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A large ion Larmor radius plasma undergoes a particularly robust form of Rayleigh–Taylor instability when sub-Alfvénically expanding into a magnetic field. Results from an experimental study of this instability are reported and compared with theory, notably a magnetohydrodynamic (MHD) treatment that includes the Hall term, a generalized kinetic lower-hybrid drift theory, and with computer simulations. Many theoretical predictions are confirmed while several features remain unexplained. New and unusual features appear in the development of this instability. In the linear stage there is an onset criterion insensitive to the magnetic field, initial density clumping (versus interchange), linear growth rate much higher than in the ‘‘classic’’ MHD regime, and dominant instability wavelength of order of the plasma density scale length. In the nonlinear limit free-streaming flutes, apparent splitting (bifurcation) of flutes, curling of flutes in the electron cyclotron sense, and a highly asymmetric expansion are found. Also examined is the effect on the instability of the following: an ambient background plasma (that adds collisionality and raises the expansion speed/Alfvén speed ratio), magnetic-field line tying, and expansion asymmetries (that promotes plasma cross-field jetting).

Laser-plasma Magnetic fields Faraday rotation Laser reflection ABSTRACT (Continue on revetee e!de II nmcmeemry and Identity by block number;Magnetic fields in the magagauss range have been observed in the laser-produced plasma near the focus of a high power laser pulse. Faraday rotation measurements, utilizing the light of a probing beam and the specularly reflected laser light, both show the presence of these large fields.

We observe linear and nonlinear features of a strong plasma-magnetic-field interchange RayleighTaylor instability in the limit of large ion Larmor radius. The instability undergoes rapid linear growth culminating in free-streaming flute tips.PACS numbers: 52.35. Gz, 52.35.Py, 52.50.Lp, 52.55.Lf Plasma expanding into a magnetic field can undergo Rayleigh-Taylor or interchange instability as the heavy fluid (plasma) is decelerated by the light fluid (magnetic field). 1>2 Direct observations of this instability have been made in the limit of small ion Larmor radius (compared to density gradients and wavelengths), 3 where conventional MHD theory applies. When the ion Larmor radius becomes finite the instability is predicted to stabilize. 4 However, when the ion Larmor radius becomes large compared to other characteristic plasma dimensions, i.e., when the ions are effectively unmagnetized but the electrons are effectively magnetized, a related instability is predicted with an even higher growth rate than that of the original MHD instability. 5 The recent barium-release space experiment with the Active Magnetospheric Particle Tracer Explorer satellite, which showed substantial structure, was in such a regime. 6 A previous laser-plasma experiment in a regime of moderate-sized ion Larmor radius also measured instability growth. 7 In this paper, we observe a robust interchangelike instability in the limit of very large ion Larmor radius. The instability exhibits a rapid linear phase with subsequent nonlinear free-streaming flutes and examples of density clumping, flute-tip bifurcation, and interesting late-time spirallike structures.Our experiment is comprised of an energetic laserproduced plasma expanding radially outward into a uniform magnetic field B formed by a pair of Helmholtz coils, 8 as depicted in Fig. 1. Steady-state (on the time scale of the experiment) vacuum B fields from 0 to 1 T are used. Plasma bursts are created by our focusing a beam of the Pharos III neodymium laser onto small Al (2 jum thick, 1 mm diam) disk targets. Unless noted otherwise, the nominal laser pulse has an irradiance of about 10 13 W/cm 2 , 30 J of energy, and 3-ns duration (FWHM). The principal diagnostic used to measure the plasma and instability development is a Grant Applied Physics fast-gated microchannel-plate optical camera focused onto the target midplane antiparallel (usually) to the magnetic field lines. Shutter speeds of 1 or 2 ns are used. In addition to the gated camera, we also used ion time-of-flight detectors to measure the plasma ion velocity distribution, several small (230 jj.m diam, two turn) magnetic induction probes to obtain magnetic field dynamics, small Langmuir and capacitive probes to measure density gradients and fluctuations, open-shutter photography and witness plates to see persistent structure, and fiber-optic spectroscopy to estimate density profiles during the plasma/magnetic field interaction.The velocity distribution of the expanding plasma, measured for B =0 with an ion time-of-flight detector, pe...

Highly collimated plasmas jets are produced with laser irradiation of solid barium targets. The plasma streams many Larmor radii across a strong transverse magnetic field (10 kG) with little inhibition. The plasma jet is observed to narrow or "focus" in the plane perpendicular to the field, while in the plane of the field the plasma expands along the field lines and displays flutelike striations. The narrowing of the plasma jet is understood in terms of the configuration of the plasma polarization fields, while the flute structure is identified as an electron-ion hybrid velocity-shear instability.

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The momentum, energy, and velocity characteristics of plasma ablating from planar targets irradiated by long Nd-laser pulses (4 ns,<1014 W/cm2) are measured and the dependence of ablation parameters upon absorbed irradiance is determined. Large laser spots are used in these experiments so that the results are not sensitive to boundary effects.

High-intensity laser irradiation of hollow glass cylinders immersed in a magnetic field results in plasma expansions strongly collimated in the direction transverse to both the initial flow and the magnetic field, but jetlike in the direction parallel to the initial flow. Magnetic fields from B=0 kG to B=10 kG produced plasmas with markedly different geometrical features. Fast framing camera photographs show the plasmas propagating across magnetic field lines and undergoing structuring indicative of transverse velocity shear-driven instabilities. Comparison is made between the observed instability characteristics and predictions of Rayleigh–Taylor, classical Kelvin–Helmholtz, and the electron–ion hybrid instabilities. Only the electron–ion hybrid instability is consistent with the experimental results.