Use of the S- alpha diagram for representing tokamak equilibria in profile modification research

In tokamak research involving plasma profile modification, it is proposed to represent a plasma equilibrium state as a trajectory in a diagram of shear and dimensionless pressure gradient commonly known as the S- alpha diagram. The method is a natural way of exhibiting local variations of the shear and pressure gradient and their interrelationship. A distinct equilibrium trajectory pattern may be associated with a set of experimentally observed phenomena, such as the level and kind of MHD activity or confinement, or with theoretically predicted properties such as the stability. When many qualitatively different trajectory patterns are identified with a set of observed or predicted characteristics, the diagram will serve as a tool for cataloguing the presently achievable plasma states as well as desired target states through 'pattern recognition'. Furthermore, types of trajectory changes produced by various profile modification techniques can also be represented in the same diagram. The method will help construct a desired state, starting from a given experimental state using profile modification techniques. It is demonstrated, using PBX-M experiments as examples, that distinct trajectory patterns can be associated with different operational regimes, and that some profile modification techniques and types of MHD activity produce characteristic trajectory changes.