Measurements and Modeling of Turbulent Transport in the HSX Stellarator PDF Download
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Author: Walter Allen Guttenfelder
Publisher:
ISBN:
Category :
Languages : en
Pages : 224
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Author: Walter Allen Guttenfelder
Publisher:
ISBN:
Category :
Languages : en
Pages : 224
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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 396
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It has been observed in tokamaks that temperature profiles are resilient to changes in heating, and that this effect has not been observed in conventional stellarators. Electron temperature profile resiliency is attributed to anomalous transport driven by turbulent micro-instabilities, and the resulting stiffness in the electron heat flux is measured using a combination of steady-state and perturbative experiments. In this work, stiffness measurements are presented in the quasihelically symmetric configuration of the Helically Symmetric eXperiment (HSX), in which the neoclassical transport is comparable to a tokamak and turbulent transport dominates throughout the plasma. A second gyrotron and transmission line have been installed and tested to facilitate modulated heating experiments on HSX, and a multi-pass absorption model accurately predicts the total absorption and spatial extent of the electron cyclotron resonance heating during a modulation experiment. The electron cyclotron emission measured by an absolutely calibrated 16-channel radiometer is used to measure the local electron temperature and its response to the modulated heating. The amplitude and phase of the heat wave through the foot of the steep electron temperature gradient region of the plasma, 0.2
Author: George Louis Smith
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ISBN:
Category : Atmospheric circulation
Languages : en
Pages : 44
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Author:
Publisher:
ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 886
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Author: Stefan P. Gerhardt
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Category :
Languages : en
Pages : 456
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Author: Benjamin Faber
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Category :
Languages : en
Pages : 0
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The ability to optimize stellarator geometry to reduce transport has led to renewed interest in stellarators for magnetic confinement fusion. In this thesis, turbulence in the Helically Symmetric eXperiment (HSX), a quasi-helically symmetric stellarator, is investigated through application of the gyrokinetic code GENE and the new reduced fluid model PTSM3D. Gyrokinetics provides an efficient formalism for high-fidelity plasma turbulence simulations by averaging out the unimportant fast particle gyromotion. Both GENE and PTSM3D are capable of handling three-dimensional stellarator geometries. The first comprehensive simulations of Trapped Electron Mode turbulence in HSX are presented. HSX geometry introduces a complex landscape of unstable eigenmodes, with strongly ballooning modes, non-symmetric modes, and extended modes all coexisting at the same wavelengths. Nonlinear simulations display several characteristics unique to HSX. At long wavelengths, surprisingly large transport is observed despite the corresponding linear growth rates being small, and is attributed to nonlinear mode interactions. Zonal flows are prominent, however the velocity shear is insufficient to be solely responsible for saturation. These linear and nonlinear features are the consequence of the low global magnetic shear of HSX, which allows modes to extend far along field lines and requires simulation domains spanning multiple poloidal turns to properly resolve. Subdominant modes with extended structures play an important role in nonlinear energy transfer at long wavelengths, removing energy from shorter wavelength modes to both drive long-wavelength transport and dissipate energy through transfer to stable modes. Calculations with PTSM3D of triplet correlation times, used to quantify turbulence saturation, support the gyrokinetic results and show geometry plays a crucial role in turbulence saturation. Energy transfer from unstable modes to stable modes through non-zonal modes is the dominant mechanism in quasi-helically symmetric geometry, while zonal modes catalyze transfer in quasi-axisymmetric geometry. PTSM3D triplet correlation time calculations for HSX configurations with different magnetic hill and well depths accurately reproduce the nonlinear simulation trends, demonstrating the suitability of triplet correlation times as the first nonlinearly-derived metric for turbulence optimization.
Author: Cole Darin Stephens
Publisher:
ISBN:
Category :
Languages : en
Pages : 312
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The quest to harness fusion energy requires the successful modeling of plasma turbulence and transport in magnetic confinement devices. For such modeling, the requisite length and time scales span many orders of magnitude. Integrated modeling approaches are constructed to account for the wide range of physics involved in turbulent transport by coupling separate physical models together. The primary physical models used in this work are kinetic and designed to simulate microturbulence on the smallest scales associated with turbulent transport. However, high precision nonlinear kinetic simulations often cannot be easily coupled to integrated modeling suites due to the extreme computational costs that would be involved. Model reduction which drastically reduces the computational complexity of the problem is therefore necessary. One must of course ensure that the reduced model does not severely diminish the accuracy of the calculation; the model reduction itself must be founded on more exact computational approaches as well as fundamental theoretical principles. One of the most successful approaches in model reduction is quasilinear gyrokinetics. There are two fundamental assumptions for the quasilinear model examined in this work. First, the three adiabatic invariants (the magnetic moment, the longitudinal invariant, and the poloidal flux) must be appropriately conserved and their associated single charged particle motions (the gyromotion, the bounce-transit motion, and the toroidal drift motion) must be characterized accurately. Second, the quasilinear approximation must hold such that the coherent linear response is adequate enough to compute the quasilinear fluxes without full calculation of the nonlinear physics. The particular model used, QuaLiKiz, has been proven successful in reproducing local gyrokinetic fluxes in the tokamak core while remaining computationally tractable. There are three primary goals of this dissertation project. The first is to examine the fundamental physics underlying gyrokinetic and reduced model approaches at the single charged particle scale. To achieve this goal, we examine the assumption of magnetic moment invariance in a wide variety of electromagnetic fields. We successfully identify the dimensionless parameters that determine