|Title||Direct observations of L-I-H and H-I-L transitions with the X-point reciprocating probe in ASDEX Upgrade|
|Publication Type||Journal Article|
|Year of Publication||2014|
|Authors||S.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|
|Journal||Physics of Plasmas|
|ISBN Number||1070-664X; 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 Title||Phys. Plasmas|
Go back one page.