Biomedical subjects

M G Haines

Publications and source records attributed to M G Haines.

8 recordsLinked to original sources

PubMed2006

Dynamics of cylindrically converging precursor plasma flow in wire-array Z -pinch experiments.

This paper summarizes the present understanding of the processes leading to precursor column formation in cylindrical wire arrays on the 1 MA MAGPIE generator at Imperial College London. Direct experimental measurements of the diameter variation during the collapse and formation phase of the precursor column are presented, along with soft x-ray emission, and quantitative radiography. In addition, data from twisted cylindrical arrays are presented which give additional information on the behavior of coronal plasma generated in wire array z pinches. Three stages in precursor column formation are identifiable from the data: broad initial density profile, rapid contraction to small diameter, and slow expansion after formation. The correlation of emission to column diameter variation indicates the contraction phase is a nonlinear collapse resulting from the increasing on-axis density and radiative cooling rate. The variation in the minimum diameter is measured for several array materials, and data show good agreement with a pressure balance model. Comparison of column expansion rates to analytical models allows an estimate of column temperature variation, and estimates of the current in the column are also made. Formation data are in good agreement with both fluid and kinetic modeling, but highlight the need to include collisionless flow in the early time behavior.

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PubMed2006

Ion viscous heating in a magnetohydrodynamically unstable Z pinch at over 2 x 10(9) Kelvin.

Pulsed power driven metallic wire-array Z pinches are the most powerful and efficient laboratory x-ray sources. Furthermore, under certain conditions the soft x-ray energy radiated in a 5 ns pulse at stagnation can exceed the estimated kinetic energy of the radial implosion phase by a factor of 3 to 4. A theoretical model is developed here to explain this, allowing the rapid conversion of magnetic energy to a very high ion temperature plasma through the generation of fine scale, fast-growing m = 0 interchange MHD instabilities at stagnation. These saturate nonlinearly and provide associated ion viscous heating. Next the ion energy is transferred by equipartition to the electrons and thus to soft x-ray radiation. Recent time-resolved iron spectra at Sandia confirm an ion temperature Ti of over 200 keV (2 x 10(9) degrees), as predicted by theory. These are believed to be record temperatures for a magnetically confined plasma.

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PubMed2005

X-point collapse and saturation in the nonlinear tearing mode reconnection.

We study the nonlinear evolution of the resistive tearing mode in slab geometry in two dimensions. We show that, in the strongly driven regime (large delta'), a collapse of the X point occurs once the island width exceeds a certain critical value approximately 1/delta'. A current sheet is formed and the reconnection is exponential in time with a growth rate proportional eta(1/2), where eta is the resistivity. If the aspect ratio of the current sheet is sufficiently large, the sheet can itself become tearing-mode unstable, giving rise to secondary islands, which then coalesce with the original island. The saturated state depends on the value of delta'. For small delta', the saturation amplitude is proportional delta' and quantitatively agrees with the theoretical prediction. If delta' is large enough for the X-point collapse to have occurred, the saturation amplitude increases noticeably and becomes independent of delta'.

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PubMed2005

Effect of radial-electric-field polarity on wire-array Z-pinch dynamics.

The formation of plasma in wire-array Z-pinch experiments was found to depend upon the polarity of the radial-electric field near the wires. Reversing the radial-electric field midway along the length of an array resulted in the ablation rate of one-half of the array being reduced by 50%, significantly delaying the start of its implosion and altering its acceleration towards the axis. The observed phenomena cannot be explained by the standard magnetohydrodynamic models of array behavior, suggesting that effects such as electron emission may be important, especially during wire initiation.

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PubMed2004

Laboratory measurements of 0.7 GG magnetic fields generated during high-intensity laser interactions with dense plasmas.

We report measurements of ultrahigh magnetic fields produced during intense ( approximately 10(20) Wcm(-2) micro m(2) ) laser interaction experiments with solids. We show that polarization measurements of high-order vuv laser harmonics generated during the interaction (up to the 15th order) suggest the existence of magnetic field strengths of 0.7+/-0.1 GG in the overdense plasma. Measurements using higher order harmonics indicate that denser regions of the plasma can be probed. This technique may be useful for measurements of multi- GG level magnetic fields which are predicted to occur at even higher intensities.

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PubMed2002

Measuring huge magnetic fields.

Huge magnetic fields are predicted to exist in the high-density region of plasmas produced during intense laser-matter interaction, near the critical-density surface where most laser absorption occurs, but until now these fields have never been measured. By using pulses focused to extreme intensities to investigate laser-plasma interactions, we have been able to record the highest magnetic fields ever produced in a laboratory--over 340 megagauss--by polarimetry measurements of self-generated laser harmonics.

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PubMed2001

Measurement of instability growth in a magnetized Z pinch in the finite-Larmor-radius regime.

The development of the m = 0 instability in a Z pinch was followed and the measured growth rates compared with 2D MHD simulations. Where MHD is valid, the measured growth rates agree well with simulation. Where the ions are magnetized, i.e., where the ion-cyclotron frequency is smaller than the ion-collision frequency and the ratio of the ion Larmor radius to pinch radius is of the order of 0.1, the growth rate was smaller than expected by a factor of 2.5. This is as predicted by finite-Larmor-radius theory. The product of the wave number and the pinch radius was ka approximately 2pi and was the same for all conditions. Perturbations as large as 30% of the pinch radius were observed; no nonlinear saturation was evident.

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PubMed2001

Generation of an axial magnetic field from photon spin.

In circularly polarized light the spins of the photons are aligned. When a short intense pulse of circularly polarized laser light is absorbed by a plasma, a torque is delivered initially to the electron species, resulting primarily in an opposing torque from an induced azimuthal electric field. This electric field, in general, has a curl and leads to the generation of an axial magnetic field. It also is the main means for transferring angular momentum to the ions. The time-dependent magnetic field has a magnitude proportional to the transverse gradient of the absorbed intensity but inversely proportional to the electron density, in contrast to earlier theories of the inverse Faraday effect.

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