HERA and other extensions of ERATO

The MHD stability code ERATO has been modified in order to study helically symmetric equilibria. The resulting new code is called HERA. A new formulation to calculate the vacuum contribution has been added to the spectral codes ERATO and HERA. The finite hybrid element approach is equivalent to that in the plasma region. The stability problem including a conducting wall is now done in one step. The fast angular variation of high n modes has been eliminated by a change of variables. With this new quasi-mode representation it is possible to study unstable modes with nq values up to 1000 in the case of high β, high shear equilibria. It applies equally well to internal and external modes. (orig.)