Effect of helium flux on near-surface helium accumulation in plasma-exposed tungsten.

We report results of object kinetic Monte Carlo (OKMC) simulations to understand the effect of helium flux on the near-surface helium accumulation in plasma-facing tungsten, which is initially pristine, defect-free, and has a (100) surface orientation. These OKMC simulations are performed at 933 K for fluxes ranging from 1022to 4 × 1025He/m2 s with 100 eV helium atoms impinging on a (100) surface up to a maximum fluence of 4 × 1019He/m2. In the near-surface region, helium clusters interact elastically with the free surface. The interaction is attractive and results in the drift of mobile helium clusters towards the surface as well as increased trap mutation rates. The associated kinetics and energetics of the above-mentioned processes obtained from molecular dynamics simulations are also considered. The OKMC simulations indicate that in pristine tungsten, as the flux decreases, the retention of implanted helium decreases, and its depth distribution shifts to deeper below the surface. Furthermore, the fraction of retained helium diffusing into the bulk increases as well, so much so that for the flux of 1022He/m2 s, almost all of the retained helium diffused into the bulk with minimal/negligible near-surface helium accumulation. At a given flux, with increasing fluence, the fraction of retained helium initially decreases and then starts to increase after reaching a minimum. The occurrence of the retention minimum shifts to higher fluences as the flux decreases. Although the near-surface helium accumulation spreads deeper into the material with decreasing flux and increasing fluence, the spread appears to saturate at depths between 80 and 100 nm. We present a detailed analysis of the influence of helium flux on the size and depth distribution of total helium and helium bubbles.

Exascale applications: skin in the game.

As noted in Wikipedia, skin in the game refers to having 'incurred risk by being involved in achieving a goal', where 'skin is a synecdoche for the person involved, and game is the metaphor for actions on the field of play under discussion'. For exascale applications under development in the US Department of Energy Exascale Computing Project, nothing could be more apt, with the skin being exascale applications and the game being delivering comprehensive science-based computational applications that effectively exploit exascale high-performance computing technologies to provide breakthrough modelling and simulation and data science solutions. These solutions will yield high-confidence insights and answers to the most critical problems and challenges for the USA in scientific discovery, national security, energy assurance, economic competitiveness and advanced healthcare. This article is part of a discussion meeting issue 'Numerical algorithms for high-performance computational science'.

Induced Fission of (240)Pu within a Real-Time Microscopic Framework.

We describe the fissioning dynamics of ^{240}Pu from a configuration in the proximity of the outer fission barrier to full scission and the formation of the fragments within an implementation of density functional theory extended to superfluid systems and real-time dynamics. The fission fragments emerge with properties similar to those determined experimentally, while the fission dynamics appears to be quite complex, with many excited shape and pairing modes. The evolution is found to be much slower than previously expected, and the ultimate role of the collective inertia is found to be negligible in this fully nonadiabatic treatment of nuclear dynamics, where all collective degrees of freedom (CDOF) are included (unlike adiabatic treatments with a small number of CDOF).

Quantized superfluid vortex rings in the unitary Fermi gas.

In a recent article, Yefsah et al. [Nature (London) 499, 426 (2013)] report the observation of an unusual excitation in an elongated harmonically trapped unitary Fermi gas. After phase imprinting a domain wall, they observe oscillations almost an order of magnitude slower than predicted by any theory of domain walls which they interpret as a "heavy soliton" of inertial mass some 200 times larger than the free fermion mass or 50 times larger than expected for a domain wall. We present compelling evidence that this "soliton" is instead a quantized vortex ring, by showing that the main aspects of the experiment can be naturally explained within the framework of time-dependent superfluid density functional theories.

Cooper pairing above the critical temperature in a unitary Fermi gas.

We present an ab initio determination of the spin response of the unitary Fermi gas. Based on finite temperature quantum Monte Carlo calculations and the Kubo linear-response formalism, we determine the temperature dependence of the spin susceptibility and the spin conductivity. We show that both quantities exhibit suppression above the critical temperature of the superfluid-to-normal phase transition due to Cooper pairing. The spin diffusion transport coefficient does not display a minimum in the vicinity of the critical temperature and drops to very low values D(s)≈0.8h/m in the superfluid phase. All these spin observables show a smooth and monotonic behavior with temperature when crossing the critical temperature T(c), until the Fermi liquid regime is attained at the temperature T(*), above which the pseudogap regime disappears.

Quantum shock waves and domain walls in the real-time dynamics of a superfluid unitary Fermi gas.

We show that in the collision of two superfluid fermionic atomic clouds one observes the formation of quantum shock waves as discontinuities in the number density and collective flow velocity. Domain walls, which are topological excitations of the superfluid order parameter, are also generated and exhibit abrupt phase changes by π and slower motion than the shock waves. The domain walls are distinct from the gray soliton train or number density ripples formed in the wake of the shock waves and observed in the collisions of superfluid bosonic atomic clouds. Domain walls with opposite phase jumps appear to collide elastically.

Real-time dynamics of quantized vortices in a unitary Fermi superfluid.

We introduce a comprehensive theoretical framework for the fermionic superfluid dynamics, grounded on a local extension of the time-dependent density functional theory. With this approach, we describe the generation and the real-time evolution and interaction of quantized vortices, the large-amplitude collective modes, as well as the loss of superfluidity at high flow velocities. We demonstrate the formation of vortex rings and provide a microscopic description of the crossing and reconnection of quantized vortex lines in a fermion superfluid, which provide the mechanism for the emergence of quantum turbulence at very low temperatures. We observe that superfluidity often survives when these systems are stirred with velocities far exceeding the speed of sound.

Three-dimensional myoarchitecture of the bovine tongue demonstrated by diffusion spectrum magnetic resonance imaging with tractography.

The anatomy of the mammalian tongue consists of an intricate array of variably aligned and extensively interwoven muscle fibers. As a result, it is particularly difficult to resolve the relationship between the tongue's microscopic anatomy and tissue-scale mechanical function. In order to address this question, we employed a method, diffusion spectrum imaging (DSI) with tractography, for displaying the macroscopic orientational properties of the tissue's constituting myofibers. DSI measures spatially variant proton displacement for a given 3D imaging segment (voxel), reflecting the principal orientation(s) of its myofibers. Tractography uses the angular similarity displayed by the principal fiber populations of multiple adjacent voxels to generate tract-like structures. DSI with tractography thus defines a unique set of tracts based on the net orientational behavior of the myofiber populations at different positions in the tissue. By this approach, we demonstrate a novel myoarchitectural pattern for the bovine tongue, consisting of short and orthogonally aligned crossing fiber tracts in the intrinsic core region, and longer, parallel-aligned fiber tracts on the tissue margins and in the regions of extrinsic fiber insertion. The identification of locally aligned myofiber populations by DSI with tractography allows us to reconsider lingual anatomy, not in conventional microscopic terms, but as a set of heterogeneously aligned and macroscopically resolved myofiber tracts. We postulate that the properties associated with these myofiber tracts predict the mechanical behavior of the tissue and thus constitute a method to relate structure and function for anatomically complex muscular tissues.