Laser weapons come into better focus

Press, 9 July 1983, Page 16

Laser weapons come into better focus

America’s Lawrence Livermore National Laboratory has just finished developing a machine which brings the possibility of high- energy laser and particle-beam weapons one step nearer to reality. Called the large-optics, diamond - turning machine, it should be able to make mirrors at once sufficiently large and accurate to meet the specifications of America’s space-weapons programme. Testing of the machine (which officially cost $l5 million, though ovservers suggest that the real cost may have been twice as high) will begin this year. Scientists sceptical about the possibility of developing anti- missile space weapons systems have 1 jointed to various technical probems. One of the principal ones is accuracy. Such a weapon, based on a satellite, would have to be able to hit missiles only a few metres long at ranges of several hundreds of miles. That requires not only fine targeting, to within fractions of a degree. It also demands that the beam of the weapon be focused extraordinarily tightly; otherwise its energy would be dispersed.

That is where the mirrors come in. Conventionally, mirrors are ground. That can produce high levels of accuracy, but only for small mirrors. There is a trade-off between accuracy and size. Laser

From “The Economist,” London

weapons require accuracy in mirrors more than one metre in diameter. There is one other machine in the world built to fulfil similar requirements—though for quite another purpose. Britain’s Cranfield Institute of Technology has developed one to make mirrors for the peaceful pursuit of X-ray astronomy. X-ray telescopes require mirrors around 25 times more accurate than those needed for ordinary optical astronomy. (Lawrence Livermore’s machine will also make telescope mirrors.) The two machines operate on the same principle, though the American one is built for higher accuracies. Instead of using abrasion to grind a mirror, they act like a lathe, smoothing the surface with a fine point: a gemstonequality diamond-cutting tool. The mirrors needed are highly curved, and a fine tool has a greater potential for cutting such a surface to exactly the right form. Mind, the tool must be precisionguided and both machine and mirror must be protected from vibration and from shape- distorting changes in temperature. The machines use different guidance systems, though both are computer controlled. Lawrence Livermore’s uses a laser to measure how much of the surface needs to be cut away—and feeds the information back to the com-

puter controlling the cutting tool. Cranfield’s system uses optical guidance for the first cutting stage and a laser to spot the inaccuracies this leaves. To avoid the possibility of vibrations causing faults, the Cranfield machine is made of synthetic granite, an excellent dampening material. The American machine is made of solid steel—because steel vibrates only at unusually high frequencies which should not be encountered. Nevertheless, to play it safe, the machine also sits on a sort of insulating air cushion. Both machines are sound-proofed. The chances of temperature changes causing problems are lessened by the sheer bulk of the machines (they weigh about 70 tonnes): a large mass in an enclosed space is not easily affected by air temperature changes. Air temperatures are kept within o.2deg C of 20deg C. With all these elaborate arrangements, the Lawrence Livermore machine can make mirrors up to 1.4 metres in diameter with variations of less than one tenth of a micrometre in their surface smoothness. (Surprisingly, the Cranfield machine is also approaching that performance.) This is only a first step. The long-term plans of America’s defence department envisage laser weapons using mirrors several metres in diameter.