New insights on boundary plasma turbulence and the quasi-coherent mode in Alcator C-Mod using a Mirror Langmuir Probe

Abstract

A new “Mirror Langmuir Probe” diagnostic, combined with a double-coil scanning magnetic probe, is used to interrogate Alcator C-Mod's quasi-coherent mode (QCM) with unprecedented detail. In ohmic EDA H-modes, the QCM is found to reside in a region of positive radial electric field, with a radial width (∼3 mm) that spans open and closed field line regions. Large amplitude, in-phase sinusoidal bursts (∼100 kHz) in density, electron temperature, and plasma potential are observed, with potential lagging density by ∼16°, producing an outward radial transport velocity of ∼10 m/s. Mode propagation corresponds to the sum of local E × B and electron diamagnetic drift velocities. Poloidal magnetic field fluctuations project to current filaments carrying peak current densities of ∼25 A/cm2. An evaluation of parallel electron force balance (Ohm's law) over a fluctuation cycle indicates a significant electromotive component. Interchange drive is also a contributor in the current continuity (vorticity) equation. Thus, the QCM is primarily a separatrix-spanning electron drift-wave with interchange and electromagnetic contributions.

Comparison of 3D flux-driven scrape-off layer turbulence simulations with gas-puff imaging of Alcator C-Mod inner-wall limited discharges

“…It appears that pro le formation involves physical processes not included in our analytical scaling [15]. The analysis of the background pro les will be addressed in the future using the so-called 'mirror' Langmuir probe [27], which has the accuracy needed to unequivocally resolve the separate SOL scale lengths.…”

20 years of research on the Alcator C-Mod tokamak

“…In EDA H-modes, the increase of particle and impurity transport over ELM-free discharges is due to very large amplitude, narrow-band fluctuations observed by every diagnostic capable of detecting short-wavelength fluctuations in the plasma edge, including reflectometry, 119,183 phase-contrast imaging, 121 Langmuir probes, 184 magnetic loops, 120 gas-puff imaging, 110 and polarimetry. 127 The QCM frequency is typically in the range of f $ 50-150 kHz and field aligned (kÁB ¼ 0) with an outer-midplane poloidal wavenumber, k h $ 1.5 cm À1 .…”

“…It may be notable that broadband fluctuations in the same x and k range are prevalent in Lmode discharges and believed to be a key component in boundary plasma transport. Multi-field measurements of the mode have been made recently using "mirror probe" electronics, 184 showing mode amplitudes in plasma density, dn e / n e $ 0.3; electron temperature dT e /T e $ 0.45, plasma potential d//T e $ 0.45, and magnetic field, dB $ 0.4 mT, dJ $ 25 A/cm 2 . Measurements from an active antenna (described in Sec.…”

Mean flows and blob velocities in scrape-off layer (SOLT) simulations of an L-mode discharge on Alcator C-Mod

“…24. This diagnostic yields extremely high quality time resolved data for plasma potential, density and electron temperature.…”

“…[14][15][16][17][18][19][20][21] Binormal (approximately poloidal) flows that vary in the radial direction are routinely observed in the outer midplane region of tokamaks as for example in Refs. [22][23][24]. Often, such flows are not measured directly, but rather inferred from measurements of the radial electric field E r (allowing calculation of the poloidal EB drift v E ), the density or pressure profiles (for the diamagnetic drifts), or from velocimetry of imaging diagnostics such as gas puff imaging (GPI).…”