Although very different in design, the architecture of spherical tokamaks creates some of the conditions that will be experienced in ITER. Their form, comparable to that of a cored apple, generates highly pressurised plasma at a relatively low magnetic field. One such fusion device is the Mega Amp Spherical Toka mak (MAST) in Culham which is currently undergoing major upgrades in order to become MAST Upgrade. Programme leader Andrew Kirk explains how these improvements will help pave the way for ITER and the first fusion power plant and why the European team is happy to have the National Spherical Torus Experiment in Princeton as sibling across the pond.

MAST Upgrade will help us look at some aspects of ITER operation more easily than on other devices, states Andrew Kirk enthusiastically.

Andrew Kirk, MAST Upgrade Programme Leader (Image: CCFE)

Andrew Kirk, MAST Upgrade Programme Leader (Image: CCFE)

The head of the MAST Upgrade programme can hardly wait for his upgraded tokamak to start operating. Unlike the usual doughnut shaped vessel, ‘his’ tokamak is a spherical one. It has a design which can be compared to a cored apple thus confining the plasma of a fusion experiment into a more compressed shape.

Under pressure

The advantage of a compressed tokamak for fusion experiments is its lower magnetic field which is able to confine highly pressurised plasma. These plasmas have some of the characteristics that will be produced in a burning plasma in ITER. “The special design of a spherical tokamak allows it to operate at a lower magnetic field which, since superconducting magnets are very expensive, may help to reduce the cost of future reactors.” In figures this means that Andrew Kirk and his colleagues will attempt to create a plasma pressure of 2-3 atmospheres contained using a magnetic field of 0.8 Tesla. This will result in a so called ‘high beta plasma’, whereby beta refers to the ratio of plasma pressure to magnetic pressure.

MAST on a mission

The tokamak is currently undergoing a major upgrade to serve several missions laid out in the European fusion roadmap. The makeover of the 16-year-old tokamak at Culham Centre for Fusion Energy began in 2013. It includes a novel divertor, several new power supplies, an upgraded heating system and modern diagnostic tools. Like a puzzle the tokamak is stripped down to its pieces and needs to be put together.

“In fact, with this upgrade we hope to solve three major issues in fusion research: Handling the power exhaust with the help of new divertor concepts, investigating turbulence in the plasma and exploring fast particles, which can cause energy loss,” says Kirk.

Xavier Litaudon

Xavier Litaudon (Image: EUROfusion)

All eyes on the super-X-divertor

Looking at novel divertor concepts will support Mission 2 of the European fusion roadmap: Handling the power that is exhausted during a fusion experiment. In a normal steady state fusion reaction, the charged plasma particles follow magnetic lines. In tokamaks, magnetic lines divert the particles into the bottom or the top of the vessel, called the divertor. MAST Upgrade needs to be able to handle a higher peak power density than traditional tokamaks due to the compressed shape of the device.
Consequently, the researchers are looking for a way to reduce the high particle flux. The unique Super-X-Divertor at the upgrade in Culham attempts to implement this by increasing the distance the particles must travel along an “extended leg” and thus providing a larger divertor area. “Like a river that spreads out into a larger delta, the stream slows down”, describes Kirk. Moreover, the updated spherical tokamak provides a closed divertor area, a major change from MAST. This allows the investigators to introduce gases and radiate the energy carried by the particles

MAST Upgrade will adress the challenging exhaust physics and control issues for ITER and DEMO, Xavier Litaudon, Head of EUROfusion’s ITER Physics department

A divertor for DEMO

MAST Upgrade will be the only device able to operate a closed pumped divertor with an extended leg. Thanks to a flexible divertor, it also provides the ability to compare different divertor layouts and examine which one will be most useful for DEMO, the first power plant. DEMO is intended to show how to generate electricity from fusion and introduce it into the grid. The first experimental campaign at the spherical tokamak will take place in autumn 2017 and is highly anticipated by Xavier Litaudon, Head of EUROfusion’s ITER Physics department: “MAST Upgrade will advance physics understanding to support the fusion energy development along the key missions of the European roadmap to the realisation of fusion energy, in particular by addressing the challenging exhaust physics and control issues for ITER and DEMO.”

Understanding fusion turbulences

The second goal is to investigate and control turbulence, a process that increases the rate at which plasma and heat are lost from the plasma. Understanding turbulence is essential for optimising the performance of ITER and fusion power plants. The types of plasmas and diagnostics available in MAST Upgrade give it the unique capability to study these processes.

Faster than the speed of sound

Jonathan Menard

Jonathan Menard (Image: PPPL)

Just as an airplane travelling faster than the speed of sound leads to an instability known as a “sonic boom “, fast ions in a fusion plasma lead to instabilities that may increase losses of particles from the confined plasma thus reducing its efficiency. In ITER, these will be the alpha particles produced by the fusion reaction. In the spherical tokamak, the effect of such alphas can be simulated using particles which are injected to heat the plasma. The upgrades to the heating systems will allow the scientists to study the effects of the turbulence which is caused by these fast particles in detail.

MAST U and NSTX U will identify the best approaches of using conventional and spherical tokamaks for fusion power production, Jonathan Menard, NSTX Upgrade Programme Director

A fusion sibling across the pond

The EUROfusion experiment is not the only modernised spherical fusion device that will help to explore some of the conditions encountered in ITER. On the other side of the ocean, the National Spherical Torus Experiment (NSTX) also shines in new splendour. The tokamak at the Princeton Plasma Physics Laboratory (PPPL) in the US saw its first plasma in August after undergoing a multimillion dollar upgrade. The parallel upgrades are not accidental.

Building complementary fusion devices

On the contrary, they have been carefully planned. Scientific discussions between CCFE and PPPL have been ongoing for several years. The knowledge exchange among fusion scientists from both sides of the Atlantic Ocean is dearly desired. “We prepared our upgrades more or less together. Members from both labs sit on both of the advisory boards. As a matter of fact, we decided to build the devices complementary to each other,” explains Andrew Kirk. While NSTX Upgrade (NSTX U) concentrates on looking at the core of the plasma, MAST Upgrade focuses more on examining the edge.

The core of the apple

According to Jonathan Menard, the Programme Director for the NSTX Upgrade, it will be the first spherical tokamak to try and sustain high plasma current and pressure using a combination of externally injected current and self-generated plasma current. Sustainment at high plasma current and pressure is essential for any future tokamak in order to enable it to operate continuously at high fusion power. The research team on NSTX U will inform MAST U on the subject of how to optimise its sustainment, since the two devices use somewhat different approaches.

In plasma edge research, the American tokamak is the only device capable of exploring a very advanced approach to expanding the power exhaust channel for reducing heat loads that might otherwise damage the reactor walls. Such heat load reduction is most probably essential to making smaller tokamak power plants viable.

Solving key challenges of fusion together

The MAST U approach complements that of NSTX U, which will explore liquid metals as a means of handling high edge power exhaust. “Together, MAST U and NSTX U will help identify the best approaches to solving the key challenges of using conventional and spherical tokamaks for sustained fusion power production”, states Menard.

The experimental campaigns for EUROfusion at the updated tokamak MAST Upgrade will serve Mission 1 and Mission 2 of the European fusion roadmap. Mission 1 refers to plasma regimes of operation. Due to their architecture, spherical tokamaks maintain a plasma environment that is close to ITER’s. In addition, Mission 2 tackles the problem of ‘Heat Exhaust Systems’. Such systems must be able to resist the large heat and particle fluxes of a fusion power plant. Therefore, different divertor concepts will be tested in MAST Upgrade to see what options may work best for DEMO, the first demonstration plant for fusion energy.