Gauss's principle of least constraint is used to develop nonequilibrium molecular-dynamics algo rithms for systems SUbject to constraints. The treatment not only includes "nonholonomic" constraints-those involving velocities-but it also provides a basis for simulating nonequilibrium steady states. We describe two applications of this new use of Gauss's principle. The first of these examples, the isothermal molecular dynamics of a three-particle chain, can be treated analytically. The second, the steady-state diffusiort~of a Lennard-Jones liquid, near its triple point, is studied nu merically. The measured diffusion coefficient agrees with inaependent estimates from eqUilibrium fluctuation theory and from Hamiltonian external fields.

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Supersonic, counterstreaming plasmas were produced by ablating plasma from the inside surfaces of two parallel disks made of aluminum and magnesium, respectively, with a 0.53 pm laser at an intensity of lOi W/cm2 for 1.3 nsec. Diagnostics included holographic interferometry, a time-integrated x-ray pinhole camera and a gated x-ray crystal spectrograph with imaging slits. The plasmas interpenetrate for the first half of the laser pulse but stagnate once the electron density exceeds 5 X 102' cmF3. Spectroscopic measurements suggest a coronal electron temperature of -800 eV and an ion temperature of -15 keV in the stagnated plasma. The observations are in good agreement with a two ion fluid model of interpenetrating plasmas in which the dominant slowing down process is ion-ion collisions.

We have proposed and demonstrated successfully a new approach for generating high-yield K-shell radiation with large-diameter gas-puff Z pinches. The novel load design consists of an outer region plasma that carries the current and couples energy from the driver, an inner region plasma that stabilizes the implosion, and a high-density center jet plasma that radiates. It increased the Ar K-shell yield at 3.46 MA in 200 ns implosions from 12 cm initial diameter by a factor of 2, to 21 kJ, matching the yields obtained earlier on the same accelerator with 100 ns implosions. A new "pusher-stabilizer-radiator" physical model is advanced to explain this result.

In thermal-barrier experiments in the tandem mirror experiment upgrade, axial confinement times of 50 to 100 ms have been achieved. During enhanced confinement we measured the thermal-barrier potential profile using a neutral-particle-beam probe. The experimental data agree qualitatively and quantitatively with the theory of thermal-barrier formation in a tandem mirror.