Michael MacDonald

Developed turbulence and nonlinear amplification of magnetic fields in laboratory and astrophysical plasmas

Significance Magnetic fields exist throughout the universe. Their energy density is comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter in the universe. The origin and the amplification of these magnetic fields to their observed strengths are far from being understood. The standard model for the origin of these galactic and intergalactic magnetic fields is through the amplification of seed fields via turbulent processes to the level consistent with current observations. For this process to be effective, the amplification needs to reach a strongly nonlinear phase. Experimental evidence of the initial nonlinear amplification of magnetic fields is presented in this paper. The visible matter in the universe is turbulent and magnetized. Turbulence in galaxy clusters is produced by mergers and by jets of the central galaxies and believed responsible for the amplification of magnetic fields. We report on experiments looking at the collision of two laser-produced plasma clouds, mimicking, in the laboratory, a cluster merger event. By measuring the spectrum of the density fluctuations, we infer developed, Kolmogorov-like turbulence. From spectral line broadening, we estimate a level of turbulence consistent with turbulent heating balancing radiative cooling, as it likely does in galaxy clusters. We show that the magnetic field is amplified by turbulent motions, reaching a nonlinear regime that is a precursor to turbulent dynamo. Thus, our experiment provides a promising platform for understanding the structure of turbulence and the amplification of magnetic fields in the universe.

Compound refractive lenses as prefocusing optics for X-ray FEL radiation

A prefocusing compound refractive lens was implemented for the Matter under Extreme Conditions Instrument at the Linac Coherent Light Source. A significant improvement in the beamline transmission was calculated and observed at 5 keV.

Observations of the Magnetized Disruption of Collimated Plasma Flows

The dynamics of magnetized flows is of great interest to the astrophysics community as the formation and long collimation distances of jets in accretion systems are still open questions. In many of these systems, the background magnetic field is parallel to the jet propagation direction. Recent experiments [1] performed at the Jupiter Laser Facility investigated the effects of imposing a background magnetic field aligned with a collimated jet. Plastic cone targets were irradiated by a long-pulse laser as shown schematically in Fig. 1a. When the shock emerges from the backside of the cone, accelerated material accumulates on axis producing a collimated flow. Figure 1b demonstrates the collimation of the plasma without the background field and the disruption of the flow when applying a 5 T field. Experimental results will be discussed in detail with supporting numerical work describing the mechanisms causing the jet disruption.

Reviews : Anton J. L. van Hooff, From Autothanasia to Suicide: Self-Killing in Classical Antiquity. London: Routledge, 1990. £35.00

the Bloomsbury Group. (My own first analyst was Adrian Stephen, then elderly and frail). But after that, we are in deep waters, where the writings of Virginia Woolf are subjected to a very close analysis in a language which presented me as the ’general reader’ with so much difficulty that it became off-putting. The book appears in a series entitled, ’Women in Culture and Society’ and is written in a language of feminism, feminist psychoanalysis and contemporary literary criticism. To those familiar with this style of discourse, it will be accessible, but this must be a limited readership. Elizabeth Abel states that Virginia Woolf shared an historical moment with Sigmund Freud and Melanie Klein, but that she is less concerned with influence than with intertextuality although the distinction seems arbitrary and should be argued, and not simply stated. She argues that, together with Melanie Klein, in the 1920s, Virginia Woolf offered a deep, visionary matricentric alternative to Freud’s patricentric view of the development of culture, but that in the 1930s, she turned away from this, back to the father, as she feared that the fascist celebration of