[en] A Motional Stark Effect (MSE) instrument is being installed on the Alcator C-Mod tokamak at MIT. This MSE diagnostic will provide measurements of the spatial profile of the internal poloidal magnetic field. The MSE has its primary collection optics inside the vacuum vessel. The light collected by the internal optics passes through a vacuum window and is relayed to a fiber optic array. The MSE optics are shared by a Beam Emission Spectroscopy (BES) diagnostic which measures electron density fluctuations and their spatial correlations. This optical system requires high throughput and spatial resolution of less than 1 cm at the focal plane in the plasma. The design requirements for the internal optics also include the effects associated with plasma impingement, plasma disruptions, and thermal excursions. The parameters that affect polarization measurement include the location and orientation of optical elements, the choice of substrates and optical materials. These unique design requirements led to a number of interesting optical and mechanical design features which are presented here

[en] A technical assessment of the critical issues and problem areas in the field of plasma materials interactions (PMI) in magnetic fusion devices shows these problems to be central for near-term experiments, for intermediate-range reactor devices including D-T burning physics experiments, and for long-term reactor machines. Critical technical issues are ones central to understanding and successful operation of existing and near-term experiments/reactors or devices of great importance for the long run, i.e., ones which will require an extensive, long-term development effort and thus should receive attention now. Four subgroups were formed to assess the critical PMI issues along four major lines: (1) PMI and plasma confinement physics experiments; (2) plasma-edge modelling and theory; (3) surface physics; and (4) materials technology for in-vessel components and the first wall. The report which follows is divided into four major sections, one for each of these topics

[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] A high resolution edge Thomson scattering diagnostic is used in the Alcator C-Mod tokamak to study spatial structure of the edge plasma, with emphasis on the investigation of the edge transport barrier that forms in high confinement mode (H mode) of tokamak operation. The diagnostic employs a 30 Hz Nd:YAG laser (1.5 J, 8 nsec pulses) and a four spectral channel filter polychromator. Twenty scattering volumes, defined by collection optics geometry, span the region of the transport barrier, providing measurements of the electron density and temperature profiles with ∼1.5 mm spatial resolution. The system is capable of measuring values of T_e between 15 eV and 800 eV, and covers the electron density range from 3x10¹⁹ m^-3 to 5x10²⁰ m^-3. (author)

[en] A high-resolution Thomson scattering diagnostic is in operation on the Alcator C-Mod tokamak, measuring radial profiles of electron temperature and density at the plasma edge. Photons are scattered from a Nd--yttrium--aluminum--garnet laser beam pulsed at 30 Hz (1.3 J, 8 ns pulse), and are measured by a filter polychromator with four spectral channels. The polychromator measures T_e in the range of 15--800 eV and n_e of 0.3--3x10²⁰m^-3. Twenty scattering volumes are located about the last closed flux surface, spaced for a nominal resolution of 1.3 mm in midplane radial coordinates. High resolution is essential for measuring edge T_e and n_e profiles on C-Mod, since these quantities exhibit gradient scale lengths as small as 2 mm in H mode. The steep profiles at the H mode edge are fit to a parameterized pedestal function for ease of analysis. Measured profiles are compared with edge profiles from electron cyclotron emission and visible continuum diagnostics

[en] A motional Stark effect (MSE) instrument to measure q(r) using the Torus Experiment for Technology (TEXT) neutral beam on the Alcator C-Mod tokamak has been designed and installed. The neutral beam is on the midplane, aimed radially, and designed to operate in hydrogen at 50 keV. The MSE optics view the outer half of the plasma -0.3<(R-R₀)/a<1.05. This geometry results in a spatial resolution of 1 (edge)--3 (center) cm from a beam apertured horizontally to 2 cm. Estimates of the signal levels indicate for Hα that photoelectron statistical errors in the measurement of B_θ/B_φ can be less than 0.2% for n_e0<2.0x10¹⁴ cm^-3 similar to other MSE instruments. The collection optics reside within the vacuum chamber reflecting and imaging the neutral beam through a vacuum window in the neutral beam port. The optics are designed so that beam emission spectroscopy can be carried out simultaneously with MSE measurements. Optical signals are transmitted to remote detectors by fiber optics. Low Verdet glass and dielectric mirrors are used to minimize polarization changes caused by the ambient fields. Polarization is measured by conventional techniques using photoelastic modulators, optical filters, and photomultipliers

[en] With use of x-ray imaging arrays, spatially and temporally resolved measurements were made of fully ionized and total absolute Si (Z = 14) densities following injection of Si into Alcator C. The fully stripped ions were detected through x rays resulting from ground-state radiative recombination. Central ion densities were found to be close to coronal equilibrium values. In addition, radial profiles of fully ionized and H-like Si showed large fluctuations during internal disruptions. .ID LX2087 .PG 1830 1833

