The behaviour of carbon under high pressure in the vicinity of the melt boundary has been investigated by scientists actively in the last years. Recently, the group around Marcus Knudson of the Sandia National Laboratories, Albuquerque, has been able to study this phenomenon with an order of magnitude improvement in accuracy, allowing quantitative modelling for the first time [Knudson et al., Science (2008) 322, 1822].
The pressures ranging from 550 to 1400 GPa were obtained by magnetic acceleration of small 17-by-40 mm copper pellets, called flyer plates. “It's the fastest gun in the world,” says Knudson jokingly, although thus he actually understates the performance of the so called Sandia Z machine, which can accelerate the flyer plates to propulsion velocities in excess of 20km/s – i.e. about twice the Earth's escape velocity. To generate this propulsion, several million amperes are employed to produce a magnetic field expanding in about 200 ns, by which the plates are then accelerated and shot onto a variety of diamond samples with diameters of 6 mm, backed by quartz or sapphire. “The plates are designed so that they reach a terminal velocity within a few millimetres,” explains Knudson. “At that point the plates impact the samples. It is this high velocity impact that creates the rather large shock waves that propagate through the samples.”
These shock wave impacts facilitate the study of the behaviour of carbon under highest pressures near the melt boundary, i.e. around the triple point between diamond, bc8 (a long-theorized but never-before-confirmed state of solid carbon) and liquid carbon. Experimental results are compared with ab initio molecular dynamics simulations, thus gaining and strengthening insights, valuable for different branches of physics. Astrophysicists reckon that high pressure carbon may be present in vast quantities in gas giants, particularly Uranus and Neptune (as much as 10 to 15% of the total planetary mass). On Earth, nuclear physicists are considering diamond as an ablator material for the use in inertial confinement fusion capsules, so the knowledge gained from Sandia's “fastest gun” may provide data useful in the development of fusion power.
“This work – both experimental and computational – provides rich understanding of the properties of carbon in this high energy density regime, which will aid in the design of fusion capsules and pressure pulses to drive these capsules to fusion conditions,” says Knudson.