| 1. |
- Buller, Stefan, 1991, et al.
(författare)
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Neoclassical transport in density pedestals with non-trace impurities
- 2016
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Ingår i: 58th Annual Meeting of the APS Division of Plasma Physics. ; 61:18, s. TO9.00003-
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Konferensbidrag (refereegranskat)abstract
- We study neoclassical transport in steady-state density pedestals with non-trace impurities using the Eulerian delta-f code PERFECT, with an emphasis on radially global effects and the effects of impurities. To properly describe transport in a tokamak pedestal, radial coupling must be included, which strongly affects the transport. We find that radial coupling reduces the pedestal heat flux compared to local predictions. Furthermore, the influence of the pedestal persists several orbit widths into the core. The electron flux is significant in the pedestal, and global neoclassical transport is not intrinsically ambipolar. Thus, the impurity flux is not simply opposing the ion flux. The resulting radial current gives a torque that is balanced by a non-negligible radial transport of toroidal momentum. The effective Prandtl number is comparable to typical turbulent values in the core (0.1-0.3), and is sensitive to the impurity content. Global effects have a strong contribution to the poloidal flows of low-Z ions, which give rise to larger in-out flow asymmetries.
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| 2. |
- Buller, Stefan, 1991, et al.
(författare)
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Neoclassical transport with non-trace impurities in density pedestals
- 2016
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Ingår i: 43rd European Physical Society Conference on Plasma Physics, EPS2016.
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Konferensbidrag (refereegranskat)abstract
- By increasing the global energy confinement edge transport barriers are of crucial importance for the viability of the tokamak concept in becoming an energy source. Changing from carbon to metallic plasma facing components (PFCs) in major fusion devices was accompanied by a reduction in the pedestal performance. The performance representative of carbon PFC operation could in some cases be recovered using nitrogen seeding, pointing to a possible impact of impurities on pedestal confinement. We investigate the effect of non-trace impurities on collisional transport and flows in a sharp density pedestal (characterized by an ion orbit width scale density variation and subsonic flows), using the global, δf, Eulerian neoclassical solver PERFECT. We show that the fluxes are significantly altered by the presence of the sharp density pedestal: most notably, the coupling between different flux-surfaces allows for non-ambipolar particle fluxes and non-zero transport of parallel momentum. We find the mass and momentum transported by the main ions to be significantly affected by going from trace to non-trace impurity content. In the sharp gradient region the friction of ions on electrons can compete with that on impurities leading to ion andimpurity particle fluxes with the same sign, making the temperature screening effect on impurity transport unreliable.
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| 3. |
- Catto, Peter J., et al.
(författare)
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A unified treatment of kinetic effects in a tokamak pedestal
- 2011
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 53, s. 054004-
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Tidskriftsartikel (refereegranskat)abstract
- We consider the effects of a finite pedestal radial electric field on ion orbits using a unified approach. We then employ these modified orbit results to retain finite E×B drift departures from flux surfaces in an improved drift-kinetic equation. The procedure allows us to make a clear distinction between transit averages and flux surface averages when solving this kinetic equation. The technique outlined here is intended to clarify and unify recent evaluations of the banana regime decrease and plateau regime alterations in the ion heat diffusivity; the reduction and possible reversal of the poloidal flow in the banana regime, and its augmentation in the plateau regime; the increase in the bootstrap current; and the enhancement of the residual zonal flow regulation of turbulence.
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| 4. |
- Landreman, Matt, et al.
(författare)
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Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas
- 2014
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 21:4
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we examine the validity of several common simplifying assumptions used in numerical neoclassical calculations for nonaxisymmetric plasmas, both by using a new continuum drift-kinetic code and by considering analytic properties of the kinetic equation. First, neoclassical phenomena are computed for the LHD and W7-X stellarators using several versions of the drift-kinetic equation, including the commonly used incompressible-E × B-drift approximation and two other variants, corresponding to different effective particle trajectories. It is found that for electric fields below roughly one third of the resonant value, the different formulations give nearly identical results, demonstrating the incompressible E × B-drift approximation is quite accurate in this regime. However, near the electric field resonance, the models yield substantially different results. We also compare results for various collision operators, including the full linearized Fokker-Planck operator. At low collisionality, the radial transport driven by radial gradients is nearly identical for the different operators; while in other cases, it is found to be important that collisions conserve momentum.
