Energy weapons have sparked the imagination of authors, sci-fi fans, conspiracy theory adepts and average Joes alike. Plasma weapons have been present in sci-fi productions for years, in various forms, whether installed on spaceships or as portable guns.
The US Army hopes, within a few years, to deploy a plasma shield - a machine that generates a protective screen of dazzling mid-air explosions - to stun and disorient an enemy.
Plasma is typically an ionized gas and is usually considered to be a distinct state of matter in contrast to gases because of its unique properties. "Ionized" means that at least one electron has been dissociated from, or added to, a proportion of the atoms or molecules. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields.
At present, plasma rifles are merely theoretical, having no known practical applications, as currently they need more power than any handheld device could supply. If small portable fusion reactors are made, one potential source of weapons-grade plasma sources might be a direct tap on a fusion reactor, especially a dense plasma focus, since the natural yield of such a reactor is a hot high-speed plasma beam. Making real plasma weapons will need a major scientific breakthrough, as the concept of plasma-firing weapons is scientifically difficult, for various reasons.
The device uses a technology known as dynamic pulse detonation (DPD). A short but intense laser pulse creates a ball of plasma and a second laser pulse generates a supersonic shockwave with the plasma to generate a bright flash and a loud bang.
The Plasma Acoustic Shield System will eventually combine a dynamic pulse detonation laser with a high power speaking for hailing or warning and a dazzler light source. PASS has already been demonstrated by the system's makers, Stellar Photonics.
"It uses a programmed pattern of rapid plasma events to create a sort of wall of bright lights and reports (bangs) over the coverage area," says Keith Braun of the US Army's Advanced Energy Armaments Systems Division at Picatinny Arsenal in New Jersey, US, where the system is being tested.
Braun puts the maximum range of the system at around a hundred meters. But he says the PASS laser is unlikely to be used as a weapon, in its current format, since it lacks sufficient power. Unlike other high-power lasers which burn a target, the DPD relies on a shockwave. Braun says it would take several minutes to burn through a piece of paper using the laser.
"It is fair to say that any stunning or disabling of a target individual would require additional force on target," says Braun. "The current state-of-the-art in portable, rugged laser systems is not at the point of sufficient power."
However, he does not rule out the possibility altogether: "This type of capability is at the core of what we eventually expect from the technology."