This technology is more commonly called a plasma shield. However, its more formal name is used here in order to distinguish it from the cold plasma shields currently being developed to protect spacecraft.
The Plasma Acoustic Shield System (PASS) is a defense system being researched by the company Stellar Photonics at the US Army's Advanced Energy Armaments Systems Division at Picatinny Arsenal in New Jersey. Its operation is based on the technique of dynamic pulse detonation. A laser focused on a specific spot in the air will superheat the gasses there instantly, creating a super-hot ball of plasma called a plasmoid. A second laser focused on the same spot creates a supersonic shockwave within the plasmoid, resulting in an explosion accompanied by a bright flash of light and a loud, often disorienting bang. Focused microwaves are thought to also be able to produce this kind of effect.
The current system has a maximum range of about one hundred meters. Because of power issues, many think that PASS is unlikely to be used as an offensive weapon anytime in the near future. The lasers, by themselves, would have trouble burning through a sheet of paper.
As a potential defense against incoming projectiles, PASS may be surprisingly effective. Unlike other laser weapon systems being developed, such as the Airborne Laser, PASS does not rely on directly hitting a target and burning through a missile’s casing. Rather, rapid firing of the system in a relatively short area just ahead of the speeding projectile could produce dozens or even hundreds of small, potent shockwaves within a fraction of a second that would cause the missile to tumble and perhaps even throw it off-target.
While this would not necessarily prevent the missile from detonating or even hitting the target, causing the projectile to tumble could still prevent a great deal of armor penetration, as most anti-armor warheads depend on precise alignment with the target to work properly. While currently being developed as a vehicle defense, when PASS technology properly matures (and the mobile power available to it becomes sufficiently potent,) it can stripped down to protect individual soldiers from bullets, or scaled up to target ballistic missiles and aircraft kilometers away.
In order to be effective, PASS needs to be coupled with an advanced and highly efficient targeting system that can recognize and respond to incoming threats in time to intercept them, which would be an impressive engineering accomplishment on its own. However, given the time scales involved—usually milliseconds or less—this would mean the human operator would be completely out of the operational loop; the system would be fully automated, for better or ill.
The technology is also being considered for use as a small-scale alternative flash-bang grenades, as each individual plasmoid shockwave is about as loud and bright as a firecracker. Several hundred going off all at once near an enemy combatant could have a disorienting stun effect. The plasmoid detonations can also be precisely aligned, forming a ‘wall’ or other simple shape in the air to function as active deterrents.
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