A one-dimensional two-fluid model is presented and used for numerical analysis of the asymptotic two-scale limit of the plasma-wall transition. Numerical results confirm that when the problem is ...treated on the pre-sheath scale, the sheath edge is determined by the electric field singularity. When the problem is approached on the sheath scale, electric field at the sheath edge must be larger than zero in order to obtain any solutions of the model equations. In this case the Bohm criterion is determined by two parameters: the electric field and the ion velocity at the sheath edge.
Presheath and sheath structures of collisional two ion species (helium and argon) plasma in the presence of bi-Maxwellian electrons have been investigated by using a fluid model. As the thermal ...energy of hot electrons is higher than cold electrons, the electron impact ionization process is governed by the concentration of hot electrons. The velocity of positive ions at the sheath boundary, i.e., the Bohm criterion, gets modified in the presence of ion–neutral drag force, source term, and bi-Maxwellian electrons. It is found that the ion–neutral drag force, ionization rates, and volumetric composition of electrons affect the characteristics of the presheath and sheath. The scale length of the sheath region widens from about 1.09 mm to 5.80 mm with the increase in the concentration of hot electrons. The acoustic speed of helium ions at the sheath boundary is higher than its common speed, whereas the acoustic speed of argon ions is lower than its common speed. The common speed of positive ions at the sheath boundary is slower by 13% in magnitude than in the collisionless case. Furthermore, the effect of ion–neutral drag force on streaming instability for two ion species plasma has been systematically presented.
It was recently pointed out and demonstrated experimentally by Lundeen et al. that the wave function of a particle (more precisely, the wave function possessed by each member of an ensemble of ...identically-prepared particles) can be “directly measured” using weak measurement. Here it is shown that if this same technique is applied, with appropriate post-selection, to one particle from a perhaps entangled multi-particle system, the result is precisely the so-called “conditional wave function” of Bohmian mechanics. Thus, a plausibly operationalist method for defining the wave function of a quantum mechanical sub-system corresponds to the natural definition of a sub-system wave function which Bohmian mechanics uniquely makes possible. Similarly, a weak-measurement-based procedure for directly measuring a sub-system’s density matrix should yield, under appropriate circumstances, the Bohmian “conditional density matrix” as opposed to the standard reduced density matrix. Experimental arrangements to demonstrate this behavior–and also thereby reveal the non-local dependence of sub-system state functions on distant interventions–are suggested and discussed.
•We study a “direct measurement” protocol for wave functions and density matrices.•Weakly measured states of entangled particles correspond to Bohmian conditional states.•Novel method of observing quantum non-locality is proposed.
•A supersonic plasma flow caused by the magnetic nozzle effect is successfully reproduced.•The sonic-transition point can have bifurcation characteristics.•The Braginskii model creates various ...unphysical profiles in the supersonic plasma flow region.•A particle source/sink in front of the target makes the plasma flow subsonic/supersonic.•A discontinuous change in the sonic-transition point and corresponding plasma flow can happen.
Plasma flow patterns in a super-X-divertor-like configuration are investigated by using a one-dimensional plasma fluid model with the anisotropic-ion pressure (AIP model). The AIP model enables to treat supersonic plasma flows self-consistently by describing the parallel plasma momentum transport with a hyperbolic equation keeping the finite effect of the parallel viscosity. A supersonic plasma flow in the diverging-magnetic-field divertor region is generated due to the magnetic nozzle effect. It is found that the sonic-transition point has bifurcation characteristics. A numerical solution from the AIP model agrees well with an analytical one. The Braginskii’s plasma fluid model, on the other hand, creates various unphysical profiles in the supersonic plasma flow region for different boundary conditions of the plasma flow at the sheath entrance. It is also found that a particle source/sink in front of the target brings about generations of subsonic/supersonic plasma flow profiles. Moving the target position, it is found that a discontinuous change in the sonic-transition point and corresponding profile of plasma flow can happen due to the bifurcating characteristics of the sonic-transition point.
Electrons emitted from a solid surface can noticeably affect characteristics of plasma
sheath surrounding that surface by modifying current balance at wall, charge separation in
sheath region and ...Bohm criterion at sheath edge. We establish a static sheath model with
kinetic electrons and cold ions to emphasize the effect of different total emitted
electron velocity distribution functions (EEVDFs) on classic sheath solution and its
structure transition. Four total EEVDFs with same average energy are considered
separately. It is found that total EEVDFs influence the sheath solution and the threshold
of total electron emission coefficient (EEC) for classic sheath dramatically, and can
cause no solution for critical space-charge limited (SCL) sheath. These results indicate
that, as EEC increases from zero gradually, the sheath will not transit from classic
sheath to SCL sheath structure for some special total EEVDFs.
Quantifying main chamber wall recycling, erosion and resulting material migration, at least on the basis of known or empirical far scrape-off layer (SOL) processes, is still highly uncertain, despite ...its relevance for ITER and fusion reactor design studies. This affects, for example, the design problem of first mirror performance of many optical diagnostics in the harsh ITER environment. Poor computational access is not least due to a fundamental technical limitation in apparently all current tokamak edge plasma fluid codes, which implicates a wide computationally unresolved gap between the outermost plasma layer treated in codes and the real vessel wall. We show how the current ITER version of the B2-EIRENE code (SOLPS-4.3) can be extended to cover also this far SOL, on the same footing as the rest of the plasma transport model. We discuss consequences of this new model for estimating plasma power and particle sink terms caused by a fairly realistic wall in ITER based on the conventional Bohm criterion along all plasma–wall interfaces.
Corrections were made to this article on 14 July 2011. The authors have been assigned to the correct affiliations.
The results obtained to date by the one-dimensional theory of the emission of positive ions by a plasma are reviewed from a unified point of view. Simple and generalized forms of the Bohm criterion ...are discussed. The unipolar sheath model and the two-scale theory are outlined. Kinetic and hydrodynamic models for the formation of the Debye sheath and of the ionization zone in the presheath are described. It is shown that, except for some artificial ionization models, the Bohm criterion is fulfilled in the equality form. The emission current from the plasma emitter is calculated in the hydrodynamic and kinetic approximations. Existing numerical codes for the simulation of positive ion sources in two and three dimensions are briefly described.
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