R. L. Morse

NUMERICAL SIMULATION OF THE WEIBEL INSTABILITY IN ONE AND TWO DIMENSIONS.

Particle‐in‐cell simulation methods for doing full electromagnetic simulations of collisionless plasma phenomena are described and applied to the Weibel instability in one and two dimensions. Magnetic particle trapping and subsequent mode coalescing are seen. Magnetic field energy is seen to reach 10% of the total particle energy. The different electromagnetic simulation methods are compared.

Thermonuclear burn characteristics of compressed deuterium‐tritium microspheres

The phenomenology of thermonuclear burn in deuterium‐tritium microspheres at high densities is described, and numerical results characterizing the burn for a broad range of initial conditions are given. The fractional burnup, bootstrap‐heating, and depletion of the DT fuel, its expansive disassembly, and thermonuclear ignition by propagating burn from central hot spots in the microspheres are discussed. Extensive numerical results from a 3 T Lagrangian simulation code are presented. The yields Y0 from uniform 10, 1, and 0.1 μg microspheres with densities ρ = 1 to 4 × 104 g/cm3 and temperatures Te = Ti = 1.8 to 100 keV are given. It is shown that Y0 ∼ ρR, ρR < 0.3 (R is the microsphere radius) or, equivalently, Y0 ∼ ρ2/3 for spheres of fixed mass m. The gain‐factor G0 ≡ Y0/mI0 (I0 is the internal energy) is shown to measure burn efficiency in uniform microspheres. More than a four‐fold increment in the gain factor is shown to derive from apportionment of the internal energy in a central hot spot. The limit...

Numerical Simulation of Warm Two‐Beam Plasma

One‐dimensional numerical simulation of plasmas consisting of two unequal warm beams has shown flattening of the small beam and, in most cases, a strong single mode structure at about the wavelength of largest linear growth rate, accompanied by an eddylike arrangement of trapped particles in phase space. The total electrostatic field energy first increases at the linear growth rate and then, after saturating abruptly, decreases at about the same rate to an intermediate level where a much slower decay begins.

Occurrence of high‐energy electrons and surface expansion in laser‐heated target plasmas

It is shown that limitations on collisionless electron thermal conduction place a lower limit on the range of energies of electrons in the surface of target plasmas which are heated by incident electromagnetic radiation, and that in cases of contemporary experimental interest these energies can be hundreds of keV. It is then shown that these high‐energy electrons can cause a much faster expansion of plasma from the heated surface than would be predicted by a single fluid theory. Numerical simulations demonstrate both effects.

Numerical Simulation of Axisymmetric, Collisionless, Finite‐β Plasma

A numerical simulation method for time‐dependent, axisymmetric, collisionless, finite‐β plasma is described and applied to mirror modes in θ pinches and to rotational and tearing modes in Astron. It is shown that the mirror modes cause an increase of axial thermal velocities and the long time development of a gross bumpy structure in the θ‐pinch plasma. The Astron plasma layer first develops tearing modes in agreement with linear theory, but at later times the wavelength of this mode increases by coalescence until the plasma is completely reassembled.

Structure and Scaling Laws of Laser-Driven Ablative Implosions

A stationary, spherical flow model gives the form of laser‐driven ablation fronts and scaling laws for the dependence of implosion parameters on laser wavelength, pusher atomic number, and other input quantities.

Coaxial Snowplow Discharge

The development of a coaxial plasma discharge in an initially uniform gas filling has been studied experimentally and theoretically. In the experiment, the usual outer coaxial electrode was replaced by a current return at the rear of the system, permitting unobstructed access to the discharge for the diagnostics. These diagnostics were image converter photography, magnetic probes, and holographic interferometry. The theory consists of two parts: two‐dimensional (axisymmetric) time‐dependent fluid computations and a supplementary analytic model, both of which assume that the vacuum magnetic field and the plasma are separated by a sharp interface; the experiments and computations are in good agreement on all gross features of the discharge leading up to the formation of a plasma focus and bubblelike structure in the shock front at the outer end of this focus.

Rigid Drift Model of High‐Temperature Plasma Containment

A model is proposed which appears to be consistent with certain recent high‐temperature plasma heating and confinement experiments, whose behavior is not adequately described by magnetohydrodynamics.

Hydrodynamics and burn of optimally imploded deuterium‐tritium spheres

The phenomenology of optimized laser‐driven DT sphere implosions leading to efficient thermonuclear burn is reviewed. The optimal laser deposition profile for spheres is heuristically derived. The performance of a 7.5 μg sphere, exposed to its optimal 5.3 kJ pulse, is scrutinized in detail. The timing requirements for efficient central ignition of propagating burn in the sphere are carefully explored. The difficulties stemming from superthermal electron production and thermal flux limitation are discussed. The hydro‐burn performance of spheres is characterized as a function of the pulse energy, peak power, time scale, pulse exponent, wavelength, and on the degree of flux limitation. The optimal pulse parameters are determined for spheres with masses ranging from 40 ng to 250 μg, requiring from 50 J to 150 kJ of input energy, and the corresponding optimal performance levels are calculated. Discussion is given to the hydro‐burn performance of new structured fusion targets, in which the DT is contained as a ...

Electron Cyclotron Drift Instability and Turbulence

The linear theory of the electron‐cyclotron drift instability, including off angle and collisional effects, is discussed in the context of applications to controlled fusion and collisionless shocks. With the help of numerical simulations it is then shown in detail that a strong nonlinear development of the instability causes anomalous resistance to current flow perpendicular to magnetic field lines, which is large enough to be consistent with the anomalous diffusion seen in these applications. In particular a repeated trapping and untrapping of electrons in combined electrostatic‐magnetic wells is an essential part of the nonlinear resistive heating process. A relatively steady resistance and turbulence level is seen, in contrast with the short burst of turbulence produced by many magnetic field free beam plasma instabilities.