[en] A multi-channel FIR scattering system has been installed on TEXT as part of an overall TEXT program to study anomalous transport in Tokamaks. The multiangle scattering system allows single shot measurement of the wavenumber spectra. At present, homodyne detection is being employed utilizing an optically pumped far-infrared laser operating at 245 GHz as the radiation source. Quasi-optical Schottky diode mixers, possessing noise temperatures of about 5000⁰K, form the basis of the low noise receiver system. The scattering apparatus is capable of both horizontal and vertical spatial scans thus enabling the entire plasma cross-section to be probed. In order to accurately obtain the wavenumber spectra as well as the absolute level of the density fluctuations, considerable effort has been devoted to the system calibration. To achieve this goal, system parameters such as relative collection efficiency detector responsivity and LO power assignment factors have been carefully calibrated using an amplitudemodulated FIR beam. Modulation is achieved by launching acoustic waves of the appropriate frequency transversely into a FIR cavity using a PZT. The gain of the FIR laser medium is modified and the FIR emission amplitude-modulated. Finally, it should be noted that the injection of small levels of scandium via the laser ablation technique was found to significantly modify the microturbulence spectra. Fluctuation levels decreased by greater than 20% and frequency spectra were harrowed by apparent partial quenching of the higher frequency components

[en] Resolution and contrast recovery in planar nuclear images at depth with a high purity germanium (HPGe) camera has been achieved through use of a weighted back projection (WBP) method. The algorithm can be derived from Bayes' theorem using the point spread function of the camera. Three variations in the formulation of WBP (one single pass and two iterative procedures) are presented with the characteristics and performance of each method. The evaluation procedure determines the behaviour of signal-to-noise ratio, contrast and texture after application of the algorithm. Both real and simulated cold lesions obtained with the HPGe camera are studied with sizes ranging from 3 mm to 17 mm and background count densities from 100 to 6400 counts cm². Application of WBP is shown to increase spatial resolution and contrast without a concomitant reduction in signal-to-noise ratio. Images obtained with the HPGe camera and processed with WBP are presented. The algorithm has been applied to the scintillation camera, yielding significant resolution and contrast recovery despite the presence of scatter and textured noise not present in the HPGe images. (author)

[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] The cylindrical slab was the first and simplest model of intrinsically unstable microturbulence. The Helimak is an experimental realization of this model. Although finite, it is sufficiently large to escape boundary effects, with dimensionless parameters similar to those of a tokamak edge or scrape off layer. The essential drive is interchange-like, a pressure gradient with unfavorable magnetic curvature, leading to a non-linearly saturated state of large-amplitude turbulence, Δn_rms/n ∼ 0.5. The nonlinear processes governing this saturation are unique, unlike any of those posited for the much weaker turbulence typical of confined plasma, e.g., in a tokamak. Neither linear stability theory, quasi-linear theory, zonal flows, nor flow shear stabilization is consistent with the observations. The mechanisms determining the non-linearly saturated state constitute an important challenge to our understanding of strongly nonlinear systems

[en] In an experimental realization of the sheared cylindrical slab, the level of plasma turbulence is strongly reduced by application of a sufficient bias potential difference in the radial direction. Density fluctuation levels Δn_rms/n decrease by more than a factor of five. The ion flow velocity profile is measured spectroscopically from the Doppler shift of an argon ion line. Comparison of the shearing rates with the turbulent amplitudes as a function of bias show no relation between the shearing rate and turbulence reduction, contrary to expectations.

[en] Comparison of experiments in the Texas Experimental Tokamak (TEXT) with theory leads to a reasonably complete understanding of impurity rotation in Ohmic plasmas. In the experiments, the components of the impurity rotation velocity, v_σΦ and V _σ Θ ( σ represents impurity species) were derived from measurements of the Doppler shift of C and O spectral emission. Simultaneously, quantities that might affect rotation were measured: the neutral concentration and the gradients of θ, T_i and n_σ, and n_i. The striking results of these comparisons are that plasma toroidal rotation is damped (not quenched) by momentum loss due to ion-neutral charge exchange and that the damping can now be accurately predicted. In the initial stage of this study, it was found that rotation velocity measurements were consistent with then-current theory only for plasma regions with low neutral concentration. This observation led to the conclusion that charge exchange damping should be incorporated into transport theory. Appropriate theory was developed and is successfully compared with experiment here. The present work will demonstrate the predicative capability of current theory, identify the principal mechanisms for rotation at the plasma edge, and demonstrate a significant modification of transport by an atomic process. (author) 2 refs., 4 figs

[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] This diagnostic measures temperature, density, and rotation for the fully stripped boron ion between the pedestal top and the plasma core with resolution consistent with the profile gradients. The diagnostic neutral beam used for the measurements generates a 50 keV, 6 A hydrogen beam. The optical systems provide views in both poloidal and toroidal directions. The imaging spectrometer is optimized to simultaneously accept 45 views as input with minimum cross-talk. In situ calibration techniques are applied for spatial location, spectral intensity, and wavelength. In the analysis, measured spectra are fitted to a model constructed from a detailed description of the emission physics. Methods for removal of interfering spectra are included. Applications include impurity and thermal transport.

[en] The results of impurity transport experiments in the Texas Experimental Tokamak (TEXT) [Nucl. Technol./Fusion 1, 479 (1981)] are compared with the predictions of a turbulence-based transport model. In the experiments, Sc was injected into the plasma using laser ablation and the time-resolved profiles of critical Sc ionization stages were measured along with the potential fluctuation profile. The experiment was simulated using a one-dimensional (1-D) radial transport code with the standard transport flux Γ=-D(∂n/∂r)+nV. The diffusion coefficient D and convective velocity V parameters were varied until the time-dependent 1-D simulations reproduce the data. This representation for the empirical impurity transport is compared with the ExB turbulent diffusivities and mobility based on the fluctuation data and the measured radial electric field. The agreement is best with the ExB diffusivity taken in the strong turbulence regime, where D=c_D(φ/B_T), while the comparison with the weak turbulence diffusivity and the collisional (no fluctuations) diffusivity results in qualitative disagreements. The percolation theory diffusivity (φ/B_T)^7/10(Δω/k²_{perpendicular})^3/10 is also briefly discussed

[en] Charge-exchange recombination spectroscopy can be used as a diagnostic for local densities of fully stripped impurities. For example, impurity profiles are valuable for transport studies. A major impediment to analysis is the availability of cross sections which relate the observed spectroscopic emission to the impurity densities. In this work, this impediment is removed for boron ions. Emission cross sections for B⁺⁴ states excited by charge exchange between beams of neutrals and fully stripped B⁺⁵ impurities in high density plasmas are computed for beams with multiple energy components. We show that even the low-energy neutral beam components make significant contributions to the emission for high-density plasmas which are commonly generated in the laboratory. Excited states as high as n=3 are important. This implies that a detailed description of the beam components including density, excited state population, and penetration is required for prediction of intensities of spectral lines excited by diagnostic neutral beams in high-density plasmas. Plasma experiments performed on the Alcator C-Mod tokamak will be described which clearly show the efficacy of our analysis method. This technique can be extended to species other than boron