"Modelling and diagnosis of Beta induced Alfven eigenmodes in the strongly shaped H-1NF stellarator"

 

 

Recent advances in the modelling, analysis, and measurement of fluctuations in strongly shaped 3D magnetic configurations have significantly improved the diagnosis and understanding of Alfven eigenmodes in the H-1 heliac. Visible light emissions from H-1 reveal low frequency (10-40 kHz) electron density fluctuations correlated with magnetic measurements indicative of Alfven eigenmodes. Full 3D tomographic inversion of the emission was accomplished using a novel synchronous imaging technique, which achieves high SNR and aliases high frequency modes to frequencies that can be easily imaged. Excellent agreement is found between both the frequency and spatial structure of the mode and ideal MHD calculations of Beta-induced Alfven Eigenmodes using the CAS3D code. These results show experimentally that Alfven eigenmodes can exist in the acoustic range of frequencies even at low beta (approx. 10^−4) due to strong shaping in stellarators such as H-1, unlike in tokamaks where significantly higher beta is required. Importantly, these modes are observed in the absence of confined ions near the Alfven velocity.