An analysis is made of the stabilizing effect of a radial ponderomotive force on the interchange mode in a double-cusp mirror plasma which has unfavourable curvature on internal flux surfaces. A cavity mode in the long central cell region is set up by exciting electromagnetic ion cyclotron waves and it provides the radial force which opposes the curvature drive. Application of this method to RFC-XX shows that the ICRH power needed for stabilization is comparable to that needed for heating.