| 1. |
- Abel, Ian, 1985, et al.
(författare)
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Multiscale modelling for tokamak pedestals
- 2018
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:2
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Tidskriftsartikel (refereegranskat)abstract
- Pedestal modelling is crucial to predict the performance of future fusion devices. Current modelling efforts suffer either from a lack of kinetic physics, or an excess of computational complexity. To ameliorate these problems, we take a first-principles multiscale approach to the pedestal. We will present three separate sets of equations, covering the dynamics of edge localised modes (ELMs), the inter-ELM pedestal and pedestal turbulence, respectively. Precisely how these equations should be coupled to each other is covered in detail. This framework is completely self-consistent; it is derived from first principles by means of an asymptotic expansion of the fundamental Vlasov-Landau-Maxwell system in appropriate small parameters. The derivation exploits the narrowness of the pedestal region, the smallness of the thermal gyroradius and the low plasma beta (the ratio of thermal to magnetic pressures) typical of current pedestal operation to achieve its simplifications. The relationship between this framework and gyrokinetics is analysed, and possibilities to directly match our systems of equations onto multiscale gyrokinetics are explored. A detailed comparison between our model and other models in the literature is performed. Finally, the potential for matching this framework onto an open-field-line region is briefly discussed.
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| 2. |
- Anderson, D., et al.
(författare)
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Self-induced erosion and spectral breaking of high-power microwave pulses
- 2000
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 63:4, s. 329-341
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Tidskriftsartikel (refereegranskat)abstract
- An analysis is made of the phenomenon of self-induced erosion and spectral breaking of ionizing high-power microwave pulses propagating in a gas. The analysis describes in an analytically explicit and physically clear way the consistent interaction between the microwave pulse and the self-induced breakdown plasma. In particular, it clarifies, both qualitatively and quantitatively, the mechanisms behind the pulse erosion and the spectral breaking phenomenon, i.e. the splitting of the pulse spectrum into a redshifted and a blueshifted peak as observed in numerical simulation results as well as in experiments.
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| 3. |
- Andres, N., et al.
(författare)
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Energy cascade rate in isothermal compressible magnetohydrodynamic turbulence
- 2018
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Ingår i: Journal of Plasma Physics. - : CAMBRIDGE UNIV PRESS. - 0022-3778 .- 1469-7807. ; 84:4
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Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional direct numerical simulations are used to study the energy cascade rate in isothermal compressible magnetohydrodynamic turbulence. Our analysis is guided by a two-point exact law derived recently for this problem in which flux, source, hybrid and mixed terms are present. The relative importance of each term is studied for different initial subsonic Mach numbers M-S and different magnetic guide fields B-0. The dominant contribution to the energy cascade rate comes from the compressible flux, which depends weakly on the magnetic guide field B-0, unlike the other terms whose moduli increase significantly with M s and B-0. In particular, for strong B-0 the source and hybrid terms are dominant at small scales with almost the same amplitude but with a different sign. A statistical analysis undertaken with an isotropic decomposition based on the SO(3) rotation group is shown to generate spurious results in the presence of B-0, when compared with an axisymmetric decomposition better suited to the geometry of the problem. Our numerical results are compared with previous analyses made with in situ measurements in the solar wind and the terrestrial magnetosheath.
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| 4. |
- Andrushchenko, Zhanna N., et al.
(författare)
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Self-consistent model of electron drift mode turbulence
- 2008
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 74:1, s. 21-33
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Tidskriftsartikel (refereegranskat)abstract
- The nonlinear dynamics of magnetic electron drift mode turbulence are outlined and the generation of large-scale magnetic Structures in a non-uniform magnetized plasma by turbulent Reynolds stress is demonstrated. The loop-back of large-scale flows on the microturbulence is elucidated and the modulation of the electron drift mode turbulence spectrum in a, medium with slowly varying parameters is presented. The wave kinetic equation in the presence of large-scale flows is derived and it can be seen that the small-scale turbulence and the large-scale structures form a, self-regulating system. Finally. it is shown by the use of quasilinear theory that the shearing of microturbulence by the flows can be described by a diffusion equation in k-space and the corresponding diffusion coefficients are calculated.
