The influence of magnetic topology on plasma performance has attracted considerable interest in magnetic fusion research. In Stellarators, local (and therefore global) confinement and the passage between different confinement regimes is strongly linked to magnetic topology. In particular, low order rational surfaces have been found to affect the generation of Reynolds stress and sheared flows [1-3]. In Reversed Field Pinches (RFP), a spontaneous transition from the multiple-helicity states (higher level of magnetic chaos) to the single-helicity state (lower level of magnetic chaos) at sufficiently high currents indicates the role of magnetic topology in modifying plasma confinement properties [4]. Although extensive researches have been carried out on this topic, it is not fully clear yet how the magnetic topology may affect the formation of large coherent structures (e.g. zonal flows), and transport behaviour. In this work, we will present a study on the influence of magnetic topology on long-range temporal correlations (Hurst exponent [5]) and spatial correlations (LRC) using a multi-Langmuir probe system in TJ-II and in the RFX-mod. In TJ-II in the ECRH plasma around the critical density (0.6×10&' m*+), a clear change in LRC features accordingly to the magnetic topology was found during the dynamic configuration scan. The propagation of the rational surfaces was captured by the spatial-temporal evolution of Hurst exponents of floating potentials measured by a rake probe. In RFX-mod, LRC was detected both inwards and outwards with respect to the m=0 island radial position, which is complementary to previous studies [6, 7]. A preliminary analysis of Hurst exponent at the edge during helical boundary perturbations shows that there might be small transport barriers around the m=0 island region. Results from TJ-II and RFX-mod show that magnetic topology plays a role in both the formation of large coherent structures and affecting local particle dynamics.
Influence of magnetic topology on long-range spatial and temporal correlation in Stellarator and Reversed Field Pinch plasmas
Spolaore M;De Masi G;Martines E;Terranova D;
2017
Abstract
The influence of magnetic topology on plasma performance has attracted considerable interest in magnetic fusion research. In Stellarators, local (and therefore global) confinement and the passage between different confinement regimes is strongly linked to magnetic topology. In particular, low order rational surfaces have been found to affect the generation of Reynolds stress and sheared flows [1-3]. In Reversed Field Pinches (RFP), a spontaneous transition from the multiple-helicity states (higher level of magnetic chaos) to the single-helicity state (lower level of magnetic chaos) at sufficiently high currents indicates the role of magnetic topology in modifying plasma confinement properties [4]. Although extensive researches have been carried out on this topic, it is not fully clear yet how the magnetic topology may affect the formation of large coherent structures (e.g. zonal flows), and transport behaviour. In this work, we will present a study on the influence of magnetic topology on long-range temporal correlations (Hurst exponent [5]) and spatial correlations (LRC) using a multi-Langmuir probe system in TJ-II and in the RFX-mod. In TJ-II in the ECRH plasma around the critical density (0.6×10&' m*+), a clear change in LRC features accordingly to the magnetic topology was found during the dynamic configuration scan. The propagation of the rational surfaces was captured by the spatial-temporal evolution of Hurst exponents of floating potentials measured by a rake probe. In RFX-mod, LRC was detected both inwards and outwards with respect to the m=0 island radial position, which is complementary to previous studies [6, 7]. A preliminary analysis of Hurst exponent at the edge during helical boundary perturbations shows that there might be small transport barriers around the m=0 island region. Results from TJ-II and RFX-mod show that magnetic topology plays a role in both the formation of large coherent structures and affecting local particle dynamics.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
