Improved Marx Pulse Generator for High Power Microwave (HPM) Systems

RT&L FOCUS AREA(S): Directed Energy (DE) TECHNOLOGY AREA(S): Electronics;Ground / Sea Vehicles;Weapons OBJECTIVE: Develop a sub-10 nanosecond rise time, GW-class Marx generator with long-lifetime insulated spark gap switches with preferred air (or other gas) as the primary insulation medium to support the deployment of next-generation HPM weapons. DESCRIPTION: High power microwave (HPM) systems frequently require high voltage, high current, nanosecond duration pulses in order to generate radio frequency (RF) signals [Ref 1]. Such pulses are frequently generated by Marx generators [Refs 2, 4], which consist of multiple stages of capacitors charged in parallel and rapidly connected in series, usually by gas spark gap switches [Ref 5], to boost the voltage and provide an output pulse [Ref 6]. The spark gap switches consist of at least two electrodes, often contained within a high-pressure body filled with gas, and often utilize a trigger electrode situated between the two primary electrodes. A high voltage pulse is sent to the trigger, which breaks down to one of the primary electrodes, causing breakdown between both primary electrodes, thereby electrically connecting stages in series. Several failure mechanisms exist for these switches, making lifetime and maintenance a frequent issue. This is particularly true when compact designs are required. Problems include surface tracking between electrodes along the body of the switch and electrode erosion. High repetition rates can also be problematic, as gas flow is required to clear ionization and heat from previous shots. If sufficient gas purging is not accomplished between shots, thermal damage may be incurred or hold-off decreases, allowing breakdown to occur before full voltage is achieved [Ref 7]. Meanwhile, the Marx stages must be insulated from each other, which is usually accomplished with gas or transformer oil. In compact Marx generators, oil is often the insulator of choice due to its superior voltage hold-off qualities at Standard Temperature and Pressure (STP). However, oil insulation creates several issues. First, the use of insulating oil complicates Marx maintenance in deployed environments because of onerous drain and fill processes and OCONUS supply chain challenges. Second, high pressure switches leak into the oil at finite rates. Gas leakage occurs through switch body or gas fitting failures or by gas diffusion through the switch body and routing lines over long-term depot storage periods. Gas leaking into the oil results in dissolved gas or bubbles that significantly reduce the voltage insulating capability of the oil, and can cause ill-formed pulses or catastrophic failure of the Marx. Finally, oil insulation may result in Marx designs heavier than those insulated by gas. A compact, fast rise time Marx design that utilizes air as its insulation and long life gas switches is therefore desired. KEY MARX GENERATOR PARAMETERS: 300 kV peak pulse voltage or more into a matched load 13-20 O (ohm) output impedance 10 ns 10 90% voltage rise time or less 15 50 ns FWHM Threshold 10,000 / Objective 50,000 shot lifetime without maintenance or better 0.50 m diameter or smaller 1.25 m length or smaller Threshold 50 Hz / Objective 300 Hz repetition rate for 10 shots every 2 seconds for 5 bursts, or better 200 kg weight or less Air insulated (or other gases such as SF6 replacement mixture) Tolerant of up to 40% reversal of the output PHASE I: Develop a concept for a compact Marx generator meeting the above Key Parameters that utilizes air as its insulation medium for both the switch and primary Marx insulation. Simulate switch electrostatics, and establish switch-purging methodology. Simulate Marx pressure vessel hydrostatics and electrostatics. Prepare a report to ONR and AFRL on designs and simulations and a Phase II testing plan. PHASE II: Fabricate switch design(s) developed during Phase I for high voltage and pressure testing and gas flow analysis. These tests are to be performed under the temperatures outlined in MIL-STD-810H Test Methods 501.7 (Hot Dry) and 502.7 (Basic Cold). Measure switch lifetime under representative conditions and refine design as necessary to meet Key Parameters. Fabricate Marx pressure vessel and test at pressure. Provide switch prototype, Marx pressure vessel prototype, and report containing designs and testing results, and a Phase III plan to ONR/AFRL/NSWCDD for prototype evaluation. PHASE III DUAL USE APPLICATIONS: Assemble full gas-insulated Marx generator and demonstrate output meeting Key Parameters. Demonstrate full Marx lifetime operating into a matched load. Deliver Marx prototype and a report containing designs and testing data to ONR/AFRL/NSWCDD. The development of a compact, high reliably pulsed power system would expand the present state of the art for Directed Energy and enable component level exploitation for commercial applications in the pulsed power industry. REFERENCES: 1. Korovin, S. D. et. al. Pulsed Power-Driven High-Power Microwave Sources. Proc. of the IEEE, Vol. 92, No. 7, 2004. 2. Walterx, J. et. al. An energy efficient vircator-based HPM system. 2011 IEEE Pulsed Power Conference. DOI 10.1109/PPC.2011.6191558. 3. Sharma, A. et. al. Development and Characterization of Repetitive 1-kJ Marx-Generator-Driven Reflex Triode System for High-Power Microwave Generation. IEEE Trans. On Plasma Science, Vol. 39, Issue 5, May 2011. 4. Pouncey, J.C.; Lehr, J.M.; and Giri, D. V. "Erection of Compact Marx Generators." IEEE Transactions on Plasma Science, vol. 47, no. 6, June 2019, pp. 2902-2909. doi: 10.1109/TPS.2019.2915034. 5. Pouncey, J.C. and Lehr, J. M. A parametric SPICE model for the simulation of spark gap switches. Rev. of Sci. Instr., 91, 034704, 2020. 6. Fitch, R. and Crewson, W. Marx generator and triggering circuitry therefor. US3746881A, United States Patent and Trademark Office, 17 July 1973. 7. Winands, G. J. J. et. al. Long lifetime, triggered, spark-gap switch for repetitive pulsed power applications. Rev. of Sci. Instr., 76, 085107, 2005.