TY - JOUR
T1 - An experimental characterization of core turbulence regimes in Wendelstein 7-X
AU - Carralero, D.
AU - Estrada, T.
AU - Maragkoudakis, E.
AU - Windisch, T.
AU - Alonso, J. A.
AU - Beurskens, M.
AU - Bozhenkov, S.
AU - Calvo, I.
AU - Damm, H.
AU - Ford, O.
AU - Fuchert, G.
AU - García-Regaña, J. M.
AU - Pablant, N.
AU - Sánchez, E.
AU - Pasch, E.
AU - Velasco, J. L.
N1 - Publisher Copyright:
© 2021 Institute of Physics Publishing. All rights reserved.
PY - 2021/9
Y1 - 2021/9
N2 - First results from the optimized helias Wendelstein 7-X stellarator (W7-X) have shown that core transport is no longer mostly neoclassical, as is the case in previous kinds of stellarators. Instead, power balance analysis has shown that turbulent transport poses a serious limitation to the global performance of the machine. Several studies have found this particularly relevant for ion transport, with core ion temperatures becoming clamped at relatively low values of Ti _ 1.7 keV, except in the few scenarios in which turbulence can be suppressed. In order to understand the precise turbulent mechanisms at play and thus design improved performance scenarios, it is important to have a clear understanding of the parametric dependencies of turbulent fluctuations, and the relation between them and turbulent transport. As a first step in this direction, in this work we use Doppler reflectometry measurements carried out during a number of relevant operational scenarios to provide a systematic characterization of ion-scale (k⊥ρi _ 1) density fluctuations in the core of W7-X. Then, we study the relation between fluctuation amplitude and plasma profiles and show how distinct regimes can be defined for the former, depending on normalized gradients a/Ln and a/LTi . Furthermore, we discuss the importance of other potentially relevant parameters such as Te/Ti, Er or collisionality. Comparing the different regimes, we find that turbulence amplitude depends generally on the gradient ratio ηi = Ln/LTi , as would be expected for ITG modes, with the exception of a range of discharges, for which turbulence suppression may be better explained by an ITG to TEM transition triggered by a drop in collisionality. Finally, we show a number of scenarios under which Ti,core > 1.7 keV is achieved and how core fluctuations are suppressed in all of them, thus providing experimental evidence of microturbulence being the main responsible for the limited ion confinement in W7-X EURATOM 2021.
AB - First results from the optimized helias Wendelstein 7-X stellarator (W7-X) have shown that core transport is no longer mostly neoclassical, as is the case in previous kinds of stellarators. Instead, power balance analysis has shown that turbulent transport poses a serious limitation to the global performance of the machine. Several studies have found this particularly relevant for ion transport, with core ion temperatures becoming clamped at relatively low values of Ti _ 1.7 keV, except in the few scenarios in which turbulence can be suppressed. In order to understand the precise turbulent mechanisms at play and thus design improved performance scenarios, it is important to have a clear understanding of the parametric dependencies of turbulent fluctuations, and the relation between them and turbulent transport. As a first step in this direction, in this work we use Doppler reflectometry measurements carried out during a number of relevant operational scenarios to provide a systematic characterization of ion-scale (k⊥ρi _ 1) density fluctuations in the core of W7-X. Then, we study the relation between fluctuation amplitude and plasma profiles and show how distinct regimes can be defined for the former, depending on normalized gradients a/Ln and a/LTi . Furthermore, we discuss the importance of other potentially relevant parameters such as Te/Ti, Er or collisionality. Comparing the different regimes, we find that turbulence amplitude depends generally on the gradient ratio ηi = Ln/LTi , as would be expected for ITG modes, with the exception of a range of discharges, for which turbulence suppression may be better explained by an ITG to TEM transition triggered by a drop in collisionality. Finally, we show a number of scenarios under which Ti,core > 1.7 keV is achieved and how core fluctuations are suppressed in all of them, thus providing experimental evidence of microturbulence being the main responsible for the limited ion confinement in W7-X EURATOM 2021.
KW - Doppler reflectometry
KW - Stellarator
KW - Turbulence
KW - Wendelstein 7-x
UR - https://www.scopus.com/pages/publications/85108397610
UR - https://www.scopus.com/pages/publications/85108397610#tab=citedBy
U2 - 10.1088/1741-4326/ac112f
DO - 10.1088/1741-4326/ac112f
M3 - Article
AN - SCOPUS:85108397610
SN - 0029-5515
VL - 61
JO - Nuclear Fusion
JF - Nuclear Fusion
IS - 9
M1 - 096015
ER -