Electromagnetic Burst Generation during Annihilation of Magnetic Field in Relativistic Laser-Plasma Interaction.

We present the results of theoretical studies of formation and evolution of the current sheet in a colliosionless plasma during magnetic reconnection in relativistic limit. Relativistic magnetic reconnection is driven by parallel laser pulses interacting with underdense plasma target. Annihilation of laser created magnetic field of opposite polarity generates strong non-stationary electric field formed in between the region with opposite polarity magnetic field accelerating charged particles within the current sheet. This laser-plasma target configuration is discussed in regard with the laboratory modeling of charged particle acceleration and gamma flash generation in astrophysics. We present the results of 3-dimensional kinetic simulations and theoretical studies on the formation and evolution of the current sheet in a collisionless plasma during magnetic field annihilation in the ultra-relativistic limit. Annihilation of oppositively directed magnetic fields driven by two laser pulses interacting with underdense plasma target is accompanied by an electromagnetic burst generation. The induced strong non-stationary longitudinal electric field accelerates charged particles within the current sheet. Properties of the laser-plasma target configuration are discussed in the context of the laboratory modeling for charged particle acceleration and gamma flash generation in astrophysics.

High-resolution stigmatic spectrograph for a wavelength range of 12.5-30 nm.

We describe a broadband (12.5-30 nm) extreme ultraviolet (XUV) spectrograph, which is stigmatic throughout its operating range. The instrument employs a near-normal-incidence aperiodic Mo/Si multilayer mirror and a grazing-incidence plane varied line-space (VLS) grating. Strict stigmatism is fulfilled simultaneously at two wavelengths and the condition of practical stigmatism is fulfilled over two octaves in wavelength. The vertically space-resolved line spectra of multiple charge ions from laser plasma were recorded to demonstrate a spectral resolving power of 103 and a spatial resolution of ~26 µm, both figures corresponding to two detector pixels. The electron density was evaluated from the Stark broadening of the Balmer line Hß (135 Å) of C VI in the plasma excited by 0.5 J, 8 ns, 1.06 µm pulses.

Active Plasma Lensing for Relativistic Laser-Plasma-Accelerated Electron Beams.

Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of 3000 T/m, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

Time-resolved extinction rates of stochastic populations.

Extinction of a long-lived isolated stochastic population can be described as an exponentially slow decay of quasistationary probability distribution of the population size. We address extinction of a population in a two-population system in the case when the population turnover-renewal and removal--is much slower than all other processes. In this case there is a time-scale separation in the system which enables one to introduce a short-time quasistationary extinction rate W1 and a long-time quasistationary extinction rate W2, and to develop a time-dependent theory of the transition between the two rates. It is shown that W1 and W2 coincide with the extinction rates when the population turnover is absent and present, but very slow, respectively. The exponentially large disparity between the two rates reflects fragility of the extinction rate in the population dynamics without turnover.

Implosion dynamics and x-ray generation in small-diameter wire-array Z pinches.

It is known from experiments that the radiated x-ray energy appears to exceed the calculated implosion kinetic energy and Spitzer resistive heating [C. Deeney, Phys. Rev. A 44, 6762 (1991)] but possible mechanisms of the enhanced x-ray production are still being discussed. Enhanced plasma heating in small-diameter wire arrays with decreased calculated kinetic energy was investigated, and a review of experiments with cylindrical arrays of 1-16 mm in diameter on the 1 MA Zebra generator is presented in this paper. The implosion and x-ray generation in cylindrical wire arrays with different diameters were compared to find a transition from a regime where thermalization of the kinetic energy is the prevailing heating mechanism to regimes with other dominant mechanisms of plasma heating. Loads of 3-8 mm in diameter generate the highest x-ray power at the Zebra generator. The x-ray power falls in 1-2 mm loads which can be linked to the lower efficiency of plasma heating with the lack of kinetic energy. The electron temperature and density of the pinches also depend on the array diameter. In small-diameter arrays, 1-3 mm in diameter, ablating plasma accumulates in the inner volume much faster than in loads of 12-16 mm in diameter. Correlated bubblelike implosions were observed with multiframe shadowgraphy. Investigation of energy balance provides evidence for mechanisms of nonkinetic plasma heating in Z pinches. Formation and evolution of bright spots in Z pinches were studied with a time-gated pinhole camera. A comparison of x-ray images with shadowgrams shows that implosion bubbles can initiate bright spots in the pinch. Features of the implosions in small-diameter wire arrays are discussed to identify mechanisms of energy dissipation.

Wire initiation critical for radiation symmetry in z-pinch-driven dynamic hohlraums.

Axial symmetry in x-ray radiation of wire-array z pinches is important for the creation of dynamic hohlraums used to compress inertial-confinement-fusion capsules. We present the first evidence that this symmetry is directly correlated with the magnitude of the negative radial electric field along the wire surface. This field (in turn) is inferred to control the initial energy deposition into the wire cores, as well as any current shorting to the return conductor.

Polarity effect for exploding wires in a vacuum.

Experimental evidence for a strong influence of the radial electric field on energy deposition into thin metal wires during their electrical explosion in vacuum is presented. Explosion of the metal wire with a positive polarity when the radial electric field "pushes" electrons into the wire results in twice as much deposited energy than with the negative polarity when the radial field "expels" electrons from the wires. Moreover, the axial structure of the deposited energy changes. This effect can be explained by the influence of radial electric field on electronic emission and on vapor breakdown along the wire surface.

Phase separation and coarsening in electrostatically driven granular media.

A continuum model for the phase separation and coarsening in electrostatically driven granular media is formulated in terms of a Ginzburg-Landau equation subject to conservation of the total number of grains. In the regime of well-developed clusters, the continuum model is used to derive "sharp-interface" equations that govern the dynamics of the interphase boundary. The model captures the essential physics of this system.

Simulations of a hydrogen-filled capillary discharge waveguide.

A one-dimensional dissipative magnetohydrodynamics code is used to investigate the discharge dynamics of a waveguide for high-intensity laser pulses: the gas-filled capillary discharge waveguide. Simulations are performed for the conditions of a recent experimental measurement of the electron density profile in hydrogen-filled capillaries [D. J. Spence et al., Phys. Rev. E 63, 015401 (R) (2001)], and are found to be in good agreement with those results. The evolution of the discharge in this device is found to be substantially different to that found in Z-pinch capillary discharges, owing to the fact that the plasma pressure is always much higher than the magnetic pressure. Three stages of the capillary discharge are identified. During the last of these the distribution of plasma inside the capillary is determined by the balance between ohmic heating, and cooling due to electron heat conduction. A simple analytical model of the discharge during the final stage is presented, and shown to be in good agreement with the magnetohydrodynamic simulations.