W. Kerner
Max Planck Society
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Featured researches published by W. Kerner.
Nuclear Fusion | 1999
J.G. Cordey; B. Balet; D.V. Bartlett; R.V. Budny; J.P. Christiansen; G. D. Conway; Lars-Gunnar Eriksson; G.M. Fishpool; C. Gowers; J.C.M. de Haas; P.J. Harbour; L. D. Horton; A. Howman; J.J. Jacquinot; W. Kerner; C.G. Lowry; R.D. Monk; P. Nielsen; E. Righi; F. Rimini; G. Saibene; R. Sartori; B. Schunke; A. C. C. Sips; R.J. Smith; M. Stamp; D.F.H. Start; K. Thomsen; B.J.D. Tubbing; M von Hellermann
The scaling of the energy confinement in H mode plasmas with different hydrogenic isotopes (hydrogen, deuterium, DT and tritium) is investigated in JET. For ELM-free H modes the thermal energy confinement time τth is found to decrease weakly with the isotope mass (τth ~M-0.25±0.22), whilst in ELMy H modes the energy confinement time shows practically no mass dependence (τth ~M0.03±0.1). Detailed local transport analysis of the ELMy H mode plasmas reveals that the confinement in the edge region increases strongly with the isotope mass, whereas the confinement in the core region decreases with mass (τthcore ∝ M-0.16), in approximate agreement with theoretical models of the gyro-Bohm type (τgB ~M-0.2).
Computer Physics Communications | 1981
R. Gruber; F. Troyon; D. Berger; L. C. Bernard; S. Rousset; R. Schreiber; W. Kerner; W. Schneider; K.V. Roberts
Reference CRPP-ARTICLE-1981-020doi:10.1016/0010-4655(81)90013-8View record in Web of Science Record created on 2008-04-16, modified on 2017-05-12
Nuclear Fusion | 1999
M. Keilhacker; A. Gibson; C. Gormezano; P. Lomas; P.R. Thomas; M.L. Watkins; P. Andrew; B. Balet; D. Borba; C. Challis; I. Coffey; G.A. Cottrell; H.P.L. de Esch; N. Deliyanakis; A. Fasoli; C. Gowers; H.Y. Guo; G. Huysmans; T.T.C. Jones; W. Kerner; R. König; M.J. Loughlin; A. Maas; F.B. Marcus; M. F. F. Nave; F. Rimini; G. Sadler; S. E. Sharapov; G. Sips; P. Smeulders
High fusion power experiments using DT mixtures in ELM-free H mode and optimized shear regimes in JET are reported. A fusion power of 16.1 MW has been produced in an ELM-free H mode at 4.2 MA/3.6 T. The transient value of the fusion amplification factor was 0.95±0.17, consistent with the high value of nDT(0)τEdiaTi(0) = 8.7 × 1020±20% m-3 s keV, and was maintained for about half an energy confinement time until excessive edge pressure gradients resulted in discharge termination by MHD instabilities. The ratio of DD to DT fusion powers (from separate but otherwise similar discharges) showed the expected factor of 210, validating DD projections of DT performance for similar pressure profiles and good plasma mixture control, which was achieved by loading the vessel walls with the appropriate DT mix. Magnetic fluctuation spectra showed no evidence of Alfvenic instabilities driven by alpha particles, in agreement with theoretical model calculations. Alpha particle heating has been unambiguously observed, its effect being separated successfully from possible isotope effects on energy confinement by varying the tritium concentration in otherwise similar discharges. The scan showed that there was no, or at most a very weak, isotope effect on the energy confinement time. The highest electron temperature was clearly correlated with the maximum alpha particle heating power and the optimum DT mixture; the maximum increase was 1.3±0.23 keV with 1.3 MW of alpha particle heating power, consistent with classical expectations for alpha particle confinement and heating. In the optimized shear regime, clear internal transport barriers were established for the first time in DT, with a power similar to that required in DD. The ion thermal conductivity in the plasma core approached neoclassical levels. Real time power control maintained the plasma core close to limits set by pressure gradient driven MHD instabilities, allowing 8.2 MW of DT fusion power with nDT(0)τEdiaTi(0) ≈ 1021 m-3 s keV, even though full optimization was not possible within the imposed neutron budget. In addition, quasi-steady-state discharges with simultaneous internal and edge transport barriers have been produced with high confinement and a fusion power of up to 7 MW; these double barrier discharges show a great potential for steady state operation.
Solar Physics | 1989
Stefaan Poedts; Marcel Goossens; W. Kerner
The heating of coronal loops by resonant absorption of Alfvén waves is studied in compressible, resistive magnetohydrodynamics. The loops are approximated by straight cylindrical, axisymmetric plasma columns and the incident waves which excite the coronal loops are modelled by a periodic external driver. The stationary state of this system is determined with a numerical code based on the finite element method. Since the power spectrum of the incident waves is not well known, the intrinsic dissipation is computed. The intrinsic dissipation spectrum is independent of the external driver and reflects the intrinsic ability of the coronal loops to extract energy from incident waves by the mechanism of resonant absorption.The numerical results show that resonant absorption is very efficient for typical parameter values occurring in the loops of the solar corona. A considerable part of the energy supplied by the external driver, is actually dissipated Ohmically and converted into heat. The heating of the plasma is localized in a narrow resonant layer with a width proportional to η1/3. The energy dissipation rate is almost independent of the resistivity for the relevant values of this parameter. The efficiency of the heating mechanism and the localization of the heating strongly depend on the frequency of the external driver. Resonant absorption is extremely efficient when the plasma is excited with a frequency near the frequency of a so-called ‘collective mode’.
