The U.S. Navy wants to put powerful lasers on its ships to shoot down artillery shells and even cruise missiles at the speed of light (and really, who wouldn't). But there are a few scientific details to sort out before sailors can deploy the beams. "First we want to make sure the physics is right before throwing buckets of salt water over the thing," says Ed Pogue.
Pogue is the program manager for Boeing's free electron laser (FEL) program, potentially the most ambitious laser weapons program since the Pentagon's controversial airborne laser. In that program, the Missile Defense Agency spent billions of dollars and over a decade to get a laser-rigged jumbo jet to destroy a ballistic missile in its boost phase of flight. They eventually succeeded in February 2010, but the Obama administration nixed plans to develop the experiment into a battle-ready weapon.
Maybe the Navy's project will meet a better fate. In 5 years, at a cost of $163 million, Boeing hopes to get the physics right and demonstrate an extremely powerful--and hopefully seaworthy--giant laser. It's no small task, in part because the laser they're using is powered by several particle accelerators.
Here's an overview of how the Navy's free electron laser works. (You may notice that the pictures are not on a boat; for now, researchers are working with a landlubbing laser based at Jefferson Lab in Newport News, Virginia.)
George Neil, who leads the Jefferson Laboratory's free electron laser program, says some insiders question whether a free electron laser is even a laser, since its created via a different method than your typical laser. Neil's machine produces coherent laser light directly from an electron beam, allowing it to create any wavelength of light--in other words, any color on the spectrum. Traditional lasers are made from materials that have been pumped with enough energy to make them spontaneously burst with photons. They're confined to just one wavelength of light, which is determined by the molecular structure of the laser material that stores and releases energy.
Because a free electron laser is made out of an electron beam instead of one particular type of material, it is not a prisoner to a molecular structure. An electron beam can be manipulated with magnets to produce a beam of any wavelength. This would allow technicians to adjust the laser to suit the changing marine environment. For example, they could avoid problems with salty, misty air that can interfere with the infrared lasers often used in military laser research.
Once researchers have the electron beam, they inject it into a superconducting particle accelerator to give it a clean, efficient energy boost. Then the beam travels through a device called a wiggler, which literally wiggles the electrons to make them produce a precise type of electromagnetic wave. Voila, you have a laser beam! And you don't want to be in front of one of these beams of light. The Jefferson Laboratory FEL holds the power record at 14 kilowatts--enough to quickly burn through stainless steel.
These are the guts of the high voltage power supply, which provides juice to the electron gun. When the system is fully assembled, the six-foot-high metallic coils will be sealed in a pressurized chamber filled with a gas called sulfur hexafluoride. The massive amounts of power--hundreds of kilovolts--that the coils produce can cause arc discharges, when energy is discharged into the air. The sulfur hexafluoride prevents that from happening because it doesn't conduct electricity as well as air does.
Just as we use lenses to focus and reshape visible light, magnets are used to focus and reshape electron beams. The red devices shown in the picture are called quadrupoles. Each has four powerful magnets that keep the beam in line as it travels down the path. There are almost 70 of these focusing magnets along the football-field-length track of the beam. Precise alignment is key to getting the prettiest, strongest laser possible.
Like the red-painted quadropoles, the green-painted sextupoles focus the electron beam. Each sextupole consists of six magnets, and more magnets means stronger focusing. As the electron beam travels down its path, it naturally spreads out; the magnets prevent that from happening by re-shaping the beam. In this image, the sextupoles wait on the sidelines to be installed in the beam line.
The superconducting accelerator takes an electron beam and gives it a shot of energy using microwaves. It's not much different from the way your microwave oven works. Only instead of heating food, the microwave energy is channeled directly into the electron beam to make it travel faster. The microwaves are injected through a vertical waveguide--the central, silver-colored thing with a kink in it. The whole accelerator is cooled to just 2 kelvins above absolute zero; the frigid temperature keeps the material's electrical resistance down and prevents energy from escaping. Between five and seven of these eight-foot-long devices are placed end-to-end to get the electron beam up to speed.
Once the accelerator is in place, it looks like a big old pipe. The accelerator excites the electron beam up to about 100 million volts to prep it for lasing. In a conventional laser, an energy source such as another laser or a power supply excites atoms so that they lase. With a free electron laser, it's the accelerator that gives the electron beam the boost it needs to directly emit photons. If the Navy gets its way, this contraption will be in the hull of a ship someday.
Once the electron beam reaches just the right speed, it enters the wiggler. The wiggler directs the electron beam back and forth, or "wiggles" the beam, to make it produce electromagnetic radiation. "It's just like surfing, like catching a wave," says Henry Freund, a long-time free electron laser scientist and vice president at Science Applications International Corporation.
Inside the wiggler, very strong, oppositely charged magnets are placed side-by-side. As the electron beam passes through the magnets, it is first attracted to the positive pole of a magnet. But then the negative magnetic pole right next to it kicks it to the other side of the wiggler. This back and forth motion of the electron beam causes it to emit electromagnetic radiation. The wavelength of that radiation--the laser beam--depends on the size and spacing of the magnets in the wiggler.
In the last step before the laser beam is ready for the world, the light beam enters this giant silver cylinder holding the optical cavity mirrors. The photons bounce between these mirrors, passing through the wiggler until they build up enough energy to escape through one partially transparent mirror.
Laser scientists control the experiment from a safe distance inside a control room filled with monitors (and snacks). The poster on the back wall shows a complete diagram of the 240-by-40-foot experiment. If all goes according to the Navy's plan, one day they'll be operating their laser from a ship. By 2016, Boeing is scheduled to transfer its free electron laser technology from Jefferson Laboratory and other participating labs, in order to demonstrate a 100-kilowatt prototype that is compatible with operation on a ship. After that, plans will begin to design something that will actually go out to sea.
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