Princeton Weekly Bulletin, April 13, 1998
Physicists use lasers to interrogate fibers
By JoAnn Gutin
When Principal Research Physicist Dennis Mansfield snaps off the light in his lab at the Princeton Plasma Physics Lab, the room plunges into a darkness so thick it would give a spelunker pause. The only thing visible is a pinprick of red laser light trained on a hair- thin strand of fiber.
The reason for the blackness?
"We're interrogating that fiber," says Mansfield cheerfully.
In this sense, interrogation means discovering the physical properties of a material. And until last year Mansfield was using lasers to interrogate the plasma, or superheated gas, that circulated through the Tokamak Fusion Test Reactor (TFTR). Now his lasers are pointed in a different direction.
The shift is part of an effort by the U. S. Department of Energy (DOE), which funds the lab, to bring the scientific expertise of universities and national labs to bear on a new set of problems: revitalizing U. S. industry. In the case of Mansfield's group, the industry in question is textile manufacturing.
Not just any industry
Textile manufacturing isn't just any industry, comments Philip Efthimion, principal research scientist and project manager for the fiber project. It's been a way of life in the South and an important piece of the U.S. economy. But in recent years this piece of the economy has been eroding, as companies have moved abroad in search of cheap labor.
One way to counteract the lure of cheap labor is by using high technology to improve efficiency and to lower production cost. The steel industry and the automotive industry have had success in reviving domestic production with this tactic, Efthimion pointed out, and in 1994 the textile industry banded together to do the same. So when Lewis Meixler, head of technology transfer, attended a meeting arranged by DOE and the government and industry consortium of textile manufacturers known as AMTEX, it occurred to him that the laser experts at the Plasma Physics Lab had something to offer.
In the manufacture of synthetic fibers, molten plastic is extruded from a device that looks a lot like a shower-head. The tiny filaments cool from liquid to solid as they fall; when they finish dropping they're wound around large spools, or spinners, rotating at speeds of up to 30 mph. Minute variations in the properties of this molten material can have large consequences in the quality of the finished product. The alignment of microscopic crystals affect sheen, for example, or the ability to absorb dye evenly.
But getting the mix of factors -- polymer composition, speed of spindles, tension on the filaments -- just right can be a tricky business. It's the kind of thing where "you need to worry about the phase of the moon and the angle of the cars in the parking lot," jokes Mansfield.
And the cost of getting it wrong can be substantial. As things now stand, the only way to check the quality of the fibers is to shut down the machines, cut a fiber and study it in an on- site factory lab. The machines (and a football- field- sized factory can contain hundreds of spindles) may idle for hours to days. But if all goes well, the laser technology being worked out by Mansfield and theoretical physicist Hideo Okuda will permit operators to assess fiber quality and correct it while the machines are running.
180-degree backscatter
You can get information about a fiber by shining a laser through it and seeing how it scatters the light, says Mansfield. The problem is that on a factory floor there's no room to do that, because another machine is usually right behind the first one. But Mansfield's predecessor at the Plasma Physics Lab, Boris Grek, discovered there was also information in the light that bounced back -- what optics scientists call 180- degree backscatter. It was this discovery that paved the way for the current project.
In his pitch- black room, Mansfield and his colleagues are beginning to design a prototype version of a rugged, compact instrument that will use the backscatter to interrogate fiber as it is being made. The device will feed the results into a computer on the factory floor, where an ordinary worker will be able to fine- tune production.
Designing a machine that can be mass-produced will be a new experience for the scientists, but it's a challenge they're looking forward to. This device has to involve high tech elements -- computers and lasers -- yet be simple to operate and tough enough to keep on working if somebody drops it, Mansfield says. That's a big change from designing and using one- of- a kind instruments, but in an odd way TFTR prepared the scientists for it.
"The factory floor is a pretty harsh environment," Mansfield observes, "but no harsher than right next to a fusion device!"