A toroidal plasma configuration with simultaneously improved stability to themagnetohydrodynamic (MHD) modes and to the temperature and pressure gradientdriven drift modes has been used. It is characterized by reverse magneticshear in the plasma interior and a peaked density profile. The effects ofreverse shear on the toroidal ion temperature gradient (ηi) mode havebeen investigated by a multi-pole fluid model (i.e. it includes thepolarization drift, compressibility effects due to field curvature, inparticular the divergence of the diamagnetic heat flow, and finite Larmorradius effects to first order). Reverse and normal shear are compared.Short-wavelength ion temperature gradient modes are stabilized by access tothe second stability limit in ηi. The ηi mode is stronglystabilized by reverse shear effects well below the MHD ballooning modestability threshold. This plasma configuration seems promising for improvedballooning mode stability in advanced tokamaks.
Toroidal mode structure in weak and reversed magnetic shear plasmas and its role in the internal transport barrier