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- Catto, P. J., et al.
(författare)
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Kinetic effects on a tokamak pedestal ion flow, ion heat transport and bootstrap current
- 2013
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 55:4
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Tidskriftsartikel (refereegranskat)abstract
- We consider the effects of a finite radial electric field on ion orbits in a subsonic pedestal. Using a procedure that makes a clear distinction between a transit average and a flux surface average we are able to solve the kinetic equation to retain the modifications due to finite E X B drift orbit departures from flux surfaces. Our approach properly determines the velocity space localized, as well as the nonlocal, portion of the ion distribution function in the banana and plateau regimes in the small aspect ratio limit. The rapid variation of the poloidal ion flow coefficient and the electrostatic potential in the total energy modify previous banana regime evaluations of the ion flow, the bootstrap current, and the radial ion heat flux in a subsonic pedestal. In the plateau regime, the rapid variation of the poloidal flow coefficient alters earlier results for the ion flow and bootstrap current, while leaving the ion heat flux unchanged since the rapid poloidal variation of the total energy was properly retained.
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- Fülöp, Tünde, 1970, et al.
(författare)
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Ion runaway in lightning discharges
- 2013
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 111:1, s. 015006-
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Tidskriftsartikel (refereegranskat)abstract
- Runaway ions can be produced in plasmas with large electric fields, where the accelerating electric force is augmented by the low mean ionic charge due to the imbalance between the number of electrons and ions. Here we derive an expression for the high-energy tail of the ion distribution function in lightning discharges and investigate the energy range that the ions can reach. We also estimate the corresponding energetic proton and neutron production due to fusion reactions.
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- Kagan, G., et al.
(författare)
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Neoclassical Theory of Pedestal Flows and Comparison with Alcator C-Mod Measurements
- 2012
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Ingår i: Contributions to Plasma Physics. - : Wiley. - 0863-1042 .- 1521-3986. ; 52:5-6, s. 365 - 371
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Tidskriftsartikel (refereegranskat)abstract
- Neoclassical implications of the strong radial electric field, inherently present in an H-mode tokamak pedestal, are considered. The main ion poloidal flow in the pedestal is predicted to be reduced in magnitude, or even reversed, compared with its core counterpart. The resulting change in the neoclassical formula for the impurity flow is shown to result in improved agreement with boron measurements in the Alcator C-Mod pedestal. In addition, due to the ion flow being modified, the bootstrap current is expected to be enhanced in the pedestal over conventional predictions.
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- Landreman, M., et al.
(författare)
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Global delta-f neoclassical calculations in a pedestal
- 2013
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Ingår i: 55th Annual Meeting of the APS Division of Plasma Physics, Denver CO, USA, Bulletin of the American Physical Society. ; 58:16, s. PP8.00036-
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Konferensbidrag (refereegranskat)abstract
- Conventional calculations of neoclassical flows, current, and fluxes may not be valid in the pedestal since the strong gradients violate the assumed ordering, yet accurate calculation of these quantities is important for understanding edge stability and confinement. We have therefore developed a new radially global continuum neoclassical code PERFECT [1] which allows some radial scale lengths to be as small as the poloidal ion gyroradius. A strong radial electric field with strong shear is also included. In contrast to conventional neoclassical calculations, sources of particles and energy must be determined self-consistently to find the correction to the Maxwellian. The full linearized Fokker-Planck collision operator is implemented, arbitrary collisionality is allowed, and an arbitrary number of species are permitted. Efficiency is aided by a new spectral discretization for velocity space [2] and a preconditioned Krylov-space solver. At large aspect ratio, precise agreement is obtained between the code and recent analytic theory that accounts for finite orbit width effects. At realistic aspect ratio, strong poloidal asymmetries can arise in the flow, breaking the usual form for flows on a flux surface. [1] PPCF 54, 115006 (2012). [2] J Comp Phys 243, 130 (2013).
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- Landreman, M, et al.
(författare)
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Impurity flows and plateau-regime poloidal density variation in a tokamak pedestal
- 2011
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 18:9, s. 092507-
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Tidskriftsartikel (refereegranskat)abstract
- In the pedestal of a tokamak, the sharp radial gradients of density and temperature can give rise to poloidal variation in the density of impurities. At the same time, the flow of the impurity species is modified relative to the conventional neoclassical result. In this paper, these changes to the density and flow of a collisional impurity species are calculated for the case when the main ions are in the plateau regime. In this regime, it is found that the impurity density can be higher at either the inboard or outboard side. This finding differs from earlier results for banana- or Pfirsch-Schlüter-regime main ions, in which case the impurity density is always higher at the inboard side in the absence of rotation. Finally, the modifications to the impurity flow are also given for the other regimes of main-ion collisionality.
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- Landreman, M., et al.
(författare)
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Numerical calculation of the runaway electron distribution function and associated synchrotron emission
- 2014
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 185:3, s. 847-855
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Tidskriftsartikel (refereegranskat)abstract
- Synchrotron emission from runaway electrons may be used to diagnose plasma conditions during a tokamak disruption, but solving this inverse problem requires rapid simulation of the electron distribution function and associated synchrotron emission as a function of plasma parameters. Here we detail a framework for this forward calculation, beginning with an efficient numerical method for solving the Fokker-Planck equation in the presence of an electric field of arbitrary strength. The approach is continuum (Eulerian), and we employ a relativistic collision operator, valid for arbitrary energies. Both primary and secondary runaway electron generation are included. For cases in which primary generation dominates, a time-independent formulation of the problem is described, requiring only the solution of a single sparse linear system. In the limit of dominant secondary generation, we present the first numerical verification of an analytic model for the distribution function. The numerical electron distribution function in the presence of both primary and secondary generation is then used for calculating the synchrotron emission spectrum of the runaways. It is found that the average synchrotron spectra emitted from realistic distribution functions are not well approximated by the emission of a single electron at the maximum energy. © 2013 Elsevier B.V.
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| 9. |
- Landreman, M., et al.
(författare)
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Radially global δf computation of neoclassical phenomena in a tokamak pedestal
- 2014
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Ingår i: Plasma Physics and Controlled Fusion. - 1361-6587 .- 0741-3335. ; 56:4, s. 045005-
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Tidskriftsartikel (refereegranskat)abstract
- Conventional radially-local neoclassical calculations become inadequate if the radial gradient scale lengths of the H-mode pedestal become as small as the poloidal ion gyroradius. Here, we describe a radially global δf continuum code that generalizes neoclassical calculations to allow for stronger gradients. As with conventional neoclassical calculations, the formulation is time-independent and requires only the solution of a single sparse linear system. We demonstrate precise agreement with an asymptotic analytic solution of the radially global kinetic equation in the appropriate limits of aspect ratio and collisionality. This agreement depends crucially on accurate treatment of finite orbit width effects.
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