Study of MHD Equilibrium and Stability with Improved Stellarator Expansion Approach (STEP Code)

Two-dimensional MHD equilibrium and stability codes based on the stellarator expansion provide a good description of the properties of three-dimensional stellarator configurations. Improvements and extensions of the two-dimensional stellarator expansion of such codes with a particular emphasis on the STEP code are discussed. Three-dimensional equilibria of the ATF Torsatron at zero and finite beta are given. The relation between the Mercier stability criterion and low- n stability limit with respect to pressure driven modes is clarified, where n is a toroidal mode number. Whenever \(DI{}0.2\), the STEP code predicts global unstable modes numerically, where D I is the Mercier parameter whose shear stabilization term is normalized as -1/4. The second stability regime is realized by controlling the magnetic axis position by the vertical field in an l =2 and M =12 torsatron/heliotron configuration, where l is a pole number and M is a helical period number of the magnetic field.