs) and leads to mode transitions from ITG to TEM with increasing Mach number. However, the destabilization effect on TEMs can be quickly reduced by detrapping due to collisions.

June 02, 2017

Orso Meneghini attended the Second IAEA Technical Meeting on Fusion Data Processing, Validation and Analysis at MIT this week. He is also taking the opportunity to work with MIT researchers on using OMFIT at MIT. Brendan Lyons is visiting PPPL this week to work with Nate Ferraro on the M3D-C1 code.

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