What's Inside Darpa's $21 Million 'Liquid' Laser?

Things are really beginning to heat up in the field of laser weapons. The giant Airborne Laser is finally gearing up for real-life, missile-zapping tests. New solid-state lasers like Raytheon's Phalanx can already shoot down mortar rounds. Now, Darpa is weighing in with an yet another technology: HELLADS or "High Energy Liquid Laser Area Defense System." Weaponeer Textron will get $21 million in Darpa bucks "to design, fabricate and test a Unit Cell Module for a 150 kilowatt (kW) Laser Weapon System," according to a company press release.

As you'd expect from Darpa, the new laser is aiming for quantum leap, with a power-to-weight ratio ten times better than existing laser systems. The end product will be the size of a large refrigerator and weight of 1650 pounds. And a hundred and fifty kilowatts is real weapons-grade, compared to existing solid state lasers which still zap in the tens of kilowatts.

The agency's website says that "HELLADS will enable high-energy lasers (HELs) to be integrated onto tactical aircraft and will significantly increase engagement ranges compared to ground-based systems." The 150 kW laser is meant to be integrated with an existing fire-control system and will demonstrate the ability to shoot down tactical targets including surface-to-air missiles and rockets. That would be quite something: a fighter jet which can shoot down any missiles fired at it. Suddenly stealth doesn’t seem quite so important, when the air defenses can see you but can't touch you. Cannon fire from other aircraft might still be a threat, but what are the chances of anyone getting close enough before you laser them?

Despite Washington chatter of programmatic stumbles, HELLADS seems to be growing rapidly, from with a 1 kilowatt version in 2004 to a 15 kW version in 2006. Now, they are confident enough to start on the full-size model. However, the $21 million deal will not cover building the whole thing, just one module, plus the design for the entire system; the finished product will cost something extra. Still, it's small change compared to the $7+ billion spent on the Airborne laser.

Lasers all work in pretty much the same way: Excite certain kinds of atoms, and light particles — photons — radiate out. Reflect that light back into the excited atoms, and more photons appear. But performance varies wildly, depending on the kind of "gain medium" — the type of atoms — you use to generate the beam. The Airborne Lasers uses vats of chemicals. Raytheon's Phalanx uses solid-state materials. But those kind of lasers can rapidly overheat and suffer damage (a laser with a 50% efficiency generates the same amount of waste heat as the energy in the beam). Liquid lasers (like HELLADS, presumably) are less vulnerable to this, since the liquid can be cooled by circulation.

But is HELLADS still a liquid laser, really? However, Textron say that their HELLADS design is based on "proprietary ThinZag Ceramic solid-state laser technology"; earlier Textron HELLADS work also refers to ThinZag solid state technology and does not mention the liquid aspect. ThinZag itself appears to be an advanced type of slab laser -- an entirely different way of solving the problem of waste heat.

DARPA were cagey about how it works, offering only this statement:

"We are not able to provide details of the HELLADS laser technology other than to say that HELLADS performance is enabled by a novel laser design that combines the high energy density of a solid state laser with the efficient thermal management of a liquid laser. "

A perusal of some technical literature suggests that HELLADS is composed of a series of thin ceramic slabs, bathed in rapidly circulating coolant. The clever feature is that the laser beam actually passes through the coolant, which makes engineering much more straightforward.

Phil Coyle, a senior advisor at the Center for Defense Information, is not convinced it will work. He suggests that previous anti-missile laser tests have not been carried out under realistic operational conditions." The challenge is to achieve militarily effective damage against an unpredictable and fast-moving target. Damaging an enemy surface-to-air missile with a laser is like trying to set flying wet logs on fire with a match," he says.

Then again, perhaps designers are not counting on HELLADS damaging the missile itself. Anti-aircraft missiles (especially heat-seeking ones) have delicate sensors which are vulnerable to laser damage, and there are already laser countermeasures systems which are capable of putting a beam on to an incoming missile. So HELLADS may still be successful as a defensive system, even if the amount of energy it puts on target is less than awesome in the offensive role. And if it comes to it, HELLADS is certainly powerful enough to take out the other pilot's eyeballs in a dogfight.

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