MHD instability of the noncircular cross-section plasma is investigated by using a step current profile model. A new criterion for the instability is obtained. It is a generalization of Zakharov's criterion for circular cross-section plasma. The effects of elliptical and triangular distortion, current gradient and the value of safety factor q on β value are indicated. The optimistic value of q for achieving the largest β value is also given.The critical β value for various plasma parameters is calculated and compared with the numerical results from the local instability and ballooning modes criteria. It is noted here that the current gradient affects the β value seriously. The noncircular cross-section plasma has advantage for obtaining higher β value.
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