The plasma-sheath matching problem has attracted new interest during the last few years. It is complicated by the singular structure of the asymptotic (λ D /L → 0) plasma and sheath solutions and by a coupling with the eigenvalue problem originating from the plasma balance of bounded plasmas. Due to these difficulties the existence of a matched asymptotic expression uniformly valid from the plasma core to the wall is widely questioned. The issue is clarified both analytically and numerically by the explicit construction of a matched asymptotic expression and comparison with exact solutions for the hydrodynamic plane Tonks-Langmuir problem. The approximations obtained by consistent matching show excellent agreement with numerical potential curves. The singularities of the asymptotic components are reflected by small discontinuities in the derivatives that vanish in the limit λ D /L → 0.

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The influence of energetic electrons on magnetized plasma sheaths is studied for different current regimes and different secondary-electron emission coefficients. (Here, the term “plasma sheath” denotes the collisionless region consisting of the non-neutral Debye sheath and the quasi-neutral magnetic presheath.) It is shown that the presence of even a small population of energetic electrons can significantly influence the potential drop across the sheath and the energy flux to the wall. For example, for plasma parameters typical of contemporary tokamaks, the presence of a fast-electron population with density smaller than 0.1% (!) can double the potential drop across the sheath and the energy flux to the wall, and the presence of a few percent of fast electrons can enhance these values by up to one order. The effect of fast electrons decreases with increasing secondary-electron emission coefficient and increasing current to the wall. Analytical results obtained are checked against particle-in-cell (PIC) simulations for different current regimes and different secondary-electron emission coefficients, showing good agreement except in some cases where the simulation results exhibit strong fluctuations.

In this paper the general problem of linking fluid and kinetic plasma parameters, with special attention devoted to the plasma boundaries where, due to strong deviations from thermodynamic equilibrium, there are intrinsic difficulties regarding the closure of the hydrodynamic equations, is considered. This problem is demonstrated by means of two examples for which the solutions of the kinetic equations are known. These examples are the collision-free Tonks-Langmuir model [Phys. Rev. 34, 876 (1929)] and Riemann’s presheath model [Phys. Fluids 24, 2163 (1981)] dominated by charge-exchange collisions. It is found that in the vicinity of the sheath edge the “polytropic” coefficient γ(x) shows an unexpected behavior that contradicts the commonly used hydrodynamic approaches assuming γ=const. In spite of all differences, the two models investigated exhibit quite similar behavior of the hydrodynamic quantities and of the polytropic coefficient in the presheath and sheath regions. This rises to hopes that the results presented in this paper can be generalized to models characterizing other physical scenarios of plasma production and confinement. In particular, the basic findings presented here will, in suitably adopted form, be of importance, e.g., in properly formulating boundary conditions for fluid codes simulating bounded plasmas.

We consider the plasma‐wall boundary conditions in the presence of a strong magnetic field and discuss realistic examples when they can strongly deviate from the well‐accepted classical ones. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)