Alex Sitzman

Design, Construction, and Testing of an Inductive Pulsed-Power Supply for a Small Railgun

Advances in high-power-density batteries have rekindled interest in using inductive store as a pulse compression system. Although these batteries are considered very power dense, they lack over an order of magnitude of power density to drive a deployable electric gun. However, one can add an inductive circuit to a battery bank to make a hybrid system that has a much higher power density than batteries alone. A battery-inductor hybrid pulsed-power supply boasts several advantages over pulsed alternators, as inductors are static and relatively easy to cool. Inductors are potentially more energy dense than capacitors, making a battery-inductor hybrid pulsed-power supply an attractive alternative to capacitor-based pulsed-power supplies. The opening switch has been a major obstacle in previous inductive store projects, but in simulation, a new circuit topology-the Slow Transfer of Energy Through Capacitive Hybrid (STRETCH) meat grinder-greatly attenuates the problem. This paper discusses the design, construction, and testing of a small-scale STRETCH meat grinder system, which was successfully used to power a miniature railgun

Stretch Meat Grinder: A Novel Circuit Topology for Reducing Opening Switch Voltage Stress

The slow transfer of energy through capacitive hybrid (STRETCH) meat grinder is an inductive- capacitive current multiplication circuit that reduces switching requirements and achieves a high degree of current multiplication while possessing an energy density approaching that of a purely inductive system. Initially, the STRETCH meat grinder operates like a single-stage meat grinder; it increases the current through an inductor by switching out a coupled inductor. However, during switching in generic meat grinder circuits, leakage flux caused by imperfect coupling and the sudden change in current induces a voltage across the opening switch well beyond what modern solid-state switches can handle. The STRETCH meat grinder mitigates these problems by using a capacitor to recapture the energy in the leakage flux and to slow down the turnoff of current in one of the inductors. The energy from the leakage flux is then used to reverse the current on the turned-off inductor, thereby further increasing the current multiplication. A system comprising several STRETCH meat grinders in parallel can develop currents in the mega-ampere range without exceeding the capabilities of solid-state switches. Such a system could be used to power a railgun.

Modification and Testing of a Battery-Inductor Repetitive Pulsed Power Supply for a Small Railgun

Pulsed power supplies for electromagnetic launch have unique requirements, including very low impedance, relatively long pulse discharge times, and high stored energy. The three primary types of pulsed power systems for electromagnetic launch have been capacitive, inductive, and rotating machine. This paper describes recent results in the design, construction, and testing of a pulsed power system based on an inductor that is charged by batteries.

Operational Limits of a Commercial Gate Turn-Off Thyristor for Inductive-Store Systems

This paper details the testing of a commercially available gate turn-off thyristor (GTO) for application in inductive-store power supplies. GTOs have been used as a component of inductive-store systems directly and as part of compound switches. These GTOs are generally available commercially and typically have extensive datasheets. However, the inductive-store application differs significantly from conventional switching applications, which significantly reduces the utility of the datasheets; GTOs are typically used at 100-1000 Hz in a fairly continuous manner, whereas in an inductive-store system, a GTO may be conducting for a relatively long time before doing a single turn off.

Testing a Lithium Ion Battery as a Pulsed Power Source

Experiments were performed on a prototype very high power lithium-ion battery cell from Saft Batteries for its pulsed power capacity. The experiments were centered on a series of tests designed to exercise the battery for maximum power transfer and maximum charge/current transfer from battery to a load. The pulsing periods ranged from 0.5 ms to 50 ms. The maximum power obtained was 14.6 kW/kg for a 5 ms pulse. Battery output current measured at 5000 A was obtained for a 5 ms pulse under a 245 Omega load.

Use of the Magnetic Saw Effect for Manufacturing

This paper describes a novel technique for making fine cuts in metal plates and foils. The process is based on a variant of the magnetic saw effect, a magnetic instability associated with high pulsed magnetic fields. When not controlled, the magnetic saw effect can lead to failure in high-field pulsed conductors. By limiting energy and pulse duration, the authors have produced controlled cuts with kerfs as fine as 0.01 mm. These promising results motivated a systematic study of the parameters that govern the quality and reproducibility of magnetic saw cuts. This paper provides an overview of the technique, of some key experiments, and of the practical issues associated with using pulsed magnetic fields for manufacturing.

The electrical specific action to melt of structural copper and aluminum alloys

This paper describes an exploding wire experiment to measure the electrical specific action to melt of structural alloys of copper and aluminum, including C10100, C11000, C18000, C18200, Al6061, and Al7075. These alloys, which are commonly used in railguns and other pulsed power devices, are not produced in fine wire form. Instead of wires, we developed a technique to test macroscopic samples (0.25 mm × 0.5 mm cross section) manufactured with wire electrical discharge machining. This paper includes a description of the design considerations for such macroscopic exploding wire experiments.

The Effect of Geometric Enhancement on the Magnetic Saw Effect

This paper proposes that in some instances, the magnetic saw effect is purely a melt phenomenon and describes a series of experiments where the magnetic saw effect was studied by varying the submillimeter radius of the enhancement point on a set of highly controlled samples. The magnetic saw effect is an instability that can occur when high currents are required to make sharp bends, thereby greatly enhancing the magnetic field and current densities. These sharp bends generally occur at joints, cuts, or other flaws and can cause failure of high-current conductors. The magnetic saw effect has also been proposed as a possible metal-cutting mechanism for manufacturing purposes. Due to the very small length scales of the features associated with magnetic sawing, it is difficult to make samples that are controlled enough to get reproducible experimental results for systematic studies. In these experiments, printed circuit board technology was used to accurately make samples with very fine enhancement features. The conditions for the magnetic saw effect were modeled using an electromagnetic code to establish the important parameters that predict the onset of damage.

The effect of geometric enhancement on the magnetic saw effect

This paper proposes that in some instances, the magnetic saw effect is purely a melt phenomena and describes a series of experiments where the magnetic saw effect was studied by varying the sub-millimeter radius of the enhancement point on a set of highly-controlled samples. The magnetic saw effect is an instability that can occur when high currents are required to make sharp bends, thereby greatly enhancing the magnetic field and current densities. These sharp bends generally occur at joints, cuts, or other flaws and can cause failure of high-current conductors. The magnetic saw effect has also been proposed as a possible metal-cutting mechanism for manufacturing purposes. Due to the very small length scales of the features associated with magnetic sawing, it is difficult to make samples that are controlled enough to get reproducible experimental results for systematic studies. In these experiments, printed circuit board technology was used to accurately make samples with very fine enhancement features. The conditions for the magnetic saw effect were modeled using an electromagnetic code to establish the important parameters that predict the onset of damage.

The electrical specific-action-to-melt of structural copper and aluminum alloys

This paper describes an exploding wire experiment to measure the electrical specific action to melt of structural alloys of copper and aluminum, including C10100, C11000, C18000, C18200, Al6061, and Al7075. These alloys, which are commonly used in railguns and other pulsed power devices, are not produced in fine wire form. Instead of wires, we developed a technique to test macroscopic samples (0.25 mm × 0.5 mm cross section) manufactured with wire electrical discharge machining. This paper includes a description of the design considerations for such macroscopic exploding wire experiments.