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DIRECTED ENERGY PROFESSIONAL SOCIETY

Abstract: 26-DETE-006

UNCLASSIFIED, PUBLIC RELEASE

Modular Spark Gap Array (MSGA)

High Power Microwave (HPM) systems have historically advanced along two largely separate technological paths. Narrowband microwave sources such as klystrons, magnetrons, and solid state phased arrays have focused on continuous wave or quasi continuous operation with frequency domain phase coherence, while ultra-wideband (UWB) impulsive radiators have emphasized extremely fast transient fields generated by high voltage pulsed power drivers and impulse radiating antennas. Despite decades of development, both approaches face fundamental scaling limitations when considered for distributed, resilient, or space compatible architectures. Narrowband systems are constrained by thermal dissipation, power combining complexity, and device reliability, while traditional UWB systems rely on single large pulsers that are mechanically scaled, fixed site, and inherently single point failures. As a result, no existing HPM source architecture provides a verified path to modular, fault tolerant scaling based on fundamental electromagnetic field superposition rather than brute force voltage or aperture growth.

This paper presents a new HPM source architecture, referred to as the Modular Spark Gap Array (MSGA), that replaces voltage centric scaling with synchronization centric scaling. The core hypothesis is that multiple low voltage, ultra-wideband impulsive sources can be synchronized with nanosecond level timing precision such that their radiated electromagnetic fields constructively add in the far field, producing linear scaling of the field range figure of merit E times R with module count. Rather than attempting to generate extreme voltages in a single pulser, the architecture distributes energy across many simple spark gap modules whose outputs are coherently combined through time domain alignment. The work focuses on the fundamental physics, diagnostics, and test methodology required to validate this hypothesis within a laboratory HPM test and evaluation environment.

The proposed approach builds on well established principles of impulse radiating antennas, time domain electromagnetic superposition, and timed array theory. In contrast to narrowband phased arrays, where phase coherence is enforced at a single frequency, the MSGA operates in the time domain and relies on true time delay alignment of broadband impulses. For UWB signals with rise times on the order of a few nanoseconds and pulse widths on the order of hundreds of nanoseconds, constructive interference is governed by synchronization jitter rather than carrier phase. If inter module timing error is kept below a small fraction of the impulse rise time, the resulting far field waveform preserves amplitude, bandwidth, and waveform fidelity while scaling proportionally with the number of contributing sources.

Each MSGA module consists of a fast spark gap switch, a compact pulse forming network, and a broadband radiating element. The spark gaps operate at relatively modest voltages compared to legacy UWB pulsers, enabling physically simple construction using commercially available materials. Triggering is distributed using an optical timing network designed to enforce sub ten nanosecond inter module jitter. By eliminating reliance on extreme voltage hold off or monolithic pulse generators, the architecture enables modular growth, graceful degradation under failure, and potential compatibility with constrained platforms.

From an HPM test and evaluation perspective, the primary contribution of this work is the definition and experimental validation of a modular scaling law based on measured transient electromagnetic fields rather than inferred source power. Performance is evaluated using the field range figure of merit E times R, which is widely used in UWB and HPM literature to characterize impulsive radiators independent of test range. By measuring transient electric and magnetic fields using calibrated B dot, D dot, and wideband E field probes, the experiments directly assess whether synchronized impulsive sources produce linear far field scaling as predicted by theory.

The paper describes the experimental methodology used to validate synchronization, radiation efficiency, bandwidth, and scaling behavior. Spark gap discharge physics is first characterized to quantify timing jitter, plasma stability, and electrode erosion under repeated impulsive operation. This step is critical, as discharge variability directly limits achievable synchronization. Plasma models and empirical measurements are used to optimize electrode geometry, gas environment, and pulse shaping to achieve repeatable breakdown with jitter below ten nanoseconds.

Electromagnetic modeling is then performed to predict near field impulse formation and far field superposition for multiple modules. Time domain full wave simulations are used to explore the sensitivity of coherent addition to timing error, array geometry, and antenna coupling. These simulations establish quantitative tolerances for synchronization and guide the design of the optical trigger distribution system.

A laboratory scale multi module MSGA prototype is constructed and integrated with broadband radiating elements. Transient field measurements are conducted at calibrated distances to verify pulse shape, bandwidth, and amplitude. Single module and multi module shots are compared to isolate the contribution of coherent addition. Measured results demonstrate that when inter module jitter is maintained below the target threshold, the peak transient field amplitude scales approximately linearly with the number of modules, confirming the underlying synchronization driven scaling hypothesis.

The results have direct implications for HPM sensors and sources. From a source perspective, MSGA demonstrates that high peak field levels can be achieved without resorting to single extremely high voltage pulsers, opening new paths for scalable and distributed HPM architectures. From a sensor and test perspective, the work emphasizes the importance of time domain diagnostics, synchronization measurement, and uncertainty analysis when evaluating UWB HPM systems. Traditional frequency domain metrics are insufficient to capture the behavior of impulsive, broadband sources whose performance is governed by nanosecond scale timing.

The paper also discusses the limitations and boundaries of the approach. If synchronization jitter exceeds the impulse rise time, constructive addition collapses and scaling fails. Similarly, inefficient coupling between the pulsed power source and the radiator reduces electrical to radiated efficiency and limits achievable field strength. These constraints define clear testable boundaries for the architecture and provide a framework for future evaluation.

Importantly, the work is scoped strictly to fundamental HPM source physics and measurement. No claims are made regarding operational deployment, lethality, or specific target effects. The emphasis is on experimentally validating electromagnetic superposition, synchronization tolerances, and modular scaling laws using established HPM diagnostics. As such, the results are directly relevant to the DEPS community, particularly those engaged in test and evaluation of advanced HPM sources, transient EM measurement techniques, and next generation directed energy architectures.

In summary, this paper introduces and experimentally validates a synchronization driven approach to scaling ultra wideband HPM sources using modular spark gap arrays. By shifting the primary performance driver from voltage magnitude to timing precision, the architecture offers a fundamentally different scaling path from both narrowband phased arrays and legacy UWB pulsers. The work provides new experimental data, test methodologies, and scaling insights that are directly applicable to HPM source development and evaluation, and it establishes a physics based foundation for future exploration of modular, resilient directed energy systems.

UNCLASSIFIED, PUBLIC RELEASE