Perturbed plasma equilibria

The computation of perturbed plasma equilibria is required to assess the pr operties of resistive wall modes and to investigate the driving of magnetic islands in toroidal plasmas by external field errors. The computation of p erturbed equilibria using the so-called delta W techniques of magnetohydrod ynamic (MHD) stability codes is explained. The theory is developed that is required to use the results of a perturbed equilibrium calculation for eith er a study of resistive wall modes or of the elimination of magnetic island s by the use of trim coils. (C) 1999 American Institute of Physics. [S1070- 664X(99)00403-6].