Fusion experiments

In operation since 1983, JET was explicitly designed to study plasma behaviour in conditions and dimensions approaching those required in a fusion reactor. Today, its primary task is to prepare for the construction and operation of ITER, acting as a test bed for ITER technologies and plasma operating scenarios and to perform deuterium-tritium (D-T) operation. Usually fusion laboratories operate with D-D plasmas, which is more practical for extensive plasma physics investigations. The D-T mixture, which enabes high fusion power production, will be the fuel for fusion power plants. ITER will carry out its ultimate experiments with D-T plasmas.

JET is the only device operating that has been licensed for tritium. After a successful experimental campaign in 2015–2016, JET is currently preparing for the D-T experiments in 2021. Several Finnish fusion researchers will participate in the JET D-T experiments and perform analyses after the experiment.

Several FinnFusion members, including scientists from VTT, Aalto University, and the University of Helsinki have contributed to the scientific program on the EUROfusion medium-size tokamaks (MST) since the advent of the MST program in 2014. The MST framework presently contains four devices – ASDEX Upgrade (Garching, Germany) TCV (Lausanne, Switzerland), MAST Upgrade (Culham, UK), and WEST (Cadarache, France) – all having their unique features. In particular, the entire inner walls of ASDEX Upgrade and WEST have been made of tungsten while on TCV and MAST Upgrade a huge mixture of conventional and novel plasma geometries can be investigated.

The role of FinnFusion is particularly prominent in areas related to plasma-surface interactions, physics in the core and edge parts of the fusion plasma, and understanding the behaviour of high-energy ions in the plasma and their impact on the integrity of the reactor. Several people have paid visits to the MST devices to take part in experiments or analyses while in home laboratories modelling by state-of-the-art numerical tools and surface analyses by our unique device pool have been carried out.

In addition, within a long-standing collaboration with ASDEX Upgrade and DIARC-Technology Oy (presently Oerlikon Balzers Coating Finland), VTT has produced marker samples and components for dedicated plasma experiments. FinnFusion scientists have had several leading positions in the MST programme, both as scientific coordinators and as task force leaders in the EUROfusion level.

Now that production of fusion power by magnetically confined plasmas has been demonstrated in several experiments, the focus is being shifted to more technological problems that need solving before a power-producing fusion plant can be built. A fundamental problem in the tokamak-branch of reactors is the pulsed nature of operation. Stellarators represent an alternative branch of toroidal devices, with advantages and disadvantages pretty much complementary to tokamaks, the major advantage of a stellarator being continuous operation that makes it directly power-plant compatible. However, before the advent of supercomputers, the stellarator magnetic cage could not be designed as plasma-tight.

Wendelstein 7-X (W7-X), at Max Planck Institute in Greifswald, Germany, is currently the world’s largest stellarator. Its numerically optimised magnetic configuration proved to fit the predictions perfectly, and three experimental campaigns have already been carried out since 2016, with significant enhancements in between. The aim is to bring stellarator plasmas to the same level as their tokamak counterparts.

The stellarator plasma is heated up by either neutral beam injection or RF waves. Both methods produce fast ions that have to be confined by the complex magnetic geometry of W7-X. With the Finnish ASCOT code, the behaviour of fast ions from beam injection were carefully studied before the first beams were commissioned, and it is fair to stay that the ASCOT work was instrumental in identifying vulnerabilities in the wall components, so that preventive measures could be taken in time.

The ASCOT work continues with the W7-X team as a new beam box as well as an antenna for ion cyclotron heating (ICRH) are being installed.