W.C. Nunnally

Efficiency and Scaling of Constant Inductance Gradient DC Electromagnetic Launchers

We present efficiency and scaling relationships for dc (i.e., noninduction) constant inductance gradient electromagnetic launchers.We derive expressions for electromagnetic force, efficiency, back-voltage, and kinetic power in terms of electrical circuit parameters. We show that launcher efficiency is a simple function of armature velocity and the launcher’s characteristic velocity. The characteristic velocity characterizes the launcher and is the product of two new parameters: the mode constant and launcher constant. Mathematically, the launcher must operate at its characteristic velocity for 50% maximum efficiency. The mode constant reflects the manner in which the launcher is powered and its maximum efficiency. The launcher constant reflects the geometry of the launcher. We consider two modes of operation: constant current and zero exit current operation. We develop the ideal electromagnetic launcher concept and define it as operation at 100% maximum efficiency at all velocities.We also develop the concept of same-scale comparisons, that is, that electromagnetic launcher comparisons should be done with equal bore diameter, launcher length, projectile mass, and velocity. Finally, we present a comparative analysis based on experimental data of same-scale constant gradient electromagnetic launchers for conventional railgun, augmented railgun, and helical gun launchers in terms of the launcher constant, inductance gradient, bore diameter, bore length, system resistance, and armature (i.e., projectile) velocity.

Energy conversion and high power pulse production using miniature piezoelectric compressors

The design, construction, and testing of miniature, high-power piezoelectric compression generators are presented and discussed. The piezoelectric compression generators are located inside high speed, 30-mm projectiles that are launched with a high pressure helium gun to velocities of approximately 300 m/s. The large deceleration force created when the projectile imparts the ground is used to power the piezoelectric compression generator. The peak output power is approximately 25 kW into a 10-/spl Omega/ load with the output pulse length on the order of 1 /spl mu/s. Good agreement is found between the experimental results and the theoretical predictions.

Silicon carbide photoconductive switch for high-power, linear-mode operations through sub-band-gap triggering

The analysis of a 6H silicon carbide (SiC) photoconductive switch, designed and packaged for high-power, linear-mode operations, is presented. The switch, fabricated from semi-insulating compensated SiC, is triggered by an optical source with photon energy less than the band-gap energy. Simulation models incorporating the effects of vanadium trap and nitrogen dopant in the compensation material show I-V characteristics that agree with measured values. The photoconductive switch has improved rise-time characteristics as compared to a gallium arsenide (GaAs) switch. The analysis also shows that improved performance at high power is possible through passivation using high-permittivity dielectric near the contact-semiconductor interface and by placing a p+ layer next to the cathode.

The Marx generator as an ultra wideband source

Traditional uses of the Marx generator have been limited to energy storage and delivery systems, such as charging capacitors or pulse forming lines. However, low energy, high peak power Marx generators are finding applications in ultra wideband radar and high power microwave systems. This paper discusses the compact generator as well as an assortment of impulse antennas. Experimental results, including those of the Marx generator and antenna range measurements are presented and discussed.

Prediction and verification of electromagnetic forces in helical coil launchers

Calculating the circuital parameters and electromagnetic force production between two coupled coils is critical when modeling helical coil launchers (HCLs). Computer tools that calculate the self inductance, mutual inductance, and inductance gradient of a coupled-coil pair are developed for the PSpice circuit simulator. The inductance values and gradient are used to model a short (16.3-mm length), small-bore (19.4-mm diameter) HCL. The HCL is constructed in the laboratory and its performance is measured. The HCL accelerated nominal 16-g projectiles up to 170 m/s using a 6-kJ capacitive energy store. The highest overall electric-to-kinetic conversion efficiency was 5.4%. Comparisons are made between the theoretical and experimentally measured launcher parameters. In general, there is good agreement between the predicted and measured HCL parameters.

