Nonlinear simulation with strong Internal transport barrier and weak External transport barrier under the BOUT++ Gyro-Landau-Fluid code

BOUT++ GLF simulations are performed for ITER’s Q=10 high β_P scenario with strong ITB and weak ETB. Here the ITB stands for internal transport barrier and the ETB stands for external transport barrier. In the linear simulation the KBMs are 2^nd stable inside ITB and ETB. However, the KBMs are unstable in the region between ITB and ETB. We find that the mode number of the instabilities is mainly in the region n>50 and the growth rate decreases slowly vs k_⊥ρ_i only in the region where k_⊥ρ_i~1, because of the weak stabilizing finite Larmor radius effect for high-n instabilities. In the nonlinear simulation, the KBMs develop in the region between the ITB and ETB, therefore flattening the profile there which is consistent with the linear local TGLF results. With the flattening of the pressure profile, the turbulence spreads to the ITB bottom and the whole ETB region. The large bootstrap current in the ITB contributes to the large negative magnetic shear, which makes the KBM in deep 2^nd stable region and prevent the turbulence spread into the ITB region. But the bootstrap current in the ETB is not so large as that of the ITB, and the magnetic shear decreases to a small positive number. So the turbulence can spread into the whole ETB region which contributes to the transport in the ETB.