Magnetic well scan and confinement in the TJ-II stellarator

Abstract

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 Mercierunstable 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.

Kinetic ballooning modes in tokamaks and stellarators

“…The TEH configuration is characterised by a small rotational transform, low iota <5/6 and very large mirror ratio = 24 %, whereas the KJM configuration corresponds to a more typical case with mirror ratio = 10 % and standard iota = 1. For the sake of completeness, it is important to note that, although the magnetic well of the configuration is taken as the figure of merit for MHD stability, there are experimental results that show stable configurations with a magnetic hill (de Aguilera et al 2015). We see that the 'diamagnetically-modified iMHD' ballooning mode description (based on our ω r condition, blue line figure 7b) is valid for all KBMs presented on this plot.…”

“…For the sake of completeness, it is important to note that, although the magnetic well of the configuration is taken as the figure of merit for MHD stability, there are experimental results that show stable configurations with a magnetic hill (de Aguilera et al. 2015). We see that the ‘diamagnetically-modified iMHD’ ballooning mode description (based on our condition, blue line figure 7 b ) is valid for all KBMs presented on this plot.…”

Magnetic well and Mercier stability of stellarators near the magnetic axis

“…2006; de Aguilera et al. 2015). These studies provide some indications that turbulence may increase in Mercier-unstable regimes.…”

Free-boundary ideal MHD stability of W7-X divertor equilibria

“…In experiments, however, several stellarators showed quiescent plasmas in scenarios predicted to be unstable according to ideal MHD theory or close to the MHD stability limit. Examples are very quiescent ECRH-heated plasmas in Wendelstein 7-AS [1], and the conclusion that W7-AS was not limited by pressure-driven MHD instabilities, neither in the achievable plasma pressures nor in the performance of the high-plasma-pressure discharges [2,3], and almost shearless configurations in TJ-II [4]. In the Large Helical Device (LHD), explicitly MHDunstable scenarios were experimentally investigated [5], by making use of the flexibility of its magnetic configuration.…”