Free boundary MHD equilibrium properties of Heliotron J are investigated by VMEC, HINT and PIES codes, and ideal MHD stability properties are studied by the Mercier criterion, the ballooning mode equation and the CAS3D global stability code. It is shown by the equilibrium calculations that the change of the plasma boundary shape is substantial in a low shear helical system even if β is relatively low. Preliminary comparison between PIES results and HINT results shows that the β value at which the magnetic island begins to be perceptible is almost the same in both codes, but the island width seems to be different. It is considered that the difference comes from a difference of pressure distribution in real space which is determined through each numerical algorithm. In the stability analysis, the effect of the global magnetic shear on the three-dimensional or helical ballooning mode whose mode structure shows strong poloidal and toroidal mode (helical mode) coupling is investigated. It is found that the positive shear of the rotational transform is favourable for the three-dimensional ballooning mode stability in a low shear helical system.
Effects of net toroidal current on the Mercier criterion in the Large Helical Device