Fusion Burn Regulation via Deuterium Tritium Mixture Control in JET.
The first generation of nuclear fusion reactors is expected to operate using a mixture of deuterium(D) and tritium(T) fuel. Controlling the D:T ratio is a promising option to control the fusion burn rate. The Joint European Torus (JET), as the only operational tokamak which can use tritium, is uniquely placed to test the feasibility of such control. Experiments carried out in 2023, during the 3rd JET Deuterium-Tritium (DT) campaign, have demonstrated effective feedback control of the D:T ratio in H-mode conditions. The D:T ratio was measured using visible spectroscopy and the tritium was injected via gas valves, while the deuterium was injected either via gas valves or pellets. In these experiments the fusion power, measured via the neutron rate, responded promptly to variations in the measured D:T ratio. This demonstrates that, although the plasma is fuelled mainly at the edge, rapid mixing of the isotopes occurs throughout the plasma and that controlling the D:T ratio is an effective way of controlling the burn rate. In order to sustain a stable type-I ELMy H-mode plasma it is desirable to maintain a given ELM frequency. However, both the total fuelling rate and the D:T ratio influence this ELM frequency, with higher fuelling rates and higher D:T ratios both resulting more frequent ELMs. For this reason, the D:T ratio controller was combined with an ELM frequency controller in a multi-input multi-output (MIMO) controller. Successful simultaneous, decoupled, control of D:T ratio and ELM frequency was demonstrated using a combination of pellet and gas fuelling. This is the first and, for the time being, only demonstration of such an advanced burn control scheme in a DT plasma.