Direct observations of L-I-H and H-I-L transitions with the X-point reciprocating probe in ASDEX Upgrade

TitleDirect observations of L-I-H and H-I-L transitions with the X-point reciprocating probe in ASDEX Upgrade
Publication TypeJournal Article
Year of Publication2014
AuthorsS.H Mueller, G.D Conway, G. Birkenmeier, D. Carralero, T. Happel, A. Herrmann, P. Manz, P. de Marne, A. Mlynek, H.W. Müller, S. Potzel, V. Rohde, U. Stroth, M. Tsalas, G.R Tynan, E. Wolfrum, ASDEX Upgrade Team
JournalPhysics of Plasmas
Date PublishedApr
ISBN Number1070-664X; 1089-7674

A reciprocating Langmuir probe was used to directly measure the behavior of turbulence and flows in the X-point region during transitions between low-(L) and high-confinement (H) mode in ASDEX Upgrade. The probe traverses the divertor horizontally in 140 ms, typically 2-5 cm below the X-point. Toroidal Mach number, density, floating potential (phi(f)), and electron temperature (T-e) are measured. In the regime accessible to the probe (P-inj < 1.5 MW, line-integrated core density <4 x 10(19) m(-2)), the L-H transition features an intermediate phase (I-phase), characterized by limit-cycle oscillations at 0.5-3 kHz [Conway et al., Phys. Rev. Lett. 106, 065001 (2011)]. The probe measurements reveal that this pulsing affects both the density and the toroidal Mach number. It is present in both the low-(LFS) and high-field sides (HFS) of the scrape-off layer, while high-amplitude broadband turbulence usually dominates the private-flux region. Profile comparisons between L-mode and I-phase show lower density in pulsing regions and small shifts in T-e, directed oppositely on LFS and HFS, which are compensated by shifts in phi(f) to yield a surprisingly unchanged plasma potential profile. Directly observed L-I-phase transitions reveal that the onset of the pulsing is preceded by a fast 50% density drop in the HFS X-point region. Back transitions to L-mode occur essentially symmetrically, with the pulsing stopping first, followed by a fast recovery to L-mode density levels in the divertor. (C) 2014 AIP Publishing LLC.







Alternate TitlePhys. Plasmas

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