Electrostatic drift instabilities, turbulence and anomalous transport: introduction and basic theory

Experts consider that the enhanced transport observed in fusion devices is the consequence of low frequency instabilities which are elongated along the magnetic field lines and whose free energy sources are the gradients inherent to confinement. The cases k(parallel to)qR >> 1 and k(parallel to)qR similar to 1 will be considered, where k(parallel to) is the parallel wave number and qR the connection length. The first limit is relevant to plasmas with large parallel flow velocity gradients (\partial derivative U-r(phi,i) \similar to c(i)\partial derivative(r) ln N-i\, where (U) over bar (i) and c(i) are the ion bulk and thermal velocities; N-i is the density); here, a local dispersion relation leads to exact stability criteria with the help of the Nyquist diagram technique. The ordering k(parallel to)qR similar to 1 applies otherwise; it requires a non-local analysis leading to a second order differential equation whose complex eigenvalues provide the wave frequencies and growth / decay rates; two sub-cases must be considered according to whether the eigenfunctions are radially or poloidally localised. After reviewing some of the most important micro-instabilities, the non-linear saturation mechanism and ultimate turbulence level will be discussed, as well as the various aspects of anomalous transport theories

Nonlinear structures in interchange mode turbulence

Generation of linearly unstable streamers and nonlinearly saturated zonal flows have been investigated using two-dimensional simulations of interchange mode turbulence. It has been shown that nonlinear diamagnetic interactions tend to inhibit spectral cascades towards larger length scales and consequently suppress the nonlinear excitation of zonal flows. The latter has also been found to be in qualitative agreement with a theoretical analysis based on the reductive perturbation method

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

Dynamics of modal power distribution in a multimode semiconductor laser with optical feedback

The dynamics of power distribution between longitudinal modes of a multimode semiconductor laser subjected to external optical feedback is experimentally analyzed in the low-frequency fluctuation regime. Power dropouts in the total light intensity are invariably accompanied by sudden activations of several longitudinal modes. These activations are seen not to be simultaneous to the dropouts, but to occur after them. The phenomenon is statistically analysed in a systematic way, and the corresponding delay is estimated.Comment: 3 pages, 4 figures, revte

Dissipative Dynamics of Collisionless Nonlinear Alfven Wave Trains

The nonlinear dynamics of collisionless Alfven trains, including resonant particle effects is studied using the kinetic nonlinear Schroedinger (KNLS) equation model. Numerical solutions of the KNLS reveal the dynamics of Alfven waves to be sensitive to the sense of polarization as well as the angle of propagation with respect to the ambient magnetic field. The combined effects of both wave nonlinearity and Landau damping result in the evolutionary formation of stationaryOA S- and arc-polarized directional and rotational discontinuities. These waveforms are freqently observed in the interplanetary plasma.Comment: REVTeX, 6 pages (including 5 figures). This and other papers may be found at http://sdphpd.ucsd.edu/~medvedev/papers.htm

Towards a Simple Model of Compressible Alfvenic Turbulence

A simple model collisionless, dissipative, compressible MHD (Alfvenic) turbulence in a magnetized system is investigated. In contrast to more familiar paradigms of turbulence, dissipation arises from Landau damping, enters via nonlinearity, and is distributed over all scales. The theory predicts that two different regimes or phases of turbulence are possible, depending on the ratio of steepening to damping coefficient (m_1/m_2). For strong damping (|m_1/m_2|<1), a regime of smooth, hydrodynamic turbulence is predicted. For |m_1/m_2|>1, steady state turbulence does not exist in the hydrodynamic limit. Rather, spikey, small scale structure is predicted.Comment: 6 pages, one figure, REVTeX; this version to be published in PRE. For related papers, see http://sdphpd.ucsd.edu/~medvedev/papers.htm

Semi-Dense 3D Reconstruction with a Stereo Event Camera

Event cameras are bio-inspired sensors that offer several advantages, such as low latency, high-speed and high dynamic range, to tackle challenging scenarios in computer vision. This paper presents a solution to the problem of 3D reconstruction from data captured by a stereo event-camera rig moving in a static scene, such as in the context of stereo Simultaneous Localization and Mapping. The proposed method consists of the optimization of an energy function designed to exploit small-baseline spatio-temporal consistency of events triggered across both stereo image planes. To improve the density of the reconstruction and to reduce the uncertainty of the estimation, a probabilistic depth-fusion strategy is also developed. The resulting method has no special requirements on either the motion of the stereo event-camera rig or on prior knowledge about the scene. Experiments demonstrate our method can deal with both texture-rich scenes as well as sparse scenes, outperforming state-of-the-art stereo methods based on event data image representations.Comment: 19 pages, 8 figures, Video: https://youtu.be/Qrnpj2FD1e