Nuclear Fusion | 1999
E. Righi; D.V. Bartlett; J.P. Christiansen; G. D. Conway; J.G. Cordey; Lars-Gunnar Eriksson; H.P.L. de Esch; G.M. Fishpool; C. Gowers; J.C.M. de Haas; P.J. Harbour; N. Hawkes; J. Jacquinot; T.T.C. Jones; W. Kerner; Q.A. King; C.G. Lowry; R.D. Monk; P. Nielsen; F. Rimini; G. Saibene; R. Sartori; B. Schunke; A. C. C. Sips; R.J. Smith; M. Stamp; D.F.H. Start; K. Thomsen; B.J.D. Tubbing; N. Zornig
Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.
The Astrophysical Journal | 1990
Stefaan Poedts; Marcel Goossens; W. Kerner
The heating of solar coronal loops by resonant absorption of Alfven waves is investigated in the framework of linearized compressible resistive MHD. The resonant absorption of the waves incident on the coronal loops is numerically simulated in straight cylindrical, axisymmetric loop models externally excited by a periodic source. The stationary state of this driven system and the ohmic dissipation rate in this state are determined by a very accurate code based on the finite element technique. The efficiency of the heating mechanism and the energy deposition profile in this stationary state strongly depend on the characteristics of both the external driver and the equilibrium. It is shown that resonant absorption is very efficient for typical coronal loops as a considerable part of the energy supplied by the external source is actually dissipated ohmically and converted into heat. The heating rate is proportional to the square of the magnitude of the background magnetic field. 19 refs.
Journal of Computational Physics | 1985
Douglas S. Harned; W. Kerner
Abstract A semi-implicit method for solving the full compressible magnetohydrodynamic equations in three dimensions is presented. The method is unconditionally stable with respect to the fast compressional modes. The time step is limited instead by the slower shear Alfven motion. The computing time required for one time step is essentially the same as for explicit methods. Linear stability limits are derived and verified by three-dimensional tests on linear waves in slab geometry.
Computer Physics Communications | 1998
S. D. Pinches; L. C. Appel; J. Candy; S. E. Sharapov; H. L. Berk; D. Borba; Boris N. Breizman; T. C. Hender; K I Hopcraft; G. Huysmans; W. Kerner
Abstract The problem of modelling the self-consistent interaction of an energetic particle ensemble with a wave spectrum specific to magnetically confined plasmas in a torus is discussed. Particle motion in a magnetic field coordinate system, whose surfaces are perturbed by a spectrum of finite amplitude magnetohydrodynamical (MHD) waves, is described using a Hamiltonian formulation. Employing the δƒ method enables the simulation particles to only represent the change in the total particle distribution function and consequently possesses significant computational advantages over standard techniques. Changes to the particle distribution function subsequently affect the wave spectrum through wave-particle interactions. The model is validated using large aspect-ratio asymptotic limits as well as through a comparison with other numerical work. A consideration of the Kinetic Toroidal Alfven Eigenmode instability driven by fusion born α -particles in a D-T JET plasma illustrates a use of the code and demonstrates nonlinear saturation of the instability, together with the resultant redistribution of particles both in energy and across the plasma cross section.
Physics of Plasmas | 1994
Francesco Porcelli; R. Stankiewicz; W. Kerner; H. L. Berk
The response of a collisionless plasma to global electromagnetic perturbations of an axisymmetric toroidal equilibrium is derived. By adopting a variational formulation for guiding center motion, the perturbed distribution function is expressed in terms of the linearized guiding center Lagrangian. Finite orbit widths are retained. In particular, the high particle energy limit where mirror‐trapped banana orbits are distorted into ‘‘potato‐shaped’’ orbits is considered. In this limit, the time scales associated with the drift and bounce motions of a mirror‐trapped orbit become comparable, yielding important consequences on plasma stability. Quadratic forms are constructed in the context of kinetic‐magnetohydrodynamic (MHD) models of plasmas composed of a thermal component obeying fluid‐like equations and a high‐energy component described in terms of the collisionless drift‐kinetic equation. Relevant applications include improved modeling of energetic ion effects on toroidicity‐induced Alfven gap modes and i...
Computer Physics Communications | 1981
R. Gruber; S. Semenzato; F. Troyon; T. Tsunematsu; W. Kerner; P. Merkel; Wolfgang Schneider
Abstract The MHD stability code ERATO has been modified in order to study helically symmetric equilibria. The resulting new code is called HERA. A new formulation to calculate the vacuum contribution has been added to the spectral codes ERATO and HERA. The finite hybrid element approach is equivalent to that in the plasma region. The stability problem including a conducting wall is now done in one step. The fast angular variation of high n modes has been eliminated by a change of variables. With this new quasi-mode representation it is possible to study unstable modes with nq values up to 1000 in the case of high β, high shear equilibria. It applies equally well to internal and external modes.