Turning the tables on turbulence

CCFE | 03/09/2015

Plasma turbulence has been the bane of fusion scientists for decades. But now they’re getting their own back – images of plasma inside the MAST tokamak at Culham are showing how turbulence could actually tackle one of the hottest issues in fusion reactor design.

Plasma is a fascinating but frustrating fact of life for researchers developing fusion energy. The fourth state of matter, despite making up most of the universe, still holds many secrets for Earth-bound physicists. Controlling this incredibly hot ionised gas in a magnetic field within a tokamak is a proven way of triggering fusion reactions, but the downside is that the plasma becomes turbulent and unstable, making it difficult to confine – analogous to the creation of blobs in a lava lamp, or the break-up of clouds in the sky.

The MAST videos provide the closest view yet of plasma in the tokamak’s exhaust system, the divertor, and may hold the key to dealing with the intense heat ejected from the fusion chamber onto surrounding surfaces. This is a major concern for researchers designing full-scale tokamak power plants. The divertor, made from extremely tough materials, acts as a target for the waste plasma, and pumps helium ash and impurities out of the tokamak. But in a fusion power plant the divertor will be exposed to power loads of tens of megawatts per square metre (many times greater than a spacecraft re-entering the atmosphere), putting a strain on even the toughest of structures.

Fusion scientists think that one answer to the problem is to deliberately make the plasma turbulent. By stirring it up in the divertor region, instabilities are triggered which can spread the plasma over a wider ‘footprint’, reducing the concentration of particles being deposited on solid surfaces in the machine.

“There are two ways of dealing with the power load on the divertor,” explains CCFE’s James Harrison. “One is to decrease the load itself, and the other is to disperse it over a larger area – that’s what we’re looking at here.”

Simulating turbulent movement of plasma in the divertor of a tokamak is notoriously difficult, due to the complex interactions which occur in that region. That’s where the high-resolution video footage, recorded during the last MAST experiments in 2013, is proving invaluable. For the first time it provides a detailed picture of the whole divertor area. Filmed at 120,000 frames per second over the period of a 0.5 second MAST pulse, it shows previously unknown instabilities. The wealth of data yielded is helping physicists to understand how the instabilities form and work out ways to encourage them in a controlled fashion, with the aim of spreading out the plasma and thus the power being exhausted from it.

James, part of the team that produced the videos, explains what is happening in the video clip below: “Although this is in a relatively quiet period for the plasma, there is still a lot of turbulence. For example, we can see ‘blobs’ of plasma emerge at the edge and fly off. Some form in the divertor itself; others leak down from the core (at the top right of the screen) and get stretched into ‘tails’ as they go. Other activity is visible too – what looks like out of control hula hoops near the bottom are actually another form of plasma instability.”

The images were made possible by the loan of a fast camera by CCFE’s funding body, the Engineering & Physical Sciences Research Council. The success of the trial installation on MAST is being followed up by similar tests on Swiss tokamak TCV, whose flexible divertor geometry should lead to a whole new range of insights. And the footage will come into its own on the MAST Upgrade device, currently being assembled at Culham. MAST Upgrade will specialise in divertor studies and its ‘Super-X’ exhaust system, designed to cool down particles being ejected from the plasma before they collide with surfaces, is seen as a candidate for use in fusion powerplants. Hi-res video footage like this will tell researchers how well Super-X is doing its job and inform the design of the fusion reactors of the future.


Top right: image of plasma fluctuations in the MAST divertor, showing where the filaments are brightest and instabilities are strongest

Above right: diagram plotting the apparent direction of motion of the plasma filaments
Below: slow-motion video of plasma in the MAST divertor, described above