Harold Hjalmarson - Academia.edu (original) (raw)

Papers by Harold Hjalmarson

Optically activated, high gain GaAs switches are being tested for many different pulsed power app... more Optically activated, high gain GaAs switches are being tested for many different pulsed power applications that require long lifetime (longevity). The switches have p and n contact metallization (with intentional or unintentional dopants) configured in such a way as to produce p-in or ni-n switches. The longevity of the switches is determined by circuit parameters and by the ability of the contacts to resist erosion. This paper will describe how the switches performed in test-beds designed to measure switch longevity. The best longevity was achieved with switches made with diffused contacts, achieving over 50 million pulses at 10 A and over 2 million pulses at 80 A. * Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.

Conference Record of the 2000 Twenty-fourth International Power Modulator Symposium

Physical Review E, 2002

An electromechanical analysis based on thin-shell theory is presented to analyze cell shape chang... more An electromechanical analysis based on thin-shell theory is presented to analyze cell shape changes in response to external electric fields. This approach can be extended to include osmotic-pressure changes. Our calculations demonstrate that at large fields, the spherical cell geometry can be significantly modified, and even ellipsoidal forms would be inappropriate to account for the deformation. Values of the surface forces obtained from our calculations are in very good agreement with the 1-10 mN/m range for membrane rupture reported in the literature. The results, in keeping with reports in the literature, demonstrate that the final shape depends on membrane thickness. This has direct implications for tissues in which significant molecular restructuring can occur. It is also shown that, at least for the smaller electric fields, both the cellular surface area and volume change roughly in a quadratic manner with the electric field. Finally, it is shown that the bending moments are generally quite small and can be neglected for a simpler analysis.

AIP Conference Proceedings, 2005

Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia, a wholl... more Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000 Controlling and preventing electrical breakdown is a key issue for many electrical components. Disruptive devices (e.g., lightning arresters, insulators) are used to protect electrical systems in the presence of sufficiently large voltages. Predicting the behavior of disruptive devices in extreme environments, particularly for stockpile and other critical applications, is a key issue of national importance. Science-based modeling of electrical breakdown has had limited success in predicting many of the observed characteristics without the help of diagnostic discovery experiments. A new diagnostic technique is being developed, using pulse terahertz (THz) radiation to image electron distributions in conventional and electron-hole plasmas. This innovative technique will provide previously unavailable information to help model and predict electrical breakdown phenomena, which are present in all aspects of science and engineering, for research and commercial applications. Images of electrical breakdown phenomena often show the recombination of electrons and ions (or holes in a semiconductor). Recombination radiation requires the presence of both polarities of charge. Thus, in cases where electron densities exceed ion or hole densities, recombination radiation is improbable and will not reveal the highly mobile electronic charge or current distributions. Because visible or shorter wavelength light does not couple strongly to electron densities in plasmas-which are common to the initiation of electrical breakdown phenomena-this new technique utilizes THz radiation, which has a strong coupling to these densities. In fact, the plasma frequencies of electron densities from 10 12 to 10 cm-3 are in the 0.01 to 10 THz range, so they radiate and strongly absorb radiation in the THz regime.

2005 IEEE Pulsed Power Conference, 2005

Digest of Technical Papers. Tenth IEEE International Pulsed Power Conference, 1995

Optically activated GaAs switches operated in their high gain mode are being used or tested for p... more Optically activated GaAs switches operated in their high gain mode are being used or tested for pulsed power applications as diverse as low impedance, high current pulsers, and high impedance, low current Pockels cell or Q switch drivers. These are important to firing sets in munitions, lasers used in detonation of munitions, and lasers used in large weapons effects simulators (such as Jupiter). For firing sets we have switched 2.8 kA at 3 kV dc charge in a very compact package. For driving Q switches, the load is the small (30 pF) capacitance of the Q switch which is charged to 6 kV. We have demonstrated that we can modulate a laser beam with a sub ns risetime. Some aspects of the switches that are relevant to most of these applications are lifetime (longevity), leakage resistance, jitter, and trigger energy. This paper will describe the specific project requirements and switch parameters in all of these applications, and emphasize the.switch research and development that is being pursued to address the important issues.

Ninth IEEE International Pulsed Power Conference, 1993

Digest of Technical Papers. 11th IEEE International Pulsed Power Conference (Cat. No.97CH36127)

High gain GaAs photoconductive semiconductor switches (PCSS) are being used in a variety of elect... more High gain GaAs photoconductive semiconductor switches (PCSS) are being used in a variety of electrical and optical short pulse applications. The highest power application, which we are developing, is a compact, repetitive, short pulse linear induction accelerator. The array of PCSS, which drive the accelerator, will switch 75 kA and 250 kV in 30 ns long pulses at 50 Hz. The accelerator will produce a 700 kV, 7kA electron beam for industrial and military applications. In the low power regime, these switches are being used to switch 400 A and 5 kV to drive laser diode arrays which produce 100 ps optical pulses. These short optical pulses are for military and commercial applications in optical and electrical range sensing, 3D laser radar, and high speed imaging. Both types of these applications demand a better understanding of the switch properties to increase switch lifetime, reduce jitter, optimize optical triggering, and improve overall switch performance. These applications and experiments on the fundamental behavior of high gain GaAs switches will be discussed. Open shutter, infra-red images and time-resolved Schlieren images of the current filaments, which form during high gain switching, will be presented. Results from optical triggering experiments to produce multiple, diffuse filaments for high current repetitive switching will be described. * This work was supported by the U.S. Department Of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.

