Dr. Ksenia Aleynikova, Max-Planck-Institut für Plasmaphysik, EURATOM Association, Greifswald, Germany

"Kinetic ballooning modes: from theory to high density operation in W7-X"

Abstract: Kinetic ballooning modes (KBMs) are investigated by means of analytical theory and linear electromagnetic gyrokinetic (GK) numerical simulations for different geometries. A physics-based ordering for beta (the ratio of kinetic to magnetic plasma pressure) with small asymptotic parameters is found. This allows us to derive several simplified limits of previously known theory [1] and to identify regimes where quantitative agreement between theory and numerical simulations can be achieved.

For the axisymmetric case, in simple s-alpha geometry, it is found that, for large pressure gradients, the growth rate and frequencies computed by the gyrokinetic code GENE show excellent agreement with those evaluated by using a diamagnetic modification of ideal MHD.

For the stellarator geometry, we extend the analyses of KBMs reported in Refs. [2, 3], to cover high-density regimes in W7-X. In recent W7-X experiments (reported in [4]), hydrogen pellet core fuelling was realized in order to study effects of plasma pressure, plasma stability and confinement. The central β (the ratio of the thermal to magnetic pressure) of above 3.5% were demonstrated. Such plasmas feature peaked density profiles and may potentially be prone to KBMs generation.Linear electromagneticgyrokinetic simulations of finite-βplasmas in W7-X predict a stability limit for strongly destabilized KBMs, which is consistent with the experimental observations of, so-called, MHD-like events during the high-density operation.
References
[1] W.M. Tang, J.W. Connor, and R. J. Hastie, Nucl. Fusion 20 , 1439 (1980)
[2] K. Aleynikova and A. Zocco, Physics of Plasmas 24 , 092106 (2017)
[3] K. Aleynikova, et al, Journal of Plasma Physics 84 (6), 2018.
[4] S. Bozhenkov et al, 27th IAEA Fusion Energy Conference, EX/P8-8, 2018.