Abstract
A study of the toroidal ion temperature gradient (ITG) orηi mode [ηi =Ln/LT,Lj = j | dj/dr | -1(characteristic scale length)] at larger ηi values,ηi << 1, has been performed using a two-fluid model. Itis shown that the upper ηi stability threshold(ηiup) is in the range ηiup ≈ 10 orlarger and that it is sensitive to FLR (finite Larmor radius) effectsand toεn = 2Ln/LB.Comparisons are made with the results of a gyrokinetic model reportedearlier. The two-fluid description is found to reproduce the resultsof the gyrokinetic description with a good accuracy in the regimesb0 = k2ρi2 << 1and εn << 1 (k = perpendicular wavenumber,ρi = ion Larmor radius) whereas for largerb0 and εn the fluid model gives alower ηiup than the gyrokinetic model. In the lowb0/low εn regime the densitygradient independent relationεT = LT/LB ≈ b02τ/8at the upper ηi stability threshold is derived(τ = Te/Ti).
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