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| 5. |
- Asp, Elina, et al.
(författare)
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Stability of the Landau Resonance for Drift Modes in Rotating Tokamak Plasma
- 2003
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 60:5, s. 371-
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Tidskriftsartikel (refereegranskat)abstract
- The linear stability of drift waves in a poloidally rotating tokamak plasma is considered. The derived dispersion relation features a peaking of the diamagnetic frequency which gives the drift modes an irreducible two-dimensional character. We then show that inverse Landau damping can be suppressed and even stabilized, if the flow's shear is strong. Even though the instability, excited by the Landau resonance, is stronger at a high velocity shear for positive rotation velocities, effects due to the rotation of the plasma can reverse the sign and induce damping of the two-dimensional drift modes. This stabilizing mechanism works only for positive rotation velocities. For negative rotation velocities, we show that only modes with high poloidal mode numbers are unstable.
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| 6. |
- Berger, Esmée, 1998, et al.
(författare)
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Runaway dynamics in reactor-scale spherical tokamak disruptions
- 2022
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 88:6
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Tidskriftsartikel (refereegranskat)abstract
- Understanding generation and mitigation of runaway electrons in disruptions is important for the safe operation of future tokamaks. In this paper we investigate the runaway dynamics in reactor-scale spherical tokamaks, focusing on a compact nominal design with a plasma current of 21 megaamperes (MA), 1.8 T magnetic field on axis and major radius of approximately 3 m. We study both the severity of runaway generation during unmitigated disruptions, and the effect that typical mitigation schemes based on massive material injection have on runaway production. The study is conducted using the numerical framework DREAM (Disruption Runaway Electron Analysis Model). We find that, in many cases, mitigation strategies are necessary to prevent the runaway current from reaching multi-MA levels. Our results indicate that, with a suitably chosen deuterium–neon mixture for mitigation, it is possible to achieve a tolerable runaway current and ohmic current evolution. However, this does not account for the runaway source due to wall activation, which has been found to severely limit successful mitigation at conventional aspect ratios, but whose definition requires a more complete wall specification. Furthermore, the majority of the thermal energy loss is found to happen through radial transport rather than radiation, which poses a risk of unacceptable localised heat loads.
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| 7. |
- Bingham, Robert, et al.
(författare)
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Solar coronal heating by plasma waves
- 2010
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Ingår i: Journal of Plasma Physics. - : Cambridge university. - 0022-3778 .- 1469-7807. ; 76:2, s. 135-158
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Tidskriftsartikel (refereegranskat)abstract
- The solar coronal plasma is maintained at temperatures of millions of degrees, much hotter than the photosphere, which is at a temperature of just 6000 K. In this paper, the plasma particle heating based on the kinetic theory of wave–particle interactions involving kinetic Alfvén waves and lower-hybrid drift modes is presented. The solar coronal plasma is collisionless and therefore the heating must rely on turbulent wave heating models, such as lower-hybrid drift models at reconnection sites or the kinetic Alfvén waves. These turbulent wave modes are created by a variety of instabilities driven from below. The transition region at altitudes of about 2000 km is an important boundary chromosphere, since it separates the collision-dominated photosphere/chromosphere and the collisionless corona. The collisionless plasma of the corona is ideal for supporting kinetic wave–plasma interactions. Wave–particle interactions lead to anisotropic non-Maxwellian plasma distribution functions, which may be investigated by using spectral analysis procedures being developed at the present time.
