No abstract

C:0ntrolled, ultrahigh axial :naglletic fields have been produced and measured in a gas-puff Z pmch. A O.S-MA. 2-cm-radlus annular gas-puff Z pinch with a 3-min repetition rate was imploded radially onto an axial seed field, causing the field to compress. Axial magnetic field com~ressions up to 180 and peak ~agnetic fields up to 1.6 MG were measured. Faraday rotation of an Argon laser (5154 A) in a quartz fiber on-axis was the principal magnetic field diagnostic. Other diagnostics included a nitrogen laser interferometer, x-ray diodes, and magnetic field probes, The magnetic field compression results are consistent with simple snowplow and self-similar analytic models, which are presented here. Even small axial fields help stabilize the pinches, some of which exhibit several stable radial bounces during a current pulse. The method of compressing axial fields in a gas-puff Z pinch is extrapolable to the order of 100 MG. Scaling laws are presented. Potential applications of ultrahigh axial fields in Z pinches are discussed for x~ray lasers, inertial confinement fusion, gamma-ray generators, and atomic physics studies. 3831

Detailed spatial and temporal measurements of the total vector electric field E and current density J in a plasma with dynamic magnetic field line reconnection have been made. The resistivity calculated via the generalized Ohm's law is found to be spatially inhomogeneous with values exceeding the classical resistivity by 1 to 2 orders of magnitude. Resistivity and current density do not maximize in the same locations. The dissipation E · J is determined and analyzed in terms of particle heating and fluid acceleration. Electron heating is found to be the dominant dissipation process in the diffusion region. Independent measurements of the divergence of the Poynting vector ▽ · (E × H), the change in stored magnetic field energy ∂/∂t(B²/2µo), and the dissipation give a consistent, detailed picture of the energy flow. Efficient conversion of electromagnetic energy into particle heating is observed.

In a large laboratory plasma a current sheet is generated in the process of magnetic field line reconnection. The stability of the sheet with respect to local current increases is investigated. When the current density in the center of the sheet exceeds a critical value, spontaneous local current disruptions are observed. The current from the center of the sheet moves out to the sides. Magnetic flux variations in regions remote from the current sheet generate an inductive voltage in the current loop that drops off inside the plasma in the form of a potential double layer. This leads to particle acceleration with velocities much larger than those expected from the steady state electric fields in the plasma. The particle beams acquire their energy at the expense of the stored magnetic field energy of the current system. Beam‐plasma instabilities are generated that dissipate some of the directed kinetic energy and heat the background plasma. A model for the mechanism of the current disruptions is formulated. The potential structure leads to ion expulsion creating a localized density drop. The associated current drop in an inductive circuit drives the potential structure, thereby providing feedback for the disruptive instability. It saturates at a total current loss upon which the current system recovers, and the process repeats randomly. Similarities and differences to magnetospheric substorm phenomena are pointed out.

A directional velocity analyzer has been developed for measuring electron distribution functions in plasmas. It contains a collimating aperture which selects particles from a narrow cone in velocity space and a retarding potential analyzer. The distribution function f(v, θ, φ) is obtained from a large number of analyzer traces taken at different angles θ, φ. In addition, the small analyzer can be moved in space and the measurements are time resolved so as to obtain the complete phase space information f (v, r, t). The large data flow of this seven-variable function is processed with a high-speed digital data-acquisition system. The new electron velocity analyzer is applicable over a wide parameter range in electron energies and densities. Various cases of anisotropic distributions such as beams, shells, tails, and drifts have been successfully investigated.