In the last 30 days

• Open/close Thermodynamics of neoclassical and turbulent transport

  X Garbet et al 2012 Plasma Phys. Control. Fusion 54 055007 Tag this article

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  A variational principle based on the calculation of the entropy production rate is derived, which covers particle, momentum and heat transport. This principle is used to define proper thermodynamical forces and fluxes. When turbulent parallel wavenumbers are small, and fluctuations are ballooned, it is found that the forces are the gradients of density, velocity and temperature normalized to canonical profiles, which are power laws of the magnetic field. The transport matrix is symmetrical for a given background of fluctuations, i.e. if the dependence of the turbulence intensity on gradients is ignored. Minimization of the entropy production rate implies that the profiles tend to relax towards their canonical values, though these values are never reached simultaneously since they are linearly stable. Also it turns out that parallel and perpendicular canonical temperatures are not the same, so that the equilibrium distribution function relaxes towards a two-temperature Maxwellian. When finite turbulence parallel wavenumbers are accounted for, residual fluxes appear. Forces can be redefined to preserve Onsager symmetry but residual heat and momentum sources remain, which correspond to turbulent heating and momentum transfer.

• Open/close First implosion experiments with cryogenic thermonuclear fuel on the National Ignition Facility

  Siegfried H Glenzer et al 2012 Plasma Phys. Control. Fusion 54 045013 Tag this article

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  Non-burning thermonuclear fuel implosion experiments have been fielded on the National Ignition Facility to assess progress toward ignition by indirect drive inertial confinement fusion. These experiments use cryogenic fuel ice layers, consisting of mixtures of tritium and deuterium with large amounts of hydrogen to control the neutron yield and to allow fielding of an extensive suite of optical, x-ray and nuclear diagnostics. The thermonuclear fuel layer is contained in a spherical plastic capsule that is fielded in the center of a cylindrical gold hohlraum. Heating the hohlraum with 1.3 MJ of energy delivered by 192 laser beams produces a soft x-ray drive spectrum with a radiation temperature of 300 eV. The radiation field produces an ablation pressure of 100 Mbar which compresses the capsule to a spherical dense fuel shell that contains a hot plasma core 80 µm in diameter. The implosion core is observed with x-ray imaging diagnostics that provide size, shape, the absolute x-ray emission along with bangtime and hot plasma lifetime. Nuclear measurements provide the 14.1 MeV neutron yield from fusion of deuterium and tritium nuclei along with down-scattered neutrons at energies of 10–12 MeV due to energy loss by scattering in the dense fuel that surrounds the central hot-spot plasma. Neutron time-of-flight spectra allow the inference of the ion temperature while gamma-ray measurements provide the duration of nuclear activity. The fusion yield from deuterium–tritium reactions scales with ion temperature, which is in agreement with modeling over more than one order of magnitude to a neutron yield in excess of 10 ¹⁴ neutrons, indicating large confinement parameters on these first experiments.

• Open/close Low-frequency Alfvén eigenmodes during the sawtooth cycle at ASDEX Upgrade

  D Curran et al 2012 Plasma Phys. Control. Fusion 54 055001 Tag this article

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  The confinement of fast particles, present in tokamak plasmas as nuclear fusion products and through external heating, will be essential for any future reactor. Fast particles can be expelled from the plasma through their interaction with Alfvén eigenmode (AE) instabilities. AEs can exist in gaps in the Alfvén continuum created by plasma equilibrium non-uniformities. In ASDEX Upgrade low-frequency modes in the Alfvén-acoustic frequency regime, including beta-induced Alfvén eigenmodes (BAEs) and lower frequency modes with mixed Alfvén and acoustic polarizations, have been observed. They exist in gaps in the Alfvén continuum opened up by geodesic curvature and finite plasma compressibility. In this paper a kinetic dispersion relation (Lauber et al 2009 Plasma Phys. Control. Fusion 51 124009) is solved numerically to investigate the influence of diamagnetic effects on the evolution of these low-frequency modes during the sawtooth cycle. Other distinct but potentially related modes which sweep significantly upwards in frequency towards the end of the sawtooth cycle are also considered. Using information gained from soft x-ray measurements (Igochine et al 2010 IPP Report 1/338) and electron temperature information from electron cyclotron emission to constrain the safety factor profiles, realistic equilibrium reconstructions for the analysis are obtained using the CLISTE code (Mc Carthy 2012 Plasma Phys. Control. Fusion 54 015010). The results for the mode frequency evolution are then compared with experimental results from ASDEX Upgrade.

