Tokamak resistive magnetohydrodynamic ballooning instability in the negative shear regime

Iproved confinement of tokamak plasma with central negative shear is checked against the resistive ballooning mode. In the negative shear regime, the plasma is always unstable for purely growing resistive ballooning mode. For a simplest tokamak equilibrium model, the s-\alpha model, characteristics of this kind of instability are fully clarified by numerically solving the high n resistive magnetohydrodynamic ballooning eigen-equation. Dependences of the growth rate on the resistivity, the absolute shear value, the pressure gradient are scanned in detail. It is found that the growth rate is a monotonically increasing function of \alpha while it is not sensitive to the changes of the shear s, the initial phase \theta_0 and the resistivity parameter \varepsilon_\rm R.