JET’s upgraded Neutral Beam Injection heating is gradually being turned up, always in step with commissioning of the wall-protection systems. This makes sure that the power does not harm the new ITER-Like-Wall. Along the way, the system has been setting record after record.

Neutral Beam Injection (NBI) is the principal external heating system on JET. It was upgraded along with the installation of the ITER-Like-Wall in order to increase the flexibility for higher power experiments. Two Neutral Injection Boxes, each delivering eight beams of energetic particles, provide the power to the plasma. Each box was originally designed, over 20 years ago, to provide 7.5 megawatt (MW) for up to ten seconds. Long ago that performance has been exceeded, and the whole system used to routinely deliver over 20 MW. The upgraded design aims for an overall power of 34 MW for 20 seconds.

The NBI heating has been turned up step by step and now, at the end of May, we have about 30 MW available for use, although the full power has not yet been applied to the plasma. The ITER-Like-Wall protection systems are being commissioned systematically as the power levels rise. Without this protection working, there would be a significant risk of over-heating a few plasma-facing components that are particularly exposed, notably the inner wall guard limiters and the divertor.

Long, powerful beams

Meanwhile the system has reached its previous power record of 23.8 MW and routinely supplies more than 20 MW to the plasma. Other records have been achieved almost un-noticed: 14.3 MW was delivered to the JET plasma from just one injection box, beating the previous record of 13.7 MW, and approaching double the value of the original design. A record beam pulse length of 15 seconds, kept up by four of the 16 beams, has also been achieved. Previously the beam pulses lasted at most ten seconds. The long pulse lengths also demonstrate that the water-cooled duct liners work as expected. These were installed to protect the walls of the narrow port through which the beams are injected.

Robust power supply

The Neutral Beam Local Manager software is now commissioned. This controls the 16 individual injection beams and attempts to keep up the pre-set amount of neutral beam power during a plasma pulse, even if one beam happens to fail. It thus makes the experiments more robust. With this system in place, scientists control the delivered power in real time for the first time since operations restarted. This factor, along with the availability of a substantial amount of heating, has opened up the opportunity for some interesting experiments. The sense of excitement in the JET Control Room suggests that good results are being achieved.
Nick Balshaw, CCFE