The arcs and sparks in Sandia's Z machine travel between metal conductors.

The arcs and sparks in Sandia's Z machine travel between metal conductors.

Sandia's Z machine, built to generate radiation fields for validating models of nuclear weapons, has produced plasmas that exceed 2 billion degrees Kelvin — hotter than the interiors of stars.

The unexpectedly hot output, if its cause were understood and harnessed, could lead to smaller, less costly nuclear-fusion plants. The phenomenon may also explain how astrophysical entities like solar flares maintain their extreme temperatures.

"At first, we didn't believe the results," says Sandia project lead Chris Deeney. "We repeated the experiment many times to make sure we had a true result and not an 'Oops!'" The results, recorded by spectrometers and confirmed by computer models have held up over 14 months of tests.

Z's energies raised several questions. First, the X-ray output was as much as four times the expected kinetic energy input. In nonnuclear reactions, output energies are less, not greater, than the total input energies. Second, and even more surprising, temperatures remained high even after the plasma ions had presumably lost motion and, therefore, energy and heat.

Sandia's Z machine normally sends 20 million amps of electricity through a core of vertical tungsten wires about the size of a spool of thread. The wires dissolve into a cloud of charged particles or plasma.

The plasma, caught in the magnetic field accompanying the electrical current, is rapidly compressedto the thickness of pencil lead. The ions and electrons, having nowhere further to go, stop suddenly and release energy in the form of X-rays with temperatures of several million degrees — the temperature of solar flares.

The unusual output happened after researchers substituted 55 to 80-mm steel wire in cylindrical arrays for the usual 20-mm tungsten wire.

Researchers theorize that the machine's magnetic energies create microturbulences that increase the kinetic energies of ions caught in the field. Already hot, the extra jolt of kinetic energy leads to the increased heat. Previously, it was assumed high temperatures were produced solely by kinetic flight and intersection of ions and electrons.