Dynamics of exploding plasmas in a magnetic field

Abstract

The expansion of laser-ablation plasmas in a magnetic field is studied with a new Faraday-rotation magnetic imaging probe and Fourier-analyzed optical plasma images over a wide range of ion magnetization. Plasma instabilities are observed during the plasma expansion which evolve from short to long wavelengths and significantly affect the magnetic structure.PACS numbers: 52.35.Qz, 52.35.Py, 52.50.Lp, 52.55.Lf The collisionless expansion of energetic plasmas in a magnetic field is a rich and complicated phenomenon occurring in laboratory and astrophysical environments. Initially, a diamagnetic cavity forms and expands until the plasma and magnetic pressures equalize. The magnetic containment radius is of order Rtj=(3EkjHo/ ITCBQ) '^^ in mks units, where JHQ is the free-space permeability, Bo is the magnetic field, and Ek is the plasma kinetic energy. As the plasma decelerates, Rayleigh-Taylor or lower-hybrid-drift-(LHD-) type instabilities grow to large amplitude, producing flutes which extend well beyond the main plasma cloud. The electrons are magnetized and carry most of the plasma current, whereas the ions supply the kinetic energy and are not necessarily magnetized. Thus, an important dynamical parameter is the directed ion Larmor radius p, relative to /?/,.Complex plasma cavities have been investigated in the laboratory [1-4], the ionosphere [5], and near the Earth's bow shock [6]. The maximum radius of the plasma cloud Rp was found to increase with Ek and decrease with ^o, consistent with magnetic containment [2], but with Rp--0,5Rb.Jets extending beyond the main plasma cloud were observed and associated with the magnetic deceleration [1,3,4]. The wave number k perpendicular to B of the flutes increased with ^o in Japanese experiments [3], whereas k was insensitive to Bo in NRL experiments [4]. According to linear LHD theory [7], the wave number of the most unstable mode A: max and the associated frequency both increase with ^o, but their calculated values are much larger than observed. Similar discrepancies occurred in the AMPTE (active magnetospheric particle tracer explorer) chemical release experiments [5]. The shortcomings of linear theory have motivated particle simulations [8] and nonlinear theories of the plasma evolution [9].To clarify the discrepancies in the containment radius and instability wavelengths, we have experimentally investigated the expansion of laser-ablation plasmas in a magnetic field. We developed a magneto-optic imaging probe (MIP) using Faraday rotation to measure the magnetic profile continuously in radius and time. Plasma instabilities are investigated by Fourier analyzing digitized optical images. The wave-number spectrum and the magnetic profile are found to evolve significantly during the plasma expansion. The role of ion magnetization is studied by varying E^, BQ, the ion species, and expansion velocity.The experiment is shown schematically in Fig. 1. We use the Janus laser at LLNL with energy Ej < 200 J at 1.06 jum and a 25-ns pulse length. Counterstreaming beams are fo...

Optical emission in magnetically confined laser-induced breakdown spectroscopy

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Laboratory experiments on plasma jets in a magnetic field using high-power lasers

“…Additionally, the plasma pressure was ∼5 × 10 7 kg/s 2 m, and the magnetic pressure was 1 × 10 4 kg/s 2 m. The plasma , the ratio between the plasma and magnetic pressures was 1. In this region of 1, the magnetic field is pushed by plasma flow, and in the region of high magnetic Reynolds number, the magnetic field is frozen and considered to move with plasma flow [10]. In our experiment, the plasma flow moved with a magnetic field, and distorted the magnetic field along the plasma flow.…”

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).…”