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A novel particle simulation code is described that self-consistently models certain classes of laser-plasma interactions without resolving the optical cycles of the laser. This is accomplished by separating the electromagnetic field into a laser component and a wake component. Although the wake component is treated as in a fully explicit particle-in-cell (PIC) code, the laser component is treated in the high-frequency limit, which allows the optical cycles to be averaged out. This leads to enormous reductions in computer time when the laser frequency is much greater than all other frequencies of interest. This work is an extension of the work of Mora and Antonsen, Jr. [1], [2], who derived the time-averaged equations coupling the laser with the particles and developed a code to solve these equations in the quasi-static limit. The code presented here is distinguished by the fact that it is useful when the plasma length is much less than the laser pulse length. Also, it is already parallelized and should be straightforward to extend to three dimensions.

A model for the rate of density rise observed when neutral gas is fed into a plasma-containing chamber is presented for regimes where known collisional transport processes do not provide an adequate explanation. A dense layer of cold plasma produced at the edge of the plasma column and the resulting relatively sharp ion temperature gradient, as compared with the local density gradient, can lead to the excitation of electron temperature fluctuations driven by ion drift modes. The net inflow of electrons and ions that is produced by these modes has been included in a one-dimensional transport code used to simulate experiments performed by the Alcator device. The linear and quasi-linear theories of these modes are given for the regimes of interest. The cold-plasma-layer model is also consistent with the presence of an outflow of impurity ions, due to impurity driven modes, that balance the inflow produced by discrete collisions.

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The heating of a single argon (Ar) cluster by a strong laser field is studied using an electrostatic particle-in-cell code for a range of intensities and cluster sizes. Heating is dominated by a nonlinear resonant absorption process involving energetic electrons transiting through the cluster. This process gives rise to a threshold in field strength for strong absorption and controls the dielectric properties of the cluster.

The passive convection of scalar fields by an incompressible fluid flow in two dimensions is investigated numerically. The prescribed flow is chaotic meaning that nearby fluid elements diverge exponentially with time. The gradient of the convected scalar field is of primary interest, and a measure is defined, reflecting the spatial distribution of the regions having large gradient. The dimension spectrum for this measure is computed by the standard box counting technique, and it is found to be fractal. A recent theory proposes that the fractal structure of the scalar gradient can be related to the nonuniform stretching properties of the flow. Using this theory, the fractal dimension spectrum is computed from the distribution of finite time Lyapunov exponents of the flow, and it is found to be in reasonable agreement with the dimension spectrum computed directly by means of box counting.

Exploiting the time-reversal invariance and reciprocal properties of the lossless wave equation enables elegantly simple solutions to complex wave-scattering problems, and is embodied in the time-reversal mirror. Here we demonstrate the implementation of an electromagnetic time-reversal mirror in a wave chaotic system containing a discrete nonlinearity. We demonstrate that the timereversed nonlinear excitations reconstruct exclusively upon the source of the nonlinearity. As an example of its utility, we demonstrate a new form of secure communication, and point out other applications.PACS numbers: 05.45. Mt, 05.45.Vx, 41.20.Jb, 42.25.Dd Wave chaos concerns the study of solutions to linear wave equations that display classical chaos in their short-wavelength limit. Such systems are endowed with many universal wave properties, such as eigenvalue and scattering-matrix statistics, by virtue of their classically chaotic counterparts.[1] Although wave chaotic systems are strongly scattering and have complex behavior, they can be elegantly studied by exploiting the time-reversal invariance and reciprocal properties of the linear wave equation. [2-9] Adding objects with complex nonlinear dynamics to linear wave chaotic systems has only recently been considered, [10] and represents an exciting new direction of research. Here we examine a wave chaotic system with a single discrete nonlinear element, and create a new nonlinear electromagnetic time-reversal mirror that shows promise for both fundamental studies and novel applications.A time-reversal mirror works by taking advantage of the invariance of the lossless wave equation under timereversal; for a time-forward solution of the wave equation representing a wave travelling in a given direction, there is a corresponding time-reversed solution representing a wave travelling in the same direction backwards in time, or in the opposite direction forward in time. This can be realized by transmitting a waveform at a particular source location and recording the reverberating waveforms (sona) with an array of receivers; the recorded waveforms are reversed in time and retransmitted back from the receivers, propagating to and reconstructing a time-reversed version of the original waveform at the source [3]. Time-reversal mirrors have been demonstrated for both acoustic [2-9, 11, 12] and electromagnetic waves [6,8,13], and exploited for applications such as lithotripsy [2,4], underwater communication [2, 14, 15], sensing perturbations [11,12], and achieving sub-wavelength imaging [6][7][8]16].An ideal time-reversal mirror in an open environment would collect the forward-propagating wave at every point on a closed surface enclosing the transmitter, requiring a very large number of receivers. The receiving array can be simplified, without significant loss of fidelity of the reconstruction, if there is a closed, ray-chaotic environment where a propagating wave (with wavelength much smaller than the size of the enclosure) will eventually reach every point in the environment, allowin...

The shape gradient quantifies the change in some figure of merit resulting from differential perturbations to a shape. Shape gradients can be applied to gradient-based optimization, sensitivity analysis, and tolerance calculation. An efficient method for computing the shape gradient for toroidal 3D MHD equilibria is presented. The method is based on the self-adjoint property of the equations for driven perturbations of MHD equilibria and is similar to the Onsager symmetry of transport coefficients. Two versions of the shape gradient are considered. One describes the change in a figure of merit due to an arbitrary displacement of the outer flux surface; the other describes the change in the figure of merit due to the displacement of a coil. The method is implemented for several example figures of merit and compared with direct calculation of the shape gradient. In these examples the adjoint method reduces the number of equilibrium computations by factors of O(N ), where N is the number of parameters used to describe the outer flux surface or coil shapes.