A compact, repetitive accelerator for military and industrial applications

Abstract

A compact, short pulse, repetitive accelerator has many useful military and commercial applications in biological counter-proliferation , materials processing, radiography, and sterilization (medical instruments, waste, and food). The goal of this project was to develop and demonstrate a small, 700 kV accelerator, which can produce 7 kA particle beams with pulse lengths of 10-30 ns at rates up to 50 Hz. At reduced power levels, longer pulses or higher repetition rates (up to 10 kHz) could be achieved. Two switching technologies were tested: (1) spark gaps, which have been used to build low repetition rate accelerators for many years; and (2) high gain photoconductive semiconductor switches (PCSS), a new solid state switching technology. This plan was economical, because it used existing hardware for the accelerator, and the PCSS material and fabrication for one module was relatively inexpensive. It was research oriented, because it provided a test bed to examine the utility of other emerging switching technologies, such as magnetic switches. At full power, the accelerator will produce 700 kV and 7 l~4 with either the spark gap or PCSS pulser.Mature spark gap technology was used to demonstrate operation of the accelerator. Later multiple PCSS were tested with a PFL, which could drive 1/8" of the accelerator. The spark gap based system is pulse charged with a low impedance pulse forming line (PFL) which is switched by a single high current spark gap. The PFL drives four small linear induction accelerator (LIA) cavities. In FY95 and 96, the spark gap system was assembled and tested. A field-enhanced diode was tested, and the LIA was analyzed for modifications to operate continuously at 50 Hz. Prolonged testing at 500 kV and 5.5 kA produced several cable breakdown problems, which were repaired, but eventually, the cables will have to be replaced with higher breakdown strength cables for continuous operation. The longest burst tested was 3000 pulses at 30 Hz.The main advantages of PCSS versus spark gaps for the LIA are lower inductance and greater design flexibility due to the distributed switching that is possible with precisely triggered PCSS. For the LIA and the diode, this translates to faster rise-times, more rectangular pulses, the capability of shorter pulses, and potentially higher diode efficiency. The PCSS-based pulser is composed of 8 identical modules. Each module is switched with six 2-inch diameter PCSS. In FY97, 30 simultaneous current filaments were initiated on a 3-cm wide section of a single switch to limit the current per filament to less than 50 A (500 Ncm) and improve switch lifetime. The first module was assembled and tested to 210 kV and 8 kA. Further optimization of the optical trigger delivery system is necessary to reach 250 kV and 10 kA (full power for 1 module) with reasonable PCSS lifetimes (>10,000 pulses). Improved PCSS contacts were developed which demonstrated switch lifetime greater than lo7 pulses.. However, further resources are necessary to incorporate the new switches, ...

High current, multi-filament photoconductive semiconductor switching

“…This research addresses this limitation by seeking methods to efficiently and reliably generate large numbers of current sharing, linear filaments on a single PCSS. One approach to multi-filament high gain PCSS triggering is to form lines of light across the PCSS with the trigger laser or lasers and a suitable optical delivery system [3]. This approach results in the creation of multiple, current sharing, linear filaments along the lines of the light.…”

“…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 micro lens 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.…”

“…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].…”

Photoconductive, semiconductor switch technology for short pulse electromagnetics and lasers

“…These goals can be achieved with banks of high gain PCSS producing many independent current filaments [3].…”

“…This paper will provide a brief discussion of PCSS switching properties and general applications, and then focus on optical pulse generation and control with PCSS technology. Specific electromagnetic pulse applications have been demonstrated and documented in internal reports [2,3].…”