Susan Heidger

Review of Cold Cathode Research at the Air Force Research Laboratory

Over the last decade, the Air Force Research Laboratory, Directed Energy Directorate (AFRL/DE) has engaged in a high current density field emission cathode research program. This program explored the aspects of cathode materials as well as the details of cathode geometries and emission physics. This paper summarizes the results of this ongoing research effort to date. We review the history and motivation for the program, which provide insight into the physics issues of concern for various vacuum electronic sources. One important aspect of the program consists of the investigation of new cathode materials. For many high power microwave (HPM) sources, neutral out-gassing, which ties critically with cathode materials, plays a key role in the effective operation of the source. These material properties influence plasma formation, which in turn dictates the operation of an HPM device. For a cathode material, AFRL chose to focus on cesium-iodide-coated carbon fiber cathodes, which we discuss in detail here. A second important aspect of the program consists of understanding emission physics and the optimum geometries for a cathode. This aspect couples closely with electron beam quality, which in turns effects the electron beam interaction with microwaves in the HPM structure. This paper concludes with a discussion of the implementation of the cathode material on both a Magnetically Insulated transmission Line Oscillator and a relativistic magnetron.

Submicrosecond Pulsed Power Capacitors Based on Novel Ceramic Technologies

Capacitor energy density for submicrosecond discharge applications was investigated for capacitors based on the following: 1) polymer-ceramic nanocomposite (PCNC); 2) antiferroelectric (AFE); and 3) paraelectric (PE) ceramic dielectrics. The developmental PCNC dielectric enabled design, fabrication, and testing iterations to be completed relatively rapidly. The PCNC capacitors were nominally 4 nF and were tested to dc potentials of at least 75 kV. The capacitors were then charged from 20 to 48 kV with a dc high-voltage power supply and discharged into a nearly critically damped test circuit of up to 5 pulses/s (pps) repetition rate for lifetime testing. The discharge time was 65 ns. Shot life as a function of the charge voltage was compared for three design iterations. Changes in the manufacturing of the PCNC capacitors have yielded up to 100× improvements in pulse discharge life. The 1-2-kV prototype, nonlinear (antiferroelectric and paraelectric) multilayer ceramic capacitors had zero-voltage capacitance ratings of between 60 and 300 nF. They were charged to their operating voltage and discharged into a nearly critically damped load in 2-6 μs, depending on their capacitance, at repetition rates of up to 75 pps. Their operating voltage for fast, repetitive discharge was determined for lifetimes consistently over 105 shots. Discharge energy densities of 0.27-1.80 J/cc and energy losses of 7.9-36.8% were obtained for the packaged multilayer capacitors with different formulations of nonlinear dielectrics. Increased field-induced strain was correlated with increased permittivity and contributed to the limitations on the operating voltage. Multilayer ceramic capacitors fabricated from AFE and PE ceramic dielectrics have the potential to achieve high energy density owing to their high relative permittivities that vary with applied electric field, assuming they can be scaled up to sufficiently high voltages.

Materials characteristics and surface morphology of a cesium iodide coated carbon velvet cathode

Cesium iodide (CsI) coated carbon fiber cathodes have shown promise as a cold cathode for microwave and x-ray devices. In particular, the cathodes have demonstrated over 1 000 000 shots lifetime at operating voltages at or in excess of 165 kV and current densities greater than 25A/cm2. While the vacuum emission characteristics have been well studied, the materials characteristics of the cathodes themselves, particularly after operation, have received little attention. Furthermore, while researchers at University of Wisconsin have demonstrated a reduction in work function of carbon due to the CsI coating, the details of the emission mechanism remain poorly understood. This article gives results of a series of materials diagnostics investigating the cathode surface morphology as well as the changes in the carbon fiber structure with cathode shot history. We demonstrate that the cathode surface undergoes several changes in relation to the bond line along the fiber-substrate interface as well as at the fiber ...

Design, modeling, and verification of a high-pressure liquid dielectric switch for directed energy applications

A high-power liquid dielectric switch is being developed to satisfy the requirements for future directed energy applications. A flowing, high-pressure liquid dielectric was chosen for the design of a megavolt class switch operating at 100 pps. This paper reports on the design philosophy, modeling, and experimental results of a full size, single-shot prototype 250-300 kV concept validation test (CVT) switch which can transfer kilojoules per pulse. Analysis of design criteria and scaling for a compact, 100-pps, kilojoule, high-voltage switch are presented. Optimization studies indicate that a pressure range of 6.9-13.8 MPa (1000-2000 psi) appears to be ideally suited to a flowing dielectric rep-rate switch.

Dielectric nonlinear transmission line

A parallel plate nonlinear transmission line (NLTL) was constructed. Periodic loading of nonlinear dielectric slabs provides the nonlinear capacitance and the gaps between provide linear inductive interconnects, this is essentially the same design used by Ikezi [1],[2]. The NLTL was modeled in a circuit simulation code using the experimentally measured form of the nonlinear capacitance. Dielectric loss included in the model as an equivalent series resistance derived from the measured loss tangent data affects the formation of RF oscillations. The diagnostics used on the experimental system are Bdots along the line and a current viewing resistor at the load. The diagnostics provide a measurement of the pulse evolution as it travels down the line. The waveforms from the experimental line qualitatively agree with the circuit model, showing no strong RF formation as a result of the loss.

