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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- 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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| 8. |
- 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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| 12. |
- Stahl, Adam, 1985, et al.
(författare)
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Kinetic modelling of runaway electrons in dynamic scenarios
- 2016
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 56:11, s. 112009-
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Tidskriftsartikel (refereegranskat)abstract
- Improved understanding of runaway-electron formation and decay processes are of prime interest for the safe operation of large tokamaks, and the dynamics of the runaway electrons during dynamical scenarios such as disruptions are of particular concern. In this paper, we present kinetic modelling of scenarios with time-dependent plasma parameters; in particular, we investigate hot-tail runaway generation during a rapid drop in plasma temperature. With the goal of studying runaway-electron generation with a self-consistent electric-field evolution, we also discuss the implementation of a conservative collision operator and demonstrate its properties. An operator for avalanche runaway-electron generation, which takes the energy dependence of the scattering cross section and the runaway distribution into account, is investigated. We show that the simpler avalanche model of Rosenbluth & Putvinskii [Nucl. Fusion 37, 1355 (1997)] can give very inaccurate results for the avalanche growth rate (either lower or higher) for many parameters, especially when the average runaway energy is modest, such as during the initial phase of the avalanche multiplication. The developments presented pave the way for an improved modelling of runaway-electron dynamics during disruptions or other dynamic events.
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| 13. |
- Stahl, Adam, 1985, et al.
(författare)
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NORSE: A solver for the relativistic non-linear Fokker-Planck equation for electrons in a homogeneous plasma
- 2017
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 212, s. 269-279
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Tidskriftsartikel (refereegranskat)abstract
- Energetic electrons are of interest in many types of plasmas, howe ver previous modelling of their properties have been restricted to the use of linear Fokker-Planck collision ope rators or non-relativistic formulations. Here, we describe a fully non-linear kinetic-equation solver, capable of handling large electric-field strengths (compared to the Dreicer field) and relativistic temperatures. This tool allows modelling of the momentum- space dynamics of the electrons in cases where strong departure s from Maxwellian distributions may arise. As an example, we consider electron runaway in magnetic-confinement fusion plasmas and describe a transition to electron slide-away at field strengths significantly lower than pre viously predicted.
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| 14. |
- Stahl, Adam, 1985, et al.
(författare)
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Runaway-electron formation and electron slide-away in an ITER post-disruption scenario
- 2016
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 775:1
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Konferensbidrag (refereegranskat)abstract
- Mitigation of runaway electrons is one of the outstanding issues for a reliable operation of ITER and other large tokamaks, and accurate estimates for the expected runaway- electron energies and current are needed. Previously, linearized tools, assuming the runaway population to be small, have been used to study the runaway dynamics, but these tools are not valid in the cases of most interest, i.e. when the runaway population becomes substantial. We study runaway-electron formation in a post-disruption ITER plasma using the newly developed non-linear code NORSE , and nd that the entire electron population is converted to runaways in the scenario considered. A new non-linear feedback mechanism is also described, by which a transition to electron slide-away can be induced at eld strengths signi cantly lower than previously expected. We nd the exact time to the transition to be highly dependent on the details of the mechanisms removing heat from the thermal electron population.
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| 15. |
- Stahl, Adam, 1985, et al.
(författare)
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Self-consistent nonlinear kinetic modeling of runaway-electron dynamics
- 2017
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Konferensbidrag (refereegranskat)abstract
- Runaway electrons represent the greatest threat to the plasma-facing components of atokamak when they are highly energetic and constitute a significant fraction of the electronpopulation, a regime which has not been previously accessible in modelling since it requiresa nonlinear relativistic treatment. To address this problem, we present an efficient numericaltool called NORSE for the study of runaway-electron momentum-space dynamics.The kinetic equation solved in NORSE includes a fully nonlinear relativistic collision operator, making it possible to consider scenarios where the electric field is comparableto the Dreicer field (or larger), or the electron distribution function is otherwise far froma Maxwellian (which can be the case already in present-day runaway experiments). Thiscapability makes NORSE unique in the field of runaway-electron studies.Using NORSE, we investigate the transition to a regime where the entire electronpopulation experiences continuous acceleration, so-called electron slide-away. Forthe first time, we apply a nonlinear kinetic-equation solver to study the evolution ofthe electron distribution in an ITER disruption. We use an electric field calculated self consistently,and show that the runaway-electron density becomes substantial, makingthe nonlinear treatment essential. In addition, we find that Ohmic heating and the rateof heat loss play an important role in determining the electron dynamics, with the latteraffecting the average energy reached by the runaways by several orders of magnitude.
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| 16. |
- Stahl, Adam, 1985, et al.
(författare)
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Synchrotron radiation from a runaway electron distribution in tokamaks
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 20:9, s. 093302-
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Tidskriftsartikel (refereegranskat)abstract
- The synchrotron radiation emitted by runaway electrons in a fusion plasma provides information regarding the particle momenta and pitch-angles of the runaway electron population through the strong dependence of the synchrotron spectrum on these parameters. Information about the runaway density and its spatial distribution, as well as the time evolution of the above quantities, can also be deduced. In this paper, we present the synchrotron radiation spectra for typical avalanching runaway electron distributions. Spectra obtained for a distribution of electrons are compared with the emission of mono-energetic electrons with a prescribed pitch-angle. We also examine the effects of magnetic field curvature and analyse the sensitivity of the resulting spectrum to perturbations to the runaway distribution. The implications for the deduced runaway electron parameters are discussed. We compare our calculations to experimental data from DIII-D and estimate the maximum observed runaway energy.
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| 17. |
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| 18. |
- Wilkie, George, 1983, et al.
(författare)
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Transport and deceleration of fusion products in microturbulence
- 2016
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:6, s. 060703-
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Tidskriftsartikel (refereegranskat)abstract
- The velocity-space distribution of alpha particles born in fusion devices is subject to modification at moderateenergies due to turbulent transport. Therefore, one must calculate the evolution of an equilibrium distributionwhose functional form is not known a priori. Using a novel technique, applicable to any trace impurity, wehave made this calculation for fully nonlinear gyrokinetic simulations not only possible, but particularlyefficient. We demonstrate a microturbulence-induced departure from the local slowing-down distribution, aninversion of the energy distribution, and associated modifications to the alpha heating and pressure profilesin an ITER-like scenario.
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