Unexpected transport phenomena in low-β plasmas of L-2M stellarator

Earlier it was commonly accepted that magnetohydrodynamic (MHD) theory explains (and predicts) experimental results. In more recent experiments L-2M usually followed predictions of magnetohydrodynamic (MHD) theory. However, here we present and discuss L-2M stellarator experimental results that can not be explained within the standard MHD approach. Stellarator L-2M is a medium size high shear, large aspect ratio classical stellarator with nearly planar geometrical axis. All the experiments presented here are performed with a small β ∼ (0.1-0.2)% electron cyclotron heated plasmas, β is the ratio of the plasma pressure to the pressure of magnetic field. At such values of β the ratio of plasma induced shift of magnetic axis ξ≤0.25ap, where ap is the average plasma radius. Plasma is stable with respect to ideal MHD modes due to self-stabilizing effect. The phenomenon that we present here start with the fast (∼ 200μsec) small drop of total plasma energy. For the first time such drops were indicated by diamagnetic measurements and further on related signals were indicated by diamagnetic measurements and further on related signals were indicated by other diagnostics. For example, the drop in diamagnetic signal correlate with a sudden drop of floating potential measured by Langmuir probe. The duration of the event definitely point that the cause of the event can be an instability. Transport events were indicated only at sufficiently high densities, and for the given value of average density they appear only at higher heating power. The search of reason for the phenomenon caused a pedantic analysis of different signals. It occurred that spectra of Mirnov signals before and after event practically coincide. The only hardly visible difference is that in the spectrum of signal after the event some enhancement of high frequency components can be observed. The amplitudes of floating potential fluctuations after the event are significantly lower than that of fluctuations before but the spectra practically coincide. After the event plasma energy slowly monotonically increase up to the value ∼ (5-10)% larger compared to the value before the event. The highest available values of plasma energy at given heating power are obtained in the discharges where transport event happens. It is pertinent to point another observation. The transport event occur at the stationary plasma energy. But the transition to stationary phase (from the beginning of the discharge) depending on the value of heating power and plasma density may be fast or slow. But in all the variety of cases the graphic of total energy flattens (or in another words energy become stationary) when in the spectrum of Mirnov oscillations occur high frequencies (40-100 kHz). We present here an analysis based on the model of coupling of ion-temperature-gradient modes to Alfven and acoustic modes using simplified model that take into account only averaged curvature of the magnetic field lines and high mode number approximation. Several types of stable and unstable electromagnetic modes were found. Their relevance to the observed phenomena is discussed. (author)