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We describe a direct magneto-optical approach to measuring the magnetic field driven by a narrow pulse width (<10 ns), 20 kA electrical current flow in the transmission line of a high energy pulsed power accelerator. The magnetic field and electrical current are among the most important operating parameters in a pulsed power accelerator and are critical to understanding the properties of the radiation output. However, accurately measuring these fields and electrical currents using conventional pulsed power diagnostics is difficult due to the strength of ionizing radiation and electromagnetic interference. Our approach uses a fiber coupled laser beam with a rare earth element sensing crystal sensor that is highly resistant to electromagnetic interference and does not require external calibration. Here, we focus on device theory, operating parameters, results from an experiment on a high energy pulsed power accelerator, and comparison to a conventional electrical current shunt sensor. [ABSTRACT FROM AUTHOR]

Field reversed theta pinch technology is employed with programmed cusp fields at the theta coil ends to form high-density field-reversed configuration (FRC) plasmas. The well-formed FRC plasmas have volume-averaged density of 2 - 4 X [10.sup.22] [m.sup.-3], total temperature ([T.sub.e] + [T.sub.i]) of 300-500 eV, and plasma lifetime between 10-20 [micro]s in 50-70 mtorr of deuterium static gas fill. The achieved FRC parameters are very close to the desired target plasma requirements for magnetized target fusion. Index Terms--Field-reversed configuration, fusion reactors, magnetized target fusion, plasma confinement, plasma generation, plasma measurements, plasma pinch, plasmas.

We describe a program to demonstrate the scientific basis of magnetized target fusion (MTF). MTF is a potentially low-cost path to fusion which is intermediate in plasma regime between magnetic (MFE) and inertial fusion energy (IFE). MTF involves the compression of a magnetized target plasma and pressure times volume (PdV) heating to fusion relevant conditions inside a converging flux conserving boundary. We have chosen to demonstrate MTF by using a field-reversed configuration (FRC) as our magnetized target plasma and an imploding metal liner for compression. These choices take advantage of significant past scientific and technical accomplishments in MFE and defense programs research and should yield substantial plasma performance (n[tau] > [10.sup.13] s-[cm.sup.-3] T > 5 keV) using an available pulsed-power implosion facility at modest cost. We have recently shown the density, temperature, and lifetime of this FRC to be within a factor of 2-3 of that required for use as a suitable target plasma for MTF compression for a fusion demonstration. Index Terms--Field-reversed configuration, fusion energy, magnetized target fusion (MTF).

The design and test results of a crowbar switch developed for the formation of long-lifetime field-reversed configurations are presented. These research efforts are being pursued at the FRX-L facility at Los Alamos National Laboratory using the 'Colt' capacitor bank (a 36 [micro]F Shiva Star bank module capable of storing up to 250 kJ) and at the Air Force Research Laboratory using the 'Formation' capacitor bank (consisting of three parallel banks identical to Colt). The crowbar switch design includes four Maxwell rail-gap switches mounted on a cable header that transitions from the capacitor bank bus plates to 48 RG 17/14 coaxial cables. For the testing performed at AFRL, a dummy load was set up to simulate the magnetic field coils of the actual experiment. Tests thus far have demonstrated the crowbarring of peak currents up to 1.25 MA. Breakdown within the cable header due to the initial high voltage applied from the bank has been successfully suppressed by the cable feed-through design, proper placement of Mylar sheets around the switch for insulation, and replacement of air in the header with S[F.sub.6]. Timing for the triggering of the crowbar is somewhat critical, as inductance in the switch increases when the switch is triggered with lower voltages across the switch rails. At the higher bank charge voltages, the charge-flow ratings on the rail-gap switches are exceeded; however, other than requiring that the rail electrodes in the switches be cleaned more frequently, no detrimental effects have been observed from the excessive charge flow. Index Terms--Cable feedthroughs, crowbar switches, pulsed power, rail-gap switches, field-reversed configurations.

