Beta-induced Alfven eigenmodes and geodesic acoustic modes in the presence of strong tearing activity during the current ramp-down on JET

The analysis of the current ramp-down phase of JET plasmas has revealed the occurrence of additional magnetic oscillations in pulses characterized by large magnetic islands. The frequencies of these oscillations range from 5 to 20 kHz , being well below the toroidal gap in the Alfven continuum and of the same order as the low-frequency gap opened by plasma compressibility. The additional oscillations only appear when the magnetic island width exceeds a critical threshold, suggesting that the oscillations could tap their energy from the tearing mode (TM) by a non-linear coupling mechanism. A possible role of fast ions in the excitation process can be excluded, being the pulse phase considered in the observations characterized by very low additional heating. The calculation of the coupled Alfven-acoustic continuum in toroidal geometry suggests the possibility of beta-induced Alfven eigenmodes (BAEs) rather than beta-induced Alfven-acoustic eigenmodes. As a main novelty compared to previous work, the analysis of the electron temperature profiles from electron cyclotron emission has shown the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple magnetic oscillations in the low-frequency gap of the Alfven continuum. This observation supports the hypothesis of different BAE with toroidal mode number n = 1 associated with different magnetic islands. As another novelty, the observation of magnetic oscillations with n = 2 in the BAE range is reported for the first time in this work. Some pulses, characterized by slowly rotating magnetic islands, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes (GAMs), and a cross-spectral bicoherence analysis has confirmed a non-linear interaction between TM, BAE and GAM, with the novelty of the observation of multiple triplets (twin BAEs plus GAM), due to the simultaneous presence of several magnetic islands in the plasma.

Automated summary

The analysis of JET plasmas during the current ramp-down phase has found additional magnetic oscillations in pulses with large magnetic islands. These oscillations have frequencies ranging from 5 to 20 kHz, which are below the Alfven continuum gap and of the same order as the low-frequency gap caused by plasma compressibility. The presence of these oscillations is linked to the width of the magnetic island, suggesting a non-linear coupling mechanism with the tearing mode. The analysis also reveals the simultaneous presence of magnetic islands on different rational surfaces, supporting the hypothesis of different beta-induced Alfven eigenmodes (BAEs) associated with different magnetic islands.