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

A novel “Shoelace” antenna has been used to inductively excite a short-wavelength edge fluctuation in a tokamak boundary layer for the first time. The principal design parameters, k[subscript ⊥] = 1.5 ± 0.1 cm[superscript −1] and 45 < f < 300 kHz, match the Quasi-Coherent Mode (QCM, k[subscript ⊥] ∼ 1.5 cm[superscript −1], f ∼ 50−150 kHz) in Alcator C-Mod, responsible for exhausting impurities in the steady-state, ELM-free Enhanced D[subscript α] H-mode. In H-mode, whether or not there is a QCM, the antenna drives coherent, field-aligned perturbations in density, [˜ over n][subscript e], and field, [˜ over B][subscript θ], which are guided by field lines, propagate in the electron diamagnetic drift direction, and exhibit a weakly damped (γ/ω[subscript 0] ∼ 5%−10%) resonance near the natural QCM frequency. This result is significant, offering the possibility that externally driven modes may be used to enhance particle transport. In L-mode, the antenna drives only a non-resonant [˜ over B][subscript θ] response. The facts that the driven mode has the same wave number and propagation direction as the QCM, and is resonant at the QCM frequency, suggest the antenna may couple to this mode, which we have shown elsewhere to be predominantly drift-mode-like [B. LaBombard et al., Phys. Plasmas 21, 056108 (2014)].United States. Dept. of Energy (Cooperative Agreement DE-FC02-99ER54512

Correlation ECE diagnostic in Alcator C-Mod

Correlation ECE (CECE) is a diagnostic technique that allows measurement of small amplitude electron temperature, T[subscript e], fluctuations through standard cross-correlation analysis methods. In Alcator C-Mod, a new CECE diagnostic has been installed[Sung RSI 2012], and interesting phenomena have been observed in various plasma conditions. We find that local T[subscript e] fluctuations near the edge (ρ ~ 0:8) decrease across the linearto- saturated ohmic confinement transition, with fluctuations decreasing with increasing plasma density[Sung NF 2013], which occurs simultaneously with rotation reversals[Rice NF 2011]. T[subscript e] fluctuations are also reduced across core rotation reversals with an increase of plasma density in RF heated L-mode plasmas, which implies that the same physics related to the reduction of T[subscript e] fluctuations may be applied to both ohmic and RF heated L-mode plasmas. In I-mode plasmas, we observe the reduction of core T[subscript e] fluctuations, which indicates changes of turbulence occur not only in the pedestal region but also in the core across the L/I transition[White NF 2014]. The present CECE diagnostic system in C-Mod and these experimental results are described in this paper

The flush-mounted rail Langmuir probe array designed for the Alcator C-Mod vertical target plate divertor

An array of flush-mounted and toroidally elongated Langmuir probes (henceforth called rail probes) have been specifically designed for the Alcator C-Mod’s vertical target plate divertor and operated over multiple campaigns. The “flush” geometry enables the tungsten electrodes to survive high heat flux conditions in which traditional “proud” tungsten electrodes suffer damage from melting. The toroidally elongated rail-like geometry reduces the influence of sheath expansion, which is an important effect to consider in the design and interpretation of flush-mounted Langmuir probes. The new rail probes successfully operated during C-Mod’s FY2015 and FY2016 experimental campaigns with no evidence of damage, despite being regularly subjected to heat flux densities parallel to the magnetic field exceeding ∼1 GW per square meter for short periods of time. A comparison between rail and proud probe data indicates that sheath expansion effects were successfully mitigated by the rail design, extending the use of these Langmuir probes to incident magnetic field line angles as low as 0.5 degree