Multiple ion temperature gradient driven modes in transport barriers

摘要

The ion temperature gradient (ITG) modes in transport barriers (TBs) of tokamak plasmas are numerically studied with a code solving gyrokinetic integral eigenvalue equations in toroidal configurations. It is found that multiple ITG modes with conventional and unconventional ballooning mode structures can be excited simultaneously in TBs with steep gradients of ion temperature and density. The characteristics of the modes, including the dependence of the mode frequencies, growth rate and structure on plasma parameters, are systematically investigated. Unconventional modes with large mode-number l (where / denotes a certain parity and peak number in ballooning space) dominate in the large k_θps region (k_θps ≥1.2), while the conventional mode with / = 0 dominates in the medium k_θps region (0.4≤k_θps < 1.2), and unconventional modes with small mode-number I dominate in the small k_θps region (k_θps < 0.4). Thus, the k_θps spectra of these conventional and unconventional modes at steep gradients are qualitatively different from those of the conventional ITG modes at small or medium gradients, in which the growth rate of the only ITG mode with I = 0 reaches maximum at the medium value k_θps = 0.6. Through scanning ion temperature gradient εTi and density gradient ε_n separately, it is proven that the synergetic effect of ε_Ti and ε_n, rather than ε_Ti alone, drives the unconventional ITG modes in TBs. Moreover, it is found that the critical value of ε_n for driving the unconventional ITG modes with large I number increases with increasing k_θps. In addition, the effects of magnetic shear on conventional and unconventional ITG modes in the high confinement regime (H-mode) are analyzed in detail, and compared with equivalent effects on conventional modes in the low and intermediate gradient regimes (L- and I- modes). Finally, the effects of the poloidal wave number and gradients of ion temperature and density on radial transport are analyzed based on quasi-linear mixing length estimations.