Fast Ion Effects on Fishbones and n = 1 Kinks in JET Simulated by a Non-Perturbative NOVA-KN Code

A new global nonperturbative hybrid code, NOVA-KN, and its application to simulate resonant type modes in JET plasmas driven by energetic H-minority ions are presented. The NOVA-KN code employs the ideal MHD description for the background plasma and treats non-perturbatively the fast particle kinetic response, which includes the fast ion finite orbit width (FOW) effect. In particular, the n = 1 fishbone mode, which is in precession drift resonance with fast ions, is studied. The NOVA-KN code is applied to model the n =1 (f = 50 - 80kHz) MHD activity observed recently in JET low density plasma discharges with high fast ion (H-minority) energy content generated during the ICRH. This n = 1 MHD activity is interpreted as the instability of the n = 1 precession drift frequency fishbone modes. Low frequency n = 1 MHD activity was observed in JET discharges with high fast-ion energy content comparable to or exceeding the energy content of the background plasma [1]. The frequency of such activity is between 50 and 80kHz. The period of sawteeth in those discharges was changed by a factor of five by controlling the plasma density. The energetic H-minority beta builds up strongly in low density plasmas, because the slowing down time of fast ions is inversely proportional to the electron density.
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