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The optimised Wendelstein 7-X stellarator, which went into operation five years ago, is intended to demonstrate that stellarator-type fusion plants are suitable for power plants. The magnetic field, which encloses the hot plasma and keeps it away from the vessel walls, was planned with great theoretical and computational effort in such a way that the disadvantages of earlier stellarators are avoided. One of the most important goals was to reduce the energy losses of the plasma, which are caused by the ripple of the magnetic field. This is responsible for plasma particles drifting outwards and being lost despite being bound to the magnetic field lines.

Unlike in the competing tokamak-type devices, for which this so-called "neo-classical" energy and particle loss is not a major problem, it is a serious weakness in conventional stellarators. It causes the losses to increase so much with rising plasma temperature that a power plant designed on this basis would be very large and thus very expensive.

In tokamaks, on the other hand -- thanks to their symmetrical shape -- the losses due to the magnetic field ripple are only small. Here, the energy losses are mainly determined by small vortex movements in the plasma, by turbulence -- which is also added as a loss channel in stellarators. Therefore, in order to catch up with the good confinement properties of the tokamaks, lowering the neoclassical losses is an important task for stellarator optimisation. Accordingly, the magnetic field of Wendelstein 7-X was designed to minimise those losses.

In a detailed analysis of the experimental results of Wendelstein 7-X, scientists led by Dr. Craig Beidler from IPP's Stellarator Theory Division have now investigated whether this optimisation leads to the desired effect. With the heating devices available so far, Wendelstein 7-X has already been able to generate high-temperature plasmas and set the stellarator world record for the "fusion product" at high temperature. This product of temperature, plasma density and energy confinement time indicates how close you get to the values for a burning plasma.

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