The Plasma Acoustic Shield System (PASS)

is a crackling, flashing wall of light hanging in the air up to 100 meters away. And while it's the stuff of sci-fi, the laser-powered PASS already exists in prototype form, built by Stellar Photonics of Redmond, Wash., with funding from the Pentagon's Joint Nonlethal Weapons Directorate ( JNLWD ).

Initially intended to shield soldiers by creating a high-intensity distraction and provide a highly visible warning signal, PASS may develop into a weapon in its own right, producing a barrage of explosions with the power of flash–bang stun grenades. It may also evolve into a shield capable of absorbing laser and microwave blasts from directed energy weapons.

The technology is based on short pulses from a high-power laser. If you focus a laser beam down to a point, the intensity can become so great that the molecules of air itself break down. As electrons separate from their atoms to produce ions, the gas becomes plasma, an effect seen at the core of a lightning bolt. This ionization produces a miniature explosion with a flash of light and a sharp pop.

W. Alexander Long, vice president and co-founder of Stellar Photonics, says the company developed a technique called dynamic pulse detonation, designed to enhance this blast. The initial laser pulse produces a small cloud of plasma; another pulse strikes the plasma a fraction of a second later. The plasma cloud absorbs the laser energy and expands rapidly in a supersonic shock wave, creating a bigger bang than the first pulse's. The laser can scan and rapid-fire a series of pulses to build up a wall of plasma bursts, in much the same way that an old cathode-ray television builds up a picture by scanning across the screen.

The PASS demonstrator is mounted in a turret attached to a Hummer. It is a nonlethal option that might be used, for example, if a vehicle approaches a checkpoint without slowing down. If the driver ignores verbal and other warnings (such as a flash of ultrabright light) to slow down, soldiers could use PASS to produce a barrage of flash–bangs right in front of the car. Anyone who proceeded might be assumed to be hostile. In addition, the bright laser bursts are likely to interfere with attempts to aim a weapon, though PASS cannot inflict damage at its current power levels.

"At the current state of the art, the Plasma Shield System can only be used for warnings," Long says. "More work is needed on the next generation of eye-safe, high-energy pulse lasers."

The demonstration system produces 10 flash–bangs per second. Long says other currently available lasers could raise this rate to 40 per second, but they are not eye-safe. Stellar Photonics plans a next-generation system capable of 200 flash–bangs per second, and this rate will increase as more powerful solid-state lasers become available. Ensuring that the lasers cannot cause eye damage or blindness has been one of the major challenges.

Higher scan rates would allow the plasma shield to take more advanced shapes—for example, to put a horizontal barrier across a road. Long says that once the scan rate reaches kilohertz, thousands of flash–bangs per second, it would be possible generate letters or graphics and literally paint in the air.

Stellar Photonics has been working on the PASS since 2005 with partners Orca Photonic Systems and Laser Guidance. Previously, the Pentagon's JNLWD funded the Pulsed Energy Projectile, a chemical laser that produced a flash–bang effect at the surface of the target. The intent was to provide a precise, long-range alternative to rubber bullets. But, as with the giant antimissile Airborne Laser, the chemical laser for PEP was very big, weighing about 500 pounds, and carried a limited supply of fuel for laser shots. After years of development, the PEP program disappeared around 2008.

Another Navy program, Plasma Point Defense, begun in 2004, looked at generating a plasma shield as a way to deflect incoming missiles. This was not feasible with existing technology, and the military dropped the idea.

PASS, modest as it looks now, has the potential to keep growing and succeed where Plasma Point Defense and the Pulsed Energy Projectile did not. Solid-state lasers mean that PASS is compact and rugged and can keep firing for as long as there is a power supply. The Navy is already exploring how they can get more bang out of laser technology with contracts to two new developers whose systems will rival the one built by Stellar Photonics.

RadiaBeam Technologies of Santa Monica, Calif., specializes in components for particle accelerators. They are working on MILI-Flash, for Mid-Infrared Laser Induced Flashbang system. This also uses a double-pulse scheme, with an ultrashort ignition pulse to ionize the air, followed by longer pulse to heat and expand the plasma and create the desired flash–bang. Meanwhile Physical Optics Corporation of Torrance, Calif., is offering RESLIFE (Ranged Eye-Safe Laser-Induced Flashbang Enabling System). This uses two lasers operating at different wavelengths for the two pulses, a picosecond Erbium-doped laser for the initial pulse, and a nanosecond Thulium-doped laser for the followup. Both companies are on a nine-month development contract; the successful one will then produce a prototype with a range in excess of a hundred meters.

It's easy to see the immediate military applications of PASS technology for warning, signalling, and shielding. Networked together, a number of PASS systems would produce quite an impressive spectacle. The plasma wall is a barrier to both visible and infrared viewing. Because plasma can absorb microwaves and radar, PASS may also prove a viable technology for screening from radar, as well as blocking directed microwave pulses that can damage electronics.

While the flash–bangs will always be less effective than kinetic weapons, they could serve as a nonlethal option for crowd control or for hitting small, fast-moving targets such as miniature drones at the speed of light and with laser accuracy.

Similar technology may be used in the civilian world for 3D displays. Japanese company Burton Inc. has demonstrated a laboratory system that can sketch crude moving images of a stick man and a bouncing ball floating in the air. Something the size of PASS should enable stadium-scale plasma displays.

However, according to the Department of Defense, it's too early to say what applications the technology will find, or what the timeline is for possible deployment. "The technology is still in its infancy to appropriately respond to its applicability," said Kelley Hughes, spokesperson for the Joint Non-Lethal Weapons Directorate.

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