COLLISIONALITY DEPENDENCE OF MERCIER STABILITY IN LHD EQUILIBRIA WITHBOOTSTRAP CURRENTS

The Mercier stability of the plasmas carrying bootstrap currents with different plasma collisionality is studied in the Large Helical Device (LHD). In the LHD configuration, the direction of the bootstrap curre nt depends on the collisionality of the plasma through the change in t he sign of the geometrical factor When the beta value is raised by inc reasing the density of the plasma with a fixed low temperature. the pl asma becomes more collisional and the collisionality approaches the pl ateau regime. In this case, the bootstrap current can flow in the dire ction so as to decrease the rotational transform. Then, the large Shaf ranov shift enhances the magnetic well and the magnetic shear, and the refore, the Mercier stability is improved. On the other hand, when the beta value is raised by increasing the temperature of the plasma with a fixed low density, the plasma collisionality becomes reduced to ent er the 1/nu collisionality regime. In this case, the bootstrap current flows so that the rotational transform should be increased, which is unfavourable for the Mercier stability.