Scientists and engineers start up the “new” JET machine.

Just before the end of August JET was ready for operation and so the torus hall was locked up to attempt for plasma. Achieving stable plasma operation in the past immediately after a long shut down has never completely been straight-forward and the operations team was expecting a few attempts might have to be made.

However, to their delight, the first pulse for 22 months ran at the initial attempt, at 18.30 on 24th August. Plasma was created and controlled for 15 seconds, reaching a very respectable current of 1.1 mega amperes. After only a few days of commissioning by the engineers, the machine was handed over to the scientists so that experiments could begin. The initial phase tested the behaviour of the new wall in ohmic configuration, in which the only heating of the plasma comes from the current flowing in it. The total ohmic heating power is only about one megawatt and limits the plasma temperature to a few tens of millions of degrees. That might seem hot by every-day standards, but for JET it represents a careful and considered period of operation.
Early experiments studied the migration of beryllium from some of the plasma facing tiles and measured the amount of fuel retained in the tiles. Many people are eager to find out how much the fuel retention has been reduced compared with similar experiments done with the old carbon tiles. Some experimentalists have also been studying the spectrum of light emitted by the plasma to learn about the role of impurities from the new tiles. In parallel with operating the machine for ten shifts per week, other staff continued to install more new systems during nights and weekends. The priority was to have the new camera-based protection system in place, to monitor the wall temperature.

The second phase of commissioning thoroughly tested the updated algorithms used to control the plasma position and plasma current. Another new feature is an enhancement to the power supplies of the coils, which allows the plasma to touch the divertor earlier in a pulse than was previously possible. This system is now commissioned and in use. After the installation of a camera with partial coverage of the radio frequency antennae, the first additional heating system was brought into operation. The heating power and pulse lengths have been slowly increased, while the camera operators watched the antennae for unexpected hot spots. The protection system has been successfully commissioned for powers of several megawatts.
JET’s main additional heating comes from the Neutral Beam Injection system, whose power supplies and beam sources have been considerably enhanced. Bringing the system back into operation has taken a little longer than planned, but beams have been produced and conditioned successfully by aiming them at a target inside the Neutral Injection Box. The beams will initially be run carefully in short bursts until the computer models, which predict how the in-vessel components heat up, are verified. Only then will full power beams be injected.

Each of these steps brings the machine closer to producing new physics that will be relevant for ITER. The current routine of interspersing commissioning sessions with experimental work is expected to continue until the end of the year.

Nick Balshaw, CCFE