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  • 1. Arber, T. D.
    et al.
    Coppins, M.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Large Larmor Radius Stability of the Z-pinch1994In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 72, p. 2399-Article in journal (Refereed)
    Abstract [en]

    The linear m=0 stability of the z pinch in the collisionless, large ion Larmor radius regime is examined using the Vlasov fluid model. The results reveal a strong equilibrium dependence. The uniform current density equilibrium shows a reduction in growth rate when the average ion Larmor radius is about one-fifth of the pinch radius. However, finite Larmor radius effects cannot in themselves produce a stabilized z pinch.

  • 2. Arber, T. D.
    et al.
    Russell, P. G. F.
    Coppins, M.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Linear Stability of the High Temperature, Dense Z-pinch1995In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 74, p. 2698-Article in journal (Refereed)
    Abstract [en]

    Results are presented on the linear stability of the collisionless m=1 mode in a dense Z pinch. It is shown that a reduction in growth rate by a factor of about 10 (when compared to the zero Larmor radius result) is possible by initializing the Z pinch with a sufficiently low line density. With the completion of this work we conclude that linear, large Larmor radius effects cannot stabilize the high temperature, dense Z pinch. Such pinches will always exhibit linear m=0 or m=1 instabilities with growth times comparable to the radial Alfvén transit time.

  • 3. Bonnevier, B.
    et al.
    Bures, M.
    Drake, J. R.
    Faghihi, M.
    Haslbrunner, D.
    Hellsten, T.
    Karlsson, P.
    Lehnert, B.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Tendler, M.
    Tennfors, E.
    Wilner, B.
    Toroidal EXTRAP-T1 Device1982Report (Other academic)
  • 4. Bonnevier, B.
    et al.
    Dalhed, H. E.
    Drake, J.
    Hellsten, T.
    Karlsson, P.
    Landberg, R.
    Lehnert, B.
    Scheffel, Jan
    Tendler, E.
    Tennfors, E.
    Wilner, B.
    Equilibrium, Stability and Heating of Plasmas in Linear and Toroidal Extrap Pinches1982In: Ninth International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Baltimore, USA, 1 - 8 September, (1982), 1982, article id IAEA-CN-41/J-5Conference paper (Refereed)
  • 5. Brunsell, P.
    et al.
    Hellblom, G.
    Karlsson, P.
    Mazur, S.
    Nordlund, P.
    Scheffel, Jan
    Extrap L-1 Experimental Stability1990In: XVII European Conference on Controlled Fusion and Plasma Heating, Amsterdam, Netherlands, June 25-29, 1990, 1990, p. 610-, article id Contribute Papers IIConference paper (Refereed)
  • 6.
    Brunsell, P R
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Bergsåker, H
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Brzozowski, J H
    KTH, Superseded Departments, Alfvén Laboratory.
    Cecconello, M
    KTH, Superseded Departments, Alfvén Laboratory.
    Drake, J R
    KTH, Superseded Departments, Alfvén Laboratory.
    Malmberg, J-A
    KTH, Superseded Departments, Alfvén Laboratory.
    Scheffel, J
    KTH, Superseded Departments, Alfvén Laboratory.
    Schnack, D D
    Mode dynamics and confinement in the reversed-field pinch2000In: IAEA-CN-77: Fusion Energy 2000, 2000, p. Paper EXP3/14-Conference paper (Refereed)
  • 7.
    Brunsell, Per R.
    et al.
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Bergsåker, Henric
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Brzozowski, Jerzy
    Cecconello, Marco
    Drake, James R.
    Malmberg, Jenny-Ann
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Schnack, Dalton
    Mode Dynamics and Confinement in the Reversed-field Pinch2000In: 18th IAEA Fusion Energy Conference in Sorrento, Italy, 4-10 Oct. 2000. Paper IAEA-CN-77/EXP3/14, 2000Conference paper (Refereed)
    Abstract [en]

    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.

  • 8. Coppins, M.
    et al.
    Arber, T. D.
    Russell, P. G. F.
    Scheffel, Jan
    Recent progress on large Larmor radius theory1997In: Fourth International Conference on Dense Z-pinches, Vancouver, Canada, 28-31 May 1997, 1997, Vol. 409, p. 585-Conference paper (Refereed)
    Abstract [en]

    An overview of theoretical work on large Larmor radius stability of the z-pinch is presented, highlighting two recent innovations. Firstly, finite electron temperature has been included for the linear m=0 instability. Compared to the usual cold electron case, growth rates are increased and are closer to those of ideal MHD. Secondly, a 2-D hybrid code has been written to study the non-linear development of the m=0 instability. First results provide no evidence of instability saturation.

