Experimental characterization of inter-ELM modes in type-I ELMy scenarios at TCV using the thermal helium beam diagnostic

Abstract The high confinement regime (H-mode) is considered fundamental for high-performance operation in next-step tokamaks. It is generally characterized by the occurrence of edge localized modes (ELMs), even though several no-ELM and small-ELM regimes have extended their operational space in the past years (Viezzer et al 2023 Nucl. Mater. Energy 34 101308). The control and suppression of these modes is a priority for the development of scenarios with a confinements good enough for the ITER baseline (Sips et al 2018 Nucl. Fusion 58 126010) requests, thus a better understanding of the causes of these phenomena is needed. When these modes are present, they are often preceded by inter-ELM oscillations, and this work focuses on the analysis of inter-ELM oscillations at the TCV experiment using as primary diagnostic the thermal helium beam (THB). This diagnostic is widely used in nuclear fusion devices, given its ability to measure simultaneously the electron density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) and temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) profiles and fluctuations in the plasma edge. Thanks to its fast acquisition rate (1 MHz), the THB has been used to analyze inter-ELM fluctuations observed in Type-I ELMy H-mode plasmas, showing characteristic frequencies in the range of 30–90 kHz. These coherent oscillations in the HeI emissivity have been detected and are spatially localized across the separatrix. These fluctuations are identified mostly as electron density oscillations, with less evident temperature structure. The correlation between the development of these radially localized density fluctuations and the electron profiles in the edge is studied, as well as their correlation with turbulent behavior in the scrape-off layer.