The role of BT-dependent flows on W accumulation at the edge of the confined plasma

Near-separatrix impurity accumulation between the crown and the outer midplane of tokamaks is a common feature in results from codes such as SOLPS-ITER and DIVIMP; however, experimental evidence of accumulation has only recently been obtained and is reported here. The codes find that the poloidal distribution of impurity ions in the scrape-off layer (SOL) depends primarily on toroidal field (BT)-dependent parallel flow patterns of the background plasma and the parallel ion temperature gradient (∇||Tion) force. Experimentally, Mach probes used in L-mode plasmas with favorable (for H-mode access) BT measure fast (M~0.3-0.5) inner-target-directed (ITD) background plasma flows at the crown of single-null discharges. This study reports a set of DIVIMP simulations for two similar H-mode discharges from the DIII-D W Metal Rings Campaign differing primarily in BT-direction to assess the effect that fast ITD flows have on the distribution of W ions in the SOL. It is found that for imposed ITD flows of M = 0.3, W ions that otherwise accumulate due to the ∇||Tion-force are largely flushed out. It is also found that doubling the radial diffusion coefficient from 0.3 to 0.6 m2/s prevents accumulation due to rapid cross-field transport into the far-SOL, where background plasma flows drain W ions to the divertors. Far-SOL W distributions from DIVIMP are then used to specify input to the impurity transport code 3DLIM, which is used to interpretively model collector probe deposition patterns measured in the “wall-SOL.” It is demonstrated that the deposition patterns are consistent with the DIVIMP predictions of near-SOL accumulation for the unfavorable-BT direction, and little/no accumulation for the favorable-BT direction. The wall-SOL collector probes have thus provided the first experimental evidence, albeit indirect, of near-SOL W accumulation – finding it occurs for the unfavorable-BT direction only. For the favorable-BT direction, fast flows can largely prevent accumulation from occurring.

Methodology for calculation of radial diffusion coefficients for a relativistic electron population from hybrid-Vlasov simulation

<p>The relative importance of radial diffusion and local acceleration to the dynamics of outer radiation belt electron populations is an open question in radiation belt physics. A key component of this discussion is the calculation of the radial diffusion coefficients, which quantify the effect of radial diffusion on an electron population. However, there is currently a broad range of radial diffusion coefficient values in the literature, which presents difficulties when determining the dominant process governing radiation belt energisation. Here we develop a methodology for the calculation of radial diffusion coefficients using Vlasiator, a 5D hybrid-Vlasov simulation of near-Earth space, and calculate the radial diffusion coefficients for a 10 MeV electron population at multiple locations within the outer radiation belt.</p><p> </p><p>Vlasiator currently models ions as velocity distribution functions and electrons as a magnetohydrodynamic fluid, so the drift motion of the electron population can not be directly studied. However, the ion dynamics accurately determine the magnetic field in the inner magnetosphere, and the spatial and temporal magnetic field variations can be used to calculate the radial diffusion coefficient of a population according to principles outlined in Lejosne et. al. 2020.<span>  </span>Four magnetic field isocontours in the outer radiation belt are used to model the guiding centre drift contours of an electron population, and the corresponding Roederer L-star coordinates are calculated from the magnetic flux through each of these drift contours. The variation of the L-stars over time are calculated from population-specific variables and the Lagrangian magnetic field time derivative along the magnetic isocontours. The radial diffusion coefficients for the 10 MeV electron population are then calculated at each of these L-stars and compared to the literature. This methodology produces radial diffusion coefficients from Vlasiator that have the expected L-shell dependence and are consistent with the literature, including studies based on satellite measurements of radiation belt electrons. These results indicate that this is a valid methodology for the calculation of radial diffusion coefficients, and can therefore be extended to evaluate the radial diffusion coefficients in different solar wind conditions and at more L-stars.</p>

Convective uphill transport of NaCl from ascending thin limb of loop of Henle

In this paper we describe a mathematical model of the renal inner medulla based on a previously proposed model [A.S. Wexler, R.E. Kalaba, and D.J. Marsh. Am. J. Physiol. 260 (Renal Fluid Electrolyte Physiol. 29): F368–F383, 1991] in which in the inner medullary ascending thin limb of Henle's loop (ATL) and collecting duct (CD) exchange with a local capillary node with the reabsorbed water and solutes flowing radially toward a central vascular bundle. Our model differs in that ascending and descending vasa recta and surrounding interstitial space are replaced by a central core. Our analysis of the coupled ATL-CD system shows that it is theoretically capable of transporting NaCl out of the ATL into the central vascular space (approximated by the central core) against a concentration gradient, which in the absence of radial diffusion can be arbitrarily large. By numerical solution of the model with the radial diffusion coefficient (D(r)) for NaCl of 0, we find that the ATL can be more than 100 mosmol/l hypotonic with respect to the core. We also find that with restricted diffusion the osmolality of the CD at the papilla is significantly greater than that of the loop of Henle. As D(r) approaches the diffusion coefficient of NaCl in free solution, the osmolality of the loop increases and that of the CD decreases. Thus, overall, contrary to intuitive expectations, the radial separation and uphill transport of NaCl do not give any significant increase in loop concentration, which depends primarily on the quantity of urea reabsorbed from the CD.