| 21. |
- Anderson, Johan, 1973
(författare)
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Drift waves in non-circular tokamak geometry
- 2000
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the main concerns in fusion research is to understand the anomalously high transport in magnetically confined plasmas. In recent years, substantial progress in the understanding of transport in terms of drift waves in tokamaks has been achieved. It is at present an important issue to investigate the stability of drift waves in realistic tokamak geometries. Among the drift wave candidates for explaining the anomalous transport are the toroidal etai-modes in the core and the resistive etai-modes in the edge. In this thesis, the etai mode and the resistive edge mode stability in a non-circular tokamak geometry are studied. In particular, the effects of elongation and Shafranov shift are studied. The effects of plasma shaping on magnetohydrodynamic (MHD) modes have been thoroughly studied. However, the effects of plasma shaping on the drift waves are not well known. Empirically it is found that the overall effects of elonagtion is favourable.In the core plasma a stabilization of the etai modes with increasing elongation is found. It has been found that the spectrum of the unstable etai-modes is shifted toward shorter wavelengths with increasing elongation.In the edge (or rather for peaked density profiles) the effects of elongation on the etai-mode are, however, stabilizing. For edge parameters a stabilizing effect of elongation on the resistive modes is also found and the collisional effects on the etai-modes are rather weak. It is shown that the effects of ion temperature fluctuations on the resistive ballooning modes are stabilizing and this may further enhanced by elongation effects. In particular, it is found that the resistive ballooning mode is stabilized by finite Larmor radius effects already for etai
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| 22. |
- Anderson, Johan, 1973, et al.
(författare)
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Effects of cross-sectional elongation on the resistive edge modes
- 2001
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 8, s. 180-
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Tidskriftsartikel (refereegranskat)abstract
- Resistive edge modes in a shifted noncircular tokamak geometry are investigated in the electrostatic limit. The reduced Braghinskii equations are used as a model for the electrons and an advanced fluid model for the ions. An eigenvalue problem is derived from these equations which is solved numerically. It is found that the resistive ballooning modes are stabilized by plasma elongation forpeaked density profiles. In addition, it is found that the resistive ITG mode may be either stabilized or destabilized by elongation depending on the collision frequency.
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| 23. |
- Anderson, Johan, 1973, et al.
(författare)
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Effects of non-circular tokamak geometry on ion-temperature-gradient driven modes
- 2000
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Ingår i: Plasma Phys. Contr. Fusion. - : IOP Publishing. ; 42, s. 545-
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Tidskriftsartikel (refereegranskat)abstract
- The influence of plasma elongation and Shafranov shift on the stability of electrostatic ion-temperature-gradient driven modes (ηi-modes) is investigated. An advanced fluid model is used for the ions together with Boltzmann distributed electrons. The derived eigenvalue equation is solved both analytically, in the strong ballooning limit, and numerically. It is found that the effects of elongation change from stabilizing, for peaked density profiles, to destabilizing in the flat density regime. In addition, it is shown that the maximum growth rate is shifted towardsshorter wavelengths as the elongation increases. The effects of shaping on tokamak stability are exemplified with data from a Joint European Torus (JET) high-performance mode discharge.
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| 24. |
- Anderson, Johan, 1973, et al.
(författare)
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Effects of the Second Harmonic and Plasma Shaping on the Geodesic Acoustic Mode
- 2014
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Ingår i: 41st EPS conference, Berlin, 23.- 27.6.2014. ; :P1.056, s. 4-
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Konferensbidrag (refereegranskat)abstract
- The effects of second harmonics of the density and temperature perturbations on the linear Geodesic Acoustic Mode (GAM) frequency and non-linear generation of the GAM are investigated, using a fluid model. We show that the second harmonics contribute to the frequency through the density gradient scale length and the wave number of the GAM. In addition, the linear frequency of the GAM is generally increased by coupling to the higher harmonic.
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| 25. |
- Anderson, Johan, 1973, et al.
(författare)
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Effects of the Second Harmonic on the GAM in Electron Scale Turbulence
- 2013
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Ingår i: European Fusion Theory Conference. ; 15, s. 10-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The effects higher order harmonics have been self-consistently includedin the derivation of the electron branch of the electron Geodesic Acoustic Mode (el-GAM) in an Electron-Temperature-Gradient (ETG) turbulence background. The workis based on a two-fluid model including finite β-effects while retaining non-adiabaticions. In solving the linear dispersion relation, it is found that the due to the couplingto the m = 2 mode the real frequency may be significantly altered and yield highervalues.
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| 26. |
- Anderson, Johan, 1973, et al.