• Open/close Dynamics of particle flux in a cylindrical magnetized plasma

  S Oldenbürger et al 2012 Plasma Phys. Control. Fusion 54 055002 Tag this article

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  Fluctuation-induced cross-field convective particle flux is studied in the cylindrical magnetized plasma assembly for non-linear turbulence analysis (PANTA). The plasma presents a wave turbulence characterized by a limited number of developing modes. To study the link between mode dynamics and particle transport, frequency-dependent time evolution of transport is obtained by applying complex continuous wavelet decomposition on spatio-temporal data from electrostatic probes. The method is shown to recover transport description consistent with other analysis methods for a weakly turbulent state. For a plasma state featuring a large non-sinusoidal structure, the time evolution of transport on the second harmonic of the main fluctuation shows a slow modulation of azimuthal mode number m = 0 with similarities to a zonal perturbation.

• Open/close Zonal flows in plasma—a review

  P H Diamond et al 2005 Plasma Phys. Control. Fusion 47 R35 Tag this article

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  A comprehensive review of zonal flow phenomena in plasmas is presented. While the emphasis is on zonal flows in laboratory plasmas, planetary zonal flows are discussed as well. The review presents the status of theory, numerical simulation and experiments relevant to zonal flows. The emphasis is on developing an integrated understanding of the dynamics of drift wave–zonal flow turbulence by combining detailed studies of the generation of zonal flows by drift waves, the back-interaction of zonal flows on the drift waves, and the various feedback loops by which the system regulates and organizes itself. The implications of zonal flow phenomena for confinement in, and the phenomena of fusion devices are discussed. Special attention is given to the comparison of experiment with theory and to identifying directions for progress in future research.

• Open/close One-dimensional study of the radiation-dominated implosion of a cylindrical tungsten plasma column

  M M Basko et al 2012 Plasma Phys. Control. Fusion 54 055003 Tag this article

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  Spectral properties of the x-ray pulses, generated by perfectly uniform cylindrical implosions of tungsten plasma with parameters typical of wire array z-pinches, are investigated under the simplifying assumption that the final stage of the kinetic-to-radiant energy conversion is not affected by the magnetic field. The x-ray emission is shown to be generated within a narrow (sub-micron) radiation-dominated stagnation shock front with a ‘supercritical’ amplitude. The structure of the stagnation shock is investigated using two independent radiation-hydrodynamics codes, and by constructing an approximate analytical model. The x-ray spectra are calculated for two values of the plasma column mass, 0.3 and 6 mg cm ⁻¹, with a newly developed two-dimensional radiation-hydrodynamics code RALEF-2D. The hard component of the spectrum (with a blackbody-fit temperature of 0.5–0.6 keV for the 6 mg cm ⁻¹ mass) originates from a narrow peak of the electron temperature inside the stagnation shock. The softer main component emerges from an extended halo, where the primary shock radiation is reemitted by colder layers of the imploding plasma. Our calculated x-ray spectrum for the 6 mg cm ⁻¹ tungsten column agrees well with the published Sandia experimental data (Foord et al 2004 Phys. Rev. Lett. 93 055002).