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| 5. |
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| 6. |
- Mollén, Albert, 1985, et al.
(författare)
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Impurities in a non-axisymmetric plasma: Transport and effect on bootstrap current
- 2015
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:11, s. 112508-
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Tidskriftsartikel (refereegranskat)abstract
- Impurities cause radiation losses and plasma dilution, and in stellarator plasmas the neoclassical ambipolar radial electric field is often unfavorable for avoiding strong impurity peaking. In this work we use a new continuum drift-kinetic solver, the SFINCS code (the Stellarator Fokker-Planck Iterative Neoclassical Conservative Solver) [M. Landreman et al., Phys. Plasmas 21 (2014) 042503] which employs the full linearized Fokker-Planck-Landau operator, to calculate neoclassical impurity transport coefficients for a Wendelstein 7-X (W7-X) magnetic configuration. We compare SFINCS calculations with theoretical asymptotes in the high collisionality limit. We observe and explain a 1/nu-scaling of the inter-species radial transport coefficient at low collisionality, arising due to the field term in the inter-species collision operator, and which is not found with simplified collision models even when momentum correction is applied. However, this type of scaling disappears if a radial electric field is present. We also use SFINCS to analyze how the impurity content affects the neoclassical impurity dynamics and the bootstrap current. We show that a change in plasma effective charge Zeff of order unity can affect the bootstrap current enough to cause a deviation in the divertor strike point locations.
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| 7. |
- Mollén, Albert, 1985, et al.
(författare)
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On collisional impurity transport in nonaxisymmetric plasmas
- 2014
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 561:1, s. 012012-
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Konferensbidrag (refereegranskat)abstract
- The presence of impurity species in magnetic confinement fusion devices leads to radiation losses and plasma dilution. Thus it is important to analyze impurity dynamics, and search for means to control them. In stellarator plasmas the neoclassical ambipolar radial electric field often points radially inwards (referred to as the ion root regime), causing impurities to accumulate in the core. This can limit the performance of nonaxisymmetric devices.In the present work we analyze neoclassical impurity transport in stellarator plasmas using a recently developed continuum drift-kinetic solver, the SFINCS code (the Stellarator Fokker- Planck Iterative Neoclassical Conservative Solver). The study is performed for a case close to the edge of W7-X using the standard configuration magnetic geometry. We investigate the sensitivity of impurity transport to impurity charge, main species density and temperature gradients, as well as ion temperature.At the studied radial location we find that the neoclassical impurity peaking factor can be very large, particularly for high-Z impurities. The ambipolar radial electric field is in the ion root regime, and impurity accumulation can thus be expected. The accumulation is strengthened by the large main species density and temperature gradients. Moreover we find that the size of the bootstrap current is affected by the value of the plasma effective charge, suggesting that employing a realistic ion composition can be important when calculating the bootstrap current.
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| 8. |
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| 9. |
- Paul, Elizabeth J., et al.
