[en] Subtleties of implementing the standard perfectly conducting wall boundary condition in a general toroidal geometry are clarified for a mixed scalar magnetic field representation. An iterative scheme based on Ohm's law is given

[en] In a sheath-limited high-temperature and low-density plasma, energy and particle loss to an absorbing wall can set up a temperature anisotropy in which the normal-to-the-wall temperature is significantly lower than that of the parallel-to-the-wall directions, even for an upstream plasma source with isotropic temperature. This temperature anisotropy excites the Weibel instability and introduces a self-generated magnetic field which is parallel to the wall surface. The self-generated magnetic field modifies the sheath/presheath plasma in two ways upon saturation: (1) it suppresses the net energy loss rate to the absorbing wall, primarily through the ion channel and (2) it transfers energy between different degrees of freedom and reduces the plasma temperature anisotropy. (brief communication)

[en] A Taylor-relaxed plasma (j=kB with k a constant) under external magnetic helicity injection encounters resonances in spatial frequencies of its force-free eigenmodes. Such driven resonance underlies the physics of magnetic self-organization and determines the flux amplification in laboratory helicity injection applications. Here we show that for partially relaxed plasmas where the deviation from the fully relaxed Taylor state, for example, a flux-dependent k, is a function of the normalized flux χ/χ_a with χ_a the poloidal flux at the magnetic axis, a modified driven resonance persists even if k(χ) has an order-unity variation across the flux surfaces

[en] Full text: Radiation damage by fusion neutrons can significantly degrade material properties. In a fusion reactor, long-lasting radiation-induced defects such as vacancies, vacancy clusters, and voids introduce additional nuclear safety complication in terms of trap items for excessive tritium retention. This is especially true in the divertor/first wall of a tokamak like ITER, where extreme thermal stress is also present. Here we use molecular dynamics simulations to elucidate a self-mitigating mechanism in which the large thermal stress can facilitate the recombination of the neutron-collision-cascade-induced vacancies and interstitials through coupled grain boundary (GB) motion in a bcc tungsten under fusion reactor conditions. Specifically, our simulations reveal that for a number of tungsten GBs, absorbing the fast-moving interstitials can help activate coupled GB motion at reduced mechanical stress; the migrating GB then sweeps up the less-mobile vacancies, facilitating vacancy-interstitial recombination inside the GB. We examine the stress-induced mobility characteristics of a number of GBs in W to investigate the likelihood of this scenario. (author)

[en] While the sensitivity of the scrape-off layer and divertor plasma to the highly uncertain cross-field transport assumptions is widely recognized, the plasma is also sensitive to the details of the plasma-material interface (PMI) models used as part of comprehensive predictive simulations. Here in this paper, these PMI sensitivities are studied by varying the relevant sub-models within the SOLPS plasma transport code. Two aspects are explored: the sheath model used as a boundary condition in SOLPS, and fast particle reflection rates for ions impinging on a material surface. Both of these have been the study of recent high-fidelity simulation efforts aimed at improving the understanding and prediction of these phenomena. It is found that in both cases quantitative changes to the plasma solution result from modification of the PMI model, with a larger impact in the case of the reflection coefficient variation. Finally, this indicates the necessity to better quantify the uncertainties within the PMI models themselves, and perform thorough sensitivity analysis to propagate these throughout the boundary model; this is especially important for validation against experiment, where the error in the simulation is a critical and less-studied piece of the code-experiment comparison.

[en] The natural occurrence of small scale structures and the extreme anisotropy in the evolution of a magnetic field embedded in a conducting flow is interpreted in terms of the properties of the local Lyapunov exponents along the various local characteristic (un)stable directions for the Lagrangian flow trajectories. The local Lyapunov exponents and the characteristic directions are functions of Lagrangian coordinates and time, which are completely determined once the flow field is specified. The characteristic directions that are associated with the spatial anisotropy of the problem, are prescribed in both Lagrangian and Eulerian frames. Coordinate transformation techniques are employed to relate the spatial distributions of the magnetic field, the induced current density, and the Lorentz force, which are usually followed in Eulerian frame, to those of the local Lyapunov exponents, which are naturally defined in Lagrangian coordinates. (c) 2000 American Institute of Physics

[en] The highest performing spheromaks in the laboratory are formed by electrostatic helicity injection. Discharges with up to 1 MA plasma current and core electron temperature as high as 500 eV have been recently obtained. For such a scheme to scale-up to a reactor, however, a much higher current multiplication factor (plasma current over injector current) must be achieved. It is shown here that spheromak current multiplication is linearly proportional to flux amplification (ratio of poloidal fluxes inside and outside the separatrix of the mean field). Hence, spheromak optimization is centered around achieving high flux amplification, which is provided by linear or nonlinear resonant coupling between helicity injector and the spheromak force-free eigenmode. The nonlinear resonant field amplification is the most promising route to high flux amplification in a realistic plasma that often significantly deviates from the Taylor state. Accessing such nonlinear resonant field amplification can be facilitated by auxiliary current drive around the magnetic axis and auxiliary heating to break the electron temperature constraint on flux amplification.

[en] Current multiplication and flux amplification are two critical measures in assessing the usefulness of magnetic helicity injection for forming the plasma confining magnetic field in laboratory spheromak and spherical tokamak (ST) experiments. While the two concepts are closely related for spheromaks, they are independent for a Taylor-relaxed ST plasma, and negatively correlated for a more realistic, partially relaxed ST plasma. An important application of this understanding leads to the so-called relaxed transient coaxial helicity injection scheme for solenoid-free ST startup, which can deliver reactor-grade high current multiplication and flux amplification

[en] The fast ion tail for a spherically symmetric hot spot is computed via the solution of a simplified Fokker-Planck collision operator. Emphasis is placed on describing the energy scaling of the fast ion distribution function in the hot spot as well as the surrounding cold plasma throughout a broad range of collisionalities and temperatures. It is found that while the fast ion tail inside the hot spot is significantly depleted, leading to a reduction of the fusion yield in this region, a surplus of fast ions is observed in the neighboring cold plasma region. The presence of this surplus of fast ions in the neighboring cold region is shown to result in a partial recovery of the fusion yield lost in the hot spot

[en] The M3D (Multi-level 3D) project simulates plasmas using multiple levels of physics, geometry, and grid models in one code package. The M3D code has been extended to fundamentally nonaxisymmetric and small aspect ratio, R/a>or∼1, configurations. Applications include the nonlinear stability of the NSTX spherical torus and the spherical pinch, and the relaxation of stellarator equilibria. The fluid-level physics model has been extended to evolve the anisotropic pressures p_jparallel and p_{jperpendicular} for the ion and electron species. Results show that when the density evolves, other terms in addition to the neoclassical collisional parallel viscous force, such as B· ∇p_e in the Ohm's law, can be strongly destabilizing for nonlinear magnetic islands. (author)