  • 9. Coppins, M.
    et al.
    Scheffel, Jan
    Anisotropic Z-Pinch Equilibria and Their Stability1992In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. B4, p. 3251-Article in journal (Refereed)
    Abstract [en]

    The Chew–Goldberger–Low equations are used to study the effect of pressure anisotropy on Z‐pinches operating in the collisionless regime. The limitations on the form of accessible equilibria are investigated. The effect on the m=0 instability is asessed both by means of the energy principle and by direct solution of the eigenvalue equation for a variety of anisotropicequilibria. The results indicate that in the small Larmor radius limit pressure anisotropy offers a rather slight enhancement of the linear stability of the Z‐pinch.

  • 10. Coppins, M.
    et al.
    Scheffel, Jan
    CGL Anisotropic Equilibria and Stability1989In: 2nd International Conference on High Density Pinches, Laguna Beach, CA, USA 1989, 1989, Vol. 195, p. 211-Conference paper (Refereed)
  • 11. Coppins, M.
    et al.
    Scheffel, Jan
    FIGARO-a Large Larmor Radius Stability Code1992In: International Conference on Plasma Physics, Innsbruck, Austria, 29 June - 3 July, 1992, 1992, p. 663-, article id Contributed papers IConference paper (Refereed)
  • 12.
    Dahlin, Jon-Erik
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    A novel feedback algorithm for simulating controlled dynamics and confinement in the advanced reversed-field pinch2005In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, no 6, p. 062502-Article in journal (Refereed)
    Abstract [en]

    In the advanced reversed-field pinch (RFP), the current density profile is externally controlled to diminish tearing instabilities. Thus the scaling of energy confinement time with plasma current and density is improved substantially as compared to the conventional RFP. This may be numerically simulated by introducing an ad hoc electric field, adjusted to generate a tearing mode stable parallel current density profile. In the present work a current profile control algorithm, based on feedback of the fluctuating electric field in Ohm's law, is introduced into the resistive magnetohydrodynamic code DEBSP [D. D. Schnack and D. C. Baxter, J. Comput. Phys. 55, 485 (1984); D. D. Schnack, D. C. Barnes, Z. Mikic, D. S. Marneal, E. J. Caramana, and R. A. Nebel, Comput. Phys. Commun. 43, 17 (1986)]. The resulting radial magnetic field is decreased considerably, causing an increase in energy confinement time and poloidal beta. It is found that the parallel current density profile spontaneously becomes hollow, and that a formation, being related to persisting resistive g modes, appears close to the reversal surface.

  • 13.
    Dahlin, Jon-Erik
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Advanced Reversed-field Pinch Scaling Laws2005In: 32nd EPS Conference on Plasma Physics, Tarragona, Spain 27 June-1 July, P-1.118, 2005, 2005Conference paper (Refereed)
    Abstract [en]

    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.

  • 14.
    Dahlin, Jon-Erik
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Scheffel, Jan
    KTH, Superseded Departments, Alfvén Laboratory.
    Feedback current profile control in the advanced RFP2004In: Proceedings of the 31st EPS plasma physics conference, 2004Conference paper (Refereed)
  • 15.
    Dahlin, Jon-Erik
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Improved Computer Simulations of Energy Confinement in the Advanced Reversed-field Pinch2006In: 33rd EPS Conference on Plasma Phys, 2006Conference paper (Refereed)
    Abstract [en]

    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.

  • 16.
    Dahlin, Jon-Erik
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Numerical studies of confinement scalings for the dynamo-free reversed-field pinch2007In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 47, no 1, p. 9-16Article in journal (Refereed)
    Abstract [en]

    In the reversed-field pinch (RFP), tearing modes associated with the dynamo are responsible for reduced energy- and particle confinement. In this study, it is observed that by implementing current profile control (CPC) in the RFP, a dynamo-free state can be achieved. The effect of CPC in the RFP is examined by the use of numerical simulations, and scaling laws are presented for confinement parameters. The model is nonlinear MHD in 3D including finite resistivity and pressure. A linear regression analysis is performed on simulation data from a series of computer runs for a set of initial parameter values. Scaling laws are determined for radial magnetic field, energy confinement time, poloidal beta and temperature. Confinement is improved substantially as compared with the conventional RFP - the temperature reaches reactor relevant levels by ohmic heating alone. It is observed that the configuration spontaneously develops into a quasi single helicity state. The CPC scheme is designed to eliminate the fluctuating electric dynamo field Ef ≤ -〈v × B〉, using feedback of an externally imposed electric field. The focus of this study is on obtaining principal theoretical optimization of confinement in the RFP by implementing CPC and to formulate scaling laws for confinement parameters, thus investigating the reactor viability of the concept.

