F.E. Peterkin

High Power Density Capacitor Charging Power Supply Development for Repetitive Pulsed Power

Power modulators for compact, repetitive systems are continually faced with new requirements as the corresponding system objectives increase. Changes in pulse rate frequency or number of pulses significantly impact the design of the power conditioning system. In order to meet future power supply requirements, we have developed several high voltage (HV) capacitor charging power supplies (CCPS). This effort focuses on a volume of 6 x 6 x 14 and a weight of 25 lbs. The primary focus was to increase the effective capacitor charge rate, or power output, for the given size and weight. Although increased power output was the principal objective, efficiency and repeatability were also considered. A number of DC-DC converter topologies were compared to determine the optimal design. In order to push the limits of output power, numerous resonant converter parameters were examined. Comparisons of numerous topologies, HV transformers and rectifiers, and switching frequency ranges are presented. The impacts of the control system and integration requirements are also considered.

Investigation of Charge-Rate Increase for a Capacitor Charging Power Supply in Burst Mode Operation

Interest in rep-rate operation of pulsed power systems has led to an increase in power requirements of high voltage (HV) capacitor charging power supplies (CCPS). Compact repetitive pulse power systems provide an impetus to develop supplies with a high power density (HPD). We have an application requiring compact HPD CCPSs and have evaluated a number of options to determine their applicability to a bipolar charging scheme with a charge rate requirement of 6400 J/s. The load for our application has significantly imbalanced charge rail stray capacitances, which introduces issues with CCPS polarity dominance and no commercially available options were found to be satisfactory. This paper discusses our effort to modify a commercial CCPS to meet our requirements for charge rate and control versatility. Two Lambda Americas model 152A CCPS, of opposite polarities, were modified to operate from DC prime power and used to charge a bench load capacitance with integrated diagnostics. The primary objective was to double the effective rating of the commercial supply in a low duty cycle burst mode through modification of the operating parameters. The impact of variable switching frequency, passive resonant component value, and prime power voltage level on the charge rate is examined in detail. Burst-mode charge rate increases of greater than three times the continuous rating are reported. The impact on efficiency and reliability of the supply after modifications is also discussed.

High voltage breakdown strength of rapid prototype materials

We report measurements on the breakdown strength of plastics used in stereo lithography for rapid prototyping. Three epoxy-based photopolymer resins commonly used for stereo lithography were the focus of this work. Test samples were manufactured with an electrically smooth geometry to minimize field enhancements. The thickness of the stressed region in the samples was nominally 1 or 2 mm. Samples were tested to failure by applying a ramped and held voltage pulse at discrete levels up to a maximum of 240 kV. We confirmed the uniform field distribution of the sample geometry with electrostatic modeling and calculated the electric field stress at failure as a simple voltage/thickness ratio. These results are compared with values obtained for several typical materials often used in high voltage applications (acrylic, nylon, etc.). We find that the failure threshold for the SLA materials can be a factor of 2-3 below that of the standard materials.

Testing and Evaluation of an Ultra-Wideband Pulse Generator for HPM Weapon Simulation

The potential for high power microwave (HPM) and radio frequency weapons (RFW) to degrade or destroy electronics has been studied for decades. However, the fast pace of change in the underlying digital technologies of modern electronics makes it necessary to continually re-evaluate the susceptibilities of such systems. The Directed Energy Technology Office has developed a major test complex at the Naval Surface Warfare Center- Dahlgren Division (NSWCDD) to study the effectiveness of a variety of HPM and RFW prototypes. The facility consists of two concrete block buildings populated with a variety of modern commercial infrastructure electronics. In this paper we discuss a recent project which typifies the testing activities at the NSWCDD facility. Schriner Engineering, Inc. built a pulse generator, the S-30, to simulate an ultra-wideband RF weapon. The S- 30 operates from AC mains and uses a series of AC and pulsed transformers to charge a wide-band antenna structure up to 100s of kV. The S-30 radiates a fast risetime short pulse (<3 ns) when an oil-filled spark gap switch closes. We report our evaluation of the S-30 design and operation, measurements of the radiated output, and modeling of the antenna pattern. We also describe the electronic systems which were tested for susceptibility to the S-30 and the test protocol that was followed.

