I. M. Vitkovitsky

X radiation from high‐energy‐density exploded‐wire discharges

Exploded‐wire discharges of tungsten and titanium driven by a high‐power pulse generator have been used to produce intense x‐ray continuum and line radiation. A calibrated LiF crystal spectrograph recorded the radiation spectrum in the 3‐ to 25‐keV range. More than 20 J of x radiation are emitted in this photon energy band by tungsten plasmas in less than 50 nsec. The source of emission is less than 1 mm in diameter and about 3.5 cm long.

Recovery characteristic of exploding wire fuses in air and vacuum

The dielectric strength of exploded wire fuses during the vaporization and after recovery period and fuse resistivity are functions of time and depend on the medium surrounding the fuse channel. When fuses are used as opening switches in inductive storage systems, the above parameters determine the efficiency of power flow from the storage to the load. Specifically, design of experiments using loads with variable impedance, such as imploding plasma, requires detailed information on the fuse characteristics during the time when the load impedance is changing. To provide data that determine the interaction between fuses and variable impedance loads, inductive and recovery electric fields and fuse resistance in air and vacuum were studied. The results show that inductive field amplitude follows the dependence on time needed to vaporize the fuse in a manner similar to that established for fuses in other media. The characteristics of recovery rates of fuses in air and vacuum differ drastically due to the early...

PROPAGATION OF AN INTENSE RELATIVISTIC ELECTRON BEAM THROUGH A PLASMA BACKGROUND.

Experiments have been performed to study the propagation of an intense relativistic electron beam [input (ν/γ)≃2.5] in a drift tube filled with either neutral gas or preexisting plasma. Beam front velocity, magnetic neutralization, and energy transport are measured. Results for propagation into neutral gas confirm previous work. Propagation into a preionized background is characterized by nearly complete magnetic neutralization. However, the beam front velocity never exceeds 60% of its potential value, and a two‐stream instability is suggested as a possible explanation. The efficiency of beam energy propagation through the plasma is found to depend on the neutral background density, and a possible cause of this effect is suggested. Hollow beam profiles are observed under suitable conditions.

Inductively generated, high voltage pulse using an electron beam controlled opening switch

An electron beam controlled opening switch with a conduction period of ∼1 μs has been used in an inductive store system to generate a 280‐kV, 60‐ns full width at half‐maximum voltage pulse across an open circuit by interrupting a 10‐kA discharge. The switch was pressurized to 5 atm with a 99:1 mixture of CH4:C2F6. Degraded voltage performance was obtained with pure CH4 or N2. These results support the conjecture that a judicious choice of gas mixture can lead to optimization of such an opening switch in a parameter regime of interest to pulsed power applications.

Neutron Production by Hot Dense Plasmas Generated with High Power Pulsers

Neutron production from plasmas generated by relativistic electron beam and exploding wire discharges is surveyed. Diagnostics appropriate to distinguishing neutrons produced by acceleration mechanisms from thermonuclear sources are discussed. The results of applying these techniques to exploding wire plasmas indicate that the dominant neutron source is thermonuclear fusion.

Dense plasma discharges for solid‐target heating

Dense plasma channels formed by exploding‐wire discharges have been used to form very‐high‐current‐density electron beams. The interaction of the beam with the plasma lowers the mean electron energy relative to that to be gained in the accelerating potential of 106 V applied across the plasma. The current density of electrons incident on the anode and their energy distribution have been determined. Approximately 10% of the current at the anode contains electrons with energy greater than 25 keV. The current density associated with these electrons exceeds 107 A/cm2. The power delivered to the anode approaches 1012 W/cm2, which leads to strong anode heating. X‐ray spectroscopy of the radiation emitted from the anode and of neutron production in deuterated anodes was used to deduce a target temperature of 0.1–1.0 keV.

Interaction of Accelerating High‐Current Electron Beams with External Magnetic Fields

When the current and current density of electrons accelerated in the anode‐cathode gap of a field emission tube reach sufficiently high values, the induced self‐magnetic field begins to dominate the electron trajectories. As a result, the spatial distribution of electrons arriving at the anode becomes strongly peaked in the center of the anode. It has been found possible to prevent such beam collapse with relatively modest external magnetic fields. The field is applied parallel to the electron flow (perpendicular to the anode plane). A scaling law which determines the necessary field strength based on a simple orbit model is found to agree with experimental results over a wide range of electron‐beam parameters.

High recovery voltage switch for interruption of large currents

An opening switch for interrupting 50‐KA currents in inertial–inductive storage systems has been developed. A switch arc voltage of 2 kV (3.3 kV/cm) has been obtained using explosive cutting of the current‐carrying sheet. A single‐gap switch interrupts a current of 7 kA per cm of sheet width in ∼10 μs. Recovery fields of up to 100 kV/cm were achieved at 2 kA/cm, at 35 μs after interruption. The switch is designed for long (∼1 s) current‐carrying time associated with homopolar generator charging of inductors.

Pulsed high‐voltage and high‐current outputs from homopolar energy storage system

Pulsed energy source with a versatile output, using self‐excited homopolar generator for the initial storage of energy, has been developed. Large energy storage of this inertial–inductive system provides an attractive option for satisfying pulse power requirements associated with such applications as plasma confinement and heating, electromagnetic acceleration of projectiles, and with production of intense radiation. These applications require high rate of energy delivery to the load at specific current and voltage levels. Megajoule output pulses, alternatively, at hundreds of kilovolts or at megampere levels, have been obtained by incorporating unique current interrupting system. The overall pulser efficiency, which depends sensitively on the load characteristics was measured over a range from 10% to more than 90% for different pulser‐load circuit arrangements.

X‐Ray Emission from Exploding Wires

It is shown that the soft x radiation emitted from hollow exploding wires emanates from a region of the explodingwire plasma whose electron temperature is a few kev. The results were obtained from hollow aluminum cylinders that were exploded using a low-inductance storage system capable of operation at very high voltage (400 kev). (R.E.U.)