[en] Recent results from the compact, high field, Alcator C-Mod tokamak program are summarized. H-mode threshold studies have demonstrated that the threshold appears to be closely related to local edge plasma parameters: for fixed field and plasma current, T_e(ψ₉₅) takes on a density independent value at the transition. The Enhanced D-Alpha H-Mode (EDA) regime has been investigated. EDA is distinct from ELM free H-mode, in that there is no accumulation of impurities, and at the same time EDA does not exhibit large discrete ELMs. The energy confinement is degraded by only about 10%, compared to ELM free. Comparisons for EDA with ELMy H-Mode database scalings indicate τ_EDA ∼1.2 τ_ITER97H. Strong toroidal rotation is observed in ICRF-only auxiliary heated plasmas; the rotation increases with plasma pressure, and decreases with increasing plasma current. The inferred radial electric field reaches the order of 30 kV/m near the center of the plasma. Through feedback controlled nitrogen impurity puffing, steady state detached EDA H-Modes have been achieved with Z_eff < 1.5 τ_E is reduced by about 10% in the detached case, compared to the confinement before the N₂ puff begins. The heat load to the divertor is reduced by a factor of 4. Volume recombination rates are measured in the divertor, using 2-d tomography of Balmer series TV movies. Volume recombination can be a significant contributor to the overall reduction in ion current to the divertor plates which occurs in detachment. Particle balance measurements indicate that the divertor and main chamber plasmas are largely isolated from one another, at least with regard to particle recycling, with most of the main chamber (core plus scrape-off) fueling coming from neutrals in the main chamber volume. With the addition of Lower Hybrid Current Drive, C-Mod would be an ideal vehicle for investigation of advanced tokamak operation with fully relaxed current profiles. Detailed modeling indicates that discharges approaching the β limit (β_N ∼ 3.7) with > 70% bootstrap fraction should be achievable. (author)

[en] Recent theoretical and experimental work has focused on the importance of neutrals in the edge dynamics of Alcator C-Mod plasmas. Two new high resolution detectors have been installed on the Alcator C-Mod tokamak to measure the neutral density and ionization rate near the edge of the discharge in the main chamber. The detectors consist of a 20 channel photodiode array which views the plasma tangentially 12.5 cm below the outer midplane, and 10 cm above the inner midplane, with a nominal radial resolution of 2 and 3 mm, respectively. The spectral bandwidth is limited by a filter with a very narrow band around the neutral deuterium Lyman alpha wavelength (1215 Angstrom). The local emissivity is then obtained via a standard Abel inversion of the absolutely calibrated brightness profile. Employing well-known branching ratios, and using measured local electron density and temperature, we therefore, infer the neutral density and ionization rate with similar radial resolution. We have observed that both Lyman alpha emissivity and ionization rate are usually peaked near the separatrix with a full width, half-maximum between 1 and 2 cm. The neutral density was found to drop rapidly with decreasing minor radius, from 2-3x10¹⁷/m³ (5-10 mm outside the separatrix) to 1-2x10¹⁶/m³ (5-10 mm inside the separatrix) for a line averaged density of 2.0x10²⁰/m³. Variations in ionization rate and neutral density from low to high confinement mode (L and H mode) are also discussed

[en] Recent results from the compact, high field, Alcator C-Mod tokamak program are summarized. H-mode threshold studies have demonstrated that the threshold appears to be closely related to local edge plasma parameters: for fixed field and plasma current, T_e(ψ₉₅) takes on a density independent value at the transition. The Enhanced D-Alpha H-Mode (EDA) regime has been investigated. EDA is distinct from ELM free H mode, in that there is no accumulation of impurities, and at the same time EDA does not exhibit large discrete ELMs. The energy confinement is degraded by only about 10%, compared to ELM free. Comparisons for EDA with ELMy H-Mode database scalings indicate τEDA ∼ 1.2τ_ITER97H. Strong toroidal rotation is observed in ICRF-only auxiliary heated plasmas; the rotation increases with plasma pressure, and decreases with increasing plasma current. The inferred radial electric field reaches the order of 30kV/m near the center of the plasma. Through feedback controlled nitrogen impurity puffing, steady state detached EDA H-Modes have been achieved with Z_eff < 1.5. τ_E is reduced by about 10% in the detached case, compared to the confinement before the N₂ puff begins. The heat load to the divertor is reduced by a factor of 4. Volume recombination rates are measured in the divertor, using 2-d tomography of Balmer series TV movies. Volume recombination can be a significant contributor to the overall reduction in ion current to the divertor plates which occurs in detachment. Particle balance measurements indicate that the divertor and main chamber plasmas are largely isolated from one another, at least with regard to particle recycling, with most of the main chamber (core plus scrape-off) fueling coming from neutrals in the main chamber volume. With the addition of Lower Hybrid Current Drive, C-Mod would be an ideal vehicle for investigation of advanced tokamak operation with fully relaxed current profiles. Detailed modeling indicates that discharges approaching the β limit (β_N ∼ 3.7) with > 70% bootstrap fraction should be achievable. (author)