High current, multi-filament photoconductive semiconductor switching

High current switching is the most critical challenge remaining for photoconductive semiconductor switch (PCSS) applications in Pulsed Power. Many authors have described the advantageous properties of high gain PCSS such as, low optical trigger energy and inductance, sub-nanosecond risetime and jitter, optical isolation and control, pulsed or DC charging, and long device lifetime, provided the current per filament is limited to 20-30A for short pulse (10-20ns) applications [1,2]. Low energy optical triggering, long device lifetime, and current filaments are related features of high gain PCSS that make high current switching a challenge. Since the location and number of current filaments can be controlled with parallel “lines” of optical pulses across the insulating gap, the problem of high current, multi-filament PCSS switching is essentially the problem of producing a reliable, efficient, multi-line, optical delivery system [3]. In this paper, several classes of optical delivery systems will be discussed: line-of-sight plastic and glass microlens arrays, multi-mode fiber-optic/micro-lens combinations, single-mode fiber-optic/micro-lens array combinations, and masked PCSS alone and with other optical "concentrating" components. The application dependent advantages and disadvantages of each approach will be discussed. Results will be shown from specific examples (plastic and glass micro-lens arrays, single and multi-mode fibers and bundles, and high density optically masked PCSS) that have been tested and demonstrated. The fundamental requirements of multi-filament, high gain PCSS triggering are also being measured and will be reported. Optical trigger energy, pulse width, line-width, and spatial density are the key factors in determining the attainable switching efficiency and volume for high current pulsed power applications. Measurements of these properties and their implied trade-offs will be presented. A high voltage, high current, high gain PCSS demonstration will be described. In these tests, three 1 cm gap PCSS switch a 60 KY Blumlein conducting 2.5 kA for 6 ns with approximately 100 filaments spaced at 30 filaments/cm. This system will test the practical aspects of optical triggering with the line-of-sight micro-lens and masked PCSS approaches. Along with these results, we will also discuss our plans to demonstrate 100 kY, 25 kA, 1000 filament switching with a bank of PCSS.