  • 17.
    Dahlin, Jon-Erik
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Scaling Laws of Confinement Parameters for the Advanced Reversed-field Pinch2005In: 47th APS Division of Plasma Physics Meeting, Denver, Colorado, 24-28 October, 2005, 2005Conference paper (Refereed)
  • 18.
    Dahlin, Jon-Erik
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Scheffel, Jan
    KTH, Superseded Departments, Alfvén Laboratory.
    Self-consistent zero-dimensional numerical simulation of a Magnetized Target Fusion Configuration2004In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 70, no 5, p. 310-316Article in journal (Refereed)
    Abstract [en]

    A self-consistent zero-dimensional model of a Magnetized Target Fusion (MTF) configuration is presented. The plasma target is a Field Reversed Configuration (FRC). Model parameters were scanned using a Monte Carlo routine in order to determine an operating point that would correspond to reactor conditions. Albeit the model being intrinsically optimistic, the highest Q-values found only slightly exceed unity. The limited performance is due to the short dwell time of the liner, preventing a large portion of the fuel to burn.

  • 19.
    Dahlin, Jon-Erik
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Ultra-high beta in numerical simulations of a tearing-mode reduced reversed-field pinch2007In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 47, no 9, p. 1184-1188Article in journal (Refereed)
    Abstract [en]

    In the advanced reversed-field pinch (RFP), current profile control (CPC) enables energy confinement time and poloidal beta to increase substantially as compared with the conventional RFP due to reduced magnetic field stochasticity. Numerical simulations using the three-dimensional non-linear resistive MHD-code DEBSP are performed showing that the poloidal beta is not limited to the m ≤ 0 stability criterion βθ < 1/2. Instead, as tearing modes are diminished, it may approach unity. The beta criterion is theoretically analysed and a new, more general, criterion is derived. Analytic estimates of the resistive tearing and g-mode growth rates are derived for m ≤ 0, and it is shown that both tearing and g-mode growth rates decrease significantly as CPC is employed. Furthermore, quasi-steady state operation with increased confinement due to active control of the current profile is numerically demonstrated for the advanced RFP for a scenario with βθ < 1/2.

  • 20.
    Dahlin, Jon-Erik
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Anderson, Jay
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Numerical studies of active current profile control in the reversed-field pinch2007In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 49, no 2, p. 183-195Article in journal (Refereed)
    Abstract [en]

    Quenching of the reversed-field pinch (RFP) dynamo is observed in numerical simulations using current profile control. A novel algorithm employing active feedback of the dynamo field has been utilized. The quasi-steady state achieved represents an important improvement as compared with earlier numerical work and may indicate a direction for the design of future experiments. Both earlier and the novel schemes of feedback control result in quasi-single helicity states. The energy confinement time and poloidal beta are observed to be substantially increased, as compared with the conventional RFP, in both the cases. Different techniques for experimental implementation are discussed.

  • 21. Drake, J. R.
    et al.
    Brunsell, P.
    Brzozowski, J.
    Eninger, J.
    Hedin, E.
    Karlsson, P.
    Lehnert, B.
    Li, J.
    Scheffel, Jan
    Sätherblom, H. E.
    Tennfors, E.
    Wilner, B.
    Experimental Studies of a High-beta, Noncircular Cross-section Toroidal Pinch1989In: Plasma Physics and Controlled Nuclear Fusion Research 1988, 1989, Vol. 2, p. 751-Conference paper (Refereed)
  • 22. Drake, J. R.
    et al.
    Brunsell, P.
    Lehnert, B.
    Li, J.
    Scheffel, Jan
    Sätherblom, H. E.
    Tennfors, E.
    Wilner, B.
    ULQ Operation and Transition to RFP in Extrap T1 with a Resistive Shell1991In: Proc. of the 13th Int. IAEA Conf., Washington, D. C. (1990), Vol. II, IAEA, Vienna (1991), 1991, p. 533-Conference paper (Refereed)
  • 23. Drake, J. R.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Calculations of Toroidal Extrap Equilibria for Different Toroidal Ring-Current Configurations1985Report (Other academic)
  • 24. Elevant, T.
    et al.
    Hellblom, G.
    Holmqvist, B.
    Malmskog, S.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    A Proposed Neutron Spectrometer System for JET1980Report (Other academic)
  • 25. Elevant, T.
    et al.
    Lindén, P.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Brelén, H.
    Profile Measurements in ITER by means of Neutron Spectroscopy, Ion Temperature Diagnostics for Experimental Thermonuclear Fusion Reactors1996In: Workshop on diagnostics for ITER, Varenna, September 1995, proceedings of the International School of Plasma Physics, Edited by P. E. Stott et.al.,  Plenum Press, New York, 1996, 1996, p. 445-450Conference paper (Refereed)
    Abstract [en]

