C. Grabowski

Electron density measurements during microwave generation in a high power backward-wave oscillator

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 9.10/sup 15/ and 2.410/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.

Initial plasma-filled backward-wave oscillator experiments using a cathode-mounted plasma prefill source

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 beams 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.

Electron emission from slow-wave structure walls in a long-pulse, high-power backward wave oscillator

Pulse shortening is a phenomenon common among all long-pulse, electron-beam-driven high-power microwave sources. Although the electron beam driving the source may continue to propagate through the interaction region of the device for several microseconds or more, the duration of the emitted microwave pulse is typically no more than /spl sim/100 ns. Most explanations of this phenomenon put forth involve the introduction of plasma into the interaction region and/or the degradation of beam quality. This paper describes experiments conducted on the University of New Mexicos Long-Pulse Backward Wave Oscillator (BWO) Experiment which investigate the behavior of beam electrons in the slow-wave structure (SWS) during microwave generation. A current probe having a small aperture at a variable radius is placed within the SWS to monitor the beam current profile at different radii as a function of time. Results from these experiments reveal the appearance of electrons between SWS ripples at times corresponding to the peaks of the microwave pulses in separate experiments. A drop in the main beam current is observed shortly thereafter. The source of the electrons within the ripples is thought to be field emission or secondary electron emission from the SWS walls or emission from plasmas generated there.

FRC lifetime studies for the Field Reversed Configuration Heating Experiment (FRCHX)

The goal of the Field-Reversed Configuration Heating Experiment (FRCHX) is to demonstrate magnetized plasma compression and thereby provide a low cost approach to high energy density laboratory plasma (HEDLP) studies, which include such topics as magneto-inertial fusion (MIF). A requirement for the field-reversed configuration (FRC) plasma is that the trapped flux in the FRC must maintain confinement of the plasma within the capture region long enough for the compression process to be completed, which is approximately 20 microseconds for FRCHX. Current lifetime measurements of the FRCs formed with FRCHX show lifetimes of only 7 ∼ 9 microseconds once the FRC has entered the capture region.

Study of the effects of the prefilled-plasma parameters on the operation of a short-conduction plasma opening switch

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 POSs, suggesting that the density in short conduction POSs 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 an 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.

Addressing Short Trapped-Flux Lifetime in High-Density Field-Reversed Configuration Plasmas in FRCHX

The objective of the field-reversed configuration heating experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high-energy density laboratory plasmas (HEDLPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion, laboratory astrophysical research, and intense radiation sources, among others. To create HEDLP conditions, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum flux conserver (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically driven implosion of the solid liner. A requirement is that, once the FRC is stopped within the solid liner, the trapped flux inside the FRC must persist while the compression process is completed. With the present liner dimensions and implosion drive bank parameters, the total time required for implosion is ~25 μs. Lifetime measurements of recent FRCHX FRCs indicate that trapped lifetimes following capture are now approaching ~14 μs (and therefore, total lifetimes after formation are now approaching ~19 μs). By separating the mirror and translation coil banks into two so that the mirror fields can be set lower initially, the liner compression can now be initiated 7-9 μs before the FRC is formed. A discussion of FRC lifetime-limiting mechanisms and various experimental approaches to extending the FRC lifetime will be presented.

The time-dependent electron density and magnetic field distributions in a 70-ns plasma opening switch

The time-dependent two-dimensional electron density distribution is determined for the first time during the operation of a short-conduction plasma opening switch. The electron density, resolved in 3-D, is determined from the line intensities of different ions doped into the plasma. A rise in the electron density followed by a drop is observed to propagate from the generator towards the load at the proton Alfven velocity. Based on spectroscopic magnetic field and ion velocity measurements, the density evolution can be explained as pushing of the protons ahead of the propagating magnetic piston, followed by magnetic field penetration into the rest of the plasma composed of the lower-density heavier ions.

Operation of parallel rail-gap switches in a high-current, low-inductance crowbar switch

The Field-Reversed Configuration Heating Experiment (FRCHX) was designed to form closed-field-line magnetized target plasmas for magneto-inertial fusion and other high energy density plasma research. These plasmas are in a field-reversed configuration (FRC) and are formed via a reversed-field theta pinch on an already-magnetized background plasma. To extend the duration and uniformity of the pinch, the capacitor bank driving the reversed-field discharge is crowbarred near the current peak. Four parallel rail-gap switches are used on FRCHX for this application to ensure a low-inductance crowbar discharge path and to accommodate the large magnitude of the discharge current (often greater than 1 MA). Parallel operation of spark gap switches in a crowbarring arrangement, however, has often proved to be difficult due to the very low voltage present on the bank and across the switches at the time of peak current. This paper reports on the successful efforts made to develop a low-inductance crowbar switch for FRCHX and to ultimately enable successful triggering and operation of the four parallel rail-gap switches used in the crowbar. The design of the parallel switch assembly is presented first, followed by a description of the triggering scheme employed to ensure conduction of all four switches.

Effects of plasma filling on microwave emission in a BWO with non-uniform slow wave structure

Summary form only given. In a backward wave oscillator (BWO), an electron beam flowing in a waveguide interacts with a ripple in the waveguide wall, known as a slow wave structure (SWS), to produce high power microwaves. The introduction of either a non-uniform ripple or plasma filling in the waveguide has been shown to increase the output power of the BWO. The effect of a non-uniform ripple and plasma filling is studied using the computer code MAGIC. For a BWO with a non-uniform SWS geometry similar to that used in experiments at UNM, the effects of plasma density, plasma radius, and the amount of plasma filling on microwave emission are being simulated. Preliminary results suggest that higher power is produced only with a plasma density less than the electron beam density. In addition, our MAGIC simulations are testing the prediction made by an analytical study which suggests that higher output power will be generated with a plasma concentrated within a small radius and an annular electron beam flowing at a larger radius close to the SWS ripples. The effects of the amount of plasma filling are also studied to determine if the presence of plasma beyond the SWS inhibits the production of high power microwaves in a plasma filled BWO.

PPPS-2013: Addressing short trapped-flux lifetime in high-density field-reversed configuration plasmas in FRCHX

Summary form only given. The objective of the Field-Reversed Configuration Heating Experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high energy density plasmas (HEDPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion (MIF), laboratory astrophysical research, and intense radiation sources, among others. To create the HEDP, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum shell (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically-driven implosion of the shell. A requirement is that once the FRC is stopped within the shell, the trapped flux inside the FRC must persist while the compression process is completed. With the present shell dimensions and drive bank parameters, the total time required for implosion is ~25 microseconds. Lifetime measurements of recent FRCHX FRCs indicate trapped lifetimes now approaching ~14 microseconds, and with recent experimental modifications the liner compression can be initiated considerably earlier before formation is completed in order to close that gap further. A discussion of FRC lifetime-limiting mechanisms will be presented along with a description of FRCHX and recent changes that have been made to it. Results from recent experiments aimed at lengthening FRC lifetime will also be presented.