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...
Experiments have been developed using high powered laser facilities to study the response of materials in the solid state under extreme pressures and strain rates. Details of the target and drive development required for solid-state experiments and results from two separate experiments are presented. In the first, thin foils were compressed to a peak pressure of 180 GPa and accelerated. A pre-imposed modulation at the embedded Rayleigh–Taylor unstable interface was observed to grow. The growth rates were fluid-like at early time, but suppressed at later time. This result is suggestive of the theory of localized heating in shear bands, followed by conduction of the heat into the bulk material, allowing for recovery of the bulk material strength. In the second experiment, the response of Si was studied by dynamic x-ray diffraction. The crystal was observed to respond with uni-axial compression at a peak pressure 11.5–13.5 GPa.
Magnetic focusing of a uniform intense proton beam produced by a new magnetically insulated diode has been demonstrated using a single dipole coil. The new diode produces an intense (80 A/cm2 at 350 keV), low divergence (half-angle less than 4°), uniform proton beam that can be injected into a field-free vacuum drift region. Prompt space-charge neutralization by low energy electrons occurs almost immediately after the beam emerges from the diode. Experimental observations of the beam focal properties are in reasonable agreement with numerical calculations of the particle orbits and show that the protons follow essentially single particle trajectories. Calculations also show that for ion currents approaching the ion Alfvén current a net azimuthal diamagnetic current in the region of the dipole lens causes the beam defocusing. Finally, calculations and experimental observations indicate that the beam angular divergence may provide the most serious limitation on beam focusing for pellet fusion.
A magnetically insulated diode has been used to produce cylindrically converging intense proton beams. By providing electron neutralization along field lines, the beams can be propagated across the magnetic field to within 1 cm of the axis. Proton currents up to 5 kA have been propagated to achieve current densities up to 70 A/cm2. Divergences less than 3° have been achieved with new plasma anode designs. Calculations are presented on the extrapolation of magnetic diodes to achieve power densities needed for ion-beam pellet-fusion breakeven.
The theoretically favorable plasma-confinement properties of field-reversed magnetic field configurations have led to many reactor proposals. 1 " 5 Such configurations have been experimentally realized (i) by the injection of relativistic electron beams 6 to form a reversed-field electron ring, 7 (ii) by plasma currents induced by relativistic-electron-beam injection, 8 and (ill) by reversed-field 0 pinches. 9 However, synchrotron-radiation energy losses make a relativistic electron ring unsuitable for a fusion reactor. Indeed, Christofilos amended his original Astron concept 1 by replacing electrons with high-energy protons. Developments in intense-ion-beam 10 " 13 technology make it possible for a reversed-field ion ring to be produced by single-pulse injection. 2 This has led to two experimental programs 12 * 13 aimed at the injection and trapping of an ion ring in a mirror magnetic field.In this Letter, we report experimental results on the production and propagation of a rotating beam of the type required for ion-ring formation.
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