Description
Wendelstein 7-X (W7-X) is the first comprehensively optimised stellarator and the first magnetic fusion device to follow design principles derived from fundamental physics criteria, such as minimising neoclassical transport losses and plasma currents. As such, W7-X is a rigorous realisation of the stellarator concept, in which the plasma equilibrium depends only weakly on the plasma pressure. It is equipped with superconducting coils that demonstrate the stellarator's ability to confine plasmas in a stationary manner, ensuring plasma discharges are long enough to achieve steady-state equilibria. Thanks to neoclassical optimisation, W7-X has achieved triple product values that were previously unattainable for stellarators. With continuous pellet fuelling, plasma performance parameters can now be maintained over longer periods of time — a capability that was previously only accessible in much larger fusion experiments. These successes are based not only on a significantly better understanding of plasma transport, particularly turbulent plasma losses, but also on technical advances such as an actively cooled divertor and the power control of a multi-gyrotron ECRH facility. The future focus of the W7-X programme will be to advance the duration of high-performance plasmas further (with the aim of achieving 30 minutes at 10 MW of heating power), to improve the understanding of heat and particle exhaust (by employing a magnetic island divertor) and to investigate the crucial role of plasma edge transport and high-beta experiments in exploring stability limits and fast ion confinement. Nevertheless, the success of W7-X must not distract from the fact that many questions remain unanswered on the path to achieving burning fusion plasma or a fusion power plant, despite the conceptual advantages of stellarators. A strategy is currently being developed to address open physics questions, including those relating to divertor physics and first wall materials. The success of W7-X has significantly increased worldwide interest in the stellarator concept among governments and fusion start-ups alike. A key question is how this interest will impact stellarator development and what concrete steps are currently being considered.