[en] A new two-dimensional full-wave code has been developed to simulate ordinary (O) mode reflectometry signals caused by plasma density fluctuations. The code uses the finite-difference time-domain method with a perfectly matched layer absorption boundary to solve Maxwell's equations. Huygens wave sources are incorporated to generate Gaussian beams. The code has been used to simulate the reflectometer measurement of the quasicoherent mode (60--250 kHz) associated with enhanced D_α (EDA) H modes in the Alcator C-Mod tokamak. It is found that an analysis of the realistic experimental layout is essential for the quantitative interpretation of the mode amplitude

[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] An eight-channel beam-emission-spectroscopy (BES)1 system has been installed on the Alcator C-Mod tokamak, intended for use with a diagnostic neutral hydrogen beam (DNB). Capable of localized measurements from the plasma edge to the plasma core, the BES diagnostic collects light from the first Balmer transition (H_α) resultant from beam/plasma collisions. The H_α line splits into several components whose central wavelengths depend on the viewing geometry, the magnetic field, and the beam energy. This is due to the Doppler shifts from viewing the beam off perpendicular, the different velocities of the three mass components of the beam (H, H₂, H₃), and the large motional Stark effect. Optimal signal-to-noise requires collecting these components while attenuating all other emission: primarily bremsstrahlung and D_α radiation (from plasma D⁰/e^- collisions). Tunable bandpass filters are thus required. A BES simulation code has been developed that calculates the brightnesses (bremsstrahlung, D_α, H_α) versus wavelength using plasma profile data from the C-Mod MDSplus database,2 a computation of the beam penetration, the viewing and DNB geometries, and bandpass filter characteristics. The model was first used to estimate signal levels and choose the optimal BES bandpass filters; its ultimate purpose is to determine the shot-to-shot tuning requirements of the filters for different discharge conditions. Comparisons of measured and predicted background bremsstrahlung and D_α brightnesses are presented, as are first measurements and calculations of the beam emission. The code is written in the IDL programming language3 utilizing the ''widget'' graphical user interface. Designed for geometrical and spectral flexibility, it can be modified to simulate other beam diagnostics such as motional-Stark-effect plasma current measurements and charge-exchange recombination spectroscopy, as well as passive diagnostics measuring chord-averaged spectral emission

[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] Plasma flow at near-sonic speed is observed in the high-field region of the scrape-off layer (SOL) in Alcator C-Mod [1]. The principal drive is identified as a ballooning-like cross-field transport mechanism: plasma streams along magnetic field lines from low- to high-field regions in response to poloidal pressure variations; these are maintained by poloidally asymmetric cross-field transport. Thus, the largest component of the parallel flow is a transport-driven flow. As a result of the drive mechanism, the poloidal location of an X-point or limiter contact point determines the magnitude and direction of the transport-driven flow. In single-null discharges with BxΔB pointing toward (away from) the X-point, the flow circulates the confined plasma toroidally in the co-current (counter-current) direction. Consequently, the SOL possesses co-current (or counter-current) volume-averaged toroidal momentum. Depending on discharge conditions, the momentum couples across the separatrix and affects the toroidal rotation of the confined plasma. Thus the SOL imposes a 'flow boundary condition,' accounting for a positive (negative) increment in central plasma co-rotation in L-mode discharges when BxΔB is toward (away from) the x-point. Experiments in ICRF-heated discharges suggest that this boundary condition and its Xpoint dependence may explain the sensitivity of L-H power threshold to X-point location: in a set of otherwise similar discharges, the L-H transition is seen to be coincident with central rotation achieving roughly the same value, independent of Xpoint location. For discharges with BxΔB pointing away from the X-point (i.e., with the flow boundary condition impeding co-current rotation), the same characteristic rotation can be achieved only with higher auxiliary input power. Remarkably, L-H power thresholds in lower-limited discharges are identical to those in lower X-point discharges; SOL flows are also similar, suggesting a connection. (Author)

[en] A correlation electron cyclotron emission (CECE) diagnostic has been installed in Alcator C-Mod. In order to measure electron temperature fluctuations, this diagnostic uses a spectral decorrelation technique. Constraints obtained with nonlinear gyrokinetic simulations guided the design of the optical system and receiver. The CECE diagnostic is designed to measure temperature fluctuations which have k_θ≤ 4.8 cm⁻¹ (k_θρ_s < 0.5) using a well-focused beam pattern. Because the CECE diagnostic is a dedicated turbulence diagnostic, the optical system is also flexible, which allows for various collimating lenses and antenna to be used. The system overview and the demonstration of its operability as designed are presented in this paper.

