Posted on: 25th October 2011

When the weather man tells us there is a low pressure zone approaching, he means the air pressure around us might drop below 1000 millibar. Inside JET, at the height of a pulse the pressure peaks at a mere 20 millionths of a millibar – fifty million times less than the pressure we live and breathe in.

And that is the highest pressure JET reaches, when the deuterium gas has been injected and heated to hundreds of millions of degrees Celsius. In between pulses the plasma cools and the pumps suck the leftovers of the gas out, taking the pressure down a thousand times lower again, ready for the next squirt of fuel.

This surprises many people, who imagine fusion experiments to be pressure cookers, with the lid jammed on tight to prevent the hot soup inside from spurting up onto the kitchen ceiling. Indeed, the most successful fusion reactor in the local area, the sun, does operate at very high pressure; but earthly experiments are at the other extreme.

So never fear: in the near-impossible event that JET sprang a leak, nothing would escape. Despite the million-degree temperatures inside the vessel, no soup would be spilt. No one nearby would even get warm. Instead air would be sucked into the chamber, and kill the plasma in an instant. A disaster for the scientists, but for no one else.