High energy laser weapons already in development are powerful enough to bring down missiles (see MTHEL - Mobile Tactical High Energy Laser). However, their very large size has precluded placement on any but the largest planes. The main weight problem comes from the cooling systems needed.

HELLADS makes use of a unique cooling technique. The high-energy laser uses a liquid that has the same angle of refraction as the mirrors inside the blaster. That way, the "ray gun" can fire away, even while it's being cooled. Currently in the third of five phases of development, a 15 kilowatt subscale prototype is being tested in the laboratory. In the next phase, the demonstrator device will be scaled up to 150 kW, and will specifically be targeted to achieve the low specific weight (5 kg/kW) and compact size need to be mounted in a smaller airborne vehicle. The final phase is engineering, fabricating, integrating and demonstrating the complete HELLADS weapon system on a tactical platform. The device will be built by General Atomics and the tracking system will be built by Lockheed.

This kind of compact system is getting very close to what science fiction writers since H.G. Wells have envisioned when writing about the heat ray in War of the Worlds. More recently, Larry Niven and Jerry Pournelle wrote about laser cannon in their 1974 novel Mote in God's Eye:

..."The intruder came from here. Whoever launched it fired a laser cannon, or a set of laser cannon - probably a whole mess of them on asteroids, with mirrors to focus them - for about forty-five years, so the intruder would have a beam to travel on... (Read more about laser cannon)

Read more at MTHEL, Playing with Liquid Fire: High Energy Lasers Cool Down, HELLADS (at DARPA).