NEWPORT NEWS, Va - The Navy has played a large part in Jefferson Lab building the world's highest power free-electron laser (FEL). Researchers at the lab are now planning a series of upgrades that could allow their FEL in coming years to meet a variety of promising defense applications, including missile defense.
Officially known as the Thomas Jefferson National Accelerator Facility, Jefferson Lab or JLab is part of the Department of Energy (DOE), but it has had to rely primarily on other agencies to fund its FEL program, particularly the U.S. Navy, private industry, and the state of Virginia, in a situation referred to at the lab as the "united fund campaign."
The single largest funder of JLab's FEL work has been the Navy. The sea service contributed $11.8 million toward the current-generation laser and will cover the entire $15 million or so it will cost for the planned upgrade to 10 kilowatts (kw).
The Navy has been involved in developing directed energy technology for a number of years. It had given up on a high-powered chemical laser because of a number of technical issues but was still interested in using lasers to defend ships against cruise missiles.
"With the Navy's mission changing, where [ships] have to often sit off-shore of some country, they become very vulnerable to these kinds of weapons that can be launched off-shore," FEL Deputy Program Manager George Neil said. A ship might have 25 seconds from the time a missile appears over the horizon until impact. "The incoming missile will always have the advantage there."
Navy vessels currently fire rounds of ammunition at incoming missiles, but the missiles get fairly close to a ship before its gun has a chance to destroy it, Neil said. For the Navy's purposes, the chemical laser's chief problem has been the fact that it operates at wavelengths that are absorbed in the atmosphere before the light could reach its target. A FEL could be a better alternative, but a realistic missile defense system would require megawatt-class devices and until JLab's work, FELs had operated only at a maximum of 10 watts.
"We managed to convince [the Navy] to work with us on this, " Neil said. "We said, 'Look, we're going to take a step at a time. We're going to do a factor of 10 [improvement] each time, and each time we bring this power up, we're going to demonstrate the technologies we need to get the next order-of-magnitude [increase].'"
In other words, the JLab team demonstrated the technology needed to get to 10 kw with its current system, and when it achieves the 10-kw upgrade, the scientists will need to show what would be required to reach 100 kw. "That will allow us to have fairly high confidence that we can get there," Neil said.
Such a step-by-step approach will take time, but that fits the Navy's time scale of engineering a workable system by 2012 or 2015, Neil said. Because a FEL is an electric laser, its development also fits within the Navy's research into an all-electric ship.
"The advantage of a free-electron laser, if you can make the power, is: one, you can choose the wavelength so it will go through the atmosphere; and two, you have an essentially infinite magazine," Neil said. "As long as the generators are running on the ship, you can keep shooting. You can't say that about any of the other weapons."
That ability to continuously fire the laser means it would also be easier and cheaper for sailors to train on it. In addition, the fact that the laser is tunable means a ship needn't run it at full power all the time. That could be an advantage in a situation where a ship might encounter a civilian boater approaching too closely. Sailors could operate the laser at a lower power initially as a warning but could ratchet up to more deadly power if the boater doesn't turn away.
A FEL is a laser that makes its energy by taking it out of an accelerated electron beam. The term "free" in free-electron laser refers to the fact that the electrons used are not bound in atoms. Most lasers work by exciting electrons to higher energy levels and then causing them to relax back down, giving off photons in the process. In the case of an FEL, the electrons are not at any fixed energy levels within atoms. They are brought up to high energy levels with a particle accelerator.
At JLab, it involves 50 million volts of energy, Neil said. The 50-million-volt electron beam is sent into a device called a "wiggler," which creates a magnetic field. When the electron beam goes into that magnetic field, the field "wiggles" the electrons up and down, which causes electrons to emit photons. As the electrons are oscillating up and down, they radiate in a forward direction. The wavelengths of light in the laser become shorter, into the infrared.
The machine essentially acts as a "transformer," Neil said, converting the electron beam energy into infrared light. (JLab is also engineering the system to also produce ultraviolet light.)
The advantage of a FEL over other types of lasers is two-fold. One is that scientists can make very powerful electron beams, "so even if we take out 1 percent of the electron beam energy in a free-electron laser, that's 1 percent of a big number," potentially leading to a very powerful laser, Neil said.
The second asset of an FEL is it is tunable. Scientists can't alter the energy levels in atoms, they're fixed in nature. But they can turn the electron beam energy they are operating at up and down, changing the light wavelength produced by the laser.
"So I can dial-in whatever wavelength I want, and that's a huge advantage. It's difficult to find lasers that operate on all the wavelengths you want," Neil said.
The JLab laser operates at 2 kilowatts (kw) of power a factor of 200 more than the next highest average-power FEL in the world, Neil said. That power means researchers at JLab can do a number of applications and experiments that can't be done elsewhere.
Getting to this point hasn't been easy, however. In past decades, the Pentagon spent heavily on developing FEL technology that never went anywhere, creating "quite a hurdle to get over," FEL Program Manager Fred Dylla said.
The FEL team at JLab is in the process of upgrading its laser. Scientists are building parts for the upgrade now. After they finish the last laser run around Thanksgiving, they will take it apart, spending about a year installing new equipment. Once the upgrade is completed, it will be capable of about 10 kw, and Neil said he believes it eventually can be pushed to 100 kw.
Submitted: Monday, August 20, 2001 - 12:00am