Long-range spatial correlations in the turbulent edge plasma of the L-2M stellarator

Abstract

Long range spatial correlations in the turbulent plasma of the L 2M stellarator were revealed experimentally, and their relation to the geometry of magnetic surfaces was analyzed (Plasma Phys. Control. Fusion 50, 045001 (2008)). The operation modes of the facility in which fast transport transitions in plasma are possible were studied. Upon these transitions, the turbulence level is found to decrease substantially. It is shown that long range spatial correlations are typical of relatively narrow frequency ranges. In particular, before a transport transition, such frequency ranges are f ~ 30-40 kHz and f ~ 1-3 kHz. After the transition, long range spatial correlations in the frequency range of f ~ 30-40 kHz disappear due to a significant decrease in the turbulence level in this frequency range. At the same time, correlations in the low frequency range are retained and new correlations at frequencies of f ~ 6-12 kHz occur. It is found that global electromagnetic oscillations in the frequency range of f ~ 1-3 kHz are related to the m/n = 0/0 perturbation and its toroidal satellites (here, m and n are the poloidal and toroidal mode numbers, respectively). It is also shown that, after the transport transition, a three dimensional localized electromagnetic mode at the frequency of the geodesic acoustic mode governed by the average magnetic field curvature is excited. At higher frequencies typical of a geodesic acoustic mode related to the three dimensional curvature of the magnetic field, no long range spa tial correlations were observed both before and after the transport transition.

On plasma edge ideal MHD stability/instability condition in Mercier stable magnetic hill configurations

“…In the nearseparatrix region, they corresponded to the m/n = 5/4 mode in the frequency range of 30-40 kHz. Such a correspondence was lost when the probes were arranged deeper in the plasma, as well as in operating regimes in which fast transport transitions were impossible [13]. The results of these experiments are consistent with the calcul ations of stability of the m/n = 5/4 peeling mode and its localization along the radius.…”

“…In all cases the numerical code CAS3D [9,10] was used. Peeling modes were also observed in experiments [11][12][13]. In all cases experiments [11][12][13] and calculations [6][7][8] were performed for classical stellarators.…”

“…Therefore, such a mechanism is doubtful under the given experimental con ditions. Studies of the edge plasma with the help of Langmuir probes have shown that, in this case, the m/n = 4/3 internal mode plays no role [13]. The same can be said of m/n = 3/2, 7/5.…”

“…Let us perform estimations for the cases where peeling modes were experimentally observed [11][12][13] in Mercier stable plasmas. Edge mode activity was observed in L2M experiments, where fast drop of plasma energy at the plasma edge was found [11].…”

“…The same can be said of m/n = 3/2, 7/5. In [13], phase relationships between signals from Langmuir probes posi tioned near the separatrix in different cross sections of the facility were studied. In the nearseparatrix region, they corresponded to the m/n = 5/4 mode in the frequency range of 30-40 kHz.…”