Compact Megavolt Switch Utilizing Novel Switching Mediums
Navy STTR FY2014A - Topic N14A-T018
ONR - Steve Sullivan -
Opens: March 5, 2014 - Closes: April 9, 2014 6:00am EST

N14A-T018 TITLE: Compact Megavolt Switch Utilizing Novel Switching Mediums

TECHNOLOGY AREAS: Materials/Processes, Electronics, Weapons

ACQUISITION PROGRAM: ONR Code 35: High Power Radio Frequency (HPRF) Basic Research

OBJECTIVE: Develop, design, and build a compact (<150 in3) high voltage switch for use in High Power Radio Frequency (HPRF) applications on naval platforms utilizing novel or innovative switching mediums/dielectrics capable of switching megavolts, in 750 ps or less, pulse lengths of < 300 ns, 20 to 100 ns switch charge times, at pulse repetition frequencies of 100 Hz - 1 kHz.

DESCRIPTION: Switches are a critical component for the pulsed power systems utilized to produce the high voltages/currents for HPRF generation. HPRF systems use a wide range of opening and closing switch technologies from traditional spark gap, gas insulated switches to solid state photoconductive switches. The commonality among these switches is the capability of the switch medium to transition from insulator to conductor, to act as the connection between the energy storage system and the load, all with precision timing to produce the desired output pulse. A very traditional switching medium, because of its intrinsic and extrinsic chemical properties is Sulfur Hexafluoride (SF6). However, the main motivation of this topic began with the desire to eliminate or reduce the use of SF6 as a switching medium in HPRF systems. SF6 is not only more costly than other insulating gases, but after subjected to electrical discharges, toxic and corrosive compounds are formed. These toxins present an environmental and personnel hazard for the maintainability and disposal of the system, increasing the overall system life-cycle costs. As the Navy endeavors to minimize the use of such materials and reduce system costs, alternative dielectric mediums that not only meet our current, but future technology needs, that can also be easily integrated into naval architectures and withstand the maritime environment must be found.

The electric power industry is the dominant commercial user of SF6, accounting for 80% of the world market alone, the majority of which is utilized for circuit breaker applications (Ref 10). However, SF6 is a greenhouse gas with an estimated lifetime of 3,000+ years in the atmosphere (Ref 17). One of the primary sources of SF6 emissions is leakage from power transmission and distribution equipment. Although the EPA has developed programs such as the SF6 Emission Reduction Partnership for Electric Power Systems, the main focus has been on reducing the use and leakage of SF6. Little to no effort by either the EPA or their commercial partners has gone into finding an alternative switching medium. Thus, the need for the development of switching mediums comparable in performance to SF6 is a capability gap for not only the military, but for industry as well.

A driving factor in the transition of HPRF systems is a focus on applicability to operational use. As these systems push the envelope of increased energy and power on-target, the more robust the subsystem components must become, while remaining in a compact form. The U.S. Navy is interested in further developing switch technology hold-off capabilities through the studying of innovative and novel dielectrics that will not only advance the current state-of-the-art, but also be safer and cheaper to utilize within HPRF systems. The switching mediums of interest include, but are not limited to, gases and gas mixtures (i.e. 40%SF6-60%N2), liquids (i.e. oil, water), and solid state materials (i.e. photoconductive switches utilizing GaAs, 4H-SiC, GaN, etc.). The challenge for switch technology development is improved performance, in the 1 MV to 10 MV range, with switching times of 750 ps or less, for pulse lengths of < 300 ns, and switch charge times between 20 ns to 100 ns. In addition, repetition rates in the 100 Hz to 1 kHz regime are required, all in a footprint of less than 150 in3 (0.002 m3). The current state-of-the-art for gas, water, and oil switches ranges from 1 to 6 MV, 170 in3 to 9100 in3 in size, with 1 to 100 Hz pulse repetition frequencies, and switch charge times of 10 to 30 ns. Photoconductive switches are reaching comparable specifications with 10’s of picosecond switch times and 10’s of kV hold-off voltage for single element handling capability, but in a much smaller package than gas or oil switches. A secondary goal of the overall switch development is to minimize the secondary support systems needed to support the dielectrics such as gas mixing, oil circulation, deionization water filtration, gas expansion chambers, laser triggering units, etc. A novel switch design should aim for reducing or removing the need for these types of secondary support systems.

