Airborne laser ready for flight tests

IT SHOULD be the moment of truth for the Airborne Laser (ABL). In the coming months, the multibillion-dollar laser built into a customised Boeing 747 will try to shoot a ballistic missile as it rises above the clouds.

Don't expect instant reports of success, though. Instead, if all goes to plan, we're likely to hear about a series of incremental improvements.

Developed by the US Department of Defense's Missile Defense Agency (MDA), the ABL aims to focus a beam of laser energy in the megawatt range for several seconds onto a missile at a "militarily significant distance" - more than 100 kilometres.

So far, the laser has only operated at near full power on the ground. On 18 August it was fired successfully from the air, but at reduced power. That, however, was no mean feat: aircraft vibrations play havoc with the precisely aligned optical components needed to generate a laser beam.

Firing at full power poses other challenges too. At powers high enough to destroy missiles, any surface contamination or tiny flaw in the laser optics can absorb so much heat that they crack or shatter.

High-power laser beams also heat the air they pass through, creating perturbations that can disperse or divert the beam. To counteract those effects, the ABL uses an adaptive system that senses atmospheric changes along its path and makes optical adjustments to compensate.

To test that system, the MDA plans a series of increasingly powerful shots at modified ballistic missiles loaded with sensors to measure the distribution of laser power on the target. Engineers will assess each shot's performance and use the results to fine-tune the adaptive optics. Once this is done, the MDA will test the laser again in varying conditions, and attempt to destroy actual missiles. The first of these tests is planned to take place late this year, with two more to follow in early 2010, according to an MDA spokeswoman.

A sister project, the Advanced Tactical Laser, which aims to use an airborne high-powered laser to hit targets on the ground, recently completed its first successful test. With future funding dependent upon the success of these tests, the pressure is on the ABL team to prove its efficacy.

Military mega-lasers are too hot to handle

HIGH-ENERGY laser weapons have been hailed as the future of anti-missile defence, but they may be further from being battle-ready than military chiefs hoped.

In recent tests, several prototypes have suffered serious damage to their optics at intensities well below the expected levels of tolerance. "Optical damage has been quietly alarming upper management in most major programmes," Sean Ross of the US Air Force Research Laboratory in New Mexico told a meeting of the Directed Energy Professional Society in Newton, Massachusetts, last week. There are also big problems managing the waste heat generated by high-intensity beams.

Laser weapons require mirrors and lenses to focus powerful beams onto distant moving targets, and to compensate for atmospheric perturbations that can reduce the power they deliver. The higher the intensity of the beam, the more likely it is to damage the surface of its optical components.

Optical surfaces are designed to withstand powers up to a specific damage threshold, but tiny flaws or irregularities - which can be extremely difficult to spot - reduce this threshold by making them more vulnerable to heat. Contaminants deposited on the surface can also reduce this threshold by forcing the surface to absorb energy.

These problems have begun to stall the development of laser weapons. Earlier this year in the US, engineers halted tests of the $4.3 billion megawatt-class Airborne Laser short of full power to avoid damaging "a handful of optics in the turret", according to Mike Rinn, a Boeing vice-president who manages the programme. They realised that the optics, designed years ago, would be "frail" in the presence of any contamination, which would be virtually inevitable in flight. In the next week or so, Boeing engineers will install replacement optics and test them on the ground before running the laser at full power in flight.

Finding a way of preventing laser weapons from frying themselves is proving just as troublesome. Depending on the type of laser, generating 1 watt of laser beam produces about 4 watts of waste heat that must be dissipated. The challenge is to develop a cooling system that is both small and extremely robust.