400 Matching Results

Search Results

Advanced search parameters have been applied.

Note of the Function of the Aquadag in the Stellarator

Description: The following report introduces the aquadag, a part of the stellarator with the purposes to prevent the non-uniform accumulation of electrostatic charges on the walls of the stellerator and to free the electrons of the stellerator and provide accelerations, achieving a uniform field distribution.
Date: January 14, 1953
Creator: Witten, Louis
Partner: UNT Libraries Government Documents Department

The Quasi-Toroidal Stellarator: An Innovative Confinement Experiment

Description: To develop a new class of stellarators that exhibit improved confinement compared to conventional stellarators. This approach generally makes use of a designed symmetry of the magnetic field strength along a particular coordinate axis in the toroidal geometry of the stellarator, and is referred to as quasi-symmetry.
Date: April 1, 2001
Creator: Knowlton, S F
Partner: UNT Libraries Government Documents Department

Designs Studies of Low-Aspect Ratio Quasi-Omnigenous Stellarators

Description: Significant progress has been made in the development of new modest-size compact stellarator devices that could test optimization principles for the design of a more attractive reactor. These are 3 and 4 field period low-aspect-ratio quasi-omnigenous (QO) stellarators based on an optimization method that targets improved confinement, stability, ease of coil design, low-aspect-ratio, and low bootstrap current.
Date: October 15, 1998
Creator: Batchelor, D.B.; Carreras, B.A.; Hirshman, S.P.; Lynch, V.E.; Sanchez, R.; Spong, D.A. et al.
Partner: UNT Libraries Government Documents Department

Diagnostics for the National Compact Stellarator Experiment

Description: The status of planning of the National Compact Stellarator Experiment (NCSX) diagnostics is presented, with the emphasis on resolution of diagnostics access issues and on diagnostics required for the early phases of operation.
Date: September 16, 2003
Creator: Stratton, B.C.; Johnson, D.; Feder, R.; Fredrickson, E.; Neilson, H.; Takahashi, H. et al.
Partner: UNT Libraries Government Documents Department

Comparing Linear Microinstability of the National Compact Stellarator Expriment and a Shaped Tokamak

Description: One metric for comparing con nement properties of di erent magnetic fusion energy con gurations is the linear critical gradient of drift wave modes. The critical gradient scale length determines the ratio of the core to pedestal temperature when a plasma is limited to marginal stability in the plasma core. The gyrokinetic turbulence code GS2 was used to calculate critical temperature gradients for the linear, collisionless ion tem- perature gradient (ITG) mode in the National Compact Stellarator Experiment (NCSX) and a prototypical shaped tokamak, based on the pro les of a JET H-mode shot and the stronger shaping of ARIES-AT. While a concern was that the narrow cross section of NCSX at some toroidal locations would result in steep gradients that drive instabilities more easily, it is found that other stabilizing e ects of the stellarator con guration o set this so that the normalized critical gradients for NCSX are competitive with or even better than for the tokamak. For the adiabatic ITG mode, NCSX and the tokamak had similar critical gradients, though beyond marginal stability, NCSX had larger growth rates. However, for the kinetic ITG mode, NCSX had a higher critical gradient and lower growth rates until a/LT ≈#25; 1:5 a/LT;crit, when it surpassed the tokamak's. A discussion of the results presented with respect to a/LT vs R/LT is included.
Date: November 20, 2012
Creator: J.A. Baumgaertel, G.W. Hammett and D.R. Mikkelsen
Partner: UNT Libraries Government Documents Department

Toroidal Precession as a Geometric Phase

Description: Toroidal precession is commonly understood as the orbit-averaged toroidal drift of guiding centers in axisymmetric and quasisymmetric configurations. We give a new, more natural description of precession as a geometric phase effect. In particular, we show that the precession angle arises as the holonomy of a guiding center's poloidal trajectory relative to a principal connection. The fact that this description is physically appropriate is borne out with new, manifestly coordinate-independent expressions for the precession angle that apply to all types of orbits in tokamaks and quasisymmetric stellarators alike. We then describe how these expressions may be fruitfully employed in numerical calculations of precession.
Date: September 26, 2012
Creator: Burby, J. W. & Qin, H.
Partner: UNT Libraries Government Documents Department

