Stacked Blumlein devices generating high-power, nanosecond-wide electrical pulses.

摘要

Many important advances in modern technology will require input powers which are too high to deliver continuously. Fortunately, constant operation is not necessary for some applications. In those cases, electrical input power can be delivered in pulses with specifically tailored characteristics. The most demanding applications create the need for particular combinations of operating parameters that are important in high-energy lasers, advanced accelerators and the generation of high-power microwaves; and those lie at the focus of this work. Conventional devices had been able to provide some of the parameters in the same arrangement, but none could deliver all of them at once.;In 1993, when this work was initiated, the concept of a high-voltage solid state switch capable of being triggered on demand held promise for a solution, despite traditional obstacles. Initially, all common solid-state materials, including GaAs and Si, had been designed for low-power applications, and attempts to extend their performance to higher powers created some problems. For high-voltage operation, these switches required some sort of isolation from the command circuit. Optical triggering solved these problems in a device known as a photoconductive switch. However, for some modern applications, which require pulses with very fast risetimes, conventional photoconductive switch technology was limited by the time required to produce enough charge carriers for commutation. Carrier generation could be accomplished in one of two ways. The first involved the use of an intense laser pulse with a fast risetime to produce all carriers through photon absorption. In this case, the necessary laser had to be large and expensive, and required its own high-power pulser, similar to the one being built, making this approach impractical.;The second method involved the initiation of a fast, non-linear, avalanche process within the semiconductor, using a more conventional light pulse. This technique, however, required solutions to several problems not previously resolved, the worst of which concerned spontaneous triggering during charging. When arranged to operate for avalanche conditions, the switch was subjected to a high electric field, causing it to leak current, generating heat, and ultimately inducing unwanted triggering. Therefore, in this work, an intermediary device was introduced which applied the charging voltage for a very limited amount of time. In addition, the switching voltage was reduced by utilizing a stacked Blumlein geometry in which several transmission lines were commutated by the switch in a parallel circuit. The lines were then combined in series at the output, increasing the generated voltage. The operating parameters were studied and optimized to find the region in which a GaAs switch could avalanche, producing output voltages of 70 kV, when activated by laser diode pulse energies as low as 40 ;The successful resolution of these issues provided for the construction of a power source capable of delivering kiloamp pulses to loads dissipating 70 MW with switching times as fast as 100 ps. The level of achievement was enhanced by the fact that the device was compact and operational at high repetition rates when triggered by simple, commercially available laser diodes. The resulting device could operate with combinations of parameters which were not possible before, but which will be required by the next advances in high-energy and microwave technology.