Large-Larmor-radius interchange instability

Abstract

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...

Plasma penetration of the dayside magnetopause

“…Limits of the cross-field width of plasmoids that can penetrate through polarization appear in the literature 45 , and large plasmoids have been seen to break up into smaller plasmoids in laboratory experiments 16 . It is therefore interesting to estimate the width of the plasmoids that are observed here.…”

“…Experimental studies showed that plasmoids can penetrate magnetic fields, while the relative change of the magnetic field, caused by the plasmoid, remains small even for large values of the kinetic beta β k = W K /(0.5µ 0 B 2 ) 14 . Experiments with laser produced plasmas have shown that wide plasmoids can break up into smaller plasmoids as they move across the field 16 , and this has been attributed to the Rayleigh-Taylor instability 17 . In active space experiments, where ion beams were injected in the ionosphere, both polarization fields and magnetic field-aligned electric fields were observed 18 .…”

Collisionless interaction of an energetic laser produced plasma with a large magnetoplasma

“…Previous laboratory experiments used magneticpinch plasmas (e.g. Paul et al 1967;Stamper and DeSilva 1968;Keilhacker et al 1969;Martone and Segre 1970;Mourenas et al 2003) or laser-produced plasmas (Tsuchimory et al 1968;Borovsky et al 1984;Ripin et al 1987;Dimonte and Wiley 1991;Ditmire et al 2000), creating the first test beds where observations and models could be compared. Although limited in size and duration at scales that allow for a detailed study of shock formation and particle acceleration, these pioneering experiments made a valuable progress in understanding various aspects of collisionless shock physics (see reviews in Drake 2000 andZakharov 2003).…”

Coupling Coefficients for Ion‐Cyclotron Alfvén Waves

“…MHD waves with frequencies which are not necessarily smaller than the ion gyrofrequency w C i (EMIC waves) have been observed in astrophysical [ 11 and cometary plasmas [2], the polar cusp [3] as well as in laser-blowoff experiments [4]. As the amplitudes of these waves can be large [3] it may be of general interest to estend previous nonlinear theories for waves with frequencies with w < wCi to the frequency range where w can be comparable to, or even larger than, the ion gyrofrequency.…”