Optimizing Stellarators for Turbulent Transport

Description: 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.
Date: May 27, 2010
Creator: Mynick, H. E.; Pomphrey, N. & Xanthopoulos, P.
Partner: UNT Libraries Government Documents Department

Modular Coils and Plasma Configurations for Quasi-axisymmetric Stellarators

Description: Characteristics of modular coils for quasi-axisymmetric stellarators that are related to the plasma aspect ratio, number of field periods and rotational transform have been examined systematically. It is observed that, for a given plasma aspect ratio, the coil complexity tends to increase with the increased number of field periods. For a given number of field periods, the toroidal excursion of coil winding is reduced as the plasma aspect ratio is increased. It is also clear that the larger the coil-plasma separation is, the more complex the coils become. It is further demonstrated that it is possible to use other types of coils to complement modular coils to improve both the physics and the modular coil characteristics.
Date: September 10, 2010
Creator: Ku, L. P. & Boozer, A. H.
Partner: UNT Libraries Government Documents Department

New Classes of Quasi-helically Symmetric Stellarators

Description: New classes of quasi-helically symmetric stellarators with aspect ratios ≤ 10 have been found which are stable to the perturbation of magnetohydrodynamic modes at plasma pressures of practical interest. These configurations have large rotational transform and good quality of flux surfaces. Characteristics of some selected examples are discussed in detail. The feasibility of using modular coils for these stellarators has been investigated. It is shown that practical designs for modular coils can be achieved.
Date: August 9, 2010
Creator: Ku, L. P.
Partner: UNT Libraries Government Documents Department

Progress Toward Attractive Stellarators

Description: The quasi-axisymmetric stellarator (QAS) concept offers a promising path to a more compact stellarator reactor, closer in linear dimensions to tokamak reactors than previous stellarator designs. Concept improvements are needed, however, to make it more maintainable and more compatible with high plant availability. Using the ARIES-CS design as a starting point, compact stellarator designs with improved maintenance characteristics have been developed. While the ARIES-CS features a through-the-port maintenance scheme, we have investigated configuration changes to enable a sector-maintenance approach, as envisioned for example in ARIES AT. Three approaches are reported. The first is to make tradeoffs within the QAS design space, giving greater emphasis to maintainability criteria. The second approach is to improve the optimization tools to more accurately and efficiently target the physics properties of importance. The third is to employ a hybrid coil topology, so that the plasma shaping functions of the main coils are shared more optimally, either with passive conductors made of high-temperature superconductor or with local compensation coils, allowing the main coils to become simpler. Optimization tools are being improved to test these approaches.
Date: January 5, 2011
Creator: Neilson, G. H.; Brown, T. G.; Gates, D. A.; Lu, K. P.; Zarnstorff, M. C.; Boozer, A. H. et al.
Partner: UNT Libraries Government Documents Department

Modular Coil Design for the Ultra-low Aspect Ratio Quasi-axially Symmetric Stellarator MHH2

Description: A family of two field-period quasi-axisymmetric stellarators generally known as MHH2 with aspect ratios of only {approx}2.5 was found. These configurations have low field ripples and excellent confinement of {alpha} particles. This discovery raises the hope that a compact stellarator reactor may eventually be designed with the property of tokamak transport and stellarator stability. In this paper we demonstrate that smooth modular coils may be designed for this family of configurations that not only yield plasmas with good physics properties but also possess engineering properties desirable for compact power producing reactors. We show designs featuring 16 modular coils with ratios of major radius to minimum coil-plasma separation {approx}5.5, major radius to minimum coil-coil separation {approx}10 and the maximum field in coil bodies to the field on axis {approx}2 for 0.2 m{sup 2} conductors. These coils is expected to allow plasmas operated at 5% {beta} with {alpha} energy loss < 10% for a reactor of major radius <9 m at 5 T.
Date: September 27, 2005
Creator: Ku LP, the ARIES-CS Team
Partner: UNT Libraries Government Documents Department

