Investigation of turbulence rotation in the SOL and plasma edge of W7-X for different magnetic configurations

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 T i ≃ 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 ⊥ ρ i ≃ 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 and . Furthermore, we discuss the importance of other potentially relevant parameters such as T e/T i, E r or collisionality. Comparing the different regimes, we find that turbulence amplitude depends generally on the gradient ratio η i = , 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 T i,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.

Section: Introductionmentioning

confidence: 99%

An experimental characterization of core turbulence regimes in Wendelstein 7-X

Carralero¹,

Estrada²,

Maragkoudakis³

et al. 2021

“…The divertor control coils with two coils per module, are available to adjust the divertor islands in position and shape. Applying positive currents in both coils increases the size of the island and negative currents will decrease its size [34,35]. In addition, the inverted currents are capable of sweeping the island position, poloidally.…”

Section: Experimental Set Up On W7-xmentioning

confidence: 99%

Investigation of a low frequency coherent mode in Wendelstein 7-X with island divertor

Xiang,

Krämer-Flecken,

Han

et al. 2023

During the island divertor operation of W7-X especially in standard magnetic configuration (with 5/5 island chain in the scrape-off-layer (SOL)), a quasi- periodic electromagnetic oscillation is observed. It appears in the frequency range of 1 kHz to 2 kHz, therefore it is called low frequency mode (LFM) within this paper. It is observed by multiple diagnostics, amongst them, a poloidal correlation reflectometer (PCR) allowing a radial localization of LFM. The LFM localized in SOL and shows an obvious modulation effect on the plasma perpendicular velocity (V⊥). Furthermore, broadband turbulence observed in the fluctuation spectra of the electron density and the magnetic field, and the perpendicular correlation length of turbulence eddies, are modulated. The calculation of the poloidal flow velocity and its oscillation allows to study the affect of the LFM on the flow. Cross correlation analysis shows that the perpendicular flow oscillation and the turbulence modulation are closely correlated. The application of external control coils to adjust the island size and position, result in a strong modification of the magnetic topology at the plasma edge which affects the appearance, amplitude and frequency of LFM. Bi-cohernece analysis indicates that a nonlinear interactions among turbulence components is a possible mechanism for the generation of LFM.

“…In the Alcator C-Mod tokamak, Shikama et al observed that significant poloidal asymmetry can exist in the impurity density, ion temperature and main ion density in the edge pedestal region [9,10]. In TEXTOR [11] and Tore Supra [12], higher poloidal velocities of the density fluctuations on the outboard side have been observed and the asymmetry present in the Tore Supra tokamak is explained by considering the balloon type structure of the convective cells in the underlying turbulence. However, experimental measurements of neutral temperature on poloidal asymmetries are quite sparse.…”

Section: Nuclear Fusionmentioning

confidence: 99%

Observation of poloidal asymmetry in measured neutral temperatures in the Aditya-U tokamak plasma

Yadava¹,

Ghosh

Chowdhuri

et al. 2019

The neutral particle temperature in the edge region of the Aditya-U tokamak has been measured by recording the hydrogen Balmer alpha emission spectra at 656.28 nm from different lines of sight in both the high and low field sides. The spatial profile of the Hα emission has been recorded using a 1 m multi-track spectrometer. The neutral temperature is estimated from the Doppler broadening of the measured Hα spectrum by appropriately removing the contribution from the Zeeman splitting of the spectral lines. A computer simulation code was developed to estimate the neutral temperature by including the broadening mechanisms such as Doppler broadening and the Zeeman effect to simulate the Hα emission spectrum along with the proper convolution of the instrumental width of the diagnostic system. It has been observed that the high field side neutral temperature (~4–6 eV) during plasma flat top is almost twice that of the low field side (~2–3 eV) suggesting poloidal asymmetry in the neutral temperature.