The OSPRES (ONR Short Pulse Research, Evaluation and non-SWaP) program performed fundamental work in the areas of Silicon (Si) and Gallium Nitride (GaN) based photoconductive switch development, measurement of HPM waveform dependent effects on small unmanned aerial vehicle (sUAV), adaptive design of experiments, noise injection, RF coupling to sUAV's, minimally dispersive wave theory, and positive feedback non-linear transmission line (NLTL) development.

Notable achievements and progress of OSPRES program by quarter.

The photoconductive solid state switch (PCSS) subgroup worked to develop a fundamental understanding of the limitations of Silicon and Gallium Nitride based photoconductive switches and their application to pulsed power. With an estimated 2 megawatts (MW) peak power generation (benchtop equivalent), the Si-PCSS subgroup has succeeded in achieving one-third of the peak power required for an individual element in the envisioned phased array required to meet mission needs.

Si-PCSSs have been integrated with a rapid charge capacitor system and pulse forming transmission lines for pulse testing. Si-PCSS hold-off voltage has been pushed to >6 kV with 3 kV output pulse moving closer to the project goal of 10 kV hold-off. Currently, thermal mitigation issues are the primary issue plaguing the Si- PCSS group. Cooling system designs are changing weekly and progressively moving towards a goal of 500 W/cm2 cooling capacity.

The University of Missouri – Kansas City's (UMKC's) effects team has developed a custom UAV model for RF effects testing, written code for the automation of testing, and researched the areas of an adaptive design of experiments (DoE) and noise injection. The adaptive DoE was developed to refine/inform optimum source parameters that will result in known effects on sUAV's and further inform the requirements for a tunable HPM system with control over the policy desired levels-of-lethality. The effects team has also traveled to multiple test locations performing over 1300 effects tests with the custom UAV.

UMKC's GaN:C simulations/modeling subgroup has made progress in optimizing GaN:C for a high power, high repetition rate solid state switch. They have computed electronic structure properties of GaN:C including the optical and symmetric band structure of GaN:C, and obtained the software to perform GaN-PCSS device level calculations to model optical absorption, hold-off voltage, and thermal dissipation relevant to determining the optimum design of a GaN:C based PCSS.

The RF coupling subgroup used Characteristic Mode Analysis (CMA) to computationally quantify electromagnetic coupling and interference to UAV frames over the L-band and Sband and validated CMA coupling predictions using experimental measurements. The RF coupling group also reviewed the most common sUAV shapes, sizes, material compositions, and electronics commercially available and developed a table of occurrences and frequency in sUAVs.

The Positive Feedback NLTL group has developed a positive feedback power amplifier coupled with a nonlinear transmission line generating ones-of-kW power RF pulses in a closed-loop network coupled with a 9-section 1200 V rated Schottky diode NLTL. Several closed-loop experiments at low voltage level within a frequency range of 5 to 70-MHz have been performed and continuous RF signal outputs with positive gain at different stages of the NLTL have been performed.

Preliminary results of the Focused Wave Mode (FWM) group suggest that EM pulses with smaller rise times can effectively launch electromagnetic waves whose time-average power density shows minimal spatial decay contrary to the standard 1/r2 decay of EM waves. The group designed and simulated low-profile, ultra-wideband micro-strip patch antennas to demonstrate the potential of launching low dispersion EM waves with results demonstrating the feasibility of short EM pulses for c-sUAV ranges relevant to the project.

This work was done by E.R. Myers, T. Fields, J.A. Crow, D. Chatterjee, P. Rulis, P. Doynov, A. Hassan, J. Lancaster, F. Khan, J. Verzella, J. Beaudin, L. Moler and A.N. Caruso of the University of Missouri – Kansas City for the Naval Research Laboratory.

This Brief includes a Technical Support Package (TSP).
ONR Short Pulse Research, Evaluation and non-SWaP Demonstration for C-sUAV Study

(reference NRL-0075) is currently available for download from the TSP library.

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This article first appeared in the June, 2019 issue of Aerospace & Defense Technology Magazine.

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