The new Orion facility will be home to 12 high-powered laser beams capable of heating and compressing material to millions of degrees Celsius in less
than a nanosecond. Housed in a gleaming building the size of a soccer pitch, the laser system will provide physicists at Aldermaston with crucial data
about how components of their ageing nuclear weapons behave. Under current plans, around 15% of Orion's time will be offered to academics wanting to
study conditions on stars or inside giant planets. And in that open spirit, researchers there invited a Nature reporter in for a look around.
In most respects, Orion is the smaller cousin of the US National Ignition Facility (NIF) in Livermore, California, which is already running academic
experiments. When operating at full steam, the NIF will use 192 lasers to create around 4 million joules of energy, some 100 times more powerful than
Orion. What makes the AWE's laser notable is the exquisite precision that it will give researchers in controlling the heat and compression exerted on
the materials placed in its target chamber — and the fact that the AWE is sharing it at all.
“The defence ministry is notoriously tight-lipped about the lab's activities.”
Orion's main mission, like that of the NIF, is to explore how nuclear weapons work, particularly as they get older. In 1998, Britain ratified the
Comprehensive Nuclear-Test-Ban Treaty, an international agreement prohibiting tests of nuclear weapons. Scientists in Britain and worldwide have
therefore been busy developing computer models to simulate nuclear warheads and work out whether the weapons will still detonate after decades in
storage, and what type of detonation will result. What is missing, however, are actual data.
US scientists hope that the more powerful NIF will contribute some of those data, by generating temperatures and pressures so high that they will
spark nuclear fusion in small quantities of two hydrogen isotopes, deuterium and tritium. This fusion process would resemble conditions inside the
most powerful stage of a modern thermonuclear weapon.
If the NIF is a thermonuclear hammer, then Orion is a scalpel. The smaller facility will never achieve full-scale fusion, but it will be able to
carefully control conditions inside test materials such as uranium. Pressure and temperature usually go hand-in-hand, explains Peter Roberts, head of
the AWE's plasma-physics department. "You pump up a tyre with a bicycle pump and it gets hot," he says. But Orion can get around this. It can
compress a material with its long, nanosecond pulses then suddenly heat it with its very short, half-picosecond pulses. The result is 'isochoric
heating', an unusual condition in which a material is heated so quickly that it doesn't have time to expand. This capability allows Orion to probe
materials at wide-ranging combinations of temperatures and pressures.
In particular, researchers will use Orion to explore two key parameters for materials used in nuclear weapons: their opacity and their equation of
state. The first describes how radiation travels through a material — in this case, the two stages that make up a weapon. The first stage, or
primary, is a few kilograms of plutonium that are compressed by conventional explosives until they begin a runaway nuclear reaction. The radiation
from that reaction is then focused onto the 'secondary', the stage in which hydrogen isotopes create a much larger blast using nuclear fusion.
Researchers want to know what the opacity is and how it changes with age so that they can model radiation's flow from the primary to the secondary
and verify whether the warheads will still work. The other parameter — the equation of state — describes how a material behaves at enormous
pressures and temperatures. By generating data on these and other crucial parameters, Orion will give nuclear-weapons scientists the information they
need to ensure that their models are correct. "You can't look this stuff up," Rose says.
The researchers running the NIF often emphasize the giant laser's applications in energy production and fundamental science over its military role;
it could, for example, lead to new reactors that produce electricity using tiny fusion implosions. Orion's scientists are much less circumspect.
"We're working on weapons physics fundamentally," Roberts says.