Suppression of toroidal Alfvén eigenmodes by the electron cyclotron current drive in KSTAR plasmas

Abstract

Advanced operation scenarios such as high poloidal beta (βP) or high q min are promising concepts to achieve the steady-state high-performance fusion plasmas. However, those scenarios are prone to substantial Alfvénic activity, causing fast-ion transport and losses. Recent experiments with the advanced operation scenario on KSTAR tokamak have shown that the electron cyclotron current drive (ECCD) is able to mitigate and suppress the beam-ion driven toroidal Alfvén eigenmodes (TAEs) for over several tens of global energy confinement time. Co-current directional intermediate off-axis ECCD lowers the central safety factor slightly and tilts the central q-profile shape so that the continuum damping in the core region increases. Besides, the rise of central plasma pressure and increased thermal-ion Landau damping contribute to TAE stabilization. While the TAEs are suppressed, neutron emission rate and total stored energy increase by approximately 45% and 25%, respectively. Fast-ion transport estimated by TRANSP calculations approaches the classical level during the TAE suppression period. Substantial reduction in fast-ion loss and neutron deficit is also observed. Enhancement of fast-ion confinement by suppressing the TAEs leads to an increase of non-inductive current fraction and will benefit the sustainment of the long-pulse high-performance discharges.

Parametric study of linear stability of toroidal Alfvén eigenmode in JET and KSTAR

“…Then, we scanned those parameters of a set of discharges in KSTAR by using the measured 0D parameters to evaluate the TAE stability, as shown in figure 2. Here, q 0 's are obtained from MSE-EFIT [32] (or EFIT [33] for the cases that MSE diagnostics are not available) and the line-averaged density n l from the interferometry [34,35] or TRANSP-reconstruction [10,36]. For simplicity, we assume R/L pf = 5, r/R = 0.17, and a birth pitch angle of beam-ions, cos θ 0 = 0.75 which are typical values for r/a ≲ 0.5 region where the beam-pressure gradient is maximum.…”

“…The points for the KSTAR discharges are marked with red for the observed mode numbers of TAE and blue for mode numbers not observed (see table 2). The filled circles are from discharge #21695 where ECCD was applied to enhance the continuum damping by adjusting the q-profile [10], but this effect was not included here because the continuum damping is hard to be calculated precisely using 0D parameters. The black dashed line indicates the zero growth rate, implying the balance between the mode drive and the damping by beam-ions.…”

“…This interaction between energetic ions and shear Alfvén waves inevitably leads to the radial transport of resonant energetic ions, potentially resulting in a significant wall damage and degradation of the performance of plasmas. The observations of the fast ion losses due to the Alfvénic activity have been reported in various devices such as DIII-D [4], TFTR [5], JET [6,7], ASDEX-U [6,8] and KSTAR [9,10]. Therefore, it is essential to understand the physics of stability of AEs in a tokamak with a high content of energetic ions such as in JET DT discharges and ITER.…”

“…TAEs were not observed in the second and last cases. In discharge #21695 (the last two cases), q-profile changes by ECCD after t = 5 s[10].…”

Progress in the development and understanding of a high poloidal-beta tokamak operating scenario for an attractive fusion pilot plant

“…More importantly, various aspects of this scenario has been individually extended to reactor relevant conditions, but not yet simultaneously, e.g., stationary operation, low toroidal rotation, intermediate q 95 ∼ 6 , line-averaged density above Greenwald limit, high core confinement quality ( H 98y2 ) with fully detached divertor, etc., with deepened physics understandings (Garofalo et al 2015(Garofalo et al , 2018Ren et al 2016;Ding et al 2017aDing et al , b, 2020aDing et al , 2021aJian et al 2019Jian et al , 2021aMcClenaghan et al 2019;Huang et al 2020;Wang et al 2021a, b;Qian et al 2021). There is also recent active high-P related research on EAST (Garofalo et al 2017;Gong et al 2022), KSTAR (Na et al 2015;Park et al 2019;Kwak et al 2020;Kim et al 2022) and HL-2A (Chen et al 2022).…”