New Classes of Quasi-Axisymmetric Stellarator Configurations

Description: We have identified and developed new classes of quasi-axially symmetric configurations which have attractive properties from the standpoint of both near-term physics experiments and long-term power producing reactors. These new configurations were developed as a result of surveying the aspect ratio-rotational transform space to identify regions endowed with particularly interesting features. These include configurations with very small aspect ratios ({approx}2.5) having superior quasi-symmetry and energetic particle confinement characteristics, and configurations with strongly negative global magnetic shear from externally supplied rotational transforms so that the overall rotational transform, when combined with the transform from bootstrap currents at finite plasma pressures, will yield a small but positive shear, making the avoidance of low order rational surfaces at a given operating beta possible. Additionally, we have found configurations with NCSX-like characteristics but with the biased components in the magnetic spectrum that allow us to improve the confinement of energetic particles. For each new class of configurations, we have designed coils as well to ensure that the new configurations are realizable and engineering-wise feasible. The coil designs typically have coil aspect ratios R/{Delta}{sub min}(C-P) {le} 6 and coil separation ratios R/{Delta}{sub min}(C-C) {le} 10, where R is the plasma major radius, {Delta}{sub min}(C-P) and {Delta}{sub min}(C-C) are the minimum coil to plasma and coil to coil separations, respectively. These coil properties allow power producing reactors be designed with major radii less than 9 meters for DT plasmas with a full breeding blanket. The good quasi-axisymmetry limits the energy loss of {alpha} particles to below 10%.
Date: October 3, 2005
Creator: Ku LP, Garabedian PR
Partner: UNT Libraries Government Documents Department

Exploration of Stellarator Configuration Space with Global Search Methods

Description: An exploration of stellarator configuration space z for quasi-axisymmetric stellarator (QAS) designs is discussed, using methods which provide a more global view of that space. To this end, we have implemented a ''differential evolution'' (DE) search algorithm in an existing stellarator optimizer, which is much less prone to become trapped in local, suboptimal minima of the cost function chi than the local search methods used previously. This search algorithm is complemented by mapping studies of chi over z aimed at gaining insight into the results of the automated searches. We find that a wide range of the attractive QAS configurations previously found fall into a small number of classes, with each class corresponding to a basin of chi(z). We develop maps on which these earlier stellarators can be placed, the relations among them seen, and understanding gained into the physics differences between them. It is also found that, while still large, the region of z space containing practically realizable QAS configurations is much smaller than earlier supposed.
Date: September 10, 2001
Creator: Mynick, H.E.; Pomphrey, N. & Ethier, S.
Partner: UNT Libraries Government Documents Department

Energetic Ion Loss Diagnostic for the Wendelstein 7-AS Stellarator

Description: A diagnostic to measure the loss of energetic ions from the Wendelstein 7-AS (W7-AS) stellarator has been built. It is capable of measuring losses of both neutral beam ions and energetic ions arising from ion cyclotron resonant heating. The probe can measure losses of both clockwise and counterclockwise-going energetic ions simultaneously, and accepts a wide range of pitch angles in both directions. Initial measurements by the diagnostic are reported.
Date: December 7, 2000
Creator: Darrow, D. S.; Werner, A. & Weller, A.
Partner: UNT Libraries Government Documents Department

Neutral Beam Injection Requirements and Design Issues for the National Compact Stellarator Experiment

Description: The National Compact Stellarator Experiment (NCSX) will require 6 MW of 50 keV neutral beam injection (NBI) with initial pulse lengths of 500 msec and upgradeable to pulse lengths of 1.5 sec. This paper discusses the NCSX NBI requirements and design issues, and shows how these are provided by the candidate PBX-M [Princeton Beta Experiment-Modification] NBI system.
Date: February 11, 2002
Creator: Kugel, H. W.; Neilson, H.; Reiersen, W. & Zarnstorff, M.
Partner: UNT Libraries Government Documents Department