    Information on ion temperature profiles will be needed for burn optimisation and transport studies in ITER. The feasibility of deriving these profiles for the core plasma (r< 0.75a) directly from the width of measured 14-MeV neutron energy spectra is demonstrated for Maxwellian ion distributions. Neutron energy spectra and fluxes generated under different heating conditions are calculated by means of Monte-Carlo technique. The computation takes the reaction kinematics and the velocity distributions of the reacting ions into account and calculates the resulting neutron energy distribution and flux into a defined collimator. Energy spectra of neutrons emitted along a line-of-sight are superimposed. The associated correction factor, which depends on the measured ion temperature, can be given an analytical form when fitted to code data and is insensitive to large variations in temperature-, density and magnetic flux profile shapes. The accuracy in ion temperature evaluation is expected to be better than ± 10% and can be improved to ± 5% provided information on fuel density profiles are made available.

  • 26. Elevant, T.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Role of Neutron Spectrometers for ITER1998In: Proc. of the Int. School of Plasma Physics "Piero Calderola", Workshop on Diagnostics for Experimental Fusion Reactors, held September 4-12, 1997, in Varenna, Italy. Edited by Stott et al, Plenum Press, New York, 1998, 1998Conference paper (Refereed)
    Abstract [en]

    Conditions for retrieval of ion temperature profiles through measurement and evaluation of neutron energy spectra emitted in ITER-like fusion plasmas are discussed. Spectra from deuterium- and deuterium-tritium plasmas heated by 100 MW, 1 MeV neutral deuterium beams, and ignited H-mode plasmas are simulated by means of a Monte-Carlo Fokker-Planck code. Regions are defined where information on ion temperature clearly is preserved. The correction from line-of-sight integrated measurement to peak temperature is found to be 4–10% for deuterium plasmas of all temperatures and for deuterium-tritium plasmas of temperatures below approximately 5 keV. For higher temperatures the correction equals 10–30% depending on the density and temperature profiles. Ion temperatures can be determined with an accuracy better than 10% in this region only if profile data is made available.

    Several spectrometers have been proposed. Operating ranges in ion temperature and beam power, spatial- and temporal resolution, space requirements and life expectancy are compared with ITER measurement requirements. Necessary R&D work is also discussed.

  • 27. Elevant, Thomas
    et al.
    Brelén, Hans
    Lindén, P.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Ion Temperature Profile Measurements in ITER by means of Neutron Spectroscopy1997In: Fusion technology, ISSN 0748-1896, Vol. 32, p. 304-Article in journal (Refereed)
  • 28. Faghihi, M.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Stability of Small Axial Wavelength Internal Kink Modes of an Anisotropic Plasma1987Report (Other academic)
  • 29. Faghihi, M.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Stability of Small Axial Wavelength Internal Kink Modes of an Anisotropic Plasma1987In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 38, p. 495-Article in journal (Refereed)
    Abstract [en]

    The double adiabatic equations are used to study the stability of a cylindrical Z-pinch with respect to small axial wavelength, internal kink (m ≥ 1) modes. It is found that marginally (ideally) unstable, isotropic equilibria are stabilized. Also, constant-current-density equilibria can be stabilized for P⊥ > P∥ and large β⊥

  • 30. Faghihi, M.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Stability of Small Axial Wavelength Internal Kink Modes of an Anisotropic Plasma: Corrigendum1988In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 40, p. 603-Article in journal (Refereed)
    Abstract [en]

    A minor correction, having no major influence on our results, is reported here.