[en] A three-chord polarimeter on Alcator C-Mod will make measurements of the poloidal magnetic field and plasma fluctuations. The beams from two frequency-offset, 200 mW, FIR lasers operating at 117.73 μm are combined to produce collinear, counter-rotating, circularly polarized beams. The beams are divided into three chords which are directed into the plasma at one toroidal location. Corner cube retro-reflectors mounted on the inside wall return the beam for a double pass. The mixing product of the two beams is detected both before (reference) and after (signal) the plasma using polarization sensitive detectors that produce a beat signal at ∼ 4 MHz. During the plasma discharge, the phase delay of the signal mixer, which depends on the Faraday effect, is evaluated with respect to the reference and produces line-integrated information on the poloidal magnetic field. Measurements on C-Mod require the phase error to be at the 0.1 degree level, and great care in the design of optical mounts, polarizers, beam-splitters, focusing optics, and acoustic and magnetic shielding was required. Development of new planar diode Schottky detectors was necessary to provide high sensitivity for a diagnostic that will eventually have at least six chords. Absorption of the FIR laser light by water vapor requires that the entire beam path be purged with dry air. Six retro-reflectors on the inner wall arranged in an ITER-like configuration provide poloidally viewing chords from near the mid-plane to well into the plasma scrape off layer. A pneumatically controlled shutter protects the in-vessel optics during boronizations and during limited discharges that might accelerate damage to the retro-reflector surfaces. Tests indicate there is no measurable signal contamination from the toroidal magnetic field due to the Cotton-Mouton effect. Polarization sensitivity of the wire mesh beamsplitters necessitated system calibration. Good agreement to EFIT reconstructions has been observed along with plasma fluctuations up to 400 kHz.

[en] A four-channel compact neutral particle analyzer (CNPA) based on operating small Si diode detectors in pulse-height analysis (PHA) mode is used to measure energetic hydrogen minority ions with energies between ∼50 and 350 keV stemming from ion-cyclotron range-of-frequency heated D(H) Alcator C-Mod plasmas with both active and passive charge exchange (CX). First core minority ion distribution results from Alcator C-Mod discharges and a detailed description of the diagnostic are presented. The diagnostic employs integrated electronics and fast digitization of the shaping amplifier voltage. The digitized data are stored for postshot PHA, which removes the constraints of real-time PHA and allows for improved performance via elimination of base line shift effects and potentially relieving pileup through Gaussian fitting routines. The CNPA is insensitive to the large gamma and neutron background in Alcator C-Mod discharges but is susceptible to the plasma's soft x-ray flux. The soft x-ray flux limits the CNPA energy resolution to ∼15-20 keV. A simple model is used to interpret the active CNPA data which permits rapid estimates of the core hydrogen minority temperatures and anisotropy with a time resolution of ∼100 ms. Hydrogenlike boron is identified as an important electron donor for the CX signal

[en] X-ray spectra of n = 3–1 transitions in He-like ions (and satellites) from calcium, argon and chlorine have been measured in the core of Alcator C-Mod tokamak plasmas using high wavelength resolution x-ray spectrometer systems. The intensity ratio of the intercombination line y ₃ (1s3p ³P₁–1s^{2 1}S₀) to the resonance line w ₃ (1s3p ¹P₁–1s^{2 1}S₀) is found to be much larger than what is expected if collisional excitation out of the ground state is considered as the only population mechanism for the upper levels. This suggests that recombination and cascades from higher levels with n $⩾<!--\; ???\; -->$ 4 are important. Modeling with the MARIA code is in good agreement with the observations, demonstrating the importance of recombination population of the upper level for y ₃. The intensity ratio y ₃/w ₃ has been studied over a large range of core electron temperature and density, and radial position in the plasma. The observed ratio decreases with increasing T _e, increases with increasing Z and is independent of n _e, in agreement with modeling. (paper)

[en] X-ray spectra of n = 2 to 1 transitions in hydrogen-like Ca¹⁹⁺, helium-like Ca¹⁸⁺ and nearby satellites have been obtained from Alcator C-Mod tokamak plasmas using a spatially imaging high resolution x-ray spectrometer system. For Ca¹⁹⁺, the intensity ratio of Ly_α2 (1s ¹S_1/2–2p ²P_1/2) to Ly_α1 (1s ¹S_1/2–2p ²P_3/2) was found to be ∼0.531 ± 0.005 over a range of plasma parameters, which is somewhat greater than the ratio of the statistical weights of the upper n = 2 levels, 1/2. This difference is mainly due to interaction with the ²S_1/2 fine structure sub-level. Experimental results are compared to calculations from COLRAD, a collisional-radiative modelling code, and good agreement is shown. For Ca¹⁸⁺, the intensity ratio of the dielectronic satellite k (1s²2p ²P_1/2–1s2p²²D_3/2) to the resonance line w (1s²¹S₀–1s2p ¹P₁) is sensitive only to the electron temperature. The observed brightness ratio scaling with T_e is in good agreement with the calculated ratio of the respective dielectronic recombination to the collisional excitation rates. (paper)