Theory Weekly Highlights for November 2013

November 22, 2013

New analytic calculations of the neoclassical transport coefficients have been obtained in the Pfirsch-Schluter regime. Using the model Hirshman-Sigmar operator, exact results were derived for the (non-ambipolar) ion transport coefficients in general 3D (and 2D) equilibria. These formulae agree to very high accuracy with NEO at very large collision frequency and have been used as a critical verification tool for the 3D NEO capability. For the full Fokker-Planck operator, in 2D, the known Pfirsch-Schluter regime results have been recovered completely within the context of kinetic theory - without relying on the Braginskii fluid moment expansion. We are presently extending this self-contained kinetic analysis in general geometry to trace heavy impurities and sonic rotation, for which existing analytic theory is not adequate to recover the NEO results.

November 15, 2013

Gary Staebler, Emily Belli, and Eric Bass presented invited talks at the November 2013 APS Divistion of Plasma Physics Meeting in Denver. Staebler described the recent improvements to the TGLF model that allow the accurate prediction of the residual transport inside internal transport barriers. The presentation by Belli described the recent applications of the NEO code in precise calculations of the bootstrap current, including the effect of multiple ion species and fast ions. She also discussed the impact of non-axisymmetric magnetic field perturbations on neoclassical transport. Bass discussed predictive modeling of alpha particle transport in ITER burning plasmas, including effects from the fusion source, microturbulence, and Alfven eigenmodes.

Phil Snyder shared the 2013 John Dawson Award for Excellence in Plasma Physics Research with longtime collaborator Howard Wilson of the University of York and two DIII-D physicists, Tom Osborne and John Ferron. The award was presented at the November 2013 APS DPP Meeting in Denver, recognizing their work on understanding the H-mode pedestal and ELMs. The work involved a tight interaction between theory and experiment to develop and test a quantitative understanding of edge stability, and apply it to predict and optimize the pedestal.

November 08, 2013

In work performed under task agreement (C19TD48FU) between US-ITER and the IO, runaway electron (RE) confinement in the current quench (CQ) phase of ITER was simulated in NIMROD by orbit tracking to evaluate the feasibility of the repetitive gas injection (RGI) concept. Without additional gas injected into the CQ plasma, the overall RE confinement time in this phase is tc > 20 ms. The relatively large confinement time is due to the incomplete stochasticity of field lines at the plasma edge during decay of the residual MHD modes and low stochasticity in the core despite the re-growth and saturation of the n = m = 1 mode. In a second simulation, a deeply penetrating neon jet using the Burst Disc Cartridge Injection (BDCI) method was simulated. No change in the current density profile was found, and no additional MHD modes were excited. The jet did lead to a slightly faster decay of the core n = 1 mode, owing to the slightly higher plasma resistivity, yet tc remained large (tc > 10 ms). This study demonstrates the fundamental difficulty posed by using RGI for RE suppression in large tokamaks with large rational surface spacing and thus little island overlap and stochasticity, which leads to RE loss rates much slower than avalanche growth rates.

Vincent Chan has stepped down after 26 years as Director of the GA Theory group. Philip Snyder has been promoted as the new Director of the GA Theory and Computational Science Department. Vincent will remain in the Magnetic Fusion Energy (MFE) division as Special Program Advisor to MFE VP Tony Taylor.

November 01, 2013

Alpha confinement in a realistic ITER baseline scenario has been examined with an integrated 1D transport model that includes transport by both microturbulence and alpha-driven Alfvén eigenmodes (AEs). For the studied case, active AEs are seen to redistribute alpha particles within the core, but to cause no alpha flux from the plasma edge. The microturbulence is treated with a GYRO-fitted analytic formula and the AE transport is assumed stiff, constraining the alpha particle density gradient to stay at or below the AE linear stability threshold (calculated directly in GYRO). The edge particle flux comes entirely from microturbulence and is the same whether a slowing-down distribution or an equivalent Maxwellian is used in calculating the AE linear stability threshold in GYRO. While the edge flux depends sensitively on the boundary condition, worst case has 7% birth alpha particle loss coming mainly from the low energy end of the distribution with an average energy of about 30keV (essentially hot helium). Loss of alpha plasma heating is well below 1%. Variation of the prediction with key driving parameters is currently under investigation, with plans to extend to other ITER scenarios in the near future.

Lang Lao attended and chaired the fifth ITER Integrated Modeling Expert Group (IMEG) Annual Meeting at ITER Headquarters, Cadarache, France October 21-23, 2013. The two main goals of the meeting are to discuss progress in the ITER partners domestic integrated modeling programs and to review progress and to advice ITER on its Integrated Modeling Program to develop an Integrated Modeling Analysis Suite (IMAS) and an infrastructure to support ITER plasma operation and plasma research.

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