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A new coaxial plasma gun is described. The long term objective is to accelerate 100-200 microg of plasma with density above 10(17) cm(-3) to greater than 200 km/s with a Mach number above 10. Such high velocity dense plasma jets have a number of potential fusion applications, including plasma refueling, magnetized target fusion, injection of angular momentum into centrifugally confined mirrors, high energy density plasmas, and others. The approach uses symmetric injection of high density plasma into a coaxial electromagnetic accelerator having an annular gap geometry tailored to prevent formation of the blow-by instability. The injected plasma is generated by numerous (currently 32) radially oriented capillary discharges arranged uniformly around the circumference of the angled annular injection region of the accelerator. Magnetohydrodynamic modeling identified electrode profiles that can achieve the desired plasma jet parameters. The experimental hardware is described along with initial experimental results in which approximately 200 microg has been accelerated to 100 km/s in a half-scale prototype gun. Initial observations of 64 merging injector jets in a planar cylindrical testing array are presented. Density and velocity are presently limited by available peak current and injection sources. Steps to increase both the drive current and the injected plasma mass are described for next generation experiments.

The Maryland Centrifugal Experiment MCX [R. F. Ellis, A. B. Hassam, and S. Messer, Phys. Plasmas 8, 2057 (2000)] studies supersonic rotation and enhanced confinement produced by the application of an electric field perpendicular to an axial confining mirror magnetic field; radial shear in the rotation is predicted to stabilize magnetohydrodynamic (MHD) interchange modes. The MCX mirror field is 2.6 m in length, maximum mirror field 1.9 T, maximum midplane field 0.33 T; an inner coaxial core is driven by a 10 KV capacitor bank, producing the radial electric field which drives azimuthal rotation. MCX produces high density (n>1020m−3) fully ionized plasmas and has two operating modes. In the O (ordinary) mode the plasma rotates supersonically with azimuthal velocities in the range of 100 km/s for discharge times exceeding 8 ms. Ion temperatures are ∼30eV and momentum confinement times 100–200 μs. Sonic Mach numbers (uφ∕vti) in the range 1–2 and Alfvén Mach numbers (uφ∕vA)∼0.3 have been achieved for O mode discharges which remain steady for many milliseconds, much longer than MHD instability time scales; plasma lifetime is limited by the capacitance of the capacitor bank. MCX also has an enhanced mode of operation [higher rotation (HR) mode] with higher rotation velocities (>200km∕s), sonic Mach numbers greater than 3, Alfvén Mach numbers >∼0.5, and momentum confinement times of several hundred microseconds. HR mode occurs at higher B fields and lower discharge currents but is transient, transitioning to O mode after a few milliseconds. Both O and HR mode show spectroscopic evidence of radial velocity shear sufficient to satisfy the simplest criterion for MHD stability, but both modes also show significant fluctuations on magnetic probes.

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Order-of-magnitude anomalously high intensities for two-electron (dielectronic) satellite transitions, originating from the He-like 2s(2) 1S0 and Li-like 1s2s(2) (2)S(1/2) autoionizing states of silicon, have been observed in dense laser-produced plasmas at different laboratories. Spatially resolved, high-resolution spectra and plasma images show that these effects are correlated with an intense emission of the He-like 1s3p 1P-1s(2) 1S lines, as well as the K(alpha) lines. A time-dependent, collisional-radiative model, allowing for non-Maxwellian electron-energy distributions, has been developed for the determination of the relevant nonequilibrium level populations of the silicon ions, and a detailed analysis of the experimental data has been carried out. Taking into account electron density and temperature variations, plasma optical-depth effects, and hot-electron distributions, the spectral simulations are found to be not in agreement with the observations. We propose that highly stripped target ions (e.g., bare nuclei or H-like 1s ground-state ions) are transported into the dense, cold plasma (predominantly consisting of L- and M-shell ions) near the target surface and undergo single- and double-electron charge-transfer processes. The spectral simulations indicate that, in dense and optically thick plasmas, these charge-transfer processes may lead to an enhancement of the intensities of the two-electron transitions by up to a factor of 10 relative to those of the other emission lines, in agreement with the spectral observations.

In diagnosing the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. of Plasmas 8, 2057 (2001)], earlier spectroscopic measurements of averaged plasma rotation velocities have been upgraded to include radial distributions, using a five-channel fiber-optic collection system. Detailed information from each view is now possible with an 8-times increase in spectral resolution, by using a 2m spectrograph and a 2400lines∕mm grating. Inversion of the integrated chordal radiation into a radial dependence of local emissions is performed by two methods: (a) an iterative simulation beginning with assumed emissions in five axially concentric cylindrical zones followed by summation along the five viewing chords, and (b) inversion of a combination of dual Abel-type matrices. The radial profiles of the absolute velocities derived cover a range from 20to70km∕s for both C+ and C++ impurity ions. Previous apparent differences in velocities between ions from a single chordal observation are now explained by the measured radial dependence of velocities and relative emissions. An important result is the first direct and quantitative measurement on MCX of a radial shear in rotational flow velocity as large as 9×105s−1, 9 times a threshold of 1×105s−1 for magnetohydrodynamic stability. Stark-broadened hydrogen Balmer-series spectral lines provide both a value for electron density of Ne=(8.5±1.5)1014cm−3 and supporting data for radial particle distributions.