Full-f gyrokinetic turbulence simulation for ITB formation

Profile stiffness is a long standing problem, which may limit the overall performance of H-mode plasmas. In the JET experiment, while strong temperature profile stiffness is observed around the nonlinear threshold of ion temperature gradient, it can be greatly reduced by co-current toroidal rotation in weak magnetic shear plasma [1]. To understand such a mitigation mechanism of the stiffness, we have newly developed a 5D global gyrokinetic code GKNET [2, 3]. This enables us to simulate flux-driven ITG turbulence consistently coupled with neoclassical transport mechanism, where mean profiles are governed by radial force balance and can be adjusted to heat and momentum sources. By means of this code, it is found that a stiff temperature profile is established in the absence of momentum source. The stiffness is identified to result from not only the fast propagation of heat avalanches but also the explosive global transport coupled with the instantaneous formation of radially extended ballooning structure, whose size ranges from mezo to even macro-scale. The radial mean electric field is found to play an important role in forming such a global structure by recovering the up-down symmetry of the ballooning structure. This indicates that the mean filed can enhance the stiffness [2]. Then we introduce a momentum source in weak magnetic shear plasma. We found that only cocurrent momentum injection has a strong impact on local temperature build up in the momentum source region, where the ion thermal diffusivity decreases to the neoclassical transport level. This originates from the fact that the mean r E shear, which is triggered by toroidal rotation through the radial force balance, suppresses the turbulence, leading to ITB formation. Note that the established ITB is enough stable in the quasi-steady state. The underlying mechanism why only co-input is effective for ITB formation is identified to originate from that there exists a positive feedback loop between the enhanced mean r E shear and resultant momentum pinch only in the co-input case, signifying a favourite trend to ITB formation. Such a momentum pinch effect is also essential for ITB formation around min q surface in reversed magnetic shear plasma. This work was supported by Grants-in-Aid from JSPS with No. 25800304, 16K17844.