Modeling of Particle and Energy Transprot [sic] in the Edge Plasma of Alcator C-Mod PDF Download
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![Modeling of Particle and Energy Transprot [sic] in the Edge Plasma of Alcator C-Mod PDF](https://books.google.com/books/content/images/frontcover/9OvLHAAACAAJ?fife=w150-h200)
Author: Maxim V. Umansky
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Languages : en
Pages : 20
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![Modeling of Particle and Energy Transprot [sic] in the Edge Plasma of Alcator C-Mod PDF](https://books.google.com/books/content/images/frontcover/9OvLHAAACAAJ?fife=w80-h100)
Author: Maxim V. Umansky
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ISBN:
Category :
Languages : en
Pages : 20
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Author: Maxim V. Umansky
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Category :
Languages : en
Pages : 12
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Author: Arturo Dominguez (Ph. D.)
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ISBN:
Category :
Languages : en
Pages : 208
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The wide range of plasma parameters available on Alcator C-Mod has led to the accessibility of many regimes of operation. Since its commissioning, C-Mod has accessed the Linear ohmic confinement, Saturated ohmic confinement, L-Mode and ELM-free, ELMy and Enhanced D[alpha] H-Mode regimes. Recently, another novel regime, the IMode, has been identified[1][2][3][4]. I-modes feature the presence of steep H-Mode-like electron and ion temperature gradients at the edge of the plasma with L-Mode-like density profiles. The I-Mode, in contrast to the Hl-mode, shows very weak degradation of energy confinement with increased input power, and routinely reaches H98 > 1 while operating at low edge collisionalities ... making it a good candidate for reactor relevant tokamaks. Also relevant for reactors, this regime can be sustained in steady state for more than -15 energy confinement times without the need for ELMs to regulate particle and impurity confinement. Changes in edge density, temperature and magnetic field fluctuations accompany the L-mode to I-mode transition, with reduction of fluctuations in the 50-150kHz range as well as the appearance of a Weakly Coherent Mode (WCM) in the 200-300kHz range, analogous to the Quasi-Coherent Mode (QCM) characteristic of the Enhanced D[alpha] H-mode. Previous work[4] has established a connection between the midrange fluctuation suppression and reduction in the effective thermal diffusivity, Xye, in the pedestal region. The mechanism in I-mode for maintaining sufficient particle transport to avoid impurity accumulation and instabilities has been unclear. The O-mode reflectometry system has been extensively used for the characterization and detection of the I-mode and the WCM, in part, enhanced by upgrades to the system which enabled the baseband detection of density fluctuations at an array of cutoff locations at the edge of the plasma[5] [6] [7]. Using a novel model, the autopower signals of reflectometry channels detecting the density fluctuations have been decomposed into a broadband component and a WCM component. The latter is then used to estimate the intensity of the WCM. In parallel, the particle transport across the LCFS in I-mode plasmas has been estimated using a volume integrated particle transport model, where ionization source measurements are acquired using D[alpha] profiles measured near the outboard midplane. This model takes into account the anisotropic ionization source density around the periphery of the plasma by introducing an asymmetry factor, [sigma], which is then estimated using a study of I-Mode to H-Mode transitions. The results imply that measurements at the outboard midplane overestimate the surface-averaged influx. Finally, a comparison has been made between the particle flux across the LCFS of the I-mode and the intensity of the WCM, which shows a generally positive correlation between the two. This is supporting evidence that the WCM is, in fact, responsible for maintaining particle and impurity transport across the edge of the I-mode energy transport barrier.
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Category :
Languages : en
Pages : 171
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The goal of this thesis was to study the behavior of the plasma transport during the divertor detachment in order to explain the central electron density rise. The measurement of particle transport coefficients requires sophisticated diagnostic tools. A two color interferometer system was developed and installed on Alcator C-Mod to measure the electron density with high spatial (≈ 2 cm) and high temporal (≤ 1.0 ms) resolution. The system consists of 10 CO2 (10.6 [mu]m) and 4 HeNe (.6328 [mu]m) chords that are used to measure the line integrated density to within 0.08 CO2 degrees or 2.3 × 1016m−2 theoretically. Using the two color interferometer, a series of gas puffing experiments were conducted. The density was varied above and below the threshold density for detachment at a constant magnetic field and plasma current. Using a gas modulation technique, the particle diffusion, D, and the convective velocity, V, were determined. Profiles were inverted using a SVD inversion and the transport coefficients were extracted with a time regression analysis and a transport simulation analysis. Results from each analysis were in good agreement. Measured profiles of the coefficients increased with the radius and the values were consistent with measurements from other experiments. The values exceeded neoclassical predictions by a factor of 10. The profiles also exhibited an inverse dependence with plasma density. The scaling of both attached and detached plasmas agreed well with this inverse scaling. This result and the lack of change in the energy and impurity transport indicate that there was no change in the underlying transport processes after detachment.