PHASE I: Conceptualize and design a breadboard switch utilizing the chosen novel switching medium. Although not required, more than one switch may be developed and/or more than one switching medium evaluated. For the completion of Phase I, the switch prototype design(s) should be capable of the following performance characteristics in a single switching event (or pulse count):

Phase I Design Parameters:
• 500 kV switch voltage
• 1 ns or less switch time
• 100 ns pulse lengths
• 10 ns charge times
• 10 Hz pulse repetition frequency
• Volume of < 1200 in3

Electromagnetic and circuit modeling and simulation of the switch design should be conducted and results leading to the final design(s) should be documented and provided in the final report along with a data package on all proposed critical components in the breadboard system design. A design plan should also be submitted outlining the plans for scaling the switch and support systems for the Phase II requirements.

PHASE II: Construct and test a brassboard switch utilizing the Phase I design and the chosen novel switching medium. The use of actual hardware and empirical data collection is expected for the performance analysis of the switch and switching medium and should be provided in the final report along with a data package on all critical components in the brassboard system. At the completion of Phase I, the prototype switch should be capable of demonstrating the following performance characteristics in a single switching event (or pulse count):

Phase I Design Parameters:
• 500 kV to 1 MV switch voltage
• 1 ns or less switch time
• 100 ns pulse lengths
• = 10 ns charge times
• 10 to 100 Hz pulse repetition frequency
• Volume of < 300 in3

The Phase II switch prototype must demonstrate a clear path towards addressing the scalability challenges along with packaging the system into a relatively useful volume. At this point, the prototype should be able to demonstrate switch capabilities with minimal secondary system support, even if for a short test cycle. Furthermore, a plan should be developed clearing stating the methodology for future secondary system reduction and scalability for a fully developed switch. All data collected in the analysis of the switch and switching medium of the prototype system will be included in the final report along with a data package on all critical system components.

PHASE III: Phase III will consist of a demonstration of a fully capable, compact switch meeting the specified switch requirements (below) along with no immediate secondary system support. The final system will represent a complete solution and should be ruggedized for, at a minimum, testing in a dry, outdoor environment and be environmentally enclosed.

Phase III Parameters:
• 1 MV to 10 MV switch voltage
• 750 ns or less switch time
• < 300 ns pulse lengths
• 20 to 200 ns charge times
• 100 Hz to 1kHz pulse repetition frequency
• Volume of 150 in3

All data collected in the analysis of the switch and switching medium of the final system will be included in the final report along with a user’s manual and a data package on all critical system components. The final system shall be developed with performance specifications satisfying the targeted acquisition program requirements coordinated with technical point of contact. A preliminary design package and plan outlining the use of the switch in commercial switching applications should also be submitted with the final report.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: HPRF sources, and consequently switches, are used in a wide variety of commercial applications including electric power industry, semiconductor processing, x-ray machines, pulsed power, and medical applications. The use of SF6 in the commercial power industry for insulating high voltage equipment and/or as an arc quenching medium is abundant. From circuit breakers to gas-insulated substations, it has been estimated that 80% of the SF6 produced worldwide is utilized by the electric power industry alone (Ref 10). In addition, SF6 has been identified as a greenhouse gas with a global warming potential of ~23,000 times greater than carbon dioxide. Although it has been a priority of the EPA to reduce the usage and emission of SF6, little has been done to develop an alternative switching medium that is comparable.

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KEYWORDS: High power radio frequency; high power microwave; dielectrics; closing switches; opening switches; spark gap; semiconductor solid state switches; photoconductive switches

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