| 1. |
- Abel, I, et al.
(författare)
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Overview of the JET results with the ITER-like wall
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:10, s. 104002-
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Tidskriftsartikel (refereegranskat)abstract
- Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma-facing materials. The large reduction in the carbon content (more than a factor ten) led to a much lower Z(eff) (1.2-1.4) during L- and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that the fuel retention rate with the new wall is substantially reduced with respect to the C wall. The re-establishment of the baseline H-mode and hybrid scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type-I ELMy H-mode regimes with H-98,H-y2 close to 1 and beta(N) similar to 1.6 have been achieved using gas injection. ELM frequency is a key factor for the control of the metallic impurity accumulation. Pedestal temperatures tend to be lower with the new wall, leading to reduced confinement, but nitrogen seeding restores high pedestal temperatures and confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on Be wall plasma-facing components due to lower radiation made the routine use of massive gas injection for disruption mitigation essential.
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| 2. |
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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- 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
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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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| 8. |
- 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: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 19:8
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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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| 9. |
- Anderson, Johan, 1973, et al.
(författare)
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Fractional Fokker-Planck Equation vs Tsallis’ Statistical Mechanics
- 2013
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Ingår i: Festival-de-Theorie. ; 7, s. 4-
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Konferensbidrag (refereegranskat)abstract
- In this paper we present a study of anomalous diffusion using a Fokker-Planck descriptionwith fractional velocity derivatives. The distribution functions are foundusing numerical means for varying degree of fractionality observing the transitionfrom a Gaussian distribution to a L´evy distribution. The statistical properties of thedistribution functions are assessed by a generalized expectation measure and entropyin terms of Tsallis statistical mechanics. We find that the ratio of the generalizedentropy and expectation is increasing with decreasing fractionality towards the wellknown so-called sub-diffusive domain, indicating a self-organising behavior.
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| 10. |
- 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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