| 21. |
- 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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| 22. |
- Anderson, Johan, 1973, et al.
(författare)
-
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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| 23. |
- Anderson, Johan, 1973, et al.
(författare)
-
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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| 24. |
- Anderson, Johan, 1973, et al.
(författare)
-
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-
-
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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| 25. |
- Anderson, Johan, 1973, et al.
(författare)
-
Statistical properties of Charney-Hasegawa-Mima zonal flows
- 2015
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:052305
-
Tidskriftsartikel (refereegranskat)abstract
- A theoretical interpretation of numerically generated probability density functions (PDFs) ofintermittent plasma transport events in unforced zonal flows is provided within the Charney-Hasegawa-Mima (CHM) model. The governing equation is solved numerically with variousprescribed density gradients that are designed to produce different configurations of parallel andanti-parallel streams. Long-lasting vortices form whose flow is governed by the zonal streams. It isfound that the numerically generated PDFs can be matched with analytical predictions of PDFsbased on the instanton method by removing the autocorrelations from the time series. In manyinstances, the statistics generated by the CHM dynamics relaxes to Gaussian distributions for boththe electrostatic and vorticity perturbations, whereas in areas with strong nonlinear interactions it isfound that the PDFs are exponentially distributed.
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| 26. |
- Anderson, Johan, 1973, et al.
(författare)
-
Zonal flow generation in ITG turbulence
- 2002
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 9, s. 4500-
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work the zonal flow (ZF) growth rate in toroidal ion-temperature-gradient (ITG) mode turbulence including the effects of elongation is studied analytically. The scaling of the ZF growth with plasma parameters is examined for typical tokamak parameter values. The physical model used for the toroidal ITG driven mode is based on the ion continuity and ion temperature equations whereas the ZF evolution is described by the vorticity equation. The results indicate that a large ZFgrowth is found close to marginal stability and for peaked density profiles and these effects may be enhanced by elongation.
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| 27. |
- Angioni, C., et al.
(författare)
-
The impact of poloidal asymmetries on tungsten transport in the core of JET H-mode plasmas
- 2015
-
Ingår i: Physics of Plasmas. - : AMER INST PHYSICS. - 1070-664X .- 1089-7674. ; 22:5
-
Tidskriftsartikel (refereegranskat)abstract
- Recent progress in the understanding and prediction of the tungsten behaviour in the core of JET H-mode plasmas with ITER-like wall is presented. Particular emphasis is given to the impact of poloidal asymmetries of the impurity density. In particular, it is shown that the predicted reduction of temperature screening induced by the presence of low field side localization of the tungsten density produced by the centrifugal force is consistent with the observed tungsten behaviour in a JET discharge in H-mode baseline scenario. This provides first evidence of the role of poloidal asymmetries in reducing the strength of temperature screening. The main differences between plasma parameters in JET baseline and hybrid scenario discharges which affect the impact of poloidally asymmetric density on the tungsten radial transport are identified. This allows the conditions by which tungsten accumulation can be avoided to be more precisely defined.
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| 28. |
- Annibaldi, Silvia Valeria, et al.
(författare)
-
Strong transport reduction in the helical core of the reversed-field pinch
- 2007
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 14:11, s. 112515-
-
Tidskriftsartikel (refereegranskat)abstract
- An explanation of the strong heating observed in the core of a reversed-field pinch in the quasi-single-helicity state is presented. A magnetic island is formed, in which the heat transport coefficient is much smaller than in the surrounding chaotic sea, because of the formation of well defined magnetic surfaces. The values of the thermal conductivity obtained with the M1TEV [F. Porcelli , Phys. Rev. Lett 82, 1458 (1999)] two-dimensional transport code are in very good agreement with the estimates of the ion diffusion coefficient inside the island, given by a Hamiltonian guiding center code. Moreover, the values of thermal conductivity are in the tokamak range, and are consistent with the peak temperatures measured in the Reversed Field eXperiment [P. Sonato , Fusion Eng. Des. 66-68, 161 (2003)] at Consorzio RFX, Padova, Italy. The effect of the island width and the different powers deposited inside the island on the final temperature peak are also investigated.
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| 29. |
- Audet, T. L., et al.
(författare)
-
Investigation of ionization-induced electron injection in a wakefield driven by laser inside a gas cell
- 2016
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 23:2
-
Tidskriftsartikel (refereegranskat)abstract
- Ionization-induced electron injection was investigated experimentally by focusing a driving laser pulse with a maximum normalized potential of 1.2 at different positions along the plasma density profile inside a gas cell, filled with a gas mixture composed of 99%H2+1%N2. Changing the laser focus position relative to the gas cell entrance controls the accelerated electron bunch properties, such as the spectrum width, maximum energy, and accelerated charge. Simulations performed using the 3D particle-in-cell code WARP with a realistic density profile give results that are in good agreement with the experimental ones. The interest of this regime for optimizing the bunch charge in a selected energy window is discussed.
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| 30. |
- Aurand, B., et al.
(författare)
-
Manipulation of the spatial distribution of laser-accelerated proton beams by varying the laser intensity distribution
- 2016
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a study of the spatial profile of proton beams produced through target normal sheath acceleration using flat target foils and changing the laser intensity distribution on the target front surface. This is done by either defocusing a single laser pulse or by using a split-pulse setup and irradiating the target with two identical laser pulses with variable spatial separation. The resulting proton beam profile and the energy spectrum are recorded as functions of the focal spot size of the single laser pulse and of the separation between the two pulses. A shaping of the resulting proton beam profile, related to both an increase in flux of low-energy protons in the target normal direction and a decrease in their divergence, in one or two dimensions, is observed. The results are explained by simple modelling of rear surface sheath field expansion, ionization, and projection of the resulting proton beam.
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