magnetic moment conservation in each scenario and then proceed to quantify the degree to which magnetic moment conservation is broken. In doing so, we confirm that the magnetic moment is sufficiently conserved for a wide range of regimes relevant to tokamak plasmas. In addition, we derive new analytic formulas for quantities associated with bounce-transit motion in circular tokamak fields. We compare these new, more exact calculations to approximations commonly used in reduced models (including QuaLiKiz) and determine the conditions such that the approximations break down. We then also confirm that the approximations are valid in the tokamak core for conventional, large aspect ratio devices. The second goal of this dissertation project is to rederive and compile the model equations for QuaLiKiz from first principles. Over the years of QuaLiKiz's development, there has never been a complete manuscript that sketches the derivation of QuaLiKiz from start to finish. The lack of such a document makes it difficult to extend the physics of QuaLiKiz to new parameter regimes of interest. Various possible extensions such as including electromagnetic effects or more realistic tokamak geometries require the adjustment of several different assumptions that would affect the derivation in key ways. As such, correct implementations of new physics would require an existing derivation as a reference point lest the implementation be handled in an incoherent fashion. In addition, a step-by-step outline of how each assumption of QuaLiKiz affects the derivation can be helpful in determining which assumptions can be relaxed for a more accurate model. The successful completion of this derivation, included in this dissertation, will be immensely useful for future QuaLiKiz improvement and validation. With the derivation in hand, we proceed to the third goal of this project: improving the collisional model of QuaLiKiz. Collisions play an essential role in characterizing the transport associated with trapped electron modes. It has become evident in recent studies that the collisional model in QuaLiKiz requires improvement; in integrated modeling, the imprecise treatment of collisional trapped electron modes leads to incorrect density profile predictions near the tokamak core for highly collisional regimes. We revisit the collision model implemented in QuaLiKiz and use the more exact gyrokinetic code GENE (Gyrokinetic Electromagnetic Numerical Experiment) to make improvements to QuaLiKiz's collision operator. We then use the new version of QuaLiKiz in integrated modeling to compare density profiles predicted by the old and new collision operators. We confirm that the new collision operator leads to density profiles that more accurately match the experimental profiles.
Author:
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ISBN:
Category :
Languages : en
Pages : 199
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In a magnetic configuration that has been sufficiently optimized for quasi-symmetry, the neoclassical transport and viscosity can be small enough that other terms can compete in the momentum balance to determine the plasma rotation and radial electric field. The Reynolds stress generated by plasma turbulence is identified as the most likely candidate for non-neoclassical flow drive in the HSX stellarator. Using multi-tipped Langmuir probes in the edge of HSX in the quasi-helically symmetric (QHS) configuration, the radial electric field and parallel flows are found to deviate from the values calculated by the neoclassical transport code PENTA using the ambipolarity constraint in the absence of externally injected momentum. The local Reynolds stress in the parallel and perpendicular directions on a surface is also measured using the fluctuating components of floating potential and ion saturation current measurements. Although plasma turbulence enters the momentum balance as the flux surface averaged radial gradient of the Reynolds stress, the locally measured quantity implies a significant contribution to the momentum balance. If extrapolated to a flux surface average, this locally measured Reynolds stress gradient is calculated to result in a flow drive many times larger than the observed flows. Probe measurements made at two locations on the device in regions with different magnetic geometry indicate very different, but consistently large Reynolds stress drive terms. The large variation of the local Reynolds stress on a flux surface suggests that a small number of measurement locations is insufficient to properly sample the flux surface averaged quantity. Contrary to expectations, measurements in configurations with the quasi-symmetry intentionally degraded deviate more from the neoclassically calculated velocity profiles than those in the QHS configuration. Measured density fluctuations and the Reynolds stress are reduced in these cases, indicating that additional terms may be important in the momentum balance.
Author:
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Category :
Languages : en
Pages : 240
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Up to now, the term "transport-optimized" stellarators has meant optimized to minimize neoclassical transport, while the task of also mitigating turbulent transport, usually the dominant transport channel in such designs, has not been addressed, due to the complexity of plasma turbulence in stellarators. Here, we demonstrate that stellarators can also be designed to mitigate their turbulent transport, by making use of two powerful numerical tools not available until recently, namely gyrokinetic codes valid for 3D nonlinear simulations, and stellarator optimization codes. A first proof-of-principle configuration is obtained, reducing the level of ion temperature gradient turbulent transport from the NCSX baseline design by a factor of about 2.5.
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Publisher: ScholarlyEditions
ISBN: 1464963657
Category : Science
Languages : en
Pages : 2502
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Book Description
Issues in Nuclear, High Energy, Plasma, Particle, and Condensed Matter Physics: 2011 Edition is a ScholarlyEditions™ eBook that delivers timely, authoritative, and comprehensive information about Nuclear, High Energy, Plasma, Particle, and Condensed Matter Physics. The editors have built Issues in Nuclear, High Energy, Plasma, Particle, and Condensed Matter Physics: 2011 Edition on the vast information databases of ScholarlyNews.™ You can expect the information about Nuclear, High Energy, Plasma, Particle, and Condensed Matter Physics in this eBook to be deeper than what you can access anywhere else, as well as consistently reliable, authoritative, informed, and relevant. The content of Issues in Nuclear, High Energy, Plasma, Particle, and Condensed Matter Physics: 2011 Edition has been produced by the world’s leading scientists, engineers, analysts, research institutions, and companies. All of the content is from peer-reviewed sources, and all of it is written, assembled, and edited by the editors at ScholarlyEditions™ and available exclusively from us. You now have a source you can cite with authority, confidence, and credibility. More information is available at http://www.ScholarlyEditions.com/.
