| 1. |
- Albert, F., et al.
(författare)
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Betatron x-ray radiation from laser-plasma accelerators driven by femtosecond and picosecond laser systems
- 2018
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 25:5
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Tidskriftsartikel (refereegranskat)abstract
- A comparative experimental study of betatron x-ray radiation from laser wakefield acceleration in the blowout and self-modulated regimes is presented. Our experiments use picosecond duration laser pulses up to 150 J (self-modulated regime) and 60 fs duration laser pulses up to 10 J (blowout regime), for plasmas with electronic densities on the order of 1019cm-3. In the self-modulated regime, where betatron radiation has been very little studied compared to the blowout regime, electrons accelerated in the wake of the laser pulse are subject to both the longitudinal plasma and transverse laser electrical fields. As a result, their motion within the wake is relatively complex; consequently, the experimental and theoretical properties of the x-ray source based on self-modulation differ from the blowout regime of laser wakefield acceleration. In our experimental configuration, electrons accelerated up to about 250 MeV and betatron x-ray spectra with critical energies of about 10-20 keV and photon fluxes between 108and 1010photons/eV Sr are reported. Our experiments open the prospect of using betatron x-ray radiation for applications, and the source is competitive with current x-ray backlighting methods on multi-kilojoule laser systems.
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| 2. |
- Anderson, Johan, 1973, et al.
(författare)
-
A comparison of drift wave stability in stellarator and tokamak geometry
- 2002
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 9:5, s. 1629-
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Tidskriftsartikel (refereegranskat)abstract
- The influence of plasma geometry on the linear stability of electrostatic ion-temperature-gradient driven drift modes (ITG modes) is investigated. An advanced fluid model is used for the ions together with Boltzmann distributed electrons. The derived eigenvalue equation is solved numerically. A comparison is made between an H – 1NF [Fusion Technol. 17, 123 (1990)] like stellarator equilibrium, a numerical tokamak equilibrium and the analytical s - alpha equilibrium. The numerical and the analytical tokamak are found to be in good agreement in the low inverse aspect ratio limit. The growth rates of the tokamak and stellarator are comparable whereas the modulus of the real frequency is substantially larger in the stellarator. The threshold in Ln/LT for the stellarator is found to be somewhat larger. In addition, a stronger stabilization of the ITG mode growth is found for large L n / R in the stellarator case.
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| 3. |
- Anderson, Johan, 1973, et al.
(författare)
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A fractional Fokker-Planck model for anomalous diffusion
- 2014
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 21:12, s. aricle no: 122109-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present a study of anomalous diffusion using a Fokker-Planck description withfractional velocity derivatives. The distribution functions are found using numerical means forvarying degree of fractionality of the stable Lévy distribution. The statistical properties of thedistribution functions are assessed by a generalized normalized expectation measure and entropyin terms of Tsallis statistical mechanics. We find that the ratio of the generalized entropy andexpectation is increasing with decreasing fractionality towards the well known so-called sub-diffusivedomain, indicating a self-organising behavior.
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| 4. |
- Anderson, Johan, 1973, et al.
(författare)
-
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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| 5. |
- 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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| 6. |
- 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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| 7. |
- Anderson, Johan, 1973, et al.
(författare)
-
Secondary instability of electromagnetic ion-temperature-gradient modes for zonal flow generation
- 2011
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Ingår i: Physics of Plasmas. - 1089-7674 .- 1070-664X. ; 18:072306
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Tidskriftsartikel (refereegranskat)abstract
- An analytical model for zonal flow generation by toroidal ion-temperature-gradient (ITG) modes, including finite β electromagnetic effects, is derived. The derivation is based on a fluid model for ions and electrons and takes into account both linear and nonlinear β effects. The influence of finite plasma β on the zonal flow growth rate (γ_ZF) scaling is investigated for typical tokamak plasma parameters. The results show the importance of the zonal flows close to marginal stability where γ_ZF/γ_ITG ≫ 1 is obtained. In this region the parameter γ_ZF/γ_ITG increases with β, indicating that the ITG turbulence and associated transport would decrease with β at a faster rate than expected from a purely linear or quasi-linear analysis.
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| 8. |
- Anderson, Johan, 1973, et al.
(författare)
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Signature of a universal statistical description for drift-wave plasma turbulence
- 2010
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 17:11
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Tidskriftsartikel (refereegranskat)abstract
- This letter provides a theoretical interpretation of numerically generated probability densityfunctions ͑PDFs͒ of intermittent plasma transport events. Specifically, nonlinear gyrokineticsimulations of ion-temperature-gradient turbulence produce the time series of heat flux thatmanifestly exhibit non-Gaussian PDFs with enhanced tails. It is demonstrated that, after the removalof autocorrelations, the numerical PDFs can be matched with predictions from a fluid theoreticalsetup based on the instanton method. This result points to a universality in the modeling ofintermittent stochastic process offering a predictive capability.
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| 9. |
- Anderson, Johan, 1973, et al.
(författare)
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Statistical analysis and modeling of intermittent transport events in the tokamak scrape-off layer
- 2014
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 21:12, s. article no. 122306-
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Tidskriftsartikel (refereegranskat)abstract
- The turbulence observed in the scrape-off-layer of a tokamak is often characterized by intermittentevents of bursty nature, a feature which raises concerns about the prediction of heat loads on thephysical boundaries of the device. It appears thus necessary to delve into the statistical propertiesof turbulent physical fields such as density, electrostatic potential, and temperature, focusing on themathematical expression of tails of the probability distribution functions. The method followedhere is to generate statistical information from time-traces of the plasma density stemming fromBraginskii-type fluid simulations and check this against a first-principles theoretical model. The analysis of the numerical simulations indicates that the probability distribution function of the intermittent process contains strong exponential tails, as predicted by the analytical theory.
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| 10. |
- Anderson, Johan, 1973, et al.
(författare)
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Statistical analysis of Hasegawa - Wakatani turbulence
- 2017
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 24:6, s. 062301-
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Tidskriftsartikel (refereegranskat)abstract
- Resistive drift wave turbulence is a multipurpose paradigm that can be used to understand transport at the edge of fusion devices. The Hasegawa-Wakatani model captures the essential physics of drift turbulence while retaining the simplicity needed to gain a qualitative understanding of this process. We provide a theoretical interpretation of numerically generated probability density functions (PDFs) of intermittent events in Hasegawa-Wakatani turbulence with enforced equipartition of energy in large scale zonal flows and small scale drift turbulence. We find that for a wide range of adiabatic index values the stochastic component representing the small scale turbulent eddies of the flow, obtained from the ARIMA model, exhibits super-diffusive statistics, consistent with intermittent transport. The PDFs of large events (above one standard deviation) are well approximated by the Laplace distribution, while small events often exhibit a Gaussian character. Furthermore there exist a strong influence of zonal flows for example, via shearing and then viscous dissipation maintaining a sub-diffusive character of the fluxes.
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