Turbulent filament properties in L and H-mode regime in the RFX-mod operating as a tokamak

Abstract

The dynamics of turbulent filaments in L and ELM-free H-mode in the RFX-mod device operating as a tokamak is discussed in the present work. L-H transition and sustained H-mode have been achieved in the RFX-mod with the aid of an external electrode biasing (M. Spolaore et al. 2017 Nucl. Fusion 57 116039). Through advanced statistical techniques, turbulent filaments are detected and tracked from the edge to the Scrape-Off Layer (SOL) in a two-dimension floating potential map (measured with a 2D array of Langmuir probes) related with extreme events of a fixed ion saturation current signal in the SOL. While in L-mode filaments travel almost freely, their motion becomes restricted to the near SOL during the induced biasing H-mode. In this region, the background shear decorrelation time becomes shorter than the convective filament time, favouring its suppression. However, the experimental observation of a nearly 'trapped' monopole potential points out the possible role of the vortex selection mechanism. The sign and magnitude of the turbulent parallel vorticity are consistent with the E × B background vorticity in the SOL in this regime, while in L-mode, the inferred turbulent vorticity is always above the background one. The sparse filamentary transport in the far SOL during H-mode is further confirmed from the measurements of a poloidally symmetric array of electrostatic sensors on the wall. Lastly, the radial velocity of filaments in both regimes was discussed in the framework of analytical scalings. Filaments in L-mode exhibit a reasonable agreement with the sheath-limited regime in the cold ion approximation, with a non-negligible associated parallel current density measured with a set of 3-axial pickup coils installed in the probe head. In H-mode, the collisionality increases near the separatrix while filaments seem to follow the inertial regime before becoming trapped.

Transport barrier and spinning blob dynamics in the tokamak edge

“…(f) Recently a detailed study of blob-vortex interactions with shear layers in L and H mode has been carried out for the RFX-Mod device. The results [19] are in qualitative agreement with the present theory. Thus, the present theoretical framework for blob structure and motion is consistent with the experimentally observed behavior of blobs.…”

“…Both of these terms show explicitly the mutual interaction of two different sources of vorticity arising from the blob and the shear layer. Blob and shear layer interactions have been studied previously in slab geometry fluid simulations [11,17] and experiments [19,20]; however, the vortex analysis and interpretation turn out to be particularly transparent in the present large spin ordering.…”

“…Studies also examined blob interaction with localized background sheared flow. Non-spinning blobs could be destroyed by significant sheared flow [17,18], whereas spinning blobs could persist in weak sheared flows [19][20][21]. Blob suppression by sheared flow was demonstrated in fluid simulations with the SOLT code [22], as well as in gyrofluid simulations [23] where blobs could merge, exchange particles and heat, then disintegrate.…”

Cross-field and parallel dynamics of SOL filaments in TCV

“…TCV [2], DIII-D [13], NSTX [14], MAST [15], Alcator C-Mod [16], TEX-TOR [17] and confinement regimes [2,7,18]. More generally, filaments are also observed in magnetized plasmas [19], such as those found in TORPEX [20], RFX-mod [21], and other devices [22]. Due to the key role filaments play in setting the tokamak SOL profiles [18,23] and the level of plasma interaction with the first wall [6], a good understanding of the physics governing their cross-field motion is of great importance.…”

Positive and negative triangularity in RFX-mod2: a comparative analysis

“…Except for the three eyecatching regions of low connection length appearing, mirrored, in all the cases-the field lines are rapidly lost in the vacuum regions-layers of higher L c appear whenever the flux surfaces touch the wall in the outboard (for PT) or inboard (for NT) midplane before touching the lower boundary, or when the Xpoint is moved outwards or inwards. This can have relevant consequences in terms of local turbulence measurements in the scrape-off-layer, a topic recently discussed in [31] for some RFX-mod PT cases.…”