The excitation condition for ion-temperature-gradient (ITG) instabilities in linear devices is investigated using a gyro-fluid equation. The finite-Larmor-radius effect is included in the model, which helps to stabilize ITG instabilities. The critical values of ?, which is the ratio of the lengths of the density gradient to the temperature gradient, are obtained. Although the modenumbers of the most unstable modes are different with different discharge gases, their critical values have almost the same η level close to 1.0. The results are compared with those from the Hamaguchi-Horton model numerically and analytically.
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