Parameterizing Wall Effects for Coupled Tearing Modes in HBT-EP*

Otto Klüber’s now classic paper [1] reported coupling of multiple tearing modes and the detection of coupled helical scrape-off-layer currents. With newly installed arrays of EUV detectors and scape-off-layer detectors, HBT-EP also detects coupled mode activity. Boting Li [2] observed the coupling of rotating (m, n) = (1, 1), (2, 1), and (3, 1) modes that suppress sawteeth whenever the adjustable resistive wall was retracted from the plasma boundary. When the wall is moved inward, the amplitude of the (3, 1) mode and the other coupled modes decreased, and strong sawtooth oscillations reappeared. Similarly, Rian Chandra [3] observed the stabilizing influence of the resistive wall on sawtooth triggering of tearing modes. When the wall is positioned close to the plasma, sawteeth do not trigger tearing modes, unless the plasma current is sufficiently large. For all coupled modes, the position of the wall is important and coherent helical currents are observed in the SOL. To explore coupled tearing modes in HBT-EP experiments, we combine these with the now classical paper by Richard Fitzpatrick [4] and discuss the predicted wall effects and observations.

[1] Klüber, et al., NucFusion, 31, 907 (1991).

[2] Boting Li, et al., NucFusion, 64, 046020 (2024).

[3] Rian Chandra, Dissertation in prep, Columbia University, (2024).

[4] Fitzpatrick, et al. NucFusion, 33, 1533 (1993); and 33 1049 (1993).