  • 31. Faghihi, M.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Spies, G. O.
    Instability due to Magnetic Viscosity in Perpendicular MHD1986Report (Other academic)
  • 32. Faghihi, M.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Spies, G. O.
    Stability of the Thermodynamic Equilibrium: A Test of the Validity of Dynamic Models as Applied to Gyroviscous Perpendicular Magnetohydrodynamics1988In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, p. 1291-Article in journal (Refereed)
    Abstract [en]

    Stability of the thermodynamic equilibrium is put forward as a simple test of the validity of dynamic equations, and is applied to perpendicular gyroviscous magnetohydrodynamics (i.e., perpendicular magnetohydrodynamics with gyroviscosity added). This model turns out to be invalid because it predicts exponentially growing Alfvén waves in a spatially homogeneous static equilibrium with scalar pressure.

  • 33. Haines, M. G.
    et al.
    Dangor, A.
    Choi, P.
    Coppins, M.
    Bayley, J. M.
    Arber, T.
    Chittenden, J. P.
    Mitchell, I. H.
    Niffikeer, S.
    McCall, G. H.
    Scheffel, Jan
    The Fusion-Related Dense Z-Pinch Programme at Imperial College1990In: Proc.of 13th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Washington D.C., 1-6 Oct. 1990,  IAEA-CN-I-2-3, 1990Conference paper (Refereed)
  • 34. Haines, M. G.
    et al.
    Dangor, A. E.
    Coppins, M.
    Choi, P.
    Mitchell, I. H.
    Chittenden, J. P.
    Bayley, J. M.
    Aliaga-Rossel, R. F.
    Arber, T. D.
    Beg, F.
    Bell, A. R.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Decker, G.
    Russell, P.
    Worley, J. F.
    Fiber Z-pinch Experiments and Calculations in the Finite Larmor Radius Regime1996In: Laser and particle beams (Print), ISSN 0263-0346, E-ISSN 1469-803X, Vol. 14, p. 261-271Article in journal (Refereed)
    Abstract [en]

    The dense Z-pinch project at Imperial College is aimed at achieving radiative collapse to high density in a hydrogen plasma, and also to study plasmas close to controlled fusion conditions. To this end, the MAGPIE generator (2.4 MV, 1.25, and 200 ns) has been built and tested, and is now giving preliminary experimental data at 60% of full voltage for carbon and CD2 fibers. These discharges are characterized by an initial radial expansion followed by the occurrence of m = 0 structures with transient X-ray emission from bright spots. Late in the discharge a disruption can occur, accompanied by hard X-ray emission from the anode due to an energetic electron beam and, in the case of CD2 fibers, a neutron burst. Concomitant theoretical studies have solved the linear stability problem for a Z-pinch with large ion Larmor radii, showing that a reduction in growth rate of m = 0 and m = 1 modes to about 20% of the magnetohydrodynamic (MHD) value can occur for a parabolic density profile when the Larmor radius is optimally 20% of the pinch radius. Two dimensional MHD simulations of Z-pinches in two extremes of focussed short-pulse laserplasma interactions and of galactic jets reveal a nonlinear stabilizing effect in the presence of sheared flow. One-dimensional simulations show that at low line density the lower hybrid drift instability can lead to coronal radial expansion of a Z-pinch plasma.

  • 35. Hellsten, T.
    et al.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Continuous Double Adiabatic Spectrum in Toroidal Plasmas1983Report (Other academic)
  • 36.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. JET Joint Undertaking, Oxfordshire OX14 3EA, UK.
    Scheffel, Jan
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Continuous Double Adiabatic Spectrum in Toroidal Plasmas1984In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 30, p. 78-Article in journal (Refereed)
    Abstract [en]

    The continuous spectrum of an anisotropic and axisymmetric toroidal plasma is investigated using the double adiabatic theory. The continuum is given by an eigenvalue problem of a fourth order system of ordinary differential equations. In contrast to the magnetohydrodynamic continuum the double adiabatic continuum may become unstable. The stability depends upon the parallel and perpendicular pressure distributions along the field lines. In absence of a toroidal magnetic field, the fourth order system decouples into two second order differential equations for which specific stability criteria are derived.

