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

Abstract

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 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 × 10 19 m −2 ), the L-H transition features an intermediate phase (I -phase), characterized by limit-cycle oscillations (LCO) at 0.5-3 kHz [G.D. Conway et al., PRL 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 φ 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.

Poloidal asymmetric flow and current relaxation of ballooned transport during I-phase in ASDEX Upgrade

“…These fluctuations can result as a response to the plasma transport. A preceding fast density drop in the high-field side X-point region [23] points to an increase of ballooning at the separatrix prior to the transition to I-phase. During I-phase the poloidal magnetic field perturbations follow density perturbations in the very edge of the confined region measured via lithium beam emission spectroscopy [22].…”

“…Previous experimental results indicate an increase of ballooning of the transport in I-phase compared to L-mode [23].…”

“…This would be an indication of increased ballooning at the separatrix. Indeed, previous measurements with the X-point reciprocating probe in ASDEX Upgrade have shown that the onset of the pulsing in the I-phase is preceded by a fast 50% density drop at the high-field side X-point region [23] which lets suggest an increase of ballooning (cos s contribution) of the transport prior to the L-I transition. Therefore, also the ballooning of the transport in the confined region should be enhanced in I-phase compared to L-mode.…”

“…More details of this discharge can be found in Ref. [23]. Magnetic fluctuations measured by pick up coils sensitive to the poloidal magnetic componentḂ θ appear during Iphase in ASDEX Upgrade [20,22].…”

Magnetic structure and frequency scaling of limit-cycle oscillations close to L- to H-mode transitions

“…At ASDEX Upgrade (AUG) the LCO state was initially called 'dithering H-mode' and described as a modulation of electron temperature T e and density n e , which could qualitatively be understood in terms of a model involving a bifurcation of the radial electric field inducing a bistable state toggling between the L-and Hmode [2]. Further studies on AUG with Doppler reflectometry [3] and a divertor X-point probe [4] were focusing on the role of flow-turbulence interactions inside and outside the last closed flux surface (LCFS) and used the term 'I-phase' in order to express the intermediate character of the LCO state sharing properties of both the L-mode and the H-mode. The intermediate character was also * gregor.birkenmeier@ipp.mpg.de documented in slow L-to H-transitions at DIII-D in the so called 'IM-mode' which is a very smooth transition from almost sinusoidal LCOs into a bursty phase identified as the type-III edge localised mode (ELM) phase [5,6].…”