Performance characteristics of a 1 MV miniature Marx bank

In this paper, the authors report results of experiments with a compact Marx bank generator. The Marx was constructed with 25 spark-gap switched stages, each stage consisting of two Murata 2nF capacitors with N4700 ceramic dielectric rated at 40 kV. In order to evaluate the Marx efficiency and repetition rate, inductive isolation with 5 /spl mu/H per stage was implemented for the charging circuit. The Marx was charged with a 1500 J/s capacitor charging power supply. The system was controlled and monitored with a laptop computer through a fiber optic interface. The authors report results from operating the Marx from one quarter to full output voltage, nominal 250 kV to 1 MV open circuit (OC). Short duration testing showed that the Marx could be triggered continuously at a repetition rate varying from 20 to 50 Hz, depending on the charging voltage. Various representative loads were used for diagnostic purposes and for comparison to modeling efforts. When operated into a short circuit, the Marx current oscillated at 14 MHz with a peak amplitude of about 10 kA. The Marx was also operated with and without a coaxial ground enclosure to investigate the influence of this geometry on pulse characteristics. They compare the results of circuit modeling with the experimental measurements and show good agreement.

Modification of an EMP Facility to Support Threshold Testing of Electronic Systems

The naval ordnance transient electromagnetic simulator (NOTES) is a bounded wave test facility located at the Naval Surface Warfare Center, Dahlgren Division (NSWCDD). NOTES was designed to enable standardized electromagnetic pulse (EMP) testing, but in order to support tests to assess the vulnerability of US infrastructure to High-altitude EMP (HEMP) it was modified to enable threshold testing with peak amplitudes of 1 kV/m up to 100 kV/m while maintaining nominally the same temporal characteristics of the threat waveform. The range of amplitude was accomplished using three different voltage pulser configurations which provided nearly continuous variability. This paper provides a detailed description of the NOTES facility and the design, implementation, and results of these modifications. We present the waveforms that were used during testing and show that they provided a consistent stimulus both in terms of the overall waveform shape and in the critical risetime characteristic.

Antenna circuit model for time-domain transient analysis

When pulse-generating circuits excite antennas, the antenna often plays a major role in determining the characteristics of the radiated signal. Defining an equivalent electrical circuit for the antenna allows the simulation of the antennas transient behavior. This type of simulation, carried out with circuit analysis software, generates time-domain currents at different points on the antenna structure. From these currents, the time-domain radiated electric field can be calculated. This paper describes a procedure for developing a distributed RLC circuit model for a monopole antenna above a ground plane. The results from the modeling are compared to measurements of antenna current and radiated electric field.

Bistable optically controlled semiconductor switches in a frequency-agile RF source

The processes of persistent photoconductivity followed by photoquenching have been demonstrated at megawatt power levels in copper-compensated, silicon-doped, semi-insulating gallium arsenide. These processes allow a photoconductive switch to be developed that can be closed by the application of one laser pulse (/spl lambda/=1.06 /spl mu/m) and opened by the application of a second laser pulse with a wavelength equal to twice that of the first laser (/spl lambda/=2.13 /spl mu/m). This switch is called the bistable optically controlled semiconductor switch (BOSS). The opening phase of the BOSS requires a sufficient concentration of recombination centers (RC) in the material for opening to occur in the subnanosecond regime. These RCs are generated in the bulk GaAs material by fast-neutron irradiation (/spl sim/1 MeV). Neutron-irradiated BOSS devices have been opened against a rising average electric field of about 36 kV/cm (18 kV) in a time less than 1 ns while operating at a repetition rate, within a two-pulse burst, of about 1 GHz. The ability to modify the frequency content of the electrical pulses, by varying the time separation, is demonstrated. Results demonstrating the operation of two BOSS devices imbedded in a frequency-agile RF source configuration are also discussed.

Trend analysis of insertion loss data associated with large, significantly loaded compartments

Due to their metallic structure, it is not unreasonable to presume that the compartments of a naval vessel are appreciably reverberant when injected with an electromagnetic signal of reasonable wavelength. This assumption was put to the test when a series of measurements of the insertion loss were made in twelve ship compartments onboard a United States Naval vessel. The measurement technique assumes a reverberant environment and the test frequency range spans 200 MHz through 10 GHz. In references [2] and [7] the data is analyzed in order to assess the validity of the reverberant assumption and a summary of these finding is reported herein. Historically, for a reverberant environment the insertion loss is expected to behave linearly with the logarithm of the frequency

Fiber-Optically Isolated Instrumentation for Pulsed Power System Diagnostics

Advances in analog fiber optic telemetry and high sample rate/wide bandwidth analog-to-digital converters have enabled the development of increasingly sophisticated instrumentation suited for the highly stressing electromagnetic environment associated with many pulsed power systems. We report the development of several new capabilities, one based on analog modulation of the fiber-transmitted light and one in which a high-bandwidth digitizer is packaged in a compact configuration and the light signals are transmitted digitally using standard network protocols. Both systems are battery powered and heavily shielded to allow measurements to be made in regions of extremely high field strengths. Connection with external instrumentation and control systems is accomplished by using only fiberoptic cabling, providing completely isolated measurements.