Change of confinement properties and transition from absolute to non-normal mode microinstabilities

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Abstract
The change in confinement properties that can be induced in experiments, where the plasma is heated by an external injection system, is considered to be related to the transition from a regime where ‘‘absolute’’ microinstabilities are excited at the edge of the plasma column to a regime where these microinstabilities can exist only as non‐normal modes because of the effects of the local gradient of the E×B rotation frequency that is induced. Ion‐temperature‐gradient‐driven modes are analyzed as an important example and the threshold conditions for their disappearances as normal modes are evaluated.