Investigation of a Diagnostic Technique for Measuring Electron Densities Via Stark Broadening on the Alcator C-Mod Tokamak PDF Download
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Author: Dirk Lumma
Publisher:
ISBN:
Category :
Languages : en
Pages : 142
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Author: Dirk Lumma
Publisher:
ISBN:
Category :
Languages : en
Pages : 142
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Author: Robert Thomas Mumgaard
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Category :
Languages : en
Pages : 349
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Lower Hybrid Current Drive (LHCD) is a promising technique to sustain tokamak plasmas and provide control over the current profile--two important capabilities required for the development of tokamak fusion reactors. Upgraded measurement capabilities on the Alcator C-Mod Tokamak create a unique opportunity to study the plasma's toroidal electric current profile at magnetic fields, plasma densities, and magnetic geometries anticipated in future reactors in stationary discharges dominated by lhcd. The Motional Stark Effect (MSE) diagnostic uses polarized light to infer the plasma's internal current profile. The MSE diagnostic deployed on the Alcator C-Mod Tokamak previously experienced unacceptable calibration drift and sensitivity to partially-polarized background light that limited its ability to measure magnetic field pitch-angles. A comprehensive analytic study of the origin of polarization angle errors in MSE diagnostics and an experimental study using a robotic calibration system were conducted. Insight from this study guided the fabrication and installation of a first-of-a-kind in-situ calibration system for MSE diagnostics--a long sought capability-- and the development of thermal isolation schemes for the periscope. An experimental study of the effect of partially polarized background light identified this as a significant source of systematic error. Partial-polarization upon reflection was identified as the mechanism that leads to polarized light in a tokamak. Visible bremsstrahlung, divertor emission, and blackbody emission were identified as the dominant sources of light. A new technique, MSE multi-spectral line polarization (MSE-MSLP), was developed to measure the polarization on a single sight line in multiple wavelengths simultaneously using a high-throughput polarization polychromator. Wavelength-interpolation of the background light polarization utilizing this hardware decreases the error from background subtraction by a factor of 5-10 relative to time-interpolation, drastically improving the measurement quality while eliminating the need for neutral beam pulsing. The method also allows for simultaneous measurement of multiple polarized transitions within the Stark multiplet. The upgraded MSE diagnostic was used to measure the magnetic field pitch angle profile in plasmas with some or all of the plasma current driven by lhcd. Measurements were made across a range of single-parameter scans: lhcd power, loop voltage, plasma density, plasma current, and launched n// spectrum. The current profile is observed to broaden during lhcd, but consistently has significant on-axis current density, even in fully non-inductive plasmas. The current profile and hard x-ray (HXR) profiles are observed to be most sensitive to plasma current, with higher current yielding broader profiles. The current and HXR profiles as well as global current-drive efficiency are insensitive to changes in n// or loop voltage. Numerical simulations by the ray-tracing Fokker-Planck GENRAY/CQL3D code reproduce the total measured current in non-inductive conditions but fail to accurately predict the current and HXR profiles; the simulations consistently predict more current drive in the outer half of the plasma than is observed. This leads to a flattening of the HXR profile compared to the experimental profiles. These qualitative discrepancies persist across the range of plasma parameters scanned. Varying code inputs within their measurement uncertainties and adding experimentally-constrained levels of fast-electron diffusion do not reconcile profile discrepancies. Some qualitative profile trends in single parameter scans are reproduced by the simulations including broadening of profiles at higher current, and a weak dependence on the launched n//spectrum. However, HXR profile self-similarity across different densities and powers is not reproduced. These new comparisons between profile measurements and simulation suggest that the simulations are missing important physics in this operational regime.
Author: Alexander Turner Graf
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ISBN:
Category :
Languages : en
Pages : 390
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Author: Kirill Zhurovich
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Category :
Languages : en
Pages : 185
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Internal transport barriers (ITBs) in tokamak plasmas are characterized by the reduction of transport in one or more of the particle, momentum, or energy channels in the core plasma region. On Alcator C-Mod, significant contributions to ITB studies were made possible with the core Thomson scattering (TS) diagnostic, which measures profiles of electron temperature (0.03
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Languages : en
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X-ray diagnostic measurements have been used in a number of experiments to improve our understanding of important tokamak physics issues. The impurity content in TFTR plasmas, its sources and control have been clarified through soft x-ray pulse-height analysis (PHA) measurements. The dependence of intrinsic impurity concentrations and Z/sub eff/ on electron density, plasma current, limiter material and conditioning, and neutral-beam power have shown that the limiter is an important source of metal impurities. Neoclassical-like impurity peaking following hydrogen pellet injection into Alcator C and a strong effect of impurities on sawtooth behavior were demonstrated by x-ray imaging (XIS) measurements. Rapid inward motion of impurities and continuation of m = 1 activity following an internal disruption were demonstrated with XIS measurements on PLT using injected aluminum to enhance the signals. Ion temperatures up to 12 keV and a toroidal plasma rotation velocity up to 6 x 105 m/s have been measured by an x-ray crystal spectrometer (XCS) with up to 13 MW of 85-keV neutral-beam injection in TFTR. Precise wavelengths and relative intensities of x-ray lines in several helium-like ions and neon-like ions of silver have been measured in TFTR and PLT by the XCS. The data help to identify the important excitation processes predicted in atomic physics. Wavelengths of n = 3 to 2 silver lines of interest for x-ray lasers were measured, and precise instrument calibration techniques were developed. Electron thermal conductivity and sawtooth dynamics have been studied through XIS measurements on TFTR of heat-pulse propagation and compound sawteeth. A non-Maxwellian electron distribution function has been measured, and evidence of the Parail-Pogutse instability identified by hard x-ray PHA measurements on PLT during lower-hybrid current-drive experiments.
