| 1. |
- Anderson, Johan, 1973, et al.
(författare)
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High frequency geodesic acoustic modes in electron scale turbulence
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:12, s. article nr. 123016-
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Tidskriftsartikel (refereegranskat)abstract
- In this work the finite β-effects 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-GAMs driven non-linearly by ETG modes is derived. The ETG growth rate from the fluid model is compared with the results found from gyrokinetic simulations with good agreement. A new saturation mechanism for ETG turbulence through the interaction with el-GAMs is found, resulting in a significantly enhanced ETG turbulence saturation level compared with the mixing length estimate. It is shown that the el-GAM may be stabilized by an increase in finite β as well as by increasing non-adiabaticity. The decreased GAM growth rates is due to the inclusion of the Maxwell stress.
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| 2. |
- Anderson, Johan, 1973, et al.
(författare)
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High Frequency Geodesic Acoustic Modes in Electron Scale Turbulence
- 2012
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Ingår i: Proceedings 24th IAEA FEC, San Diego 8-13 October 2012.
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Konferensbidrag (refereegranskat)abstract
- In this work the first demonstration of a high frequency branch of the geodesic acoustic mode (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 high frequency GAMs driven nonlinearly by ETG modes is derived. A new saturation mechanism for ETG turbulence through the interaction with high frequency GAMs is found, resulting in a significantly enhanced ETG turbulence saturation level compared to the mixing length estimate.
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| 3. |
- Eriksson, Frida, 1986, et al.
(författare)
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Impact of fast ions on density peaking in JET : fluid and gyrokinetic modeling
- 2019
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 61:7
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Tidskriftsartikel (refereegranskat)abstract
- The effect of fast ions on turbulent particle transport, driven by ion temperature gradient (ITG)/trapped electron mode turbulence, is studied. Two neutral beam injection (NBI) heated JET discharges in different regimes are analyzed at the radial position rho(t) = 0.6, one of them an L-mode and the other one an H-mode discharge. Results obtained from the computationally efficient fluid model EDWM and the gyro-fluid model TGLF are compared to linear and nonlinear gyrokinetic GENE simulations as well as the experimentally obtained density peaking. In these models, the fast ions are treated as a dynamic species with a Maxwellian background distribution. The dependence of the zero particle flux density gradient (peaking factor) on fast ion density, temperature and corresponding gradients, is investigated. The simulations show that the inclusion of a fast ion species has a stabilizing influence on the ITG mode and reduces the peaking of the main ion and electron density profiles in the absence of sources. The models mostly reproduce the experimentally obtained density peaking for the L-mode discharge whereas the H-mode density peaking is significantly underpredicted, indicating the importance of the NBI particle source for the H-mode density profile.
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| 4. |
- Fazendeiro, Luis, 1976, et al.
(författare)
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Gyrokinetic simulations of turbulent transport in JET-like plasmas
- 2013
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Ingår i: 40th European Physical Society Conference on Plasma Physics. ; 2, s. 1074-1077
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this work turbulent transport in JET-like plasma discharges is analyzed, with main emphasison impurity transport driven by ITG/TEM modes. Gyrokinetic (GK) simulations are performedusing the GENE code [1, 2], in both quasilinear (QL) and nonlinear (NL) mode,1 and the resultsare compared with a computationally efficient fluid model [3, 4]. Particle transport is quantifiedby locally finding density gradients (R/Ln ) that yield zero particle flux, signifying a balancebetween convective and diffusive transport.The impact of the magnetic equilibrium (circular, s − α and realistic magnetic geometry) onthe various models is discussed, as well as the effects of collisionality (in both fluid and GK)and the inclusion of a (2%) Carbon background (in GK), as per JET CFC wall conditions. Theeffect of sheared toroidal rotation was also investigated and found to be important, although notfor the particular JET discharge studied in this work.
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| 5. |
- Nordman, Hans, 1957, et al.
