Observations of the effects of magnetic topology on the SOL characteristics of an electromagnetic coherent mode in the first experimental campaign of W7-X

Abstract

Turbulence is considered to play an important role in the edge cross field heat and particle transport in fusion devices. Scrape-off layer (SOL) turbulence characteristics were measured by the combined probe mounted on the multipurpose manipulator during the first experimental campaign of W7-X. An electromagnetic coherent mode (EMCM) at 7 kHz has been observed by multiple diagnostics in both the plasma core and the SOL and exhibits a strong dependence of the magnetic topology. As demonstrated by the measurements of the combined probe, the EMCM starts to appear at a radius of R = 6.15 m along the path of probe measurement and this location is shifted inwards in higher iota configurations. It propagates along the direction of electron diamagnetic drift in the far SOL with a poloidal velocity about 0.6 km/s while it turns to the opposite direction gradually in the near SOL in the laboratory frame, but keeps a velocity of about 0.6-0.7 km/s along the direction of electron diamagnetic drift in the plasma frame. This mode can be induced by raising the ECRH heating power in similar discharge conditions, which is probably linked to the gradient of electron temperature and pressure. The EMCM is enhanced significantly in the edge magnetic island with long connection length where the EMCM can grow up due to the long particle confinement time.

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…”

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

“…In the first W7-X campaign with the limiter configuration, rotations and radial electric fields are evaluated and compared with the neoclassical calculations by measuring the poloidal propagation of density fluctuations [18]. An electromagnetic coherent mode at f ≈ 7 kHz in the plasma edge is analysed by means of both the poloidal correlation reflectometer (PCR) and the Langmuir probes [19]. The CAS3D simulation suggests that the coherent fluctuation is attributed to the stable MHD mode and Alfvén wave [20].…”

Characteristics of the SOL turbulence structure in the first experimental campaign on W7-X with limiter configuration

“…The characteristics of the low frequency electromagnetic coherent mode (EMCM) have been investigated in detail in our previous work. 28…”

“…In the radial region of R¼6.11-6.14, the high correlation coefficient and negative phase velocity for the discharge of 160224028 are caused by the electromagnetic coherent mode which propagates in the x *,e direction in both the laboratory frame and the plasma frame. 28 The turbulence poloidal phase velocity in the lab frame is considered as the sum of electric drift V EÂB ¼ E r =B parallel and some plasma-frame phase velocity V plasma frame . 35 As a result, the turbulence phase velocity is usually used to estimate the radial electric field.…”