Brian T. Hutsel

Study of laser target triggering for spark gap switches

The use of laser targets as a method to decrease the required laser energy to trigger a laser triggered gas switch has been investigated at the University of Missouri. Target materials were identified based on durability, melting point, reactivity and reflection coefficient. Laser targets were placed into a cathode of a laser triggered gas switch. The switch was pulse charged by the Tiger pulsed power machine to between 185 kV and 330 kV. The switch was triggered by directing a 1064 nm or 266 nm wavelength laser pulse from an Nd:YAG laser onto a laser target to ablate material and create plasma, closing the switch. The goal of the project was to trigger a high voltage gas switch with less than 1 mJ of laser energy while maintaining a switch jitter comparable to present electrically triggered switches for LTD based systems. The study successfully demonstrated that triggering the switch using a 1 mJ infrared pulse and a graphite target resulted in a jitter less than 5 ns. Findings will be used in the design of switches for LTD based systems.

Effects of laser triggering parameters on runtime and jitter of a gas switch

Parameters affecting the runtime and jitter of a laser triggered gas switch have been studied. Experiments tested a variety of switch parameters including percentage of selfbreak and switch pressure. The effects of laser beam parameters were also considered, including focal length, laser energy, laser spark length, and laser wavelength. Experiments were performed on the Tiger pulsed power machine. Measurements were taken on a spark gap switch built from the trigger section of a Rimfire switch. A Marx bank consisting of 32, 3.1 uF, capacitors that fed into a 7 nF intermediate storage capacitor was used to drive the switch into a 4 ¿ resistive load. The test switch was pressurized to 306 kPa (30 psig) with SF6 and operated near 1 MV. A New Wave Tempest Nd:YAG laser was used to trigger breakdown of the switch. The laser was focused at the mid-gap between the switch electrodes using lenses with focal lengths between 30 cm and 100 cm. Focused laser energy in the switch ranged from <5 mJ to 80 mJ. The effects of switch and laser beam parameters on the runtime and jitter of a laser triggered gas switch are presented. The end goal of the research is to determine optimal conditions for improved switch performance.

Runtime and Jitter on a Laser-Triggered Spark-Gap Switch

The University of Missouri has completed a new facility, named Tiger, for pulsed-power experimentation. Tiger consists of a 2.8-MV 450-kJ Marx bank that charges up to four 7-nF intermediate storage capacitors (I-stores) in parallel. When charged, the storage capacitors are switched into a resistive load through an SF6-filled laser-triggered gas switch. This switch has been designed to study the factors affecting runtime and jitter of spark-gap switches. All experiments presented in this paper were performed with a single I-store. The test switch was operated from about 500 kV up to 1.25 MV, at switch pressures from 10 to 50 psig. A 30-mJ 266-nm Nd:YAG laser was focused between the switch electrodes to initiate breakdown in the switch. The University of Missouri has examined laser energy, percentage of self-break, and focal length to determine their relation to runtime and jitter. A short discussion of the Tiger facility is presented with experimental results of jitter and runtime tests. The end goal of this paper is to understand the factors contributing to increased jitter and runtime and, thereby, provide paths to improved switch performance.

Particle Swarm Optimization of Pulsed Power Circuit Models

Circuit modeling is ubiquitous throughout the pulsed power discipline. Both plasma processes and systems can be modeled with circuits of varying complexities. Sometimes, circuit models need to be generated to match experimental waveforms. Particle swarm optimization (PSO) is a technique which can be utilized to automatically generate a circuit model to match experimental data. This paper details the PSO algorithm as well as two case studies of the implementation.

Optimization of Piezoelectric Resonance Effect in a Piezoelectric Transformer Plasma Source

Optimization of the piezoelectric resonance effect in a piezoelectric transformer plasma source (PTPS) is investigated. The PTPS utilizes a piezoelectric transformer (PT) effect to generate large electric potentials and aid in plasma formation in the plasma source. Finite-element simulations are used in conjunction with observations of device operation in order to optimize the PT effect used by the plasma source. Simulations were used to determine optimal crystal rotation for electromechanical coupling and evaluate the geometry of the plasma source in order to maximize the generated voltage. Input impedance measurements and an estimate of the mechanical quality were used to verify the effectiveness of device construction changes to reduce mechanical clamping.