(författare)
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Fluid and gyrokinetic simulations of impurity transport at JET
- 2011
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 53:10, s. 105005-
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Tidskriftsartikel (refereegranskat)abstract
- Impurity transport coefficients due to Ion-Temperature-Gradient (ITG) mode and Trapped-Electron (TE) mode turbulence are calculated using profile data from dedicated impurity injection experiments at JET. Results obtained with a multi-fluid model are compared with quasi-linear and nonlinear gyrokinetic simulation results obtained with the code GENE. The sign of the impurity convective velocity (pinch) and its various contributions are discussed. The dependence of the impurity transport coefficients and impurity peaking factor -∇nZ/nZ on plasma parameters like impurity charge number Z, ion logarithmic temperature gradient, collisionality, ExB shearing, and charge fraction are investigated. It is found that for the studied ITG dominated JET discharges, both the fluid and gyrokinetic results show an increase of the impurity peaking factor for low Z-values followed by a saturation at moderate values of impurity peaking, much below the neoclassical predictions, for large values of Z. The results are in qualitative agreement with the experimental trends observed for the injected impurities (Ne, Ar, Ni) whereas for the background carbon species the observed flat or weakly hollow C profiles are not well reproduced by the simulations.
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| 6. |
- Nordman, Hans, 1957, et al.
(författare)
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Modelling of impurity transport experiments at the Joint European Torus
- 2010
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Ingår i: Proceedings of EPS 2010, Europhysics Conference Abstracts. - 2914771622 ; 34A
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Konferensbidrag (refereegranskat)abstract
- Impurity transport in JET is studied using interpretative analysis and predictive simulations of JET discharges. The simulations are based on transport models for Ion-Temperature-Gradient (ITG) mode and Trapped-Electron (TE) mode driven turbulence and neoclassicaltransport. The properties of the impurity transport coefficients obtained with fluid as well as quasi-linear and nonlinear gyrokinetic simulations using the code GENE are compared and discussed. In particular, the sign of the impurity convective velocity (pinch) and the scaling of the normalised impurity density peaking factor -R∇nZ/nZ with impurity charge number is investigated. Predictive simulations of temperatures (Te, Ti=TZ) and densities (ne,nZ) are performed with the JETTO/SANCO core transport code.The scaling of impurity transport with impurity charge Z is crucial for the performance and optimisation of a fusion reactor. In the present study, a set of dedicated JET impurity injection experiments are analysed. The impurities were injected by laser ablation (Ni) and gas injection (Ne, Ar) and the diffusivity DZ and convective velocity VZ were determined by matching spectroscopic data with predictive results obtained with the transport code UTC-SANCO.
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| 7. |
- Skyman, Andreas, 1982, et al.
(författare)
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Effects of the equilibrium model on impurity transport in tokamaks
- 2014
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 54:1, s. 013009-
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Tidskriftsartikel (refereegranskat)abstract
- Gyrokinetic simulations of ion temperature gradient mode and trapped electron mode driven impurity transport in a realistic tokamak geometry are presented and compared with results using simplified geometries. The gyrokinetic results, obtained with the GENE code in both linear and non-linear modes are compared with data and analysis for a dedicated impurity injection discharge at JET. The impact of several factors on heat and particle transport is discussed, lending special focus to tokamak geometry and rotational shear. To this end, results using s–α and concentric circular equilibria are compared with results with magnetic geometry from a JET experiment. To further approach experimental conditions, non-linear gyrokinetic simulations are performed with collisions and a carbon background included.The impurity peaking factors, computed by finding local density gradients corresponding to zero particle flux, are discussed. The impurity peaking factors are seen to be reduced by a factor of ~2 in realistic geometry compared with the simplified geometries, due to a reduction of the convective pinch. It is also seen that collisions reduce the peaking factor for low-Z impurities, while increasing it for high charge numbers, which is attributed to a shift in the transport spectra towards higher wavenumbers with the addition of collisions. With the addition of roto-diffusion, an overall reduction of the peaking factors is observed, but this decrease is not sufficient to explain the flat carbon profiles seen at JET.