  • 37. Lehnert, B.
    et al.
    Scheffel, Jan
    Comments on the Theory of the Rayleigh-Taylor Instability in the Limit of Large Larmor Radii1988In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 61, p. 897-Article in journal (Refereed)
    Abstract [en]

    In recent investigations Hassam and Huba as well as Huba, Lyon, and Hassam have reconsidered the theory on the Rayleigh-Taylor flute-type instability in the limit of large Larmor radius. They have performed a numerical simulation starting from a set of modified MHD equations. The deductions made by the authors on the basis of these equations seem consistent. However, the MHD equations themselves are inconsistent, making the resulting conclusions questionable indeed. 

  • 38. Lehnert, B.
    et al.
    Scheffel, Jan
    On Large Debye Distance Effects in a Fully Ionized Plasma1988In: XV European Conference on Controlled Fusion and Plasma Heating, Dubrovnik, Yugoslavia, May 16-20, 1988, 1988, p. 1241-, article id Contributed Paper IIIConference paper (Refereed)
  • 39. Lehnert, B.
    et al.
    Scheffel, Jan
    KTH, Superseded Departments, Alfvén Laboratory.
    On the minimum elementary charge of an extended electromagnetic theory2002In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 65, no 3, p. 200-207Article in journal (Refereed)
    Abstract [en]

    Steady axisymmetric equilibria of an earlier developed extended electromagnetic theory are considered. as based on a nonzero electric field divergence in the vacuum state and Lorentz invariance. The general solutions are derived from a generating function which yields electrically charged particle-like states when its radial part diverges at the origin. For a corresponding electron model, the integrated charge, magnetic moment, mass (energy), and angular momentum still become nonzero and finite, provided that the characteristic radius is made to shrink to that of a point-charge-like state. This removes the problem of an infinite self-energy, and presents a possible alternative to the conventional renormalization process. With the subsidiary quantum conditions on magnetic moment, angular momentum, and magnetic flux. a variational analysis is applied to find an extremum of the electronic charge. The resulting minimum value deviates only by about 3 percent from that of the experimentally determined elementary charge, and it depends only on the velocity of light and Planck's constant. This indicates that the electronic charge may no longer be considered as an independent constant of nature. but can be deduced in terms of these two constants. There could be several possible explanations of the obtained deviation, of which the most probable appears to be due to refinements caused by quantization of the field equations already from the outset.

  • 40. Lehnert, B.
    et al.
    Scheffel, Jan
    On the Minimum Elementary Charge of an Extended Electromagnetic Theory2001Report (Other academic)
  • 41. Lehnert, B.
    et al.
    Scheffel, Jan
    Paramagnetism and Plasma Beta in a Screw-Pinch1991Report (Other academic)
  • 42. Lehnert, B.
    et al.
    Scheffel, Jan
    Parameter Limits of Screw-Pinch Equilibria1992In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 34, p. 1113-1131Article in journal (Refereed)
    Abstract [en]

    Anisotropic resistivity causes paramagnetic effects (Bz´(r) < 0) in a screw pinch, being basically different to the self-relaxation in reversed field-pinches. We compute, analytically and numerically, the resulting influence on the plasma radius and on plasma beta in a straight cylindrical plasma. The results seem to indicate that D-D screw-pinch reactors which depend on safety factors q(a) > 1 are unattainable. Diamagnetism caused by radial particle diffusion and the Nernst effect is also discussed. In a Tokamak or reactor plasma, diamagnetism is shown to be negligible, whereas it may contribute in present ultra-low q, Extrap and RFP experiments.

    A basic relation is derived for the coupling betwecn the poloidal and axial magnetic field components with the above effects included. Of specific importance to the Extrap programme is the result that plasma current limitation can arise due to the lack equilibrium when the plasma radius tends to exceed its upper limit, which is defined by a magnetic or material limiter. Comparisons are made with stability limits depending on the safety factor, the number of contained ion Larmor radii and the line density. 

  • 43. Li., J.
    et al.
    Scheffel, Jan
    Boundary Larmor Radius Effect on Electrostatic Waves1991In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. B3, p. 3013-Article in journal (Refereed)
    Abstract [en]

    The linearized Vlasov–Poisson equations, which combine to an integrodifferential equation for the perturbed electric potential, are used to investigate the effect of finite plasma size on the stability of electrostatic waves in a homogeneous plasma slab. The distortion of the gyromotion of the particles at the plasma boundary influences wave stability, a phenomenon termed the boundary Larmor radius (BLR) effect. The integrodifferential equation, treated as an eigenvalue problem, is discretized into a matrix dispersion equation by use of the Galerkin method and is then solved numerically. It is found that the ion Bernstein wave,which is undamped in an infinite homogeneous plasma, now becomes damped with a maximum damping rate of 0.35 ωci at rG/L (ion Larmor radius over wall distance)≊0.15. In general, the damping is less pronounced at shorter perpendicular wavelengths. It implies a necessity to take into account the BLR effect in the kinetic stability studies for sufficiently large ion Larmor radius in comparison to the characteristic dimension.

