Impurity effects on kinetic ballooning instability in high regions of tokamak plasmas

The impurity effects on kinetic ballooning modes (KBMs) in high regions of tokamak plasmas are numerically investigated, using a full gyrokinetic description for main and impurity ions. The findings indicate that the critical plasma pressure ratio for KBM is significantly lower (higher) than the first (second) stable threshold predicted by MHD theory. The KBM exhibits stronger extended instabilities for low , while the instability region of KBM becomes significantly narrow for high , resulting in a more concentrated instability window (localized in narrow medium- region). In addition, the contributions from electromagnetic effect and Shafranov shift are identified in detail, i.e. the electromagnetic destabilizing effect is suppressed by the Shafranov shift, and strong stabilizing effect of Shafranov shift for high or on KBM is pronounced. The impurity ions with negative gradient ratio have stabilizing (destabilizing) effects on KBMs for high (low) . Such distinct behaviors are demonstrated to be associated with the used parameters close to the second and first unstable region, i.e. the instability reduces and enhances with the increase of , respectively. Significantly, the presence of impurities, regardless of the peaking directions of their density profiles, serves to stabilize KBMs and reduce the window of extended instability for high region, which is in significant contrast with the results for low region. Furthermore, high concentration, steep negative gradient ratio , and high mass numbers of impurity ions as well as large temperature gradient and contribute to the stabilization of KBMs and improvement of confinement in high regions of tokamaks. The region of stabilizing effect of magnetic shear on KBM, is found not belonging to the conventional magnetic shear region, but shows strong sensitivity to value.