Active probe experiments at the Wendelstein 7-AS stellarator

Abstract

The fluctuations in the edge plasma of magnetic fusion experiments play an important role in terms of anomalous energy and particle transport. Experiments on Wendelstein 7-AS (Renner H et al 1989 Plasma Phys. Control. Fusion 31 1579 were conducted to investigate the propagation of actively fed signals driven by electrical probes. The perspective of these investigations is an improved understanding of the turbulence and potentially a method to tailor the edge plasma turbulence. Two moveable poloidal probe arrays were used for the experiments, one located on the inboard side of the vessel and the other on the outboard side. A subset of probe tips was used for actively driving the plasma by different control signals, the remaining probes collected fluctuation data in the plasma boundary. With this set-up it was possible to investigate the signal propagation in the poloidal direction and also parallel to the magnetic field. Poloidally, we find a significant cross-correlation between active and passive probes. From analysis of the cross-coherency and phases of the signal with passive probe tips, a dominant influence of the background plasma rotation on the applied signals is observed. In the case of externally driven waves by several phase-locked active probes, the direction of the wave propagation with respect to the plasma rotation (co-or counter-rotating) is essential for an effective coupling to the turbulence. Also, the signal propagation parallel to the magnetic field depends on co-or counter-rotation with respect to the background plasma rotation. The parallel phase velocity was found to be compatible with the electron thermal velocity.

Biased electrodes for SOL control in NSTX

“…Thus the biasing potential has a relatively short range across the magnetic field. Similar results have been seen in previous experiments on SOL biasing [8][9][10].…”

“…In CASTOR [7] an electrode was biased +100 to +200 V in the SOL, a poloidal electric field of up to 5 kV/m was created on flux surfaces connected to the electrode, and a strong poloidal modulation of the radial particle flux was measured. In related experiments, a positive DC plate bias in DITE changed the floating potential in a probe $2.5 mm away along B [8], a positive DC and 30 kHz bias in TEXT were detected $12 m away along B [9], and a 60 KHz probe bias in W7-AS was observed at a distance up to $12 m away along B [10]. Thus several previous biasing experiments have created local poloidal electric fields in the SOL.…”

Edge turbulence and SOL transport in tokamaks

“…In CCT, TEX-TOR and KT-5C [101,116,117], an electrode was inserted in the core, generating H-mode-like core conditions; however, this method is not suitable to high power discharges. Experiments applying ergodic layers in Tore-Supra, TEXTOR and DIII-D and W7-AS [118][119][120] have produced significant changes in particle transport/balance as well as turbulence modification.…”

Local scrape-off layer control using biased electrodes in NSTX

“…A positive DC bias on a small plate in DITE changed the floating potential on a nearby probe ~0.25 cm away along B, while a negative plate bias did not change this potential even a this small distance [16]. Probe biases were also detected very far along B; for example, both a positive DC and a 30 kHz oscillating probe biasing in the SOL of TEXT were detected ~12 m away along B [17], and a 60 kHz probe bias in the SOL of W7-AS was observed at a distance up to ~12 m away along B [18]. Changes in the local floating potential were also observed near a biased divertor plate probe in Alcator C-Mod [19].…”