  • 44. Li, J.
    et al.
    Scheffel, Jan
    Boundary Larmor Radius Effect on Electrostatic Waves1991In: International Sherwood Fusion Conference, April 22-24, 1991, Seattle, Washington, 1991, article id Paper 3C34Conference paper (Refereed)
  • 45. Li, J.
    et al.
    Scheffel, Jan
    Boundary Larmor Radius Effects1990In: XVII European Conference on Controlled Fusion and Plasma Heating, Amsterdam, Netherlands, June 25-29, 1990, 1990, p. 1836-, article id Contributed papers IVConference paper (Refereed)
  • 46. Li, J.
    et al.
    Scheffel, Jan
    Boundary Larmor Radius Effects on Electrostatic Waves1990Report (Other academic)
  • 47. Li, J.
    et al.
    Scheffel, Jan
    Kinetic High-beta Equilibria and Stability in a Plasma Slab1991In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. B3, p. 2506-Article in journal (Refereed)
    Abstract [en]

    Self‐consistent, high‐beta kinetic equilibria are derived from the Vlasov equation for a plasma slab in a nonzero magnetic field with a density gradient and a gravitational field. It is shown that a magnetic field inhomogeneity and an electric field arise as necessary consequences of the high‐beta state. Hence the corresponding equilibrium forces should not be neglected in calculations of individual particle motion. Explicit expressions are derived for equilibrium quantities, useful, e.g., for large Larmor radius (LLR) studies of the Rayleigh–Taylor instability.

  • 48. Li, J.
    et al.
    Scheffel, Jan
    Kinetic High-beta Equilibria and Stability in a Plasma Slab1990Report (Other academic)
  • 49.
    Lindvall, Kristoffer
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    A time-spectral method for initial-value problems using a novel spatial subdomain scheme2018In: COGENT MATHEMATICS, ISSN 2331-1835, Vol. 5, no 1, article id 1529280Article in journal (Refereed)
    Abstract [en]

    We analyse a novel subdomain scheme for time-spectral solution of initial-value partial differential equations. In numerical modelling spectral methods are commonplace for spatially dependent systems, whereas finite difference schemes are typically applied for the temporal domain. The Generalized Weighted Residual Method (GWRM) is a fully spectral method that spectrally decomposes all specified domains, including the temporal domain, using multivariate Chebyshev polynomials. The Common Boundary-Condition method (CBC) here proposed is a spatial subdomain scheme for the GWRM. It solves the physical equations independently from the global connection of subdomains in order to reduce the total number of modes. Thus, it is a condensation procedure in the spatial domain that allows for a simultaneous global temporal solution. It is here evaluated against the finite difference methods of Crank-Nicolson and Lax-Wendroff for two example linear PDEs. The CBC-GWRM is also applied to the linearised ideal magnetohydrodynamic (MHD) equations for a screw pinch equilibrium. The growth rate of the most unstable mode was efficiently computed with an error <0.1%.

  • 50.
    Lindvall, Kristoffer
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Can the Time-Spectral Method GWRM Advance Fusion Transport Modelling?2017In: 59th Annual Meeting of the APS Division of Plasma Physics, 2017Conference paper (Refereed)
    Abstract [en]

    Transport phenomena in fusion plasma pose a daunting task for both real-time experiments and numerical modelling. The transport is driven by micro-instabilities caused by a host of unstable modes, for example ion temperature gradient and trapped electron modes. These modes can be modelled using fluid or gyrokinetic equations. However, the equations are characterised by high degrees of freedom and high temporal and spatial numerical requirements. Thus, a time-spectral method GWRM has been developed in order to efficiently solve these multiple time scale equations. The GWRM assumes a multivariate Chebyshev expansion ansatz in time, space, and parameter domain. Advantages are that time constraining CFL criteria no longer apply and that the solution accurately averages over small time-scale dynamics. For benchmarking, a two-fluid 2D drift wave turbulence model has been solved in order to study toroidal ion temperature gradient growth rates and nonlinear behaviour.

123 1 - 50 of 135
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