Posted on: 12th December 2011
Another day in JET, a pretty normal Monday. The tests of the new beam monitoring systems have been successfully completed, so the Engineer in Charge calls the day to a close at 9.51 PM. Twenty-two pulses, between ten and twenty seconds long, have been run in the fourteen hours since the first pulse of the day, a fairly standard number; but it brings the grand total of pulses to a staggering 81,374.
JET has been running now for 28 years, in which time the extraordinary has become routine. Now the JET team routinely generate plasmas with temperatures well over 100 million degrees – ten times hotter than the sun – and stabilise them for up to a minute, an unheard of achievement in 1983.
Thousands of scientists from all over Europe – and the world – have contributed to these 81,374 experiments. In the process they have amassed an encyclopedia of fusion science, plasma control and tokamak management. The original donut shape of the plasma has evolved to become more elongated, with a point at the bottom, known as the X-point. The vessel is now lined with “limiter” tiles to protect the walls and shape the plasma, and, in the bottom of the vessel sits one of the most significant developments in tokamaks, the divertor. The divertor is a set of tiles at the X-point of the plasma, just above the pumps, where the plasma is actually allowed to touch the vessel – a situation originally avoided at all costs. But although some energy is lost through this contact, the removal of the product of the fusion reaction, helium (known as “ash”), and other impurities, improves the plasma’s stability a lot.
More improvements are on the way, too. Systems that were upgraded during the 2011 shutdown are being tested: additional heating and measurement systems, and clever systems to keep the plasma even more stable, such as the pellet injection system which shoots frozen fuel into the middle of the 100 million degree plasma. And of course the ITER-Like Wall, which has such vital implications for the multi-billion euro experiment at Cadarache – every part of the ITER’s design that is tested at JET ensures a successful next step in the journey towards fusion power.
So there are still many thousands more pulses to be run at JET, each one contributing one more extraordinary grain of sand to an extraordinary mountain of knowledge.