Spectroscopic methods are used to determine the density, the temperature, the composition, the injection velocity, and the azimuthal uniformity of the flashboard-produced prefilled plasma in an 85-ns, 200-kA plasma opening switch (POS). The electron density is found to be an order of magnitude higher than that obtained by charge collectors, which are commonly used to determine the density in such POS's, suggesting that the density in short conduction POS's is significantly higher than is usually assumed. We also find that the plasma is mainly composed of protons. The spectroscopically measured plasma parameters are used here to calculate the conduction currents at the time of the opening predicted by various theoretical models for the POS operation. Comparison of these calculated currents to the measured currents indicates that the plasma behavior during conduction is governed either by plasma pushing or by magnetic-field penetration and less by sheath widening near the cathode, as described by existing models. Also, the conduction current mainly depends on the prefilled electron density and less on the plasma flux, which is inconsistent with the predictions of the erosion (four-phase) model for the switch operation. Another finding is that a better azimuthal uniformity of the prefilled plasma density shortens the load-current rise time. Index Terms: Plasma devices, plasma measurements, pulse power system switches.

The introduction of plasma into the slow-wave structure (SWS) of a backward-wave oscillator (BWO) has been shown to increase microwave power output and generation efficiency, as well as provide several other benefits. Researchers at Niigata University in Japan have performed a linear analysis that has shown that the optimal plasma-filled BWO configuration is one in which the plasma is confined close to the axis of the SWS, while the electron beam driving the device is kept close to the walls. Previous schemes for preinjecting plasma have utilized sources mounted downstream of the SWS and have relied upon the converging field lines of the electron beam's guiding magnetic field to compress the plasma close to the axis as it entered the SWS. This paper presents data from experiments using a plasma source mounted internal to the cathode structure of a high-power BWO, thus providing injection of the plasma directly on axis in the strong, uniform field region of the BWO. Data describing plasma prefill density at different axial positions within the SWS prior to operation is presented. The plasma prefill has been found to both enhance and reduce the microwave generation efficiency, depending on the plasma density. Details of the novel plasma injection system are also provided. Index Terms - High-power microwaves, plasma microwave devices, relativistic backward-wave oscillators.

Laser interferometry is used for the first time to measure plasma electron density along the slow wave structure (SWS) wall during microwave generation in a vacuum, long pulse, high power backward-wave oscillator (BWO). The University of New Mexico long pulse backward-wave oscillator, which displays the characteristic pulse shortening phenomenon, is investigated in these studies. Although pulse shortening is observed across a wide class of high power microwave devices, its origin is not definitively understood. Many hypotheses suggest that the unintentional introduction of plasma into the interaction region near the walls of the SWS is one of several likely causes of pulse shortening in intense electron beam-driven slow wave devices. This article presents initial measurements of the line-integrated, temporally resolved plasma density between an intense, relativistic, annular electron beam and SWS walls for a variety of radiated microwave peak power levels. Line-integrated electron densities, , between 9[multiplied by][10.sup.15] and 2.4[multiplied by][10.sup.16] [cm.sup.-2] for radiated microwave powers between 20 and 120 MW have been measured. The two main sources of the measured electron density are postulated to be i) plasma generated from the cutoff neck due to beam scrape off, and ii) material removed and ionized from the SWS walls during microwave generation. Index Terms - Backward-wave oscillator, high power microwave generation, Michelson laser interferometry, plasma diagnostics, pulse shortening.

Fuses, when used as an opening switch, are often connected in parallel to handle large energies. Here, an experiment using two fuses is carried out to understand the behavior of parallel fuses clearly. The waveforms of current flowing through each fuse are different. Unstable behaviors, in which the difference of the currents flowing into the two fuses increases, are observed, and the condition and probability of appearance of the unstable behavior are discussed. The cause of the unstable behavior is that the rate of change of fuse-resistivity with input energy is different depending on the state of the wire. Any differences in initial dimensions of the parallel fuses or any kind of disturbance during the fuse action could immediately trigger off the unstable behavior.

Field reversed theta pinch technology is employed with programmed cusp fields at the theta coil ends to form high-density field-reversed configuration (FRC) plasmas. The well-formed FRC plasmas have volume-averaged density of 2 – 4 × 1022 m-3, total temperature (Te + Ti) of 300–500 eV, and plasma lifetime between 10–20 μs in 50–70 mtorr of deuterium static gas fill. The achieved FRC parameters are very close to the desired target plasma requirements for magnetized target fusion. [ABSTRACT FROM AUTHOR]