Enhanced ion temperature gradient mode stability with reverse shear in toroidal plasmas

A toroidal plasma configuration with simultaneously improved stability to the magnetohydrodynamic (MHD) modes and to the temperature and pressure gradient driven drift modes has been used. It is characterized by reverse magnetic shear in the plasma interior and a peaked density profile. The effects of reverse shear on the toroidal ion temperature gradient (eta (i)) mode have been investigated by a multi-pole fluid model (i.e. it includes the polarization drift, compressibility effects due to field curvature, in particular the divergence of the diamagnetic heat flow, and finite Larmor radius effects to first order). Reverse and normal shear are compared. Short-wavelength ion temperature gradient modes are stabilized by access to the second stability limit in eta (i). The eta (i) mode is strongly stabilized by reverse shear effects well below the MHD ballooning mode stability threshold. This plasma configuration seems promising for improved ballooning mode stability in advanced tokamaks.