Flow Measurements in the Edge Region of the RFX‐Mod Experiment

Abstract

In this contribution a detailed study of the edge flow properties performed on the outer region of the RFX-mod experiment using a Gundestrup probe is presented. In particular, both parallel and perpendicular components (with respect to the magnetic field) have been evaluated in several low current discharges providing a complete reconstruction of the edge flow profiles.Different methods based on both magnetized and unmagnetized models have been used to evaluate the averaged plasma flow components and their time variations. Furthermore, configurations involving either ion saturation current signals or floating potential ones have been used to evaluate both parallel and perpendicular flow components and a good agreement has been found between different methods.Finally, the dynamic behaviour of edge flow fluctuations induced by gradient modifications following pellet injection and by varying the edge magnetic topology (the m = 0 islands radial position) have been studied.

Electrostatic properties and active magnetic topology modification in the RFX-mod edge plasma

“…Furthermore, the E r radial profile can be directly interpreted as an estimate of the (v E×B ) φ = (E r /B θ ) φ toroidal flow. In this sense the result obtained at shallow q a values is in good agreement with previous flow measurements obtained by probe insertion [20,51] and optical diagnostics [52] with the maximum of the toroidal shear flow located at around r/a = 0.97-0.98.…”

“…This seems to suggest that two different transport regimes can be found in the RFX-mod edge operating at shallow q a . The region around the q = 0 surface (r/a 0.95) is characterized by a strong sheared toroidal E × B (see previous section) and poloidal [20,51] flow and a small-scale turbulence pattern dominated by the presence of the m = 0 islands. The more internal region (r/a < 0.95), featuring a chaotic magnetic topology due to the overlapping of several flux surfaces, smaller L c and a reduced toroidal flow (a quasi-zero E r as described in the previous section).…”

Magnetic perturbations as a viable tool for edge turbulence modification

“…Some preliminary indication of the effects of helical boundary on small scale turbulence has been already reported [19] but in order to allow probe insertion and turbulence analysis the helical plasma has been forced at lower current (I p ≲ 450 kA) with an externally applied helical perturbation [20]. The insertion of probe head, extensively described elsewhere [21,22], allows for a proper characterization of the perpendicular flow, density, temperature and fluxes, plus the determination of the electromagnetic properties of small scale blobs/filaments [21]. The probe head indeed consists of a 2D array of electrostatic pins, arranged in a five-pin triple balanced probe [23] in order to provide high frequency estimate of plasma potential, electron density and electron temperature plus.…”