Faster than a speeding bullet

A magnetic field that accelerates pellets faster than anything except a nuclear explosion has been developed experimentally at the US Department of Energy’s Sandia National Laboratories.

The machine, Sandia’s Z accelerator, currently propels coin-sized pellets called flyer plates a few hundred millimetres to gain information on the effect of high-velocity impacts.

The data gained can be used to simulate the effect of flying space debris impacting the metal skin and the data is expected to aid materials scientists trying to balance lightness against strength for satellite and observatory shells.

The technique is also said to have potential to act as a hypervelocity ‘kinetic kill’ weapon that, emanating from a lighter, more mobile source than the huge Z machine, could still strike disabling blows through an adversary’s heavy armour. These more mobile sources are already said to be in development.

Perhaps most importantly, though least dramatically, the technique is believed to be the fastest, most accurate, and cheapest method to determine how materials will react under high pressures and temperatures. These characteristics are expressed in ‘equations of state’, which tell researchers precisely how materials will react if basic conditions like pressure and temperature are changed by specific amounts.

Accurate knowledge of these equations of state is said to be essential for the US in order to maintain its nuclear weapons without physically testing them.

The maintenance program, called ‘science-based stockpile stewardship, is said to use the most powerful computers in the world to predict the result of phenomenally high temperatures and pressures upon materials.

Researchers are currently unable to determine these material characteristics except by the less accurate, more expensive methods of impacting test materials with laser beams, or at lower energies with projectiles from gas-powered guns.

The propulsion technique works by applying the Z machine’s 20 million amps to produce an evolving magnetic field that expands in approximately 200 nanoseconds to reach several million atmospheres pressure.

Accelerated to 13 km/sec, the plates do not distort, melt, or vaporise, as they would be if shot from a gun.

When the plate is accelerated to a speed about 20 times faster than a bullet, or 20 km/sec, the more forceful acceleration needed to reach higher velocity causes temperatures of 2,500 K to occur in the flyer plate, which liquefies in these conditions.

Better understanding of launch configurations is expected to eliminate this problem, though liquidation still is superior to the worst alternative of vaporisation — the result if conventional acceleration could be used to reach these speeds.

In this even newer technique, staggering the firing of Z’s 36 lines is said to eliminate the shock that melts the flyers at the higher velocities.

The resultant expansion of the magnetic field would then be able to propel small objects at 20 km/sec.