| 1. |
- Dahlin, Jon-Erik, 1977-, et al.
(författare)
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Advanced Reversed-field Pinch Scaling Laws
- 2005
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Ingår i: 32nd EPS Conference on Plasma Physics, Tarragona, Spain 27 June-1 July, P-1.118, 2005.
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Konferensbidrag (refereegranskat)abstract
- A series of resistive magnetohydrodynamic numerical simulations are performed to generate scaling laws for energy confinement time τE and poloidal beta βp for the advanced reversed field-pinch (RFP). Strongly improved scaling with basic initial parameters is obtained as compared to the conventional RFP. Early results indicate an improved scaling of τE with plasma current I and line density N compared to the conventional RFP. The improved behaviour of the advanced RFP as compared to the conventional, uncontrolled RFP stems from the introduction of current profile control (CPC). In the present numerical simulations, CPC is performed by implementation of a parameter free automatic feedback algorithm, optimised to reduce the fluctuation caused v × B electric field. The scheme introduces an ad-hoc electric field within the plasma volume, automatically adjusted to dynamically control the plasma into more quiescent behaviour by eliminating current driven tearing mode instabilities and reducing resistive interchange modes.
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| 2. |
- Dahlin, Jon-Erik, 1977-, et al.
(författare)
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Improved Computer Simulations of Energy Confinement in the Advanced Reversed-field Pinch
- 2006
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Ingår i: 33rd EPS Conference on Plasma Phys. - 9781622763337
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Konferensbidrag (refereegranskat)abstract
- A revised algorithm for numerical simulations of the advanced reversed-field pinch (RFP) is presented. The results show improved scalings of magnetic fluctuations, energy confinement time τE and poloidal beta βθ with basic initial parameters as compared to what has been presented by the authors in earlier studies of the advanced RFP. The improved behaviour of the advanced RFP stems from the introduction of current profile control (CPC), implemented through a scheme of active feedback of the electric dynamo field. The work, which has an optimistic approach and sweeps over a large parameter domain reaching into the reactor relevant region, is theoretical and claims to answer the question of how far CPC can bring the RFP concept in principle. Experimental implementation is thus a later concern. With this scheme, a state with strongly suppressed tearing mode activity is achieved, which allows for a theoretical study of pressure driven resistive g-modes. This is a task that has been very hard to perform in the past, since tearing modes have always dominated the RFP dynamics. Thus it is now possible, for the first time, to investigate whether pressure driven modes, which are persistent in the RFP, are fatal for the confinement of a high-beta RFP configuration or if they can be accepted in a future reactor.
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| 3. |
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| 4. |
- Scheffel, Jan, 1954-, et al.
(författare)
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Pressure driven resistive modes in the advanced RFP
- 2008
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Ingår i: 35th EPS Conference on Plasma Physics 2008, EPS 2008 - Europhysics Conference Abstracts. - 9781622763351 ; , s. 2014-2017
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Konferensbidrag (refereegranskat)
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| 5. |
- Scheffel, Jan, 1954-, et al.
(författare)
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Resistive g-modes and RFP confinement
- 2009
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Ingår i: 51st Annual Meeting of the APS Division of Plasma Physics, Atlanta, Georgia, USA 2-6 November, 2009.
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Konferensbidrag (refereegranskat)abstract
- The role of pressure driven resistive modes in the reversed-field pinch remains unclear. It was early shown that unstable resistive g-modes would always exist for an inwardly directed pressure gradient. It now appears that pressure profile smoothing, due to incluson of heat conductivity terms in the energy equation, enables completey stable RFP states at moderate plasma beta. These calculations, apart from being restricted to linearized perturbations, suffer from the use of rather forced scalings, thus their accuracy can be questioned. Also, they have so far only been applied to conventional RFP states, where confinement-limiting tearing fluctuations maintain the reversed axial magnetic field. In the advanced RFP, current profile control has largely eliminated current driven tering modes. Fully nonlinear, numerical studies have shown that energy confinement and poloidal beta increase substantially, but that weak residual modes usually remain. The nature of these residual modes, which limit energy confinement, is studied using a novel semi-analytical, spectral scheme for solving the resistive MHD equations; the generalized weighted residual method (GWRM). Results from the analysis as well as comparisons with the competing linear resistive g-mode theories will be presented.
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| 6. |
- Scheffel, Jan, 1954-
(författare)
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Semi-analytical solution of initial-value problems
- 2007
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Ingår i: 49th APS Division of Plasma Physics Meeting, Orlando, Florida, 12-16 November, 2007.
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Konferensbidrag (refereegranskat)abstract
- A fully spectral weighted residual method for solution of general initial value partial differential equations has been developed. All time, spatial and physical parameter domains are represented by Chebyshev series, enabling global semi-analytical solutions. The method avoids time step limitations. The spectral coefficients are determined by iterative solution of a linear or nonlinear system of algebraic equations, for which a globally convergent root solver has been developed. Accuracy is controlled by the number of included Chebyshev modes in each dimension. The computational efficiency is shown to increase through the use of sub-domains. It is shown by example that the method may be used for efficient solution of nonlinear initial value problems in fluid mechanics and magnetohydrodynamics.1J. Scheffel, ”Semi-analytical solution of initial-value problems”, TRITA-ALF-2004-03, Royal Institute of Technology, Stockholm, Sweden, 2004.
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| 7. |
- Scheffel, Jan, 1954-
(författare)
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Solution of systems of nonlinear equations - a semi-implicit approach
- 2006
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- An iterative method for globally convergent solution of nonlinear equations and systems of nonlinear equations is presented. Convergence is quasi-monotonous and approaches second order in the proximity of the real roots. The algorithm is related to semi-implicit methods, earlier being applied to partial differential equations. It is shown that the Newton-Raphson and Newton methods are special cases of the method. This relationship enables efficient solution of the Jacobian matrix equations at each iteration. The degrees of freedom introduced by the semi-implicit parameters are used to control convergence. When applied to a single equation, efficient global convergence and convergence to either of the bounding roots makes the method attractive in comparison with methods as those of Newton-Raphson and van Wijngaarden-Dekker-Brent.
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| 8. |
- Scheffel, Jan, 1954-
(författare)
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The Time- and Parameter-Generalized Weighted Residual Method
- 2008
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A general, fully spectral weighted residual method for solution of initial value partial differential equations is presented. All time, spatial and physical parameter domains are represented by Chebyshev series enabling global semi-analytical, rather than purely numerical, solutions. The method avoids time step limitations. The spectral coefficients are determined by iterative solution of a system of algebraic equations, for which a globally convergent root solver has been developed. Accuracy is controlled by the number of included Chebyshev modes and by the use of spatial subdomains. It is shown by example and by comparisons with the explicit Lax-Wendroff and semi-implicit Crank-Nicholson methods that the method may be used for accurate and efficient solution of nonlinear initial value problems in fluid mechanics and magnetohydrodynamics.
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