No abstract

The potential difference between a floating emitting surface and the plasma surrounding it has been described by several sheath models, including the space-charge-limited sheath, the electron sheath with high emission current, and the inverse sheath produced by charge-exchange ion trapping. Our measurements reveal that each of these models has its own regime of validity. We determine the potential of an emissive filament relative to the plasma potential, emphasizing variations in emitted current density and neutral particle density. The potential of a filament in a diffuse plasma is first shown to vanish, consistent with the electron sheath model and increasing electron emission. In a denser plasma with ample neutral pressure, the floating filament potential is positive, as predicted by a derived ion trapping condition. Lastly, the filament floated negatively in a third plasma, where flowing ions and electrons and nonnegligible electric fields may have disrupted ion trapping. Depending on the regime chosen, emitting surfaces can float positively or negatively with respect to the plasma potential.Any solid surface in contact with a plasma is surrounded by the sheath, a potential structure that controls particle and energy transport between the plasma and the surface [1]. Sheath structure is complicated when the surface emits an electron current, which can be caused by impinging radiation or plasma particles. Emissive sheaths are present in divertors [2] and scrape-off layers [3] in magnetic fusion devices, around dust grains in laboratory [4] and astrophysical [5] plasmas, around satellites [6], in RF plasma processing devices [7] and around plasma probes [8]. In all of these cases, the interplay between emitted and background plasmas determines the structure of sheath that forms. Predicting which structure exists is essential for understanding the heat and charge flux to the surface.A surface emits a normalized currentĴ = J emit /J e in a background electron current J e . WhenĴ = 0, mobile plasma electrons charge the surface negatively so that its potential is negative with respect to the plasma potential φ P (in this work, all surface potentials called positive r/r 0 −3 −2 −1 0 ∆φ [V] T e [eV] (a)

A brief critique is presented of some different classes of magnetohydrodynamic equilibrium solutions based on their continuity properties and whether the magnetic field is integrable or not. A generalized energy functional is introduced that is comprised of alternating ideal regions, with nested flux surfaces with an irrational rotational transform, and Taylor-relaxed regions, possibly with magnetic islands and chaos. The equilibrium states have globally continuous magnetic fields, and may be constructed for arbitrary three-dimensional plasma boundaries and appropriately prescribed pressure and rotational-transform profiles.

Absolute quantitative data from atomic force microscopy (AFM)/lateral force microscopy experiments are always difficult to obtain mainly due to the need of the normal force FN and the friction force FF calibration. In this article, we developed an experimental method which allows us to extract absolute quantitative friction data without calibrating any force when the relation between FN and FF is linear or only calibrating the normal force when the relationship is nonlinear. The technique reported here, is suitable for an atomic force microscope that has the cantilever attached to the piezotube translator and an unguided incident laser beam on the cantilever. We take advantage of the piezotube bending during a large scan (5 μm×5 μm), generally considered as an undesirable effect, to calculate a detection factor that allows the determination of quantitative tribological data. The validity of our experimental method is checked on the extensively AFM studied materials, such as muscovite, silicon, and highly oriented pyrolytic graphite. The experiments are carried out in a load range where the shear stress τ can be expressed as τ=τ0+μP, where μ is the friction coefficient, P is the mean contact pressure, and τ0 is a parameter related to the tip/sample adhesion. The value of μ is found to be independent of the tip geometry and the pull-off force, and always constant for a given tip/sample couple in the load range investigated.

In three-dimensional ideal magnetohydrodynamics, closed flux surfaces cannot maintain both rational rotational-transform and pressure gradients, as these features together produce unphysical, infinite currents. A proposed set of equilibria nullifies these currents by flattening the pressure on sufficiently wide intervals around each rational surface. Such rational surfaces exist at every scale, which characterizes the pressure profile as self-similar and thus fractal. The pressure profile is approximated numerically by considering a finite number of rational regions, and analyzed mathematically by classifying the irrational numbers that support gradients into subsets. Applying these results to a given rotational-transform profile in cylindrical geometry, we find magnetic field and current density profiles compatible with the fractal pressure.