Parameter dependences of ion thermal transport due to toroidal ITG turbulence

The non-linear 3-D toroidal gyrokinetic simulation code PG3EQ is used to study toroidal ion temperature gradient (ITC) driven turbulence - a key cause of the anomalous transport that limits tokamak plasma performance. Surveys of chi (i) versus E x B and toroidal shear show that (a) the maximum growth rate is not a good predictor of the E x B shear required to suppress turbulent transport, (b) there is often a 'plateau region' in which E x B shear significantly reduces the maximum linear growth rate but not the transport and (c) the parallel velocity shear component of toroidal velocity shear can negate much of the transport reduction by the E x B shear. Simulations in which the ion temperature gradient T-i'(r) initially varies with radial position evolve towards a state without strong radial variations in T-i'(r) while approximately preserving the total (E x B + diamagnetic) ion flow. If the electrostatic potential is initialized to zero, then sheared E x B flows result which can significantly reduce or quench the transport. If, however, the total ion flow profile (or equivalently the radial force) is initialized to be radially uniform, then the initial radial variations in T-i'(r) do not result in a significant reduction in the transport. Surveys of chi (i) versus magnetic shear S show a (often very sharp) peak at S similar or equal to 0.5 - the value at which the orientation of the ITG modes is in the major radius direction and is independent of poloidal angle in the region near the outer midplane. Surveys of chi (i) versus T-i' show that in most cases the thermal flux Q (chi T-i(i)') has a linear dependence on T-i', with an intercept value of T-i' which is often significantly larger than than linear critical value.