Bursting toroidal Alfvén eigenmodes in KSTAR plasmas. (8th January 2019)

• Record Type: Journal Article
• Authors: Hole, M J; Qu, Z S; Layden, B; Michael, C A; Woo, M H; Bak, J G; Kim, J; Hezaveh, H
• Other Names: collab.
• Is Part Of: Plasma physics and controlled fusion. Volume 61:Number 2(2019)
• Publication Date: 2019-01-08
• Subjects: Alfvén eigenmode -- wave particle interaction -- nonlinear saturation; Plasma (Ionized gases) -- Periodicals; Controlled fusion -- Periodicals; 530.44
• DOI: 10.1088/1361-6587/aaf40b
• Languages: English
• ISSNs: 0741-3335

Abstract

Abstract: We report on observations of bursty mode activity during early neutral beam heating in KSTAR plasmas, before current flat top while the q profile is still evolving. The magnitude of the activity increases with early beam heating, and reduces with the addition of resonant magnetic perturbation magnetic field coils. A mode analysis yields a toroidal mode number of n = 2. The mode is observed to be downward chirping in frequency, and exists for the duration of the slowing-down of the beam. Motional Stark effect constrained equilibrium reconstructions are available at adjacent time slices: we have rescaled the total current to the measured value to obtain the q profile during the mode activity. From this we have computed the mode spectrum and identified a number of candidate gap modes. Wave-particle simulations with plausible distribution functions are computed, which demonstrate that the lowest frequency mode satisfies the condition for wave drive ω * / ω > 1, where ω * is the fast ion diamagnetic drift frequency. An interesting finding is the change from exponential growth of the mode above n f / n i 0 ≈ 0.6, whereby the mode continues to nonlinearly grow at a reduced rate over a period of 100 wave periods up to final saturated amplitude. We believe that this may be because the two spatial resonances at s = 0.4 and s = 0.8 overlap for sufficiently high fast ion density, and so the phase-space volume and fast ion density available to drive the mode increases.