MAST Upgrade: fitting the last piece of the puzzle

CCFE 15/03/2017

On 7 March, the finishing straight got one (major) step closer for the new MAST Upgrade tokamak, with the lifting and positioning of the centre column into the heart of the machine.

Nicknamed the ‘£1 million kebab’ by engineers here at Culham, the five metre-long centre column is a key component for operating MAST Upgrade. Once connected up to its power supplies, it will form part of the magnetic coil system that traps plasma within the tokamak, and will induce a current to heat the plasma up.

The column is a nested assembly made up of the centre rod and the solenoid – 324 turns of 110m-long copper conductor, which surrounds plasma-shaping coils, as well as support structures and cooling lines.

To meet the step up in performance from the original MAST device (an increase in magnetic field from 0.52 to 0.78 Tesla and pulse length from 0.5 to 5 seconds), the centre column had to be completely re-manufactured. One of the key features of this new assembly is its use of an insulation matrix of cyanate ester rather than epoxy resin, to increase the insulation mechanical strength and raise the maximum operating temperature from 70°C to around 100°C.

The installation was initially scheduled for early January, but final inspections revealed a slight rotation of the solenoid from its original position after the column had been moved from its assembly area. Over the past six weeks, MAST Upgrade engineers worked out the best way to implement a modification to the support mechanism to guarantee that the solenoid would not rotate or move laterally during operation. This setback was a double-edged sword: “Spotting this anomaly now caused a six-week delay; not spotting it could have caused at least six months delay later on, with this weakness very likely resulting in a serious failure during commissioning or first operations, when repairs are significantly harder to address,” explains MAST Upgrade Project Leader Joe Milnes.

The column was lifted into place in a ive-hour operation, with a few ‘hold your breath’ moments. One of the main challenges was to fit the column in the rather tight space within the centre tube, with an error margin as little as a couple of millimetres in a few locations.

Martin Harte, the Centre Column Team Leader, said: “Installing the centre column represents the culmination of several years of challenging work to make the coils. It also marks the start of a significant installation phase leading towards MAST Upgrade’s commissioning and start-up.”