At JET, operators, scientists and engineers are busy preparing the machine for the first experiments after the 18-month shutdown.

With the end of the shutdown, the main port doors of JET were closed and sealed. On the 18 May, after some final installations and checks, various pumps were started up. All 200 cubic metres of air were removed from the torus. As soon as they reached high vacuum, the entire assembly was tested for leaks. In order to evaporate any residual humidity, the torus was baked for about five days at 200 degrees Celsius. Heating the JET torus is quite an undertaking: Hot gas is blown between its inner and outer walls. Parts which cannot be accessed this way, for instance the ports, are heated electrically via cables welded onto them. Even as we write, the machine is being prepared for a second, harder bake at a temperature of 320 degrees Celsius and with a duration of four weeks. During this baking process, an ionized hydrogen gas is generated in the vacuum chamber. The gas reacts with the oxygen atoms adsorbed in the wall and thus cleans the vessel.

Getting the equipment ready
All large sub-systems, most of which have been extensively upgraded, are then made ready for operation. Among them are power supplies for the coils and the plasma heating systems, the ventilation systems for the torus hall and basement, and the cryogenic plant. The latter will soon be routinely producing one tonne of liquid helium per day. The power supplies for the magnet coils deliver high current at a high voltage, and their commissioning requires complex checks of the protection system. Hundreds of sensors monitor the conditions around the machine and feed into the main control or Central Interlock and Safety System (CISS) which protects JET. The functionality of the sensors, the correct transmission of their signals to sub-control units and finally to the CISS have been checked along with the CISS programmes. In addition, approximately 2,000 control cubicles, each containing ten to 50 electronic instruments, monitor and operate JET’s subsystems such as the various heating units. All of these have been tested and, if necessary, repaired.

A final inspection
At the time of writing this article, the coils had just been re-connected to the power supplies. Prior to this step, JET had undergone a final, thorough inspection. About 20 people who know JET well spent an entire afternoon hunting for faults and checking whether all of the issues raised during prior inspections, had been solved satisfactorily.

Starting the machine
In September, the first experimental phase with the ITER-Like-Wall will start. For four months, JET will be operated and commissioned step-by-step – much like one would run-in a new car engine. After the basic commissioning of the control and protection systems, the first experiments will be conducted without any external heating, primarily to check the behaviour of the new wall. The second stage of commissioning and a second round of experiments are set to follow, during which the heat will gradually be turned on. And finally, JET will be ready for full power operation.

Nick Balshaw, CCFE, Christine Rüth