391 research outputs found
Control of neoclassical tearing modes in large tokamaks
Some self-consistent effects pertaining to feedback control of neoclassical tearing modes in high temperature large tokamaks are investigated. For the ECRH scheme of local electron heating, it is shown that the self-consistent bootstrap currents created by the driven pressure gradients within the island are comparable to those due to the usually considered resistivity change mechanism. Similar self-consistent currents can also arise from pressure gradients created by density and energy deposition from neutral beams, thereby offering a new possibility for neoclassical mode control. The stabilizing current in such an application of neutral beams is estimated. It is further shown that such a feedback scheme can be made even more effective through appropriate modulation of the beam source to match the phase variation arising from the island rotation
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
Plasma rotation effects on Neoclassical tearing modes
The effect of an equilibrium sheared flow on the nonlinear evolution of a neoclassical tearing mode is investigated by estimating the influence it has on the inner and outer layer dynamics of the mode. Two complementary approaches are adopted. A generalized Rutherford model calculation is carried out to estimate the flow contributions to the polarization current term in the inner layer. For the outer layer,flow induced changes in the stability parameter Δ` are estimated with the help of a 3D initial value reduced MHD code (NEAR). For realistic parameters it is found that the outer layer modification is the dominant one and the scaling of Δ` with the flow shear parameter appears to agree with recent experimental observations
Effect of sheared flows on classical and neoclassical tearing modes
The influence of toroidal sheared equilibrium flows on the nonlinear evolution of classical and neoclassical tearing modes is studied through numerical solutions of a set of reduced generalized MHD equations that include viscous force effects based on neoclassical closures. In general, differential flow is found to have a strong stabilizing influence leading to lower saturated island widths for the classical tearing mode and reduced growth rates for the neoclassical mode. Velocity shear, on the other hand, is seen to make a destabilizing contribution
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
Mirnov coil data analysis for tokamak ADITYA
The spatial and temporal structures of magnetic signal in the tokamak ADITYA is analysed using recently developed singular value decomposition (SVD) technique. The analysis technique is first tested with simulated data and then applied to the ADITYA Mirnov coil data to determine the structure of current peturbation as the discharge progresses. It is observed that during the current rise phase, current perturbation undergoes transition from m=5 poloidal structure to m=4 and then to m=3. At the time of current termination, m=2 perturbation is observed. It is observed that the mode frequency remains nearly constant (≈10 kHz) when poloidal mode structure changes from m=4 to m=2. This may be either an indication of mode coupling or a consequences of changes in the plasma electron temperature and density scale length
A new regime of anomalous penetration of relativistically strong laser radiation into an overdense plasma
It is shown that penetration of relativistically intense laser light into an
overdense plasma, accessible by self-induced transparency, occurs over a finite
length only. The penetration length depends crucially on the overdense plasma
parameter and increases with increasing incident intensity after exceeding the
threshold for self-induced transparency. Exact analytical solutions describing
the plasma-field distributions are presented.Comment: 6 pages, 2 figures in 2 separate eps files; submitted to JETP Letter
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