| 1. |
- Kiyani, K. H., et al.
(författare)
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Global Scale-Invariant Dissipation in Collisionless Plasma Turbulence
- 2009
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 103:7, s. 075006-
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Tidskriftsartikel (refereegranskat)abstract
- A higher-order multiscale analysis of the dissipation range of collisionless plasma turbulence is presented using in situ high-frequency magnetic field measurements from the Cluster spacecraft in a stationary interval of fast ambient solar wind. The observations, spanning five decades in temporal scales, show a crossover from multifractal intermittent turbulence in the inertial range to non-Gaussian mono-scaling in the dissipation range. This presents a strong observational constraint on theories of dissipation mechanisms in turbulent collisionless plasmas.
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| 2. |
- Lorenzini, R., et al.
(författare)
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Self-organized helical equilibria as a new paradigm for ohmically heated fusion plasmas
- 2009
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Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481. ; 5:8, s. 570-574
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Tidskriftsartikel (refereegranskat)abstract
- In the quest for new energy sources, the research on controlled thermonuclear fusion has been boosted by the start of the construction phase of the International Thermonuclear Experimental Reactor (ITER). ITER is based on the tokamak magnetic configuration, which is the best performing one in terms of energy confinement. Alternative concepts are however actively researched, which in the long term could be considered for a second generation of reactors. Here, we show results concerning one of these configurations, the reversed-field pinch (RFP). By increasing the plasma current, a spontaneous transition to a helical equilibrium occurs, with a change of magnetic topology. Partially conserved magnetic flux surfaces emerge within residual magnetic chaos, resulting in the onset of a transport barrier. This is a structural change and sheds new light on the potential of the RFP as the basis for a low-magnetic-field ohmic fusion reactor.
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| 3. |
- Martin, P., et al.
(författare)
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Overview of RFX-mod results
- 2009
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 49:10, s. 104019-
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Tidskriftsartikel (refereegranskat)abstract
- With the exploration of the MA plasma current regime in up to 0.5 s long discharges, RFX-mod has opened new and very promising perspectives for the reversed field pinch (RFP) magnetic configuration, and has made significant progress in understanding and improving confinement and in controlling plasma stability. A big leap with respect to previous knowledge and expectations on RFP physics and performance has been made by RFX-mod since the last 2006 IAEA Fusion Energy Conference. A new self-organized helical equilibrium has been experimentally achieved ( the Single Helical Axis-SHAx-state), which is the preferred state at high current. Strong core electron transport barriers characterize this regime, with electron temperature gradients comparable to those achieved in tokamaks, and by a factor of 4 improvement in confinement time with respect to the standard RFP. RFX-mod is also providing leading edge results on real-time feedback control of MHD instabilities, of general interest for the fusion community.
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| 4. |
- McClements, K. G., et al.
(författare)
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Surfatron and stochastic acceleration of electrons in astrophysical plasmas
- 2005
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 71:2, s. 127-141
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Tidskriftsartikel (refereegranskat)abstract
- Electron acceleration by large amplitude electrostatic waves in astro-physical plasmas is studied using particle-in-cell (PIC) simulations. The waves are excited initially at the electron plasma frequency ωpe by a Buneman instability driven by ion beams: the parameters of the ion beams are appropriate for high Mach number astrophysical shocks, such as those associated with supernova remnants (SNRs). If ωpe is much higher than the electron cyclotron frequency Ωe, the linear phase of the instability does not depend on the magnitude of the magnetic field. However, the subsequent time evolution of particles and waves depends on both ωpe/Ωeand the size of the simulation box L. If L is equal to one wavelength, λ0, of the Buneman-unstable mode, electrons trapped by the waves undergo acceleration via the surfatron mechanism across the wave front. This occurs most efficiently when ωpe/Ω ≃ 100: in this case electrons are accelerated to speeds of up c/2 where c is the speed of light. In a simulation with L = 4λ0 and ωpe/ Ωe = 100, it is found that sideband instabilities give rise to a broad spectrum of wavenumbers, with a power law tail. Some stochastic electron acceleration is observed in this case, but not the surfatron process. Direct integration of the electron equations of motion, using parameters approximating to those of the wave modes observed in the simulations, suggests that the surfatron is compatible with the presence of a broad wave spectrum if Ωpe/Ωe > 100. It is concluded that a combination of stochastic and surfatron acceleration could provide an efficient generator of mildly relativistic electrons at SNR shocks.
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