In a recent letter published in Nuclear Fusion (“Velocity shear, turbulent saturation, and steep plasma gradients in the scrape-off layer of inner-wall limited plasmas,” Vol. 57, 034001) by F. Halpern, an explanation was provided for the narrow heat-flux decay found in the scrape-off layer of inner-wall limited discharges. The discovery of the “narrow heat-flux feature” recently prompted a redesign of the ITER inner-wall limiter panels to accommodate for the excess heat-flux. Non-linear turbulence simulations show the spontaneous formation of radially sheared poloidal flows in the near-scrape-off layer, which correlate with the steep plasma gradient region observed in Langmuir probe and infrared measurements. Turbulent transport is significantly affected by a complex interaction between the sheared flows and parallel plasma currents outflowing into the limiter plates. Analytical calculations, matching the non-linear simulation results, suggest that the “narrow feature” heat-flux decay length always remains close to the turbulent correlation length. Ongoing work seeks to compare these findings against DIII-D, C-Mod, and TCV Langmuir probe, GPI, and infrared data.

The unified theory of pellet ablation for arbitrary atomic number Z was extended to include multi-species pellets such as mixtures of deuterium and neon, and polyatomic (molecular) pellets. The dependence on Z can be traced to the pitch-angle scattering operator in the linearized (high-velocity limit of the Rosenbluth potentials) Fokker-Planck equation for the attenuation of the incident Maxwellian plasma electrons in the ablation cloud. The multi-species Fokker Planck equation for electrons scattering from multiply charged ions involves the standard definition of Zeff (see Cohen 1978 NF for example). Since the principally neutral gas near the pellet does most of the shielding, Zeff is replaced by the analogous version for neutral atoms Zeff0 and it is straightforward to find the dependence of Zeff0 in the gas dynamic flow variables. Because the boundary conditions at the base of the flow are simpler for cryogenic pellets, the scaling the mass ablation rate for any cryogenic mixture is readily available. The scaling also involves certain averages of the atomic mass of the species. The ablation rate of refractory molecular pellets is not yet complete because the surface boundary conditions are more complex, which precludes a universal solution of the scaled gas dynamic equations.

A team of researchers from UCSD, MIT, and General Atomics have won a 2017 INCITE award to carry out multiscale gyrokinetic simulations of turbulent transport in DIII-D and ITER plasmas. Titled “Understanding How Multiscale Transport Determines Confinement in Burning Plasmas”, the award provides 100 million processor hours on the TITAN Cray XK7 machine for C. Holland (UCSD, lead PI), N. Howard (MIT), and J. Candy (General Atomics) to investigate the coupling of ion and electron-scale turbulence in current and future H-mode plasma conditions using the GYRO and CGYRO codes. These new simulations build upon recent results presented at the 2016 IAEA FEC meeting demonstrating that electron transport has a strong multiscale character in both Alcator C-Mod L-mode and DIII-D ITER baseline H-mode plasmas with dominant electron heating and little or no injected torque. The new DIII-D simulations will study how multiscale turbulence impacts heavy ion impurity transport, while the ITER simulations will quantify the impact of magnetic fluctuations on the nonlinear cross-scale couplings observed in previous work.