Joshua J. Leckbee

Numerical Analysis of a Pulsed Compact LTD System for Electron Beam-Driven Radiography

This paper describes the configuration and operation of a seven-cavity linear transformer driver (LTD) system. This LTD system is configured to deliver ~1 MV and 125 kA into a critically damped load. A detailed transmission line model coupled to particle-in-cell simulations is used to assess the system electrical performance. The evolution of the electron power flow in negative polarity is simulated, and the impact of this flow on the operation of the system with a large-area hollow electron beam diode is examined. The simulation results are compared with available electrical measurements and with dose rate measurements where a flash X-ray pulse is produced by an annular electron beam diode. These comparisons suggest that the LTD system meets the design specifications and is a robust pulsed power architecture. Additionally, the positive polarity operation for the LTD system driving a rod-pinch diode load is modeled to further assess the utility of the LTD system

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.

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).

An advanced model 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 modeling efforts commensurate with the design of a full size, prototype 250-300 kV concept validation test (CVT), switch that can transfer kilojoules per pulse. The flow system required to clear the discharge bubble and byproducts is intimately tied to the dynamics of energy deposition, and bubble formation. A circuit model has been developed to predict the discharge temporal characteristics including the voltage, current, risetime, arc energy deposition profile, and time varying arc inductance, bubble formation timescales and oscillatory bubble effects. The model utilizes both the Braginskii equation and Charlie Martins equations to calculate the energy dissipated in the arc. A comparison of the two methods is presented. An integrated model also includes the hydrodynamic equations to predict the gas bubble volume and oscillation period, which are dramatically reduced with increasing pressure. Optimization studies indicate that a 1000-2000 psi switch appears to have ideal attributes including minimal dielectric flow requirements, compact size and low weight for implementation of a kilojoule, rep-rate switch.

Design and testing of a high pressure, rep-rate, liquid dielectric switch for directed energy applications

A high power, rep-rate, liquid dielectric switch has been designed to meet future requirements for a variety of directed energy applications. A flowing, high-pressure liquid dielectric was chosen for the design of a megavolt class switch intended to operate continuously at pulse repetition rates in excess of 100 pps. This paper reports on the design efforts and initial testing of a full size prototype, 250 kV switch. The design capitalizes on lessons learned from experiments with a single shot concept validation high pressure liquid dielectric switch. Design efforts include extensive electric field modeling, circuit model simulations, and fluid flow simulations. The design facilitates fast rise times, quick switch recovery, and long electrode life. Rep-rate testing of the high pressure dielectric switch includes testing at rep-rates up to 100 pps and voltages up to 250 kV. The design of the pulser used to test the rep-rate switch is described in a separate conference paper. Switch diagnostics include D-dot probes and a Rogowski current probe. Results from the experimental switch tests will be compared to circuit models and the data presented showing the operating characteristics of the switch for various pressure ranges

Design of a 7-MV Linear Transformer Driver (LTD) for down-hole flash x-ray radiography.

Pulsed power driven flash x-ray radiography is a valuable diagnostic for subcritical experiments at the Nevada Test Site. The existing dual-axis Cygnus system produces images using a 2.25 MV electron beam diode to produce intense x-rays from a small source. Future hydrodynamic experiments will likely use objects with higher areal mass, requiring increased x-ray dose and higher voltages while maintaining small source spot size. A linear transformer driver (LTD) is a compact pulsed power technology with applications ranging from pulsed power flash x-ray radiography to high current Z-pinch accelerators. This report describes the design of a 7-MV dual-axis system that occupies the same lab space as the Cygnus accelerators. The work builds on a design proposed in a previous report [1]. This new design provides increased diode voltage from a lower impedance accelerator to improve coupling to low impedance diodes such as the self magnetic pinch (SMP) diode. The design also improves the predicted reliability by operating at a lower charge voltage and removing components that have proven vulnerable to failure. Simulations of the new design and experimental results of the 1-MV prototype are presented.

Development of spray coated cathodes for RITS-6.

This report documents work conducted in FY13 to conduct a feasibility study on thermal spray coated cathodes to be used in the RITS-6 accelerator in an attempt to improve surface uniformity and repeatability. Currently, the cathodes are coated with colloidal silver by means of painting by hand. It is believed that improving the cathode coating process could simplify experimental setup and improve flash x-ray radiographic performance. This report documents the experimental setup and summarizes the results of our feasibility study. Lastly, it describes the path forward and potential challenges that must be overcome in order to improve the process for creating uniform and repeatable silver coatings for cathodes.

Investigations of Shot Reproducibility for the SMP Diode at 4.5 MV

In experiments conducted on the RITS-6 accelerator, the SMP diode exhibits sig- ni cant shot-to-shot variability. Speci cally, for identical hardware operated at the same voltage, some shots exhibit a catastrophic drop in diode impedance. A study is underway to identify sources of shot-to-shot variations which correlate with diode impedance collapse. To remove knob emission as a source, only data from a shot series conducted with a 4.5-MV peak voltage are considered. The scope of this report is limited to sources of variability which occur away from the diode, such as power ow emission and trajectory changes, variations in pulsed power, dustbin and transmission line alignment, and di erent knob shapes. We nd no changes in the transmission line hardware, alignment, or hardware preparation methods which correlate with impedance collapse. However, in classifying good versus poor shots, we nd that there is not a continuous spectrum of diode impedance behavior but that the good and poor shots can be grouped into two distinct impedance pro les. This result forms the basis of a follow-on study focusing on the variability resulting from diode physics. 3

Option study of an orthogonal X-ray radiography axis for pRad at LANSCE area C, Los Alamos.

We report on an option study of two potential x-ray systems for orthogonal radiography at Area C in the LANSCE facility at Los Alamos National Laboratory. The systems assessed are expected to be near equivalent systems to the presently existing Cygnus capability at the Nevada Test Site. Nominal dose and radiographic resolution of 4 rad (measured at one meter) and 1 mm spot are desired. Both a system study and qualitative design are presented as well as estimated cost and schedule. Each x-ray system analyzed is designed to drive a rod-pinch electron beam diode capable of producing the nominal dose and spot.