551 research outputs found
Phase Mixing of Nonlinear Plasma Oscillations in an Arbitrary Mass Ratio Cold Plasma
Nonlinear plasma oscillations in an arbitrary mass ratio cold plasma have
been studied using 1-D particle-in-cell simulation. In contrast to earlier work
for infinitely massive ion plasmas it has been found that the oscillations
phase mix away at any amplitude and that the rate at which phase mixing occurs,
depends on the mass ratio () and the amplitude. A
perturbation theoretic calculation carried upto third order predicts that the
normalized phase mixing time depends on the amplitude
and the mass ratio as . We have confirmed this scaling in our simulations and
conclude that stable non-linear oscillations which never phase mix, exist only
for the ideal case with and . These cold plasma results
may have direct relevance to recent experiments on superintense laser beam
plasma interactions with applications to particle acceleration, fast ignitor
concept etc.Comment: pp 10 and two figures in PS forma
Evidence of Levy stable process in tokamak edge turbulence
The time series of floating potential and poloidal electric field fluctuations in the edge plasma of ohmically heated ADITYA tokamak [Phys. Plasmas 4, 4292 (1997)] are analyzed for self-similarity. It is observed that the distribution function of a sum of n data points converges to a self-similar distribution of Levy scale index, α=1.1-1.3 for n ≤ 40 and α=1.8-2.0 for larger n. This shows that the scaling properties of small scale fluctuations are non-Gaussian and those of large scale fluctuations are Gaussian. Implication of this observation to our understanding of plasma transport is discussed
Effects of poloidal sheared flow and active feedback on tokamak edge fluctuations
The influence of sheared poloidal flow and of a phase sensitive feedback source on the various edge instabilities of tokamaks are investigated. The conditions for stabilization of the rippling mode, for the drift dissipative mode and for the radiative condensation instability are obtained from detailed numerical solutions and from approximate analytic solutions of the relevant eigenmode equations
Asymmetric radiation-induced toroidal flow and improved confinement in tokamaks
The role of impurity radiation in influencing the toroidal flow and radial electric fields (parameters critical for determining turbulent transport) has been studied on the edge of a tokamak plasma. It is demonstrated for the first time that the impurities distributed in an asymmetric (poloidally) manner may lead to significant density and temperature perturbations on magnetic surfaces. These, in turn, interact with the θ dependent toroidal field variations and yield a mean divergence of the stress tensor driving strong neoclassical toroidal flows. A self-consistent theory of interplay of equilibrium, fluctuations, neoclassical flows, and E→×B→ shear rotation in a tokamak is also presented. It is shown that the resulting enhanced toroidal velocity shear on the outer radiative layers produces a stabilizing effect on the well known instabilities (which determine edge transport) such as the drift resistive ballooning mode, the drift trapped electron mode, and the ion temperature gradient mode. For various values of the radiation asymmetry parameter, investigation of the turbulent particle flux as a function of the density gradient shows that the plasma can undergo a bifurcation into a better-confined state with a peaked density
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