How high-speed iron dust does damage.

How high-speed iron dust does damage

The Van de Graaff machine is one of the oldest designs of accelerators for subatomic particles. Invented more than 50 years ago, Van de Graaffs have done yeoman work accelerating first protons and later ions. Today a few laboratories are using them to accelerate dust motes made of iron, particles between 0.1 and 1 micron in size. The most powerful machine used for this purpose, with an energy range between 6 million and 8 million electron-volts, is at the Los Alamos (N.M.) National Laboratory. Recently it succeeded in accelerating these iron particles to speeds of 50 kilometers per second.

This is a velocity never reached before in this fashion for particles of this size. It is 50 times as fast as a bullet fired from a high-powered rifle. The other two laboratories that do this work, the University of Kent in Canterbury, England, and the Max Planck Institute in Heidelberg, West Germany, use less powerful machines, according to Paul Keaton, who leads the work at Los Alamos. After some adjustments he hopes to reach 100 km/s.

Such particles each contain about a billion atoms of iron, Keaton estimates. When they come out at this speed, they carry 100 to 1,000 times the energy needed to vaporize iron. Thus if they hit a solid iron target they will vaporize 100 to 1,000 times as many atoms as they contain. The main purpose of the work is to study how this damage is done, developing "codes" that describe in detail these processes of melting and vaporization that result in pitting of the surface of the target. Experimental verification of theoretically calculated codes for particles of this mass and velocity was previously impossible, according to a Los Alamos announcement.

Even at 50 kilometers per second, these particles, which are about the size of the particles in cigarette smoke, would not go far in the atmosphere without being stopped. However, in space such particles travel long distances and are a constant hazard to spacecraft. They come from cosmic dust, meteoritic dust and cometary dust. Right now there should be a good supply left by Comet Halley.

Although there is a national defense interest in the details of such damage codes, there seems to be no practical way of using such a system to fire these particles as projectiles. It wouldn't work in the atmosphere; the accelerator would have to be in space. And, says Keaton, "Nobody is thinking of putting a Van de Graaff into space. Ours weighs 140 tons."