2013 19th IEEE Pulsed Power Conference (PPC), 2013

Methods of producing multiple current-sharing filaments (MCSF) in GaAs photoconductive semiconduc... more Methods of producing multiple current-sharing filaments (MCSF) in GaAs photoconductive semiconductor switches (PCSSs) have produced as many as 30 filaments per switch with a spacing of 330 µm. As the approaches to triggering and isolating MCSF mature, the replacement of high current, conventional pulsed power switches with banks of MCSF PCSSs capable of switching tens of kiloamps become feasible and economical. Multiple banks of MCSF PCSS can eventually produce optically controlled pulsed power systems with faster rise-times, shorter pulses, and higher peak powers using more economical switches with device lifetimes of over one million pulses. This paper will report progress from three types of MCSF approaches: (1) line-of-sight (LOS) optics focused with cylindrical micro-lens arrays into bright narrow lines of light on the surface of the PCSS; (2) high-reflectivity dielectric optical masks which produce shadows of bright narrow lines of light on the surface of the PCSS; and (3) etched-steps in the surface of the PCSS to divide up the illuminating light and isolate the filaments. These approaches will be tested to switch 2-3 kA with three 1cm square GaAs PCSS. Switching parameters, approximate device lifetimes, and current-sharing capability are being measured and evaluated for each of these triggering techniques. Many other approaches have been considered previously, and their potential for other specific applications will be discussed. Increasing PCSS current density through the development of longer-lived PCSS with higher currents per filament is research that has continued in parallel with the multi-filament triggering work. This however is primarily an interface issue where the contact metal meets the GaAs. We will discuss new electrical contact geometries capable of reducing current crowding, thereby lowering the peak current density to enable higher total filament current. Theoretical bases for improving contacts will also be discussed.

Twenty-First International Power Modulator Symposium, Conference

provided the circuit maintains the lock-on field.2 As the field increases, the switch risetime de... more provided the circuit maintains the lock-on field.2 As the field increases, the switch risetime decreases and the trigger energy is reduced.2 During A high peak power impulse pulser that is controlled with high gain, high gain switching tire switches emit bandgap radiation. When this optically triggered GaAs Photoconductive Semiconductor Switches radiation is imaged, filaments are observed, even if the triggering radiation (PCSS) has been constructed and tested. The system has a short 50,Q iiri_ is uniforrn.3,4 Table 1 shows the results from this experiment and the best that is charged to 100 kV and discharged through the switch when the results that we have achieved (in other work) with the high gain GaAs switch is triggered with as little as 90 nJ of laser energy. We have switches when triggered with either compact laser diode arrays or with demonstrated that the GaAs switches can be used to produce either a flashlamp-pumped lasers. The work of many others has been presented at monocycle or a monopulse with a period or total duration of about 3 ns. various conferences.5 For the monopulse, the voltage switched was above 100 kV, producing a peak power of about 48 MW to the 30 fl Iodd at a burst repetition rate of 1 This E_cp. Other* kHz. The laser that is used is a small laser diode array whose output is Switch Voltage (kV) 100 155 Switch Current (1_) 1.3 5.2 delivered through a fiber to the switch. The current in the system has rise ...... times of 430 ps and a pulse width of 1.4 ns when two laser diode anays are Peak Power OVIW) 48 120 used to trigger the switch. The small trigger energy and switch jitter are Rise time (ps) 430 430 due to a high gain switching mechanism in GaAs. This experiment also R-M-S jitter (ps) 150 150 shows a relationship between the rise time of the voltage across the switch ....Optical Trigger Energy (n J) 180 90 and the required trigger energy and switch jitter. Because the jitter is Repet!tion Rate (Hz) 1,000 1,000 small, we can trigger two current filaments simultaneously:i; Electric Field (kV/cm) 67 1110 The time evolution of the current filaments in an optically _ggered, Device Lifetime O_lo.pulses) NA 4,000,000' high gain Gan, s switch was studied by recording the infrared * Not all the results are simuitaneous. photoluminescence from the filaments. When the system is triggered with Table I. Results of tests with high gain GaAs switches. The first column two laser diode arrays that are activated within I ns of each other, two is this work only, the second column includes results of previous tests. current filamentsare observed. By delaying onelaser with respectto the. other, the evolution of the filament was recorded in a time resolved fashion. The filament that is triggered first crosses the switch, the voltage. Experimental Setup drops and the other filament ceases to grow. By varying the delay between

7th Pulsed Power Conference

SAR 18. NUMBER OF PAGES 3 19a. NAME OF RESPONSIBLE PERSON

In this paper, we describe a rate equation approach that leads to new insights about electrical b... more In this paper, we describe a rate equation approach that leads to new insights about electrical breakdown in insulating and semiconducting materials. In this approach, the competition between carrier generation by impact ionization and carrier recombination by Auger and defect recombination leads to steady state solutions for the carrier generation rate, and it is the accessibility of these steady state solutions, for a given electric field, that governs whether breakdown does or does not occur. This approach leads to theoretical definitions for not only the intrinsic breakdown field but also other characteristic quantities. Results obtained for GaAs using a carrier distribution function calculated by both a Maxwellian approximation and an ensemble Monte Carlo method will be discussed.

Physical Review Letters, 1980