High-efficiency, medium-caliber helical coil electromagnetic launcher

Research progress in the development of a 40 mm/spl times/750 mm helical-coil electromagnetic launcher (HCEL) is presented and discussed. Significant technical problems that have been solved in this research include efficient stator commutation methods and the ability to simultaneously implement high-inductance gradient armatures. The HCEL is able to launch a 525-gram projectile to a velocity of 140 m/s. Power for the HCEL is derived from a 62.5 kJ sequentially fired pulse forming network (PFN) of 900 V (maximum) electrolytic capacitors. The experimentally measured HCEL efficiency of 18.2% is substantially greater than a conventional or augmented railgun of similar scale (i.e., equivalent mass, bore-size, and velocity). The HCELs high launch efficiencies result from its 150 /spl mu/H/m inductance gradient, which is approximately 300 times greater than the inductance gradient of a conventional railgun. HCEL computer model predictions are given and compared to experimentally measured HCEL and PFN parameters including peak current, inductance gradient, acceleration time, parasitic mass ratios, and electrical-to-kinetic conversion efficiency. Scaling relationships for the HCEL are also presented and used to predict launcher operation at higher velocity and with a larger diameter bore size.

Design and operation of a sequentially-fired pulse forming network for non-linear loads

While the construction of a linear pulse forming network (PFN) for a constant load impedance is relatively easy, the process is more difficult for a nonlinear or time-varying load. A passive PFN can certainly be synthesized for nonlinear loads, but is usually large and lacks the flexibility to be truly useful in most practical and research applications. This investigation describes the design and construction of a sequentially-fired pulse forming network (SFPFN) that maintains constant voltage and current for a nonlinear load. Operation of the SFPFN consists of charging multiple capacitor banks (or modules) to various levels and sequentially firing these banks into the load at appropriate times. The load and its characteristics determine the module charge voltage. An added benefit with the SFPFN is real-time computer monitoring and control allowing the PFN modules to be charged from a single prime power source via a group of switching relays. The nonlinear load used in this investigation is a helical coil electromagnetic launcher (HCEL). The SFPFN is also tested with a linear load consisting of a pulsed field coil. The nonlinearity of the HCEL is well-known with a factor of 2 change in winding resistance due to joule heating and a large variation in terminal voltage due to changes in the armature back-voltage. Experimental measurements show the SFPFN can deliver a relatively constant current pulse on the order of 5-15 kA into the HCEL load for a pulse length up to 8 ms. The maximum SFPFN operating voltage is 900 V with a total stored energy of 125 kJ. Scaling the SFPFN to larger or smaller pulse amplitudes or lengths is possible.

Sub-nanosecond jitter operation of Marx generators

Low energy, high peak power Marx generators are finding applications in ultra wideband radar and high power microwave systems. In many cases, these systems require very precise control over the delivery of the pulse from the generator. For example, unique systems might be used for bi-static radar, and excessive temporal jitter between the generators may add ambiguity to the measurement. A 17 stage Marx generator was fabricated to study techniques for reducing the jitter in a multi-spark gap system. This paper presents the results of a jitter study.

Opportunities for employing silicon carbide in high power photo-switches

High electric field geometries for high power, photoconductive switches made possible by employing subbandgap energy photons and inter-bandgap dopants / defects are being investigated for compact pulse power systems. The high field, long absorption depth package reduces the required linear mode, optical closure energy and also reduces the conduction current density through the active material and at the contacts. This paper describes the opportunities for employing semi-insulating SiC wafer in the University of Missouri-Columbia, high electric field configuration. The parameters of semi-insulating SiC materials and methods of fabricating such materials into a high power photo-switch are discussed. In addition, transient modeling of the transverse injection of optical closure energy is discussed.

Photoconductive switch design for microwave applications

A procedure for choosing the dimensions of a photoconductive semiconductor switch (PCSS) for operation at microwave switching frequencies, and particularly at 10.0 GHZ, is described. The critical dimension is the switch length (electrode separation), which must be small enough to force photoinduced charge removal during switch turn-off via sweep out rather than recombination. The switch depth in the direction of turn-on optical pulse absorption must be several optical absorption depths long to ensure absorption of all the incident light, which optimizes optical to electrical signal gain. The switch width is determined in conjunction with the peak intensity of the optical pulse because the switch width-optical intensity product, which represents optical power, determines the turn-on time, the on-state switch resistance and the turn-off delay time. Simulations show that a switch with a 0.5 ¿m length, 5.0 ¿m depth, and 20 ¿m width, illuminated with 1.0 W peak power optical pulses at 10 GHz, will have a 4.8 ps turn-on time, a 0.23 ¿ on-state resistance, and a 46 ps turn-off time.