Posted on: 23rd December 2011
Good progress is now being made with JET’s additional heating systems. The ICRH system is working routinely, and significant amounts of power are also being injected using the Neutral Beam Injection systems.
As you may know, JET’s NBI is divided into two separate Neutral Injection Boxes, (NIBs), located diametrically opposite each other at octants 4 and 8. Each of these boxes contains eight separate beam sources call PINIs (Positive Ion Neutral Injectors). The latest commissioning work has concentrated on the octant 8 NIB, which now has 6 PINIs working reliably at high power.
Part of the process of restarting the neutral injection system is to ‘condition the ducts’ through which the beams pass. The energy from the beams tends to heat tiny amounts of residual gas that are on the surfaces of the ducts, and to heat the walls of the ducts. Heating surfaces in a vacuum system usually forces gas to be released, creating even more heating and thus creating an almost invisible avalanche process. The release of this gas leads to the requirement to turn off the beams. This is not as big a problem as it sounds, because in doing this the ducts are cleaned a little, and over just a few days they are back into full operation. This cleaning is almost analogous to a sneeze.
Now that the duct is no longer limiting operation the neutral beams are being used quite routinely to heat the plasma. Already they are starting to operate for longer pulses than have ever been achieved before, thanks to the upgrades implemented during the recent shutdown. At the time of writing the maximum power delivered was a healthy 9.4MW for 4 seconds, but the team should perhaps be prouder of the achievement of longer pulses which could not be achieved before. They have already achieved 4MW for 15 seconds.
Meanwhile the new wall protection system continues to be refined so that longer pulses at higher powers will be possible in the New Year.
Following on from the popular Shutdown Weekly, this series aims to give an insight into day-to-day activities at JET, from an engineering perspective. It aims to explain the technical aspects of operation of the world’s most successful tokamak. JET’s new ITER-Like Wall is being exploited for the benefit of its future successor, ITER.