skip to main content

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

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 (ϕ{sub f}), and electron temperature (T{sub e}) are measured. In the regime accessible to the probe (P{sub inj}<1.5 MW, line-integrated core density <4×10{sup 19} m{sup −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{sub e}, directed oppositely on LFS and HFS, which are compensated by shifts in ϕ{sub 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.more » 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.« less
Authors:
 [1] ; ; ; ; ; ; ; ; ; ; ;  [2] ;  [3] ;  [4] ;  [2] ;  [4] ;  [5] ;  [1] ;  [6]
  1. Center for Energy Research, University of California of San Diego, 9500 Gilman Drive, M/C 0417, La Jolla, California 92093 (United States)
  2. Max Planck Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching (Germany)
  3. Physics Department E28, Technical University of Munich, 85747 Garching (Germany)
  4. (Germany)
  5. FOM-Institute DIFFER, Association EURATOM-FOM, NL-3439MN Nieuwegein (Netherlands)
  6. (United States)
Publication Date:
OSTI Identifier:
22253351
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 21; Journal Issue: 4; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ASDEX TOKAMAK; H-MODE PLASMA CONFINEMENT; LANGMUIR PROBE; L-MODE PLASMA CONFINEMENT; MACH NUMBER; PLASMA POTENTIAL; PLASMA SCRAPE-OFF LAYER; PULSES