Inhibition of parametric decay in heating microwave beams during fluctuations of the density profile in the edge island of Wendelstein 7-X

the W7-X Team

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9 Scopus citations

Abstract

Experimental evidence of parametric decay instability (PDI) is observed in the Wendelstein 7-X stellarator, when high-power microwave beams cross a stationary magnetic island at the plasma edge. Here, trapping and build-up of upper hybrid waves within a density bump (measured within the island by alkali beam emission spectroscopy) is responsible for the reduction of the instability power threshold below the maximum gyrotron power. In this paper, we provide the first experimental evidence of the connection between the trapping mechanism in the island density bump and excitation of PDI-related signals. We show correlations of periodic crashes in the PDI-related signals with quasi-continuous fluctuations at the plasma edge, which, additionally, cause a flattening of the density profile in the island. We demonstrate that flattening of the experimental density profiles can suppress the trapping mechanism and inhibit the low-threshold PDI. PDI on the edge island could alter the power deposition profile and reduce the efficiency of the electron cyclotron resonance heating system, simultaneously posing a serious threat to the optimal operation of microwave-based diagnostics and plasma-facing components.

Original languageEnglish
Article number015001
JournalPlasma Physics and Controlled Fusion
Volume65
Issue number1
DOIs
StatePublished - Jan 2023

Funding

This work has been supported by research Grant 15483 from VILLUM FONDEN and the Enabling Research Grant ENR-MFE19.DTU-03 from the EUROfusion Consortium. This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No. 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

FundersFunder number
Villum FondenENR-MFE19
European Commission101052200 — EUROfusion

    Keywords

    • Wendelstein 7-X stellarator
    • alkali beam emission spectroscopy
    • electron cyclotron resonance heating
    • island localized modes
    • parametric decay instability
    • upper hybrid resonance

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