• Open/close Fueling characteristics of supersonic gas puffing applied to large high-temperature plasmas in the Large Helical Device

  A Murakami et al 2012 Plasma Phys. Control. Fusion 54 055006 Tag this article

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  Supersonic gas puffing (SSGP), where a high-pressure gas is ejected through a fast solenoid valve equipped with a Laval nozzle, has been applied to large high-temperature plasmas and its fueling characteristics have been investigated in the Large Helical Device. The fueling efficiency of SSGP depends on the target plasma density and decreases as the density increases. This is due to the fueling mechanism of SSGP, where the fuel particles are supplied to the plasma edge region and then transported to the core region by diffusion. SSGP locally supplies a large number of particles to the edge region within a short time on the order of milliseconds. A fueling efficiency of ∼20% can be achieved by SSGP at a low initial density of ∼1.5 × 10 ¹⁹ m ⁻³, which is more than twice as high as that of ordinary gas puffing at a similar density. Furthermore, this property leads to the additional effect of edge cooling to SSGP that will be beneficial for divertor heat load reduction.

• Open/close A kinematic model of relativistic laser absorption in an overdense plasma

  P Gibbon et al 2012 Plasma Phys. Control. Fusion 54 045001 Tag this article

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  An analytical model of laser absorption for planar radiation on steep, high density plasma surfaces is presented. Within the constraints of classical electrodynamics, the model is valid for arbitrary intensity and incidence angle, and is insensitive to the details of the absorption mechanism. At intensities beyond 10 ²⁰ W cm ⁻², consistently high (>50%) absorption with a flat angular dependence is found. The model's convenient closed form makes it widely applicable to laser-driven ion acceleration schemes, hard x-ray sources and the fast igniter fusion concept.

• Open/close Particle-in-cell simulation study of the parametric instabilities and turbulence of Alfvén-ion-cyclotron waves in a low-beta plasma

  Helen H Kaang et al 2012 Plasma Phys. Control. Fusion 54 055004 Tag this article

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  Parametric instabilities of Alfvén-ion-cyclotron (AIC) waves and the turbulence driven by them are investigated using a particle-in-cell simulation technique. By introducing anisotropic ion temperature, a broad spectrum of AIC wave is excited. In addition to the normal AIC waves, their harmonic modes and upper sideband modes are found. Through the interaction of these Alfvénic waves and longitudinal electrostatic waves, Alfvénic turbulence is developed. Excitation of density and electromagnetic waves and their mode coupling structures are investigated using the ω– k spectrum and bicoherence analysis. It is found that the ion-acoustic wave is excited by the modulational instability, and that the ion density mode with a negative group velocity is excited in the high- k region by the decay instability. In addition, a longitudinal mode whose phase velocity behaves similarly to that of the ion-acoustic mode with a shifted wave number is found. This mode is identified as the second harmonic ion-acoustic wave. The inverse cascade structures of the electromagnetic and density fluctuations exhibit an interesting behavior that the density fluctuation shows a dual spectrum, whereas the electrostatic field E _x shows a single spectrum.

• Open/close ITER: burning plasma physics experiment

  B J Green and ITER International Team and Participant Teams 2003 Plasma Phys. Control. Fusion 45 687 Tag this article

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  The ITER project has an overall programmatic objective `to demonstrate the scientific and technological feasibility of fusion energy for peaceful purposes'. The ITER tokamak design is the embodiment of the finding, endorsed by all the parties to the project, that `the full non-linear interplay between alpha-particle heating, confinement barriers and pressure and profile control, and their compatibility with a divertor can be addressed only in an integrated step'. This step involves not only the study of fusion burning plasma physics but also the demonstration of key items of fusion reactor technology.

  This paper introduces: magnetic fusion plasma (section 1), the physics goals of ITER (section 2) and the burning plasma physics which will be studied (section 3), examines the present ITER design and how it meets the plasma performance requirements in terms of extended burn at specific fusion power amplification (section 4), describes plasma measurements for ITER and some of the issues involved in their implementation (section 5), indicates how ITER provides a focus for international research into tokamak plasma physics (section 6), and presents a brief report on the status of the ITER Project (section 7). To conclude, this paper presents some final remarks (section 8).