With Much Promise For Missile Defense And Industry, Lab Develops Powerful Laser (New Technology News)
With Much Promise For Missile Defense And Industry, Lab Develops Powerful Laser
By Scott Nance, New Technology News
August 20, 2001
NEWPORT NEWS, Va. - Having cobbled together funding from disparate sources, researchers here at Jefferson Lab have built the world's highest power free-electron laser (FEL). They are now planning a series of upgrades that could allow their FEL in coming years to meet a variety of promising defense, industrial, and other applications.
Officially known at 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 FEL was part of founding director Hermann Grunder's vision in establishing JLab. Grunder, now at Argonne National Laboratory, felt that JLab's core science mission - running the Continuous Electron Beam Accelerator Facility to study the fundamental nature of atomic nuclei - "may be a little too esoteric for the general public," said FEL Program Manager Fred Dylla. To balance that, he said, Grunder believed there ought to be another, more applied component of the laboratory's program that average citizens could relate to.
"Hermann felt very strongly that, as he was designing the superconducting structures for the main [accelerator] effort, that we ought to be looking at an application outside of the field that might have more immediate payoff," Dylla said. "That's where the free-electron laser program came from."
An 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, said FEL Deputy Program Manager George Neil.
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 untraviolet light.)
"It's coherent. The coherence results from the fact that in wiggling up and down, the electrons actually end up bunching longitudinally at this infrared wavelength," Neil said. "So you get little, tiny bunches as it's going through this wiggler."
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.
As laser technology generally has improved over time, researchers have developed the means to extend the ranges of standard lasers but those are still fairly limited and are likely to remain so, Neil said.
FEL technology dates back to 1976. "Right from the start, people recognized that it had potential as a high-power laser and one which was highly tunable. So there was a lot of interest from both on the scientific side as well as Department of Defense in looking at the technology and seeing where it might go," Neil recalled.
Despite that early potential, it wasn't until the laser at JLab became operational that anyone fielded an FEL capable of more than 10 watts. "This is the first high-power free-electron laser," 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, he 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," Dylla said.
We had a credibility hurdle to get over, on two counts," he said. "One, almost $1 billion was invested in free-electron lasers in the '80s. Those programs were shut down and very little laser light was generated. Secondly, who were we - an upstart group and a relatively small lab in Virginia - and we're talking about building a free-electron laser 100 times more powerful than anyone else had?"
JLab is overcoming the gap "by the best way to overcome a credibility gap: You deliver on your promises," Dylla said. In its contracts with the military, the lab had delivered "on a very tight schedule and very tight budget, and we exceeded out technical specifications," he 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 believed it eventually can be pushed to 100 kw.
If the researchers can reach that 100-kw milestone, it will be an important sign that the technology can be applied to industrial uses. For industry to be able to use FEL technology, it has to work reliably, produce photons cheaply enough to make its use profitable, and it has to make enough light to be able to support the production of large columns of goods, Neil said.
Along with several corporate partners, JLab researchers are developing a variety of potential industrial processes, and are heading toward the right cost per photon, as measured by the cost of kilojoule at any wavelength for less than 1 cent, including capital and operating costs.
"It turns out that FELs have kind of an interesting scaling when it comes to cost. That is, you need basically all the same components to make a low-power FEL as you do to make a high-power FEL. What that means is, it costs you, relatively speaking, a lot of money to build a low-power FEL. But it doesn't cost you very much more to build a very high-power FEL," Neil said.
If the technology allows for the construction of a high-power FEL, then the cost per photon has dropped dramatically, Neil said. "We figure that each factor of 10 in power costs us about a factor of two in cost," he said.
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 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," 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.
Turning off the light
Aside from the Navy's funding and some corporate contributions, the Department of Energy allowed JLab to use some superconducting components it had fabricated on site in building the FEL, counting as a $6 million contribution. The state of Virginia kicked in $5 million for the building where the FEL is housed.
But except for $1-million annual grants from the state, JLab has not been able to find any money to actually operate its FEL. The amount of state funding allows the lab to run its laser just a quarter of the time. The lab ran the laser in March, shut it down, then ran it again recently starting in June and shut it off again earlier this month. The researchers plan to run it next starting in October, until it is shut down for the upgrade.
"Like any system, turn it on and turning it off is not the best thing for it. You'd really rather turn it up and leave it running. It would be easier for our users, " Neil said. "Right now, with some of our users: they get into the laboratory [and if] something breaks on them, they're skunked. They lose their beam time and that's it - they're out for another four months."
A budget of about %4 million would allow for continuous FEL operation, but Neil understands that DOE's budgets for its light sources are very tight.
"I think they recognize that if they start funding us, we are going to be more popular and we are going to want more funding because there are going to be more people here, who are demanding to do more things," Neil said. "While one could think, that's [DOE's] business, they could also argue, 'We are already doing our business and we're very well booked up doing that. Thank you very much. We'd love to do more, but we ain't got [the funding].' I'm sympathetic but a little frustrated."