Science in the spotlight

Metamorphosing MAST

MAST-U modelThe construction phase of the £30 million MAST Upgrade (MAST-U) project is well underway. When the first of two stages is finished in 2015, the machine will have been transformed into a user facility which will enhance MAST's role in international fusion research.

So what will be new?

The Super-X divertor is a new component that will be unique to MAST–U. The revolutionary design offers the promise of significant improvement to performance over existing divertors. On a tokamak the divertor is the region at the bottom of the chamber where the plasma is ‘diverted' to make contact with the wall surfaces before being pumped away. With the Super-X divertor, a different geometry from conventional designs will be used which aims to increase the distance travelled by the plasma before striking the divertor surfaces. This will ensure the plasma energy is reduced and damage to the divertor plates is minimised.

In the maelstrom of hot plasma swirling around MAST, one part is instrumental in holding everything together - the centre tube in the vacuum vessel. This key component is subjected to the highest stresses of any on MAST. And for MAST-U, its life is about to get even tougher with ten times more force being placed on it. The solution, a stronger tube, is being installed which will contain 24 copper wedge-shaped conductors bonded together to form a 2-tonne cylindrical rod along the five metre column.

When carrying out plasma experiments, exhausting helium from the plasma quickly is crucial for self-sustaining fusion to occur. MAST-U will use actively cooled helium cryo-condensation pumps (cryopumps) under the divertor to do this. New to the machine, the primary function of the cryopumps is to provide plasma density control and remove particles from the divertor region, ideally as near as possible to its strike points. Preventing recycling of these particles back into the plasma ensures that impurity levels in the plasma are minimised as much as possible. The pumps will carry supercooled liquid helium at -269 °C and liquid nitrogen at -196 °C.

Other changes to the vessel will include an upgrade to many of the existing diagnostic systems. These will generate more signals to measure density, temperature and electrical potential and in turn generate more data for scientists to analyse. Extra heating power will be produced by the Neutral Beam heating systems increasing their capacity through the addition of a double beam on one side of the machine.

Improved plasma performance

MAST-U will provide high performance plasmas with pulse lengths of several seconds (up to ten times longer than the present duration). Hence, plasma shape, temperature and density and current profiles will be sustained in steady state conditions, allowing the study of stable operating regimes that could be used for future fusion machines.

The metamorphosed machine will also produce improved parameters approaching fusion conditions, including plasma temperatures in excess of 50 million degrees. This performance will enable highly accurate scaling of plasma confinement models to ITER and the prototype power plant DEMO.

In addition, MAST-U will explore the suitability of the spherical tokamak as a candidate for a future Component Test Facility (CTF). Studies on a CTF would include looking at start-up, current drive, steady state behaviour, handling of high heat flux, plasma confinement and performance reliability.