Ion-cyclotron range of frequencies in the scrape-off-layer: fine structure radial electric fields

Abstract

Gas-puff-imaging techniques are utilized to detect radial electric field structures in the scrape-off-layer (SOL) of the Alcator C-Mod tokamak via the observation of poloidal motion of advected fluctuations. When the diagnostic's field of view is magnetically connected to the ion-cyclotron range of frequencies (ICRF) active antennas, large (up to 8 km s −1 ) poloidal velocities are observed in a radial region encompassing both field lines terminating on and those passing in front of the antennas. The radial electric field switches sign indicating a peak in the potential profile corresponding to the transition from piercing to passing field lines. The electric field extends a few centimeters into the SOL and its local magnitude is of order E r ≈ 20-30 kV m −1 . The corresponding plasma potentials scale as the square root of RF power; the poloidal structure is peaked when the field lines are connected to the top and bottom of the antenna. This structure is consistent with the presence of potential structures arising as a consequence of sheath rectification of the RF waves. The most striking result, however, is that the radial penetration λ ⊥ of the potential structures is an order of magnitude larger than the basic theoretical expectation (λ ⊥ ≈ 10δ e , where δ e is the skin depth). This substantial broadening is expected to have a strong impact on RF impurity physics. A (weak) power dependence observed in the width of the poloidal velocity features is explained as a competition between the RF induced and the background potential gradients.

ICRF specific plasma wall interactions in JET with the ITER-like wall

“…This implies a modification of density in front of the antennas by ICRF power [5] which could be attributed to convective cells [19,20]. The spectroscopic observations of the increased Be sputtering at the limiters are consistent with the spatial sputtering patterns moving in accordance with magnetic field connections to active ICRF antennas.…”

SOL RF physics modelling in Europe, in support of ICRF experiments

“…Even in presence of propagating Fast Waves, this DC plasma conductivity was found necessary to produce a V DC spatial distribution peaked radially at the leading edge of the side limiters, as measured experimentally [34], [43]. The amplitude and radial width of the peak was sensitive to the badly constrained value of  DC .…”

“…RF-induced DC modifications of the SOL are frequently observed very far away from the active ICRF antennas, on field lines magnetically connected near the antenna limiters [43], [34], [44]. These large parallel distances (e.g.…”

Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data

“…One system detects the emission with the 64 × 64 pixel array of a Phantom 710 fastframing camera, covering a ∼6 cm × 6 cm view of the puff, giving a Nyquist limit in poloidal wavenumber of 33 cm −1 ; the other detects emission using a 9 × 10 array of fibers that are routed to avalanche photodiodes (APDs), covering a ∼3.4 cm × 3.7 cm view of the puff, giving a Nyquist limit of 8.1 cm −1 . 15,26 In this paper we will refer to the two systems as the "Phantom-based" and "APD-based" and use data almost exclusively from the Phantom-based system. The Phantombased system includes telescope optics and a fine optic bundle; the total spatial resolution is ∼2-3 mm.…”

“…15,29,30 We will not attempt to treat the exhaustive list of techniques but rather focus on specific implementations (codes) whose results have been examined in detail and have been used in the literature recently for C-Mod data. 15,19 In the work presented here, we examine modified tracking TDE (tracking TDE) 19 and direct FA 15,26 methods, with analysis both in time and one spatial dimension. The motivation for this is that when direct FA is applied to the same C-Mod shots analyzed by Zweben et al 19 using tracking TDE, the results do not agree in many cases with those obtained using tracking TDE.…”