The relations between the threshold values of ion-temperature-gradient instabilities and the temperature parameter of plasmas are investigated analytically in the slab and toroidal geometries separately. It is found that the threshold values increase rapidly when the ion temperature becomes much higher than the electron temperature. The change of the threshold values with respect to the ion temperature is quite similar for both geometric models. Our results are consistent with the PLT observations and also agree well to that of the numerical calculations.
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