JET is the largest and most powerful tokamak in the world and currently the only machine capable of operating with the deuterium-tritium fuel mix of future commercial reactors. 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.
In the past years, JET has been upgraded to become more ITER-like in order to conduct dedicated studies for ITER. One main upgrade was to equip the vessel with an inner wall made of the same materials planned for ITER – beryllium and tungsten. Together with upgraded heating power, this ITER-Like Wall enables scientists to develop plasma scenarios that resemble as closely as possible those planned for ITER. Already JET experiments have helped ITER make the decision to begin operation with a full tungsten divertor, thus substantially reducing investment costs. The main subject of current JET research is full exploitation of the ITER-Like Wall in order to develop robust operation scenarios for ITER. Other subjects of investigation are the interaction of the plasma with the wall materials, e.g. the question of how the vessel wall erodes and how this material migrates through the plasma and redeposits on a different location of the wall. Another issue is the accumulation of tungsten from the wall in the plasma core. JET’s goal is to demonstrate an acceptable tungsten concentration for the foreseen ITER regimes.
In certain modes of operation fusion plasmas can develop potentially harmful instabilities. Examples are disruptions, powerful events during which the plasma very rapidly loses all its energy, or Edge Localised Modes (ELMs), which are short plasma outbursts which thrust large heat and particle loads onto the vessel wall. In powerful devices like ITER such events are a potential risk for the vessel wall and must be mitigated. JET develops various methods to predict and mitigate these plasma instabilities in ITER-like regimes of plasma operation.
The current plans for JET foresee a scientific campaign with deuterium-tritium (D-T) plasmas. D-T plasmas produce more fusion power and will therefore be the fuel for fusion power plants. ITER will carry out its ultimate experiments with D-T plasmas. Usually fusion laboratories operate with D-D plasmas. JET is the only device operating that has been licensed for tritium. During its last high power D-T campaign in 1997, it produced a world record amount of fusion power. Since then, JET has become a much more ITER-like machine: The ITER-Like Wall comprises the same materials as ITER and its upgraded heating systems allow it to operate with similar plasma scenarios as ITER.