Stabilization of free-boundary modes by a magnetic separatrix

A new method of stabilizing free-boundary modes in a plasma confine in a purely poloidal magnetic field is discussed here. The free-boundar modes can be stabilized if the plasma is limited by a separatrix emanating from a neutral line. In connection with recent investigations on the stability of plasmas with elongated cross-section, essential modifications of present schemes are suggested in order to improve stability and confinement. These are an elongated Extrap and an FRC having a separatrix with a neutral line. (author)