Scientists at the Sandia National Nuclear Security Administration laboratory have produced plasma at a sizzling two billion kelvins - hotter than a star's interior - although they're not quite sure how they did it.

The "unexpectedly hot output", as the Sandia blurb puts it, is the spawn of the mighty Z machine (seen firing below as "arcs and sparks formed at the water-air interface travel between metal conductors" - crikey). The beast is the world's most powerful X-ray generator tasked mainly with "new experimental environments to help validate computer codes responsible for maintaining a reliable nuclear weapons stockpile safely and securely".

Sandia's explanation of the Z machine's normal operation is this:

20 million amps of electricity pass through a small core of vertical tungsten wires finer than human hairs. The core is about the size of a spool of thread. The wires dissolve instantly into a cloud of charged particles called a plasma.

The plasma, caught in the grip of the very strong magnetic field accompanying the electrical current, is compressed to the thickness of a pencil lead.

At that point, the ions and electrons have nowhere further to go. Like a speeding car hitting a brick wall, they stop suddenly, releasing energy in the form of X-rays that reach temperatures of several million degrees — the temperature of solar flares.

The latest breakthrough, however, "was obtained in part by substituting steel wires in cylindrical arrays 55mm to 80mm in diameter for the more typical tungsten wire arrays, approximately only 20mm in diameter. The higher velocities achieved over these longer distances were part of the reason for the higher temperatures".

That partial explanation doesn't answer all of Sandia's questions, though. First up, the X-ray output was "as much as four times the expected kinetic energy input" meaning that some extra energy must have been pumped into the equation from somewhere.

Secondly, "high ion temperatures were sustained after the plasma had stagnated — that is, after its ions had presumably lost motion and therefore energy and therefore heat — as though yet again some unknown agent was providing an additional energy source to the ions".

Sandia project lead Chris Deeney summed it up with: "At first, we were disbelieving. We repeated the experiment many times to make sure we had a true result and not an 'Ooops'!"

A possible explanation for the mystery came from Sandia consultant Malcolm Haines of Imperial College in London, which appeared in the 24 February Physical Review Letters. In summary, Haines theorised that "the rapid conversion of magnetic energy to a very high ion plasma temperature was achieved by unexpected instabilities at the point of ordinary stagnation" (where the plasma's particles "should have been unable to travel further"). Haines postulates "some unknown energy" which continued to push back against the magnetic field for around 10 nanoseconds.

In other words: "Sorry, we don't exactly know what's going on but we'll get back to you soon." Watch this space. ®

Bootnote

Our regular science readers will remember that the Z machine was put forward as a possible candidate to power the theoretical hyperdrive. Looking at the photo above, we reckon we'll easily be nipping off to Mars in three hours by 2010.