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Abstract
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Since 2016 there has been considerable discussion about anomalous health incidents (AHIs), popularly called the Havana syndrome, among US personnel stationed in Havana and elsewhere. One widely discussed theory is that the incidents were the result of attacks using pulsed microwave energy resulting in nonspecific symptoms reminiscent of vestibular disturbances (1, 2). In the commented paper, Foster et al. (3) analyzed thermoacoustically (TA) induced acoustic transients produced in a simple tissue model from high intensity pulsed microwaves over a wide frequency range and concluded that microwave pulses at extreme but feasible fluences (incident energy densities per pulse) could produce physiologically significant levels of acoustic energy in the head. The possibility of attacks by microwaves was considered in detail by JASON (4), an expert group commissioned by the U.S. Department of State, which could find no evidence for the use of microwaves. The practical difficulties of beaming high peak power microwaves at subjects in a "stealthy" manner seem daunting in any event.
Thermoacoustic sound generation results when a fluid is subject to rapid heating due to thermal expansion. The generation of acoustic waves is most efficient when the heating occurs over times shorter than the stress relaxation time of the medium (about 1 μs for the present case). The theory behind the effect is simple (3) and applies equally to pulsed laser light (5) as to pulsed microwaves. Unlike high peak power microwave sources, which are typically used only in classified military environments, high peak power lasers are commercially available and used for a variety of industrial applications. Such lasers are small (can be placed on a desktop) and together with their power supply and coolant system (which are typically in cabinets of < 1 m3 volume) and power source could be fitted into a van. Such lasers could transmit beams over long distances or, via fiberoptics, into enclosed spaces.
For example, the ANL10k10 Nd:YAG laser (Ekspla, a Lithuanian firm) generates 5 ns infrared pulses at a wavelength of 1.06 μm, with an output energy of 10 J/pulse and pulse repetition rate of 10 Hz. Such energy is invisible to humans. The transmission coefficient of this radiation into skin is high, and its (1/e) energy penetration depth in tissue is about 3.5 mm (6)--similar to those of 6 GHz microwaves. The laser beam would have to be expanded (to avoid skin burns from a narrow high intensity beam) and then aimed at the target, both of which are technically easy to do. This pulse energy, if uniformly distributed in a beam of 1 m2 area, would create a radiant exposure to a targeted individual of 10 J/m2.
When this energy is absorbed in skin, the resulting thermal expansion of tissue water will generate acoustic waves that will propagate deeper into tissue. The frequency range of the acoustic energy is determined by the energy penetration depth of the radiation in tissue, and the peak sound pressure is determined by the incremental amount of energy deposited in tissue, provided that the pulsewidth is shorter than the stress relaxation time of the medium. For IR wavelengths near 1 μm, IR energy is relatively highly penetrating in tissue, which results in relatively low frequency TA signals.
Language: en |