(författare)
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An adjoint method for neoclassical stellarator optimization
- 2019
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 85:5
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Tidskriftsartikel (refereegranskat)abstract
- Stellarators are a promising route to steady-state fusion power. However, to achieve the required confinement, the magnetic geometry must be highly optimized. This optimization requires navigating high-dimensional spaces, often necessitating the use of gradient-based methods. The gradient of the neoclassical fluxes is expensive to compute with classical methods, requiring O(N) flux computations, where N is the number of parameters. To reduce the cost of the gradient computation, we present an adjoint method for computing the derivatives of moments of the neoclassical distribution function for stellarator optimization. The linear adjoint method allows derivatives of quantities which depend on solutions of a linear system, such as moments of the distribution function, to be computed with respect to many parameters from the solution of only two linear systems. This reduces the cost of computing the gradient to the point that the finite-collisionality neoclassical fluxes can be used within an optimization loop. With the neoclassical adjoint method, we compute solutions of the drift kinetic equation and an adjoint drift kinetic equation to obtain derivatives of neoclassical quantities with respect to geometric parameters. When the number of parameters in the derivative is large (O(10(2))), this adjoint method provides up to a factor of 200 reduction in cost. We demonstrate adjoint-based optimization of the field strength to obtain minimal bootstrap current on a surface. With adjoint-based derivatives, we also compute the local sensitivity to magnetic perturbations on a flux surface and identify regions where tight tolerances on error fields are required for control of the bootstrap current or radial transport. Furthermore, the solve for the ambipolar electric field is accelerated using a Newton method with derivatives obtained from the adjoint method.
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| 10. |
- Pusztai, Istvan, 1983, et al.
(författare)
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Global effects on neoclassical transport in the pedestal with impurities
- 2016
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:8, s. 085001-
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Tidskriftsartikel (refereegranskat)abstract
- We present a numerical study of collisional transport in a tokamak pedestal in the presence of non-trace impurities, using the radially global delta-f neoclassical solver PERFECT [M. Landreman et al. 2014 Plasma Phys. Control. Fusion 56 045005]. It is known that in a tokamak core with non-trace impurities present the radial impurity flux opposes the bulk ion flux to provide an ambipolar particle transport, with the electron transport being negligibly small. However, in a sharp density pedestal with sub-sonic ion flows the electron transport can be comparable to the ion and impurity flows. Furthermore, the neoclassical particle transport is not intrinsically ambipolar, and the non-ambipolarity of the fluxes extends outside the pedestal region by the radial coupling of the perturbations. The neoclassical momentum transport, which is finite in the presence of ion orbit-width scale profile variations, is significantly enhanced when impurities are present in non-trace quantities, even if the total parallel mass flow is dominated by the bulk ions.
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| 11. |
- Pusztai, Istvan, 1983, et al.
(författare)
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Radio frequency induced and neoclassical asymmetries and their effects on turbulent impurity transport in a tokamak
- 2014
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Ingår i: Contributions to Plasma Physics. - : Wiley. - 0863-1042 .- 1521-3986. ; 54:4-6, s. 534-542
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Tidskriftsartikel (refereegranskat)abstract
- Poloidal asymmetries in the impurity density can be generated by radio frequency heating in the core and by neoclassical effects in the edge of tokamak plasmas. In a pedestal case study, using global neoclassical simulations we find that finite orbit width effects can generate significant poloidal variation in the electrostatic potential, which varies on a small radial scale. Gyrokinetic modeling shows that these poloidal asymmetries can be strong enough to significantly modify turbulent impurity peaking. In the pedestal the ExB drift in the radial electric field can give a larger contribution to the poloidal motion of impurities than that of their parallel streaming. Under such circumstances we find that up-down asymmetries can also affect impurity peaking.
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| 12. |
- Stahl, Adam, 1985, et al.
(författare)
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Kinetic modelling of runaway-electron dynamics
- 2015
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Ingår i: Proceedings of the 14th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Improved understanding of runaway-electron formation and decayprocesses are of prime interest for the safe operation of large tokamaks, and theirdynamics during dynamical scenarios such as disruptions are of particular concern. Inthis contribution, we present kinetic modelling of scenarios with time-dependent plasmaparameters – in particular, we investigate hot-tail runaway generation during a rapiddrop in plasma temperature. With the goal of studying runaway-electron generationwith a self-consistent electric field-evolution, we also discuss the implementation ofa conservative collision operator and demonstrate its properties. An operator foravalanche runaway-electron generation which includes the proper energy dependenceof the runaway distribution, is investigated, and the avalanche growth rate is shownto be significantly affected in some parameter regimes. These developments all pavethe way for an improved modelling of runaway-electron dynamics during disruptionsor other dynamic events.
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| 13. |
- 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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| 14. |
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