A High Pressure Flowing Oil Switch for Gigawatt, Repetitive Applications

A repetitive oil switch for directed energy applications has been developed in a joint effort between teams at the University of Missouri - Columbia, Alpha Omega Power Technologies and the Boeing Company. The switch is operated at test pressures to 17.24 MPa (2500 psi), flow rates to 0.72 L-s-1 (11.4 gpm), charge voltages to -300 kV and discharge energies to 275 J per pulse at 20 pps. An examination of the electrodes after 250,000 shots with the original design led to the design of an insert device which resulted in higher performance fluid flow within the switch. The flow shaper-enhanced switch was tested for 150,000 shots, the results of which are presented in the following paper. Electrode lifetime has been evaluated for stainless steel under the original and enhanced fluid flow conditions and is reported.

Development of high power; high pressure, rep-rate, liquid dielectric switches

The University of Missouri-Columbia (UMC) is developing high power liquid dielectric switches intended to address future high power microwave (HPM) applications. Although requirements encompass a broad parameter space, the initial switch concept focuses on a 250-300kV output switch operated at 100 pps that will be scaled to 1MV. Failure to clear high electric field regions prior to the next charge cycle results in prefires, thereby limiting the maximum achievable repetition rate. Elevating the operating pressure, hence minimizing the bubble size and temporal properties, has alleviated this problem. This paper presents the design philosophy, modeling, and experimental results obtained from a single shot prototype operated in oil at pressures ranging from atmospheric pressure to greater than 13.8 MPa (2000 psi).

Nonlinear transmission line based electron beam driver

Gated field emission cathodes can provide short electron pulses without the requirement of laser systems or cathode heating required by photoemission or thermionic cathodes. The large electric field requirement for field emission to take place can be achieved by using a high aspect ratio cathode with a large field enhancement factor which reduces the voltage requirement for emission. In this paper, a cathode gate driver based on the output pulse train from a nonlinear transmission line is experimentally demonstrated. The application of the pulse train to a tufted carbon fiber field emission cathode generates short electron pulses. The pulses are approximately 2 ns in duration with emission currents of several mA, and the train contains up to 6 pulses at a frequency of 100 MHz. Particle-in-cell simulation is used to predict the characteristic of the current pulse train generated from a single carbon fiber field emission cathode using the same technique.

Analysis of folded pulse forming line operation

A compact pulse forming line (CPFL) concept based on a folded transmission line and high-breakdown strength dielectric was explored through an effort combining proof-of-principle experiments with electromagnetic modeling. A small-scale folded CPFL was fabricated using surface-mount ceramic multilayer capacitors. The line consisted of 150 capacitors close-packed in parallel and delivered a 300 ns flat-top pulse. The concept was carried to a 10 kV class device using a polymer-ceramic nanocomposite dielectric with a permittivity of 37.6. The line was designed for a 161 ns FWHM length pulse into a matched load. The line delivered a 110 ns FWHM pulse, and the pulse peak amplitude exceeded the matched load ideal. Transient electromagnetic analysis using the particle-in-cell code ICEPIC was conducted to examine the nature of the unexpected pulse shortening and distortion. Two-dimensional analysis failed to capture the anomalous behavior. Three-dimensional analysis replicated the pulse shape and revealed that the bends were largely responsible for the pulse shortening. The bends not only create the expected reflection of the incident TEM wave but also produce a non-zero component of the Poynting vector perpendicular to the propagation direction of the dominant electromagnetic wave, resulting in power flow largely external to the PFL. This analysis explains both the pulse shortening and the amplitude of the pulse.

Solid dielectric transmission lines for pulsed power

This paper documents recent work developing solid dielectric transmission lines for sub-microsecond, 100 kV class compact pulsed power systems. Polymer-ceramic nanocomposite materials have demonstrated sub-microsecond discharge capability in parallel plate capacitors and transmission lines [1, 2]. With a dielectric constant of approximately 50, the propagation velocity is 2.5 cm/ns, necessitating lines of several meters length to achieve > 100 ns pulse lengths. By folding the line in a fashion analogous to ceramic multilayer capacitors, the physical length of the line can be significantly shorter than the electrical length. We present the results of an experimental effort to develop a folded transmission line using a polymer-ceramic nanocomposite dielectric. The pulse length was somewhat shorter than expected based on a simple calculation using the geometry and the dielectric constant. Fully 3-D electromagnetic calculations were used to examine the role of the edges in curtailing the pulse length. Dielectric breakdown in this device occurred below the electric field threshold demonstrated in the prior work [1]. Improvements in the large scale fabrication of TiO2 beginning with nanoscale grains have opened the possibility for producing single layer high voltage devices. Given a dielectric constant approaching 140, transmission lines using nano-TiO2 can be considerably shorter than with other materials. Relatively thick, flat sheets of TiO2 have been fabricated for testing up to 50 kV. Several transmission lines, employing a serpentine electrode geometry, have been manufactured and tested. Testing up to several 10s of kV has confirmed the operation of the lines according to the design. As expected, the triple point between the TiO2, electrode, and insulating medium has proven difficult to manage for high voltage operation. Several techniques to mitigate the effects of the triple point, including resistive grading at the edges of the electrodes, are discussed. Fully 3-D electromagnetic modeling is used to examine the effects of electrode geometry and composition on the performance of the lines.