Effects of Laser Triggering Parameters on Runtime and Jitter of a Gas Switch

The University of Missouri has studied parameters affecting the runtime and jitter of a laser triggered gas switch. Experiments tested a variety of switch parameters including percentage of self-break and charge voltage. The effects of laser beam parameters were also considered, including focal length, laser energy and laser spark length. Experiments were performed on the Tiger pulsed power machine. Measurements were taken on a spark gap switch built from the trigger section of a Rimfire switch. A Marx bank consisting of 32, 3.1 uF, capacitors that feeds into a 7 nF intermediate storage capacitor drives the switch into a 4 Omega resistive load. The test switch was pressurized with SF6 and operated near 1 MV, at a switch pressure of 30 psig. A 30 mJ, 266 nm, Nd:YAG laser was used to trigger breakdown of the switch. The laser was focused at the midgap between the electrodes using lenses with focal lengths between 30 cm and 100 cm. Focused laser energy in the switch ranged from <5 mJ to 20 mJ. The effects of switch and laser beam parameters on the runtime and jitter of a laser triggered gas switch are presented. The end goal of the research is to determine optimal conditions for improved switch performance.

Charged-Particle Emission and Self-Biasing of a Piezoelectric Transformer Plasma Source

The piezoelectric transformer (PT) plasma source (PTPS) is a compact radio-frequency-driven plasma source developed for near-space and microspacecraft propulsion. The PTPS utilizes the PT effect to aid in plasma production and acceleration of charged particles to create thrust. Charged-particle emission measured with a Faraday cup and a retarding potential analyzer is presented. Low emitted ion energies are explained by self-bias of the PT output due to charged-particle transfer from the plasma to the PT surface. Finally, self-neutralization of the PTPS was investigated for a PTPS isolated from the vacuum chamber ground.

Triggering jitter of a gas switch at high voltage and low laser energy

Laser triggered gas switch experiments have been completed to determine the effect of laser parameters and laser targets on runtime and jitter. A New Wave Tempest 10, Nd:YAG laser capable of output at 266 nm, 532 nm, or 1064 nm was used to trigger the switch. The switch was pulsed charged with the Tiger pulsed power machine consisting of a 32 capacitor Marx bank and a 7 nF intermediate storage capacitor.

Development of a 200-kV Test Stand and Experimental Study of Optical Triggering Methods for LTD Switches

A 200-kV test stand is being developed at the University of Missouri to examine optical triggering techniques for LTD switches. Two capacitors, separated by an optically triggered switch, are DC charged to plusmn100-kV. The switch will be triggered by a pulse from an Nd:YAG laser at 266, 532, and 1064-nm wavelengths. The goal of the study is to find methods to reduce the optical energy required to trigger the switch while minimizing runtime and jitter. To reduce laser energy, several methods will be explored. One method is to identify candidate materials based on their respective work functions and apply them to the switch cathode. Laser light will be directed onto the target materials with intensity below the ablation threshold, liberating electrons. Another method is to create a laser spark created in the mid-gap of the switch while manipulating length, position and orientation through control of laser power, focusing lenses, lens positions, insulating gas composition and photon energy. Findings will be used in designing of future LTD switches and triggering systems.

A 2.8 MV, 600 kA pulsed power driver constructed at the University of Missouri-Columbia

Summary form only given. The University of Missouri has completed construction of a new facility for pulsed power experimentation. The pulsed driver is driven by a Marx bank consisting of 32, 100 kV, 3.1 muF capacitors. The Marx bank can store 450 kJ and is switched into any combination of one or up to four, parallel, 7 nF intermediate storage capacitors. The intermediate storage capacitors are switched with a high voltage spark gap switch into a resistive load. The driver is capable of delivering 2.8 MV at 600 kA to a 4 ohm load. Simulations indicate that the driver is capable of producing 1 MA through a load with lower impedance. Initial uses for the facility include experimentation on switch jitter and arc spectroscopy on a spark gap switch. The test results are also used in the characterization of the machine. Simulations of the driver capabilities are presented along with details of the initial results.