Gyrokinetic physics of L-H bifurcation in tokamak edge*

We have expanded our previous gyrokinetic L-H bifurcation study [1, 2] in the edge gyrokinetic code XGC to include the isotope and size ($a/\rho_i$)) effects. The interplay between the turbulence modes, Reynolds stress, and the neoclassical ExB shearing rate will be discussed in relation to the isotope and a/ρi effects. Two different neoclassical effects will be included in the study: the X-point orbit loss effect and the radial plasma gradient effect. Influence on the enhanced $E\times B$ shearing rate by reduction in the neutral particle recycling rate will also be discussed. [1] C.S. Chang, S. Ku, G.R. Tynan, R. Hager et al., Phys. Rev. Lett. 118, 175001 (2017) [2] S. Ku, C. S. Chang, R. Hager, R. M. Churchill et al. Phys. Plasmas 25, 056107 (2018)