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Fast Transport Transitions in High Shear L-2 M Stellarator : Role of Moderate-Order Rational Magnetic Surfaces

Fast transport transitions in high shear L-2M stellarator under zero net current mode of operation are analyzed. In much detail we discuss fast transport transitions to the regime with improved confinement. Each transition is easily identified by a sudden fast (< 200 μs) small drop of total plasma energy fixed by diamagnetic measurements. After that plasma energy regains its value and slowly monotonically increases up to the end of the active heating phase (just as the line average plasma density ne). In the bulk of plasma parameters evolve slowly. Drastic changes are observed in the region that is close to the plasma boundary where two moderate order rational magnetic surfaces are located with the rotational transform μ taking the values 2/3 and 3/4. The region has definite sandwich structure being subdivided by above-named moderate order rational magnetic surfaces in three smaller zones with different plasma parameters dynamics. For reference we use conventional transport transition having no sandwich structure that happens at the initial stage of the discharge. The availability of several different transport states gave a chance to draw definite conclusion on the properties of plasma edge turbulence. It is demonstrated here that experimentally measured turbulent flux cannot serve as effective tool of plasma transport analysis but can be used (being intrinsically nonlinear in nature) as sensitive qualitative indicator of the change of state. It is shown that contribution of non-Markov processes in plasma edge turbulence is significant. Role of electromagnetic effects in plasma edge turbulence is examined. Different hypotheses on the nature of the phenomena are discussed.