A photo of an ADT demonstrator (left) and a conceptual drawing of a future iteration of ADT under development (right).

Beam Propagation Model Selection for Millimeter-Wave Directed Energy Weapons

Comparing the relatively simple Fraunhofer or “far field” (FF) approximation commonly used in radar and high-powered microwave systems with the more complex near field (NF) propagation model based on the field equivalence principle demonstrates which approach achieves reasonable modeling fidelity with minimal compute power.

Modeling and Simulation (M&S) can be used to explore the design trade space of directed energy weapons. M&S can be particularly helpful when that trade space is influenced by a large number of parameters and when acquiring field data to explore those parameters requires a large amount of resources. One example involves the Active Denial Technology (ADT) system, a non-lethal, counter-personnel, directed energy weapon that outputs high-powered, millimeter wave electromagnetic energy for crowd control, patrol/convoy protection, and perimeter security. The accompanying figure shows a photograph of a current ADT demonstrator (left) and a conceptual drawing of a future iteration of ADT (right).

The ADT system subjects a targeted individual to short-duration pulses of a focused beam of directed energy operating at a frequency of approximately 95 GHz (3.2 millimeters in wavelength). At this frequency, and within a known range of doses, the energy diffuses approximately 1/64th inch (400 microns) into the skin of the targeted individual, producing no skin damage. Yet the targeted individual perceives an intense burning sensation, potentially strong enough to repel—that is, to compel the targeted individual to immediately flee the beam.

ADT systems that are currently under development can be placed into one of two broad categories: fixed- and variable-focus systems. A fixed-focus system combines a high-power source with a fixed-focus reflector to achieve operational power densities and spot sizes at relatively long ranges (500 - 1000 m). Variable-focus systems are phased arrays of relatively low power emitters with electronic phase control, allowing for dynamic beam-steering and focusing (e.g. the focal point can be varied). Such systems are expected to deliver an active denial capability in smaller form factors.

Like all weapon technologies, the effectiveness of ADT is dependent on both the system design parameters and the target properties. System design parameters include the ADT frequency and output power, among others. Target properties include the targeted individual's skin reflectivity, thermal conductivity, specific heat capacity, density of heat-sensitive neural endings, pain perception thresholds, and motivation, to name a few.

Simultaneous exploration of all of these parameters via M&S requires several different model components— some to model the ADT system's output energy, and others to model the targeted individual's physiology, cognition, and behavior. Together, these components can be used to rapidly test hypotheses about how changes to the ADT system design will ultimately lead to changes in the ADT system effectiveness. However, running such a large model can be computationally expensive and therefore each individual component must be as low-intensive as possible. This research focuses on only the first component—that which simulates the propagation of the ADT beam through the environment to the targeted individual.

Two different computational models are explored to determine for which situations each model has the necessary balance of fidelity versus computational intensity:

  • The first model uses the simple Fraunhofer approximation, also known as the far field (FF) approximation, that is common in radar and high-powered microwave (HPM) applications. This approximation is simple and is not computationally intensive. However, operational ranges for high-powered, millimeter wave systems like ADT often fall well within the Fresnel region where we cannot assume that the electromagnetic fields are purely diffractive—thus this approximation may not provide the necessary fidelity for all situations.

  • The second model is a near field (NF) extension of the FF approximation where the system is approximated by a discrete array of radiators. This approximation is slightly more complex, meaning it is more computationally intensive but may provide improved fidelity for some situations.

    The outputs of the two computational models are compared for both a fixed-and variable-focus millimeter wave system to see in which situations they differ. The models are also validated by comparing their outputs to experimental measurements taken with the variable-focus system.

This work was done by John Biddle and Shelley Cazares for the Institute for Defense Analyses. For more information, download the Technical Support Package below.

This Brief includes a Technical Support Package (TSP).
Beam Propagation Model Selection for Millimeter-Wave Directed Energy Weapons

(reference IDA-0001) is currently available for download from the TSP library.

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