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| 8. |
- Skyman, Andreas, 1982, et al.
(författare)
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Gyrokinetic modelling of stationary electron and impurity profiles in tokamaks
- 2014
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 21:9, s. 092305-
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Tidskriftsartikel (refereegranskat)abstract
- Particle transport due to Ion Temperature Gradient (ITG)/Trapped Electron Mode (TEM) turbulence is investigated using the gyrokinetic code GENE. Both a reduced quasilinear treatment and nonlinear simulations are performed for typical tokamak parameters corresponding to ITG dominated turbulence. The gyrokinetic results are compared and contrasted with results from a computationally efficient fluid model. A selfconsistent treatment is used, where the stationary local profiles are calculated corresponding to zero particle flux simultaneously for electrons and trace impurities. The scaling of the stationary profiles with magnetic shear, safety factor, electron-to-ion temperature ratio, collisionality, toroidal sheared rotation, plasma β, triangularity, and elongation is investigated. In addition, the effect of different main ion mass on the zero flux condition is discussed. The electron density gradient can significantly affect the stationary impurity profile scaling. It is therefore expected that a selfconsistent treatment will yield results more comparable to experimental results for parameter scans where the stationary background density profile is sensitive. This is shown to be the case in scans over magnetic shear, collisionality, elongation, and temperature ratio, for which the simultaneous zero flux electron and impurity profiles are calculated. A slight asymmetry between hydrogen, deuterium, and tritium with respect to profile peaking is obtained, in particular, for scans in collisionality and temperature ratio.
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| 9. |
- Skyman, Andreas, 1982
(författare)
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Gyrokinetic simulations of turbulent transport in tokamak plasmas
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the enormous growth of high performance computing (HPC) over the last few decades, plasma physicists have gained access to a valuable instrument for investigating turbulent plasma behaviour. In this thesis, these tools are utilised for the study of particle transport in fusion devices of the tokamak variety.The transport properties of impurities is a major part of the work. This is of high relevance for the performance and optimisation of magnetic fusion devices. For instance, the possible accumulation of He ash in the core of the reactor plasma will serve to dilute the fuel, thus lowering fusion power. Heavier impurity species, originating from the plasma-facing surfaces, may also accumulate in the core, and wall-impurities of relatively low density may lead to unacceptable energy losses in the form of radiation. In an operational power plant, such as the ITER device, both impurities of low and high charge numbers will be present.This thesis studies turbulent particle transport driven by two different modes of drift wave turbulence: the trapped electron (TE) and ion temperature gradient (ITG) modes. Results for ITG mode driven impurity transport are also compared with experimental results from the Joint European Torus.Principal focus is on the balance of convective and diffusive transport, as quantified by the stationary density gradient of zero flux (“peaking factor”, PF). Quasi- and nonlinear results are obtained using the gyrokinetic code GENE, and compared with results from a computationally efficient multi-fluid model. The results are scalings of PF with the driving background gradients of temperature and density, and other parameters, including plasma shape and sheared toroidal rotation.
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| 10. |
- Skyman, Andreas, 1982, et al.
(författare)
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Impurity transport in ITG and TE mode dominated turbulence
- 2010
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Ingår i: EPS 2010.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The transport properties of impurities is of high relevancefor the performance and optimisation of magnetic fusiondevices. For instance, impurities from plasma-facing surfaces accumulating in the core dilute the plasma and leadto unacceptable energy loss in the form of radiation.In the present study, turbulent impurity transport in tokamak plasmas, driven by Ion Temperature Gradient (ITG)and Trapped Electron (TE) modes, is investigated usingfluid and gyrokinetic models. Quasilinear (QL) results obtained from the GENE code are compared with fluidresults for ITG and TE mode dominated turbulence.Scalings of the peaking factor with impurity charge (Z) andvarious parameters are studied. Of particular interest areconditions favouring an outward convective impurity flux.
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