Theory Of Plasma Physics Research Articles

Etching of CVD diamond films using oxygen ions in ECR plasma

Etching with oxygen ions produced by ECR plasma with an asymmetric magnetic mirror field was investigated as a potential technique for polishing CVD diamond. The morphology, structure and roughness of the diamond film surface before and after etching were analyzed respectively using scanning electron microscope (SEM), Raman spectroscopy and surface roughness measuring instrument. It was found that the ridges on diamond surface had been preferentially etched away and thereby the surface roughness decreased from 3.061 to 1.083μm after 4h etching. Meanwhile, non-diamond phase appeared on surface and dramatically increased with the extending of etching time. In order to fundamentally understand the etching mechanism, an etching model of diamond film was reasonably proposed on the ground of the experimental results and the theory of plasma physics. The as-generated ions taking screw movement are firstly accelerated along the magnetic field lines in the plasma and collisional presheath, and then deflected from their route towards the diamond film in the MP. When coming into Debye sheath, the motion of ions will be deflected further and strongly accelerated by electric field in the direction normal to the (111) crystal facets. This process gave rise to energetic ion bombardment towards every (111) crystal face, and thereby caused preferential etching of pyramidal crystallites.

Applications of Symmetry Methods to the Theory of Plasma Physics

The theory of plasma physics offers a number of nontrivial examples of par- tial differential equations, which can be successfully treated with symmetry methods. We propose three different examples which may illustrate the reciprocal advantage of this in- teraction between plasma physics and symmetry techniques. The examples include, in particular, the complete symmetry analysis of system of two PDE's, with the determination of some conditional and partial symmetries, the construction of group-invariant solutions, and the symmetry classification of a nonlinear PDE.

Use of stochastic web patterns to control electron transport in semiconductor superlattices

Chaotic electron transport has been explored in a variety of semiconductor structures in which the transition to chaos occurs by the gradual and progressive destruction of stable orbits in response to an increasing perturbation. There is also a much rarer type of chaos, known as non-KAM dynamics, which switches on and off abruptly when the temporal frequency of the perturbation reaches certain critical values. This type of chaotic motion is of great interest due to diverse applications in the theory of plasma physics, tokamak fusion, turbulent fluid dynamics, ion traps, and quasicrystals, but has not yet been realized in experiment. Here, we show that electrons in a superlattice miniband with a tilted magnetic field provide an experimentally-accessible non-KAM system and, moreover, that this unusual type of chaotic dynamics can produce strong resonant enhancement of the electron drift velocity. The onset of chaos is characterized by the formation of intricate “stochastic web” patterns in the electron phase space. These webs delocalize the electron orbits, thereby generating strong resonant peaks in our calculated drift velocity versus electric field characteristics.

Chaotic electron diffusion through stochastic webs enhances current flow in superlattices.

Understanding how complex systems respond to change is of fundamental importance in the natural sciences. There is particular interest in systems whose classical newtonian motion becomes chaotic as an applied perturbation grows. The transition to chaos usually occurs by the gradual destruction of stable orbits in parameter space, in accordance with the Kolmogorov-Arnold-Moser (KAM) theorem--a cornerstone of nonlinear dynamics that explains, for example, gaps in the asteroid belt. By contrast, 'non-KAM' chaos switches on and off abruptly at critical values of the perturbation frequency. This type of dynamics has wide-ranging implications in the theory of plasma physics, tokamak fusion, turbulence, ion traps, and quasicrystals. Here we realize non-KAM chaos experimentally by exploiting the quantum properties of electrons in the periodic potential of a semiconductor superlattice with an applied voltage and magnetic field. The onset of chaos at discrete voltages is observed as a large increase in the current flow due to the creation of unbound electron orbits, which propagate through intricate web patterns in phase space. Non-KAM chaos therefore provides a mechanism for controlling the electrical conductivity of a condensed matter device: its extreme sensitivity could find applications in quantum electronics and photonics.

Finite element approximation of a free boundary problem arising in the theory of liquid drops ans plasma physics

Des bornes d'erreur d'ordre optimal sont obtenues pour une approximation par elements finis d'un probleme variationnel qui apparait dans la theorie des gouttes de liquide ainsi que dans la physique des plasmas. Pour un domaine borne Ω⊂R 2 nous considerons la minimisation de γ +κ 2 (1−γ) −κ 4 soumis a η≥0 et =M/2; ou designe le produit scalaire dans L 2 γ≥0, κ 2 >0 et M>0 sont des constantes donnees et #7B-G∈#7B-L(L 2 (Ω), H 0 1 (Ω)) est l'inverse du Laplacien

A Deterministic Approximation of Diffusion Equations Using Particles

A new particle method adapted to the simulation of convection-diffusion problems is presented. The method relies on the definition of a convective field associated with the heat operator, which allows the convection of the particles in a deterministic way.No rigorous error analysis is given but, instead, a detailed numerical study of the influence of the parameters is presented. This study is performed on the heat equation, then applied to a Fokker–Planck model arising in the kinetic theory of plasma physics. Finally, an extension of the method to other problems is given.

Asymptotic Behavior of Solutions of a Nonlinear Diffusion Equation

A nonlinear diffusion equation which arises in the theory of plasma physics is discussed. It is shown that the solutions of a class of initial boundary value problems converge to a similarity solution as $t \to \infty $. The optimal rate of convergence is established.