Author: Brian LaBombard
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ISBN:
Category :
Languages : en
Pages : 16
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Recent experiments in Alcator C-Mod have uncovered a direct link between the character and scaling of cross-field particle transport in the edge plasma and the density limit, nG. As ne/nG is increased from low values to values approaching [approx.] 1, an ordered progression in the cross-field edge transport physics occurs: first benign cross-field heat convection, then cross-field heat convection impacting the scrape-off layer (SOL) power loss channels and reducing the separatrix electron temperature, and finally 1bursty2 transport (normally associated with the far SOL) invading into closed flux surface regions and carrying a convective power loss that impacts the power balance of the discharge. These observations suggest that SOL transport and its scaling with plasma conditions plays a key role in setting the empirically observed density limit scaling law.
Author: B. Lipschultz
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ISBN:
Category :
Languages : en
Pages : 50
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Scrapeoff Layer (SOL) data from DIII--D and C--Mod have been acquired and analyzed for radial particle transport based on a particle balance model. This has allowed a detailed comparison for L--mode plasmas. The inferred radial particle flux, ... , is parameterized in terms of diffusive [ ... ] and convective particle transport [ ... ]. The magnitude of the inferred Deff or veff increases across the SOL for both tokamaks. The inferred Deff or veff in the 'far' SOL (one density e-folding length from the separatrix and beyond) are essentially unchanged by changes in core density by factors of 2-3. This corresponds to changes in the far SOL density, collisionality (v*), and radial fluxes of a factor of 10 or more. Thus v* does not appear to be an important parameter in determining the radial particle transport in that region. The dimensionlessly-scaled SOL plasma profiles from the two tokamaks overlay for similar dimensionless plasma parameters. The SOL density profile near the separatrix is steeper than in the 'far' SOL. The scaled Deff and veff are slightly larger on DIII-D than C-Mod. This difference appears to be within experimental uncertainties. Neutral ionization in the SOL does not appear to affect radial transport but may be related to the observed flattening of the density profiles with increasing ... .
Author: Pablo Rodríguez Fernández (Ph. D.)
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ISBN:
Category :
Languages : en
Pages : 191
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Perturbative transport experiments in magnetically confined plasmas have shown, for more than 20 years, that the injection of cold pulses at the plasma edge can trigger the fast increase of core temperature. Because no single standard local transport model tried to date has been able to reproduce satisfactorily all the observed temporal behavior in the experiments, these transient transport phenomena feature prominently as an open question in the community and as a challenge for predictive capabilities in tokamak burning plasmas, such as ITER and SPARC. For the first time after more than two decades of experimental evidence, this Thesis resolves this long-standing enigma in plasma transport, by modeling of experiments conducted on the Alcator C-Mod and DIII-D tokamaks. Predictive integrated simulations with the Trapped Gyro Landau Fluid (TGLF) quasilinear transport model demonstrate that the increase of core temperature in some regimes, and lack thereof in other regimes, can be explained by a change in dominant linear micro-instability in the plasma core. The effect of major radius, electron density and plasma current on the cold pulse are well captured by TGLF, including the relative change in position of the temperature flex point as current density changes. Linear stability analysis of simulated density and current scans in Alcator C-Mod reveals a competition between trapped electron and ion temperature gradient modes as the main driver of the core transient response. Measurements of electron density evolution during the cold-pulse propagation in DIII-D are enabled by a high time resolution density profile reflectometer. The density evolution reveals the quick propagation of a pulse from edge to core, which is the mechanism to transiently increase core temperature in low-collisionality plasmas. The work presented in this Thesis demonstrates that the existence of nonlocal heat transport phenomena is not necessary for explaining the behavior and time scales of cold-pulse experiments in tokamak plasmas.
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Category : Aeronautics
Languages : en
Pages : 1008
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Author: Réjean Louis Boivin
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Category :
Languages : en
Pages : 31
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Author: Sanjay Gangadhara
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ISBN:
Category :
Languages : en
Pages : 10
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