The quasi-coherent signature of enhanced D_αH-mode in Alcator C-Mod

Abstract

The steady-state H-mode regime found at moderate to high density in Alcator C-Mod, known as enhanced D α (EDA) H-mode, appears to be maintained by a continuous quasi-coherent (QC) mode in the steep edge gradient region. Large amplitude density and magnetic fluctuations with typical frequencies of about 100 kHz are driven by the QC mode. These fluctuations are measured in the steep edge gradient region by inserting a fast-scanning probe containing two poloidally separated Langmuir probes and a poloidal field pick-up coil. As the probe approaches the plasma edge, clear magnetic fluctuations were measured within about 2 cm of the last-closed flux surface (LCFS). The mode amplitude falls off rapidly with distance from the plasma centre with an exponential decay length of k r ≈ 1.5 cm −1 , measured 10 cm above the outboard midplane. The root-mean-square amplitude of the fluctuation extrapolated to the LCFS was Bθ ≈ 5 G. The density fluctuations, on the other hand, were visible on the Langmuir probe only when it was within a few millimetres of the LCFS. The potential and density fluctuations were sufficiently in phase to enhance particle transport at the QC mode frequency. These results show that the QC signature of the EDA H-mode is an electromagnetic mode that appears to be responsible for the enhanced particle transport in the plasma edge.

20 years of research on the Alcator C-Mod tokamak

“…94,[118][119][120][121][122][123][124][125][126][127][128] A. H-mode access and the L-H threshold Prediction of transport bifurcations, though challenging due to the complexity of the physics, is critical for extrapolation into burning plasma regimes. Without a computable, first-principles model, prediction of the threshold has been based on empirical fits to global operating parameters.…”

External excitation of a short-wavelength fluctuation in the Alcator C-Mod edge plasma and its relationship to the quasi-coherent mode

“…2π/k ⊥ ) 46 . The wires are field-aligned when the safety factor at the 95% flux surface, q 95 , is 3, the same value used in the discharges of the earlier characterization of the QCM by Snipes et al 8 The perpendicular spacing is ∆z ⊥ = 2.1 cm, giving k ⊥ = ±1.5 ± 0.1 cm in the transform, or a span of ∆n ≈ ±23 from n 0 (FWHM). These values ensure good coupling to the QCM, which typically has 8 k ⊥ = 1.5 cm −1 and n = 10 − 25 (at higher safety factor), as well as to the WCM, which has a similar k ⊥ and n spectrum 2,47 .…”

“…Several experimental characterizations of the QCM 8,10,11,27,28 , including the most recent, have described a narrow edge mode having 50 f 150 kHz and k ⊥ ∼ 1.5 cm −1 at the midplane, approximately field-aligned with k ⊥ ≫ k , absent from the high-field side, and with large amplitude fluctuations. shown.…”

Advances in measurement and modeling of the high-confinement-mode pedestal on the Alcator C-Mod tokamak

“…Impurity accumulation is kept low, and an H-mode can be maintained steady state. The mechanism for this enhanced particle transport is a quasi-coherent mode (QCM), localized to the pedestal region [18] [19]. The QCM provides a continuous means of transport across the LCFS, in contrast to the intermittent bursts associated with ELMs [20].…”