CALCULATION OF MERCIER STABILITY LIMITS OF TOROIDAL HELIACS

The Mercier stability behaviour of toroidal Heliacs can be dominated by pressure induced current parallel to the field lines which are resonant with values of the rotational transform per field period included in, or close to, the configuration. This issue is discussed with reference to a parameter survey for the H-1 Heliac. It is shown that the calculated magnitude of the critical [beta] against Mercier instability may vary, depending on whether the parallel currents are calculated directly from the curl of the magnetic field obtained from a 3-D equilibrium code or, alternatively, from an algebraic expression derived from the appropriate magnetic differential equation.