Magnetic configuration effects on the Wendelstein 7-X stellarator
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The two leading concepts for confining high-temperature fusion plasmas are the tokamak and the stellarator. Tokamaks are
rotationally symmetric and use a large plasma current to achieve confinement, whereas stellarators are non-axisymmetric and
employ three-dimensionally shaped magnetic field coils to twist the field and confine the plasma. As a result, the magnetic
field of a stellarator needs to be carefully designed to minimize the collisional transport arising from poorly confined particle
orbits, which would otherwise cause excessive power losses at high plasma temperatures. In addition, this type of transport
leads to the appearance of a net toroidal plasma current, the so-called bootstrap current. Here, we analyse results from the
first experimental campaign of the Wendelstein 7-X stellarator, showing that its magnetic-field design allows good control
of bootstrap currents and collisional transport. The energy confinement time is among the best ever achieved in stellarators,
both in absolute figures (τE > 100 ms) and relative to the stellarator confinement scaling. The bootstrap current responds as
predicted to changes in the magnetic mirror ratio. These initial experiments confirm several theoretically predicted properties
of Wendelstein 7-X plasmas, and already indicate consistency with optimization measures.
rotationally symmetric and use a large plasma current to achieve confinement, whereas stellarators are non-axisymmetric and
employ three-dimensionally shaped magnetic field coils to twist the field and confine the plasma. As a result, the magnetic
field of a stellarator needs to be carefully designed to minimize the collisional transport arising from poorly confined particle
orbits, which would otherwise cause excessive power losses at high plasma temperatures. In addition, this type of transport
leads to the appearance of a net toroidal plasma current, the so-called bootstrap current. Here, we analyse results from the
first experimental campaign of the Wendelstein 7-X stellarator, showing that its magnetic-field design allows good control
of bootstrap currents and collisional transport. The energy confinement time is among the best ever achieved in stellarators,
both in absolute figures (τE > 100 ms) and relative to the stellarator confinement scaling. The bootstrap current responds as
predicted to changes in the magnetic mirror ratio. These initial experiments confirm several theoretically predicted properties
of Wendelstein 7-X plasmas, and already indicate consistency with optimization measures.