Author:
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Category : Chemistry
Languages : en
Pages : 442
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Author: Alexander Mazurenko
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Category :
Languages : en
Pages : 320
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Phase Contrast Imaging (PCI) is a new diagnostic that was built for the Alcator C-Mod tokamak. It measures line-integrated (along 12 vertical chords) plasma density perturbations with good temporal (2-500 kHz) and wavenumber (0.5-12 /cm) resolution. The Quasi-Coherent (QC) fluctuation mode was studied using the PCI and other diagnostics. The mode was found to cause fluctuation of density, electric and magnetic field in the plasma edge with typical frequency of 100 kHz and typical poloidal wavenumber of about 5/cm. The mode was found to be responsible for confinement properties of the "Enhanced D-alpha H-mode" (a particularly favorable regime of tokamak operation). Through numerical modeling, the physical origin of the fluctuations was tentatively identified as "resistive X-point" mode (a kind of resistive ballooning mode strongly affected by the X-point configuration of magnetic field lines). The PCI system has been upgraded to detect waves in the ion cyclotron range of frequencies (ICRF, 40-80 MHz) by means of optical heterodyning - a technique based on modulation of the diagnostic laser beam near the wave frequency. The upgraded system was then used to study propagation of the Fast Magnetosonic Waves. These waves, which have never been measured in detail in past experiments, are being used to heat the tokamak plasma at the megawatt power level. The measured results were compared to the simple cold-plasma dispersion relation and to predictions of the full-wave 3D numerical modeling.
Author: Randy Michael Churchill
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ISBN:
Category :
Languages : en
Pages : 216
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In an effort to illuminate the effects of the strong plasma gradients in the pedestal region on impurity transport, research was conducted to measure complete sets of impurity density, poloidal and parallel velocity, and temperature at two separate poloidal locations in the pedestal region of the Alcator C-Mod tokamak. To this end, the diagnostic technique gas puff-CXRS was refined and expanded on, allowing for the first time in a tokamak complete measurements of impurities at the high-field side (HFS). Large in-out B5+ impurity density asymmetries were measured in H-mode plasmas with strong boundary electron density gradients, with a build-up of impurity density at the HFS. Impurity temperatures were also found to be asymmetric in the pedestal region, with larger temperatures at the low-field side (LFS). Such temperature asymmetries suggest a significant asymmetry in electron density near the separatrix. In contrast to these H-mode results, plasmas with low boundary electron density gradients, such as L-mode and I-mode, exhibit constant impurity density on a flux surface, even if strong electron temperature gradients are present. Mechanisms which could drive such poloidal asymmetries are explored. Experiments provide evidence against localized impurity sources and fluctuation-induced transport as primary causes. Particle transport timescales are compared, showing that the radial transport becomes comparable to or faster than the parallel transport in the pedestal region. Additionally, modelling of impurity transport using conventional, one-dimensional neoclassical physics fails to correctly reproduce the measured flux-surface averaged impurity density, suggesting along with the timescale estimates that a more complete two-dimensional treatment of impurity particle transport is required. The measured impurity velocities at the LFS and HFS are compared to the canonical form for particle flow velocity within the flux surface of a tokamak. Within the error bars of the measurement, agreement is found with the canonical form. The implications of exact matches to the canonical form are low radial transport, and the E x B drift dominating the perpendicular impurity flow. Further work is motivated into more precise velocity measurements to determine if the velocities exactly match this canonical form.
Author:
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Category :
Languages : en
Pages :
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By Thomas Sunn Pedersen.
Author: Baʹlint Vetoʺ
Publisher:
ISBN:
Category :
Languages : en
Pages : 176
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(Cont.) For the first time, the poloidal measurements of the plasma edge brightness with and without the D2 neutral gas puff yielded an estimate for the vertical extent of the neutral gas puff. The steady state inboard and outboard D, brightness profiles are also compared during two consecutive periods of L and H mode operation. The time lagged cross correlation calculated for neighboring views revealed quickly propagating local brightness maxima (blobs). The phase velocity of these systematically moving brightness perturbations ranges from -1 to 1 km/s inside the separatrix and becomes uniform outside the separatrix at 0.5 km/s where the positive velocity indicates radially outward motion. Downward propagating perturbations are also observed in the outboard SOL.