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| 8. |
- Brandenburg, Axel
(författare)
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Advances in mean-field dynamo theory and applications to astrophysical turbulence
- 2018
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Ingår i: Journal of Plasma Physics. - : Cambridge University Press. - 0022-3778 .- 1469-7807. ; 84:4
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Tidskriftsartikel (refereegranskat)abstract
- Recent advances in mean-field theory are reviewed and applications to the Sun, late-type stars, accretion disks, galaxies and the early Universe are discussed. We focus particularly on aspects of spatio-temporal non-locality, which provided some of the main new qualitative and quantitative insights that emerged from applying the test-field method to magnetic fields of different length and time scales. We also review the status of nonlinear quenching and the relation to magnetic helicity, which is an important observational diagnostic of modern solar dynamo theory. Both solar and some stellar dynamos seem to operate in an intermediate regime that has not yet been possible to model successfully. This regime is bracketed by antisolar-like differential rotation on one end and stellar activity cycles belonging to the superactive stars on the other. The difficulty in modelling this regime may be related to shortcomings in simulating solar/stellar convection. On galactic and extragalactic length scales, the observational constraints on dynamo theory are still less stringent and more uncertain, but recent advances both in theory and observations suggest that more conclusive comparisons may soon be possible also here. The possibility of inversely cascading magnetic helicity in the early Universe is particularly exciting in explaining the recently observed lower limits of magnetic fields on cosmological length scales. Such magnetic fields may be helical with the same sign of magnetic helicity throughout the entire Universe. This would be a manifestation of parity breaking.
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| 9. |
- Brandenburg, Axel, 1959-
(författare)
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Hosking integral in non-helical Hall cascade
- 2023
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Ingår i: Journal of Plasma Physics. - : Cambridge University Press (CUP). - 0022-3778 .- 1469-7807. ; 89:1
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Tidskriftsartikel (refereegranskat)abstract
- The Hosking integral, which characterizes magnetic helicity fluctuations in subvolumes, is known to govern the decay of magnetically dominated turbulence. Here, we show that, when the evolution of the magnetic field is controlled by the motion of electrons only, as in neutron star crusts, the decay of the magnetic field is still controlled by the Hosking integral, but now it has effectively different dimensions than in ordinary magnetohydrodynamic (MHD) turbulence. This causes the correlation length to increase with time t like t(4/13) instead of t(4/9) in MHD. The magnetic energy density decreases like t(-10/13), which is slower than in MHD, where it decays like t(-10/9). These new analytic results agree with earlier numerical simulations for the non-helical Hall cascade.
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| 10. |
- Brandenburg, Axel, 1959-, et al.
(författare)
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Inverse cascading for initial magnetohydrodynamic turbulence spectra between Saffman and Batchelor
- 2023
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Ingår i: Journal of Plasma Physics. - : Cambridge University Press (CUP). - 0022-3778 .- 1469-7807. ; 89:6
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Tidskriftsartikel (refereegranskat)abstract
- In decaying magnetohydrodynamic (MHD) turbulence with a strong magnetic field, the spectral magnetic energy density is known to increase with time at small wavenumbers, provided the spectrum at low is sufficiently steep. This process is called inverse cascading and occurs for an initial Batchelor spectrum, where the magnetic energy per linear wavenumber interval increases like. For an initial Saffman spectrum that is proportional to, however, inverse cascading has not been found in the past. We study here the case of an intermediate spectrum, which may be relevant for magnetogenesis in the early Universe during the electroweak epoch. This case is not well understood in view of the standard Taylor expansion of the magnetic energy spectrum for small. Using high resolution MHD simulations, we show that, also in this case, there is inverse cascading with a strength just as expected from the conservation of the Hosking integral, which governs the decay of an initial Batchelor spectrum. Even for shallower spectra with spectral index 3/2$, our simulations suggest a spectral increase at small with time proportional to. The critical spectral index of is related to the slope of the spectral envelope in the Hosking phenomenology. Our simulations with mesh points now suggest inverse cascading even for an initial Saffman spectrum.
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