Experimental confirmation of theory behind I-mode edge fluctuations

Edge localized modes (ELMs) are harmful for divertor and wall of fusion devices. Therefore, a reactor must be operated in ELM-free plasma regimes. One such regime is the I-mode, being characterized by fluctuations of a weakly coherent mode (WCM) at the plasma edge.

Measured phase velocity of the WCM relative to that of a simple drift-wave.

Understanding the WCM is needed to make prediction for the access of the I-mode in reactor plasmas. A theoretical work relates the WCM to the drift-wave instability [1], which has a characteristic phase velocity close to electron-diamagnetic drift value.

A recent study deduced the phase velocity from the difference between the propagation velocity of the fluctuations, measured with a Doppler reflectometer, and the background flow velocity deduced from the thermal helium beam diagnostics. The figure confirms the theoretical prediction as it shows the agreement of the measured phase velocity with the expected values for drift waves.

 

 

This work is published open access in M. Herschel et al., 2024, Nucl. Fusion 64 076052

 

[1] P. Manz et al 2020 Nucl. Fusion 60 096011

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