The effects of magnetic topology on the scrape-off layer turbulence transport in the first divertor plasma operation of Wendelstein 7-X using a new combined probe

Abstract

Wendelstein 7-X (W7-X) was operated successfully with the first divertor plasma in the operation phase 1.2a (OP1.2a). A new combined probe head, developed and installed on the multiple-purpose manipulator, is able to measure the edge plasma profiles (𝑇 𝑒 , 𝑛 𝑒 , 𝜙 𝑓 , 𝑀 ∥ ), variation of magnetic field, poloidal and radial turbulence structures. The scrape-off layer (SOL) plasma parameters in two magnetic configurations (standard and high mirror) are in good agreement with the magnetic island structure and the field line connection length calculated by the field line tracer. In both the standard and high mirror configurations, the radial turbulent heat flux and particle flux have strong dependence on the local magnetic topology, revealing two distinct transport patterns: a broadband turbulence dominant region in the outer SOL and a low frequency dominant region in the inner SOL. In the standard divertor configuration, the broadband turbulence with a frequency range of 40-120 kHz is located near the island center along the probe path, leading to outward transport. These broadband fluctuations propagate with a velocity of 2.3-4.4 km/s poloidally along the ion diamagnetic drift direction in the plasma frame, with 𝑘 𝜃 𝜌 𝑠 close to 0.1. The large radial transport induced by the broadband turbulence is accompanied by a steep electron density gradient. The low frequency (5-30 kHz) dominant transport exhibits obvious intermittent structure. Some statistical techniques are applied to the characterization of the intermittent transport.

Experimental characterization of a quasi-coherent turbulent structure in the edge plasma in Wendelstein 7-X

“…In W7-X, the magnetic configurations are achieved by adjusting the coil current setting of the non-planar coils, planar coils and trim coils. The magnetic topology has an influence on the turbulence transport in the SOL region, which has been measured by the combined probe in W7-X [21]. Our observation indicates that the magnetic topology as well as the plasma parameter can influence the turbulence structure near the separatrix.…”

“…In the island-divertor configurations, turbulence transport in the scrape-off layer has been proved to be influenced by the magnetic topology [21]. A broadband turbulence in a frequency range of 40-120 kHz is observed near the island centre.…”

Characterization of the W7-X scrape-off layer using reciprocating probes

“…Operation Phase Description FZJ-COMB1 [13,14] OP1. 1 9 Electric probes, magnetic pick-up probe FZJ-COMB2 [15,16] OP1.2a/b 9 Electric probes, magnetic pick-up probe, ion sensitive probe, material exposition, gas pipe IPP-FLUC1 OP1.2a/b 28 Electric probes (poloidal array, parallel + poloidal Mach probe) FZJ-MACH1 [17] OP1.2a Polar (Gundestrup) + radial Mach probe array (28 electrodes) FZJ-RFA1 [18] OP1.2a 6 retarding field analyzers, 2 electric probe pins FZJ-GAS1 OP1.2a 4 electric probe pins, gas pipe FZJ-GAS2 OP1.2b 4 electric probe pins, piezo valve for gas injection FZJ-MACH2 OP1.2b Polar (Gundestrup) + radial Mach probe array (28 electrodes) FZJ-RFA2 OP1.2b 6 retarding field analyzers, 4 electric probe pins, gas pipe RFX-HRP1 [19] OP1.2b 3 magnetic pick-up probes, 8 electric probe pins, 3 Mach probes NIFS-FILD1 [20] OP1.2b 8 Faraday films for fast ion loss detection FZJ-MAT1…”

Equilibrium effects on the structure of island divertor and its impact on the divertor heat flux distribution in Wendelstein 7-X

“…The right panels display the electron pressure p e and radial turbulent particle flux Γ r . The detailed derivations of these parameters can be found in [50]. As seen in figure 11, all of these parameters have non-monotonous profiles with a local maximum around R = 6.07 m. With increasing plasma beta and toroidal current, all profiles are dominated by a resulting outward shift.…”