(författare)
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Electron Geodesic Acoustic Modes in Electron Temperature Gradient Mode Turbulence
- 2012
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Ingår i: Proceedings 39th EPS/16th ICPP, Stockholm 2-6 July 2012. - 9781622769810 ; 2, s. 814-817
-
Konferensbidrag (refereegranskat)abstract
- There has been overwhelming evidence that coherent structures such as vortices, streamers and zonal flows (m = n = 0, where m and n are the poloidal and toroidal modenumbers respectively) play a critical role in determining the overall transport in magnetically confined plasmas. The Geodesic Acoustic Mode (GAM) is the oscillatory counterpart of the zonal flow (m = n = 0 in the potential perturbation, m = 1, n = 0 in the perturbations in density, temperature and parallel velocity) and thus a much weaker effect on turbulence is expected. Nevertheless experimental studies suggest that GAMs (n = 0, m = 1) are related to the L-H transition and transport barriers. The electron-temperature-gradient (ETG) mode driven by a combination of electron temperature gradients and field line curvature effects is a likely candidate for driving electron heat transport. The ETG mode driven electron heat transport is determined by short scale fluctuations that do not influence ion heat transport and is largely unaffected by the large scale flows stabilizing ion-temperature-gradient (ITG) modes. We have utilized a fluid model for the ETG mode based on the Braghinskii equations with non-adiabatic ions including impurities and finite β - effects. The ETG mode model consists of electron continuity, electron parallel momentum and energy equations in combination of an non-adiabatic ion response. The ion and electron counterparts are coupled using quasineutrality. We use the wave kinetic equation to describe the background short scale ETG turbulence and derive an dispersion relation for the GAM. In describing the large scale plasma flow dynamics it is assumed that there is a sufficient spectral gap between the small scale fluctuations and the large scale flow. We note that the linear GAM is purely oscillatory and non-linearly the electron GAM is unstable with a growth rate depending on the saturation level |φk|2. To estimate the ETG turbulent fluctuation level and GAM growth, a predator-prey model was used to describe the coupling between the GAMs and small scale ETG turbulence. The stationary point of the coupled system implies that the ETG saturation level φk can be drastically enhanced by a new saturation mechanism, stemming from a balance between the Landau damping and the GAM growth rate.
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| 27. |
- Anderson, Johan, 1973, et al.
(författare)
-
Electron geodesic acoustic modes in electron temperature gradient mode turbulence
- 2012
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 19:8
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, the first demonstration of an electron branch of the geodesic acoustic mode (el-GAM) driven by electron temperature gradient (ETG) modes is presented. The work is based on a fluid description of the ETG mode retaining non-adiabatic ions and the dispersion relation for el-GAMsdriven nonlinearly by ETG modes is derived. A new saturation mechanism for ETG turbulence through the interaction with el-GAMs is found, resulting in a significantly enhanced ETGturbulence saturation level compared to the mixing length estimate.
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| 28. |
- Anderson, Johan, 1973, et al.
(författare)
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Elucidating plasma dynamics in Hasegawa-Wakatani turbulence by information geometry
- 2020
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 27:2
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Tidskriftsartikel (refereegranskat)abstract
- The impact of adiabatic electrons on drift-wave turbulence, modeled by the Hasegawa-Wakatani equations, is studied using information length. Information length is a novel theoretical method for measuring distances between statistical states represented by different probability distribution functions (PDFs) along the path of a system and represents the total number of statistically different states that a system evolves through in time. Specifically, the time-dependent PDFs of turbulent fluctuations for a given adiabatic index A are computed. The changes in fluctuation statistics are then quantified in time by using information length. The numerical results provide time traces exhibiting intermittent plasma dynamics, and such behavior is identified by a rapid change in the information length. The effects of A are discussed.
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| 29. |
- Anderson, Johan, 1973-, et al.
(författare)
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Fire Safety of Façades
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Façade fires do not occur often (in comparison to other major structure fires) but in recent years there have been a number of spectacular façade fires in high rise building such as the recent fire in Grenfell Tower, London.Under-ventilated compartment fires may cause flames to spill out of window openings impinging the façade, thus devastating façade fires may start on one floor leap-frogging to adjacent floors. It is therefore necessary to limit or delay fire spread to higher floors. Requirements built on large scale fire testing may decrease the risk of these types of fires provided that the building is constructed according to the specifications provided by the manufacturer. Different countries have different regulations and tests for façades. New materials and façade systems are continuously introduced which might call for an update of these tests and regulations.This report summarizes experimental and modelling efforts in characterizing the fire safety of façades using the Swedish SP Fire 105 and the British BS 8414 methods. Recent experimental results and modelling is presented exploring the variations in the fire exposure, fire load and the fuel used. The fire source and the heat exposure to the façade are characterized by additional temperatures measured by plate thermometers while some other aspects are only treated in the numerical study such as a change in fuel. It is found that the results from the BS 8414 are largely affected by wind and climate since the experimental test was performed outdoors, moreover fire spread on wooden façades is also briefly discussed.In order to obtain a deeper understanding of the test methods and the results CFD (Computational Fluid Dynamics) Modelling in FDS was used. The models were based on measured input parameters including uncertainties and an assessment of the impact of said uncertainties. The models could often reproduce the experimentally found temperatures qualitatively and quantitatively. A detailed discussion on the regulations and the tests that lead to the SP Fire 105 test method is also presented. Summaries of the façade testing methods and conditions in other European countries are presented in the appendices.Finally possible ways forward in updating the façade testing and regulations are discussed.
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| 30. |
- Anderson, Johan, 1973, et al.
(författare)
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Fire Spread due to Thermal Runaway in a Lithium-ion Battery Cell
- 2014
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Ingår i: Fires in vehicles (FIVE) 2014 Conference proceedings. ; 2014, s. 267-270
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The risk of spreading of fire between Lithium-ion battery cells is assessed using Finite-Element (FE) modelling of the heat transfer initiated by a thermal runaway. The results are contrasted to experimental data where the heat release rate (HRR) is utilized as an input to the simulation. It is found that the temperature increase in a neighbouring cell can be quantitatively estimated during theearly stages of the fire taking into account the anisotropic thermal conductivity of the cells.
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