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- Brunsell, Per R., et al.
(författare)
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Mode Dynamics and Confinement in the Reversed-field Pinch
- 2000
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Ingår i: 18<sup>th</sup> IAEA Fusion Energy Conference in Sorrento, Italy, 4-10 Oct. 2000. Paper IAEA-CN-77/EXP3/14.
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Konferensbidrag (refereegranskat)abstract
- Tearing mode dynamics and toroidal plasma flow in the RFP has been experimentally studied in the Extrap T2 device. A toroidally localised, stationary magnetic field perturbation, the ``slinky mode'' is formed in nearly all discharges. There is a tendency of increased phase alignment of different toroidal Fourier modes, resulting in higher localised mode amplitudes, with higher magnetic fluctuation level. The fluctuation level increases slightly with increasing plasma current and plasma density. The toroidal plasma flow velocity and the ion temperature has been measured with Doppler spectroscopy. Both the toroidal plasma velocity and the ion temperature clearly increase with I/N. Initial, preliminary experimental results obtained very recently after a complete change of the Extrap T2 front-end system (first wall, shell, TF coil), show that an operational window with mode rotation most likely exists in the rebuilt device, in contrast to the earlier case discussed above. A numerical code DEBSP has been developed to simulate the behaviour of RFP confinement in realistic geometry, including essential transport physics. Resulting scaling laws are presented and compared with results from Extrap T2 and other RFP experiments.
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| 3. |
- Scheffel, Jan, 1954-
(författare)
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A new method for obtaining analytical eigenfunctions and growth rates
- 2001
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Ingår i: 28th EPS Conference on Controlled Fusion and Plasma Physics, Madeira, 18-22 June 2001, P4.106, p. 516.
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Konferensbidrag (refereegranskat)abstract
- By use of computer math programs, the MHD stability properties of fusion confinement plasmas can now be determined analytically, without further approximations, to high orders in time and space coordinates. We here present results from expanding the linearized ideal and resistive MHD eigenvalue equations for cylinder geometry in radius r. The effect of sheared fluid flow on stability is also included. Not only analytical growth rates, but also analytical forms of the eigenfunctions are obtained. As expected, we find that moderately large expansions provide high accuracy for mildly localized eigenfunctions.The method, for linearized eigenvalue problems, is based on expansions of the dependent perturbed variables in powers of the spatial independent variables; e.g. ur, uq, uz, br, bq, bz as functions of rn, with n ≤ nmax, the maximum order of the expansion. The basic, scalar MHD equations can thus be retained without further elimination of variables. The equilibrium state is also expanded in r, retaining desired control coefficients.By successively solving the MHD equations at higher orders of r, solutions being exact up to this order are obtained. The indicial equation (at r = 0) is automatically solved in this method. By applying boundary conditions at r = rwall, an implicit equation for the eigenvalue w as function of the mode numbers (m,k) and the equilibrium parameters is obtained. This is similar to the often used, however numerical, shooting method. The number of solutions wi are, as implied by the oscillation theorem, dependent on nmax. The roots are obtained by implicit root solvers or graphically. Compact analytical forms of eigenvalues and eigenfunctions can also be obtained by retaining dominating terms in specified parameter limits.Our method can be used for any geometry, and also for nonlinear systems of equations. It thus has a very large potential for efficient and exact analytical investigations of (one- or many-fluid) MHD equations, also including additional physical effects. The calculations require efficient math program routines and high speed and memory computer platforms, wherefore they were difficult to perform earlier.
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| 6. |
- Scheffel, Jan, 1954-, et al.
(författare)
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Energy Confinement in the Advanced RFP
- 2003
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Ingår i: 45th Annual Meeting of the Division of Plasma Physics; Albuquerque, New Mexico, USA, 27-31 October 2003.
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Konferensbidrag (refereegranskat)abstract
- In earlier numerical studies [1,2] of confinement in the optimized, conventional reversed-field pinch (RFP), the scaling of energy confinement time with plasma current and density was found to be too weak to lead into fusion relevant regimes. In the advanced RFP, however, the detrimental magnetic (dynamo) fluctuations are largely eliminated by the presence of an externally applied electric field. This field is adjusted to generate a tearing mode stable parallel current density profile. Previous studies [3,4] used a gaussian shaped electric field with given width and amplitude that was localised at some minor radius of the plasma. A threefold increase in energy confinement was found, but the three associated parameters made further optimisation difficult. In the present work a new, parameter free scheme for current profile control is introduced. An automatic control system continuously replaces the dynamo electric field. Early results indicate strong energy confinement enhancement.[1] J. Scheffel and D. D. Schnack, Phys. Rev. Lett. 85 (2000) 322.[2] J. Scheffel and D. D. Schnack, Nucl. Fusion 40 (2000) 1885.[3] C. R. Sovinec and S. C. Prager, Nucl. Fusion 39 (1999) 777.[4] J. Scheffel and D. D. Schnack, International RFP Workshop, Stockholm 2002.
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| 8. |
- Scheffel, Jan, 1954-
(författare)
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Kan människan agera rationellt?
- 2001
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Ingår i: Forskning & Framsteg, bokbilaga. ; 5
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Livets ständiga valsituationer utgör vårt tyngsta ansvar. Vårt agerande och våra tankar om det samlar vi till en moral. Men ett organiserat handlande kan aldrig bli helt konsekvent.
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| 9. |
- Scheffel, Jan, 1954-
(författare)
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Ohanterbara problem
- 2002
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Ingår i: Forskning & Framsteg. ; 4, s. 42-45
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Finns det problem som kräver så mycket beräkningar att inte ens datorer kan lösa dem? Ja. Och inte ens framtidens datorer kommer att kunna rå på så kallade intraktabla problem, trots att de kan vara lätta att formulera. Problem kan vara så komplicerade att mänskligheten inte har resurser att lösa dem. Med datorernas hjälp har lösningar till mycket komplicerade problem kunnat hittas.Väderleksprognoser är ett exempel på enormt datorkrävande problem som skulle vara hopplösa utan datorer. Men det finns uppgifter som vi faktiskt inte kan få hjälp att lösa, vare sig nu eller i framtiden - inte för att de är principiellt olösliga, utan för att de är oerhört svåra att hantera.
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