We have been talking in our earlier blog posts (here and here) about the relevance of the deuterium-tritium (D-T) fuel mixture in a magnetic confinement fusion device, such as a tokamak. The intrinsic property that makes this mixture so interesting is the higher fusion cross section as compared to other ion combinations at a relatively “low” energy (around 100 keV which is already 10 times the temperature of the centre of the Sun). In other words, D-T maximizes the number of fusion reactions, which is after all, what we are looking for.
The Joint European Torus (JET) has recently finished its second D-T campaign (DTE2 as we refer to it in more technical contexts), and the results could not have been better. This culminates a huge effort from the nuclear fusion community which makes the way towards the success of ITER smoother.
The data for DTE2 is still under revision and needs modelling and analysis in order to extract firm conclusions. However, one particular highlight result has already been resonating for a couple of weeks in almost every media in the world: the 59 megajoules (MJ) of plasma energy content which was sustained for around 5 seconds. This is an important step towards the realisation of fusion energy, as it has doubled the previous record energy content (during the first D-T campaign in JET in 1997). The duration of 5s is due to a tokamak being a pulsed machine and the magnets getting hotter as the current increases due to the Joule effect. ITER pulses will last way longer as it will possess superconducting magnets. In any case, 5s is a longer time span than what we call the plasma characteristic time, i.e, most plasma phenomena occurs in less than 5s, therefore, we are able to study it.
Among the key successes is not only the improvement in energy content but the fact that it has been obtained using a metallic wall, the so-called ITER-like wall. This is the same plasma-facing wall that ITER will possess and a priori it was not clear whether the efficiency obtained with the previous JET carbon wall would be achieved. Fortunately, after many years of efforts working with the ITER-like wall, not only we have reached the same efficiency but we have doubled the record. You can imagine our satisfaction!
The necessity to change from a carbon to a metallic wall would need another post to discuss it in detail. To keep the story short in this post, this change is mainly to avoid tritium retention in the wall. The question on whether this energy achieved is enough to sustain the fusion reactions and provide net energy is very interesting too, but we will leave it for another day as well. In the meanwhile, let’s keep in mind that the purpose of JET is not to obtain net energy or the so-called break even conditions; in fact, there are several physical constraints that would make this very hard for JET such as its size (volume and magnetic field). ITER will avoid these constraints.
The success of JET has been an international joint effort undergone by many scientists. From our Fusion group, our group leader ICREA research Prof. Mervi Mantsinen contributed as a Scientific Coordinator for one of the experiments studying heating of the fusion fuel through radio frequency waves and Dr. Dani Gallart as a Scientific Coordinator of the modelling efforts towards D-T prediction.
Heating of the fusion fuel through radio frequency waves is a process similar to that of heating food in a microwave. It is needed because the temperatures of the fusion must reach values of the order of 150 million degrees Celsius, that is ten times the temperature in the core of the Sun, in order for the fusion reaction to occur. The hot plasma must then be sustained at these extreme temperatures in a controlled way in order to extract energy. In the JET experiments coordinated by Mervi Mantsinen various ways to heat the fusion fuel consisting of different species were investigated. This included for the first time the demonstration of main heating schemes planned for ITER in JET high-performance fusion conditions. The experiments were prepared with the help of extensive numerical modelling in close collaboration with the modeling task coordinated by Dani Gallart.
Do stay tuned as the results of analysis of these historic experiments unfold. The nuclear fusion field has never been “hotter”!