Study of low n kinetic ballooning mode in spherical tokamaks

An investigation on the onset of low n kinetic ballooning mode (KBM) in a plasma environment relevant to the spherical reactors is carried out. The low n modes can be quite global in nature and can set an upper limit on the achievable β, a critical parameter for better performance and energy production in spherical reactors. The gyrokinetic simulations for a set of numerically generated equilibrium with increasing equilibrium β show that global low n KBM gets destabilized when β increases and the longest wavelength KBM is only slightly weaker than the most unstable mode. These high β equilibria are generally associated with the presence of a large number of energetic ions. The addition of the energetic particle effect in these studies reveals that the low n branch of the KBM can be destabilized at β values lower compared to the case without the energetic ions. These modes in the presence of energetic ions tend to peak towards an even further longer wavelength and hence more global.