JET, so far the only operational fusion experiment capable of producing fusion energy, is routinely operated with Deuterium only, for a number of reasons. This minimises activation (from D-T neutrons and from Tritium retention in walls etc.), enabling to upgrade JET easily and minimising decommissioning issues at the end of JETs operational life. They operate in Deuterium only to investigate the feasibility of D-D fusion – very much concentrating on D-T fusion in ITER and the first true power plants.
Why then not get D-D fusion occurring at high levels in JET? Because D-D needs much higher temperatures of 400 – 500 million degrees C than can normally be achieved. Already – plasma temperatures of 150 – 200 million degrees C will enables lots of D-T fusion – but not very much D-D fusion.