Drift Wave Simulations with Reduced Stellarator Equilibria

Description: A three-field model to study drift-resistive, low-frequency waves in low-beta, non-axisymmetric plasmas [J.L.V. Lewandowski, Phys. Plasmas, 4 (11) 4023 (1997)] is used to analyze the effect of the inhomogeneities in the stellarator magnetic field on the fastest (linear) growth rate, gamma. Extensive numerical calculations for a toroidal heliac show that not all Fourier components in the representation of the equilibrium configuration are important as far as gamma is concerned.
Date: December 10, 1999
Creator: Lewandowski, J.L.V.
Partner: UNT Libraries Government Documents Department

Visualization of Magnetically Confined Plasmas

Description: With the rapid developments in experimental and theoretical fusion energy research towards more geometric details, visualization plays an increasingly important role. In this paper we will give an overview of how visualization can be used to compare and contrast some different configurations for future fusion reactors. Specifically we will focus on the stellarator and tokamak concepts. In order to gain understanding of the underlying fundamental differences and similarities these two competing concepts are compared and contrasted by visualizing some key attributes.
Date: December 10, 1999
Creator: Lewandowski, J.L.V.
Partner: UNT Libraries Government Documents Department

A Hamiltonian-Free Description of Single Particle Dynamics for Hopelessly Complex Periodic Systems

Description: We develop a picture of periodic systems which does not rely on the Hamiltonian of the system but on maps between a finite number of time locations. Moser or Deprit-like normalizations are done directly on the maps thereby avoiding the complex time-dependent theory. We redefine linear and nonlinear Floquet variables entirely in terms of maps. This approach relies heavily on the Lie representation of maps introduced by Dragt and Finn. One might say that although we do not use the Hamiltonian in the normalization transformation, we are using Lie operators which are themselves, in some sense, pseudo-Hamiltonians for the maps they represent. Our techniques find application in accelerator dynamics or in any field where the Hamiltonian is periodic but hopelessly complex, such as magnetic field design in stellarators.
Date: January 1, 1990
Creator: Forest, E.
Partner: UNT Libraries Government Documents Department

Advanced Design of a Novel Stellarator Using the Free Boundary VMEC Magnetic Equilibrium Code. Final Technical Report for period March 1, 1999 - February 28, 2002

Description: This report describes the goals and accomplishments of a 3-year EPSCoR Laboratory Partnership award to design an advanced stellarator device for magnetic confinement of toroidal plasmas for fusion research.
Date: June 1, 2005
Creator: Knowlton, S. F.
Partner: UNT Libraries Government Documents Department

Criteria for Second Stability for Ballooning Modes in Stellarators

Description: An expression determining how variations in the pressure-gradient and average magnetic shear affect ballooning stability for a stellarator equilibrium is presented. The procedure for determining the marginal stability boundaries, for each field line, depends only on the equilibrium and a single ballooning eigenfunction calculation. This information is sufficient to determine if increasing pressure-gradient is stabilizing or destabilizing and to predict whether the configuration possess a second stable region.
Date: July 6, 2004
Creator: Hudson, S.R. & C.C.Hegna
Partner: UNT Libraries Government Documents Department

Gyrokinetic Studies of the Effect of Beta on Drift-wave Stability in NCSX

Description: The gyrokinetic turbulence code GS2 was used to investigate the effects of plasma β on linear, collisionless ion temperature gradient (ITG) modes and trapped electron modes (TEM) in National Compact Stellarator Experiment (NCSX) geometry. Plasma β affects stability in two ways: through the equilibrium and through magnetic fluctuations. The first was studied here by comparing ITG and TEM stability in two NCSX equilibria of differing β values, revealing that the high β equilibrium was marginally more stable than the low β equilibrium in the adiabatic-electron ITG mode case. However, the high β case had a lower kinetic-electron ITG mode critical gradient. Electrostatic and electromagnetic ITG and TEM mode growth rate dependencies on temperature gradient and density gradient were qualitatively similar. The second β effect is demonstrated via electromagnetic ITG growth rates' dependency on GS2's β input parameter. A linear benchmark with gyrokinetic codes GENE and GKV-X is also presented.
Date: September 25, 2012
Creator: J.A. Baumgaertel, G.W. Hammett, D.R. Mikkelsen, M. Nunami, and P. Xanthopoulos
Partner: UNT Libraries Government Documents Department