A new control system on Culham’s MAST Upgrade experiment automatically keeps the superhot plasma fuel in check within milliseconds – allowing physicists to carry out key research with the machine’s world-first Super-X divertor system.
The divertor region of ‘tokamak’ fusion devices acts as the exhaust system, steering excess heat and particles from the fuel out of the plasma chamber. MAST Upgrade has a unique divertor configuration, called Super-X, and the hope is that it will allow heat to be exhausted in a way which avoids damage to the inner tiles of future fusion power plants.
The plasma in tokamaks is held in a strong magnetic field to keep it away from the walls of the machine. However, it can sometimes jump unpredictably due to ‘instabilities’ or ‘disruptions’ within it. Without a system to reposition the plasma in real time, the ability to conduct experiments – specifically using a divertor – would be compromised.
The newly-installed Vertical Position Control System allows a greater deal of control over unpredictable vertical movements of the plasma. And it takes full advantage of the capabilities in MAST Upgrade to vertically stretch or elongate the plasma to improve plasma performance.
The Vertical Position Control System, which responds to instabilities in the plasma, can adapt automatically within a fraction of a second. It uses the plasma’s position to adjust magnetic fields and push it back into its intended position.
The new system differs substantially from its predecessor on the original MAST tokamak, and is faster and more powerful.
Plasma control engineer at UKAEA, Kim Cave-Ayland, said: “What we really want is for the plasma to ‘float’ within the machine – only touching the divertors, which are designed to cope with the heat. To do this we stretch the plasma vertically, and this causes it to become unstable in the vertical direction. Any small perturbations of the plasma will make it wobble from one end to another, and therefore we need automatic control of the vertical plasma position to make it stable.”
Particularly challenging for the new system is when the ‘high confinement’ state (H-mode) is reached, which happens when plasma is better confined. During H-modes, mini eruptions akin to solar flares, called Edge Localised Modes, often occur at the plasma’s edge, and this can lead to a sudden loss of pressure and rapid changes in the plasma’s position. These are all things the vertical control system needs to counteract – with ELMs the biggest of these challenges.
“The system aims to return the plasma to within one millimetre of its original position within 5 to 10 milliseconds of an ELM occurring”, added Kim.
“Such a precise goal is challenging for a tokamak the size of MAST Upgrade, and so our new automatic control system is absolutely vital.”