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Magnetic well scan and confinement in the TJ-II stellarator

The magnetic well is the main stabilising mechanism in the TJ-II stellarator, since this is an almost shearless device. TJ-II has the capability of varying its magnetic configuration by changing the currents of its coils, allowing one to change the magnetic well while keeping the vacuum rotational transform profile almost constant, in particular. This characteristic makes this stellarator a suitable device to study the impact of unfavourable magnetic well conditions on plasma performance and stability. The here reported experiments explored a family of ten magnetic configurations with a similar rotational transform and varying magnetic well, from positive to negative values, hence exploring Mercier-unstable configurations. We succeeded in developing reproducible NBI-heated plasmas, even when the most Mercier-unstable configurations were performed, although the turbulence level was higher in the latter configurations. The position of the plasma boundary agreed with the equilibrium calculations in all cases.