[en] Accurately predicting the turbulent transport properties of magnetically confined plasmas is a major challenge of fusion energy research. Validation of transport models is typically done by applying so-called “synthetic diagnostics” to the output of nonlinear gyrokinetic simulations, and the results are compared to experimental data. As part of the validation process, comparing two independent turbulence measurements to each other provides the opportunity to test the synthetic diagnostics themselves; a step which is rarely possible due to limited availability of redundant fluctuation measurements on magnetic confinement experiments. At Alcator C-Mod, phase-contrast imaging (PCI) is a commonly used turbulence diagnostic. PCI measures line-integrated electron density fluctuations with high sensitivity and wavenumber resolution (1.6 cm⁻¹≲|k_R|≲11 cm⁻¹). A new fast two-color interferometry (FTCI) diagnostic on the Alcator C-Mod tokamak measures long-wavelength (|k_R|≲3.0 cm⁻¹) line-integrated electron density fluctuations. Measurements of coherent and broadband fluctuations made by PCI and FTCI are compared here for the first time. Good quantitative agreement is found between the two measurements. This provides experimental validation of the low-wavenumber region of the PCI calibration, and also helps validate the low-wavenumber portions of the synthetic PCI diagnostic that has been used in gyrokinetic model validation work in the past. We discuss possibilities to upgrade FTCI, so that a similar comparison could be done at higher wavenumbers in the future

[en] The two-color interferometry diagnostic on the Alcator C-Mod tokamak has been upgraded to measure fluctuations in the electron density and density gradient for turbulence and transport studies. Diagnostic features and capabilities are described. In differential mode, fast phase demodulation electronics detect the relative phase change between ten adjacent, radially-separated (ΔR= 1.2 cm, adjustable), vertical-viewing chords, which allows for measurement of the line-integrated electron density gradient. The system can be configured to detect the absolute phase shift of each chord by comparison to a local oscillator, measuring the line-integrated density. Each chord is sensitive to density fluctuations with k_R < 20.3 cm⁻¹ and is digitized at up to 10 MS/s, resolving aspects of ion temperature gradient-driven modes and other long-wavelength turbulence. Data from C-Mod discharges is presented, including observations of the quasi-coherent mode in enhanced D-alpha H-mode plasmas and the weakly coherent mode in I-mode.

[en] Electron temperature fluctuations are correlated with the dominant (m, n) = (0, 1) edge-resonant magnetic tearing mode during sawtooth events in the Madison Symmetric Torus reversed-field pinch. Electron temperature fluctuations in phase with the (0, 1) tearing mode are measured using high-repetition-rate Thomson scattering. Immediately prior to the sawtooth the (0, 1) island structure appears to be heat-confining, while during the sawtooth it assumes an isothermal character. Core electron temperature variation is also phase correlated with the (0, 1) mode, suggesting that the edge-resonant tearing mode has an effect on core electron thermal confinement during sawtooth events. (brief communication)