In the last 30 days

• Open/close On the transition between the Weibel and the whistler instabilities

  L Palodhi et al 2010 Plasma Phys. Control. Fusion 52 095007 Tag this article

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  The transition between non-resonant (Weibel-type) and resonant (whistler) instabilities is investigated numerically in plasma configurations with an ambient magnetic field of increasing amplitudes. The Vlasov–Maxwell system is solved in a configuration where the fields have three components but depend only on one coordinate and on time. The nonlinear evolution of these instabilities is shown to lead to the excitation of electromagnetic and electrostatic modes at the first few harmonics of the plasma frequency and, in the case of a large ambient magnetic field, to a long wavelength, spatial modulation of the amplitude of the magnetic field generated by the whistler instability.

• Open/close Fast-ion redistribution due to sawtooth crash in the TEXTOR tokamak measured by collective Thomson scattering

  S K Nielsen et al 2010 Plasma Phys. Control. Fusion 52 092001 Tag this article

  Article References Full text PDF (221 KB)

  Here we present collective Thomson scattering measurements of 1D fast-ion velocity distribution functions in neutral beam heated TEXTOR plasmas with sawtooth oscillations. Up to 50% of the fast ions in the centre are redistributed as a consequence of a sawtooth crash. We resolve various directions to the magnetic field. The fast-ion distribution is found to be anisotropic as expected. For a resolved angle of 39° to the magnetic field we find a drop in the fast-ion distribution of 20–40%. For a resolved angle of 83° to the magnetic field the drop is no larger than 20%.

• Open/close Investigation on the relation between edge radial electric field asymmetries in RFX-mod and density limit

  Gianluca Spizzo et al 2010 Plasma Phys. Control. Fusion 52 095011 Tag this article

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  In all major confinement devices (tokamaks, stellarators, spheromaks and reversed-field pinches—RFPs), a density limit has been found. Results summarized in a recent work by Puiatti et al ( 2009 Nucl. Fusion 49 045012) show that in the RFP high density does not cause a disruption, but a sequence of increasingly critical phenomena. First, at intermediate density there is the disappearance of the high-confinement quasisingle helicity/single helical axis regimes. Then, at densities close to the Greenwald limit, toroidally and radially localized density accumulation and radiation condensation are observed, together with a fast resistive decay of the plasma current, which constitutes the real operative limit of the device. In this paper we discuss the effect of the magnetic ripple on test particle motion, showing that the accumulation of electrons in the X-points of the edge m = 0 islands is responsible for a modulation of the radial electric field E _r which is at the core of the density limit mechanism. These results can be also relevant for the explanation of X-point multifaceted asymmetric radiation from the edge formation, observed in L-mode density limit discharges of JET.

• Open/close A review of experimental drift turbulence studies

  G R Tynan et al 2009 Plasma Phys. Control. Fusion 51 113001 Tag this article

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  Experimental drift turbulence and zonal flow studies in magnetically confined plasma experiments are reviewed. The origins of drift waves, transition to drift turbulence and drift turbulence–zonal flow interactions in open field line and toroidal closed flux surface experiments are discussed and the free energy sources, dissipation mechanisms and nonlinear dynamics of drift turbulence in the core, edge and scrape-off layer plasma regions are examined. Evidence that turbulence across these regions is linked and that turbulence-driven zonal flows exist is presented, and evidence that these flows help regulate the turbulent scale lengths, amplitude and fluxes is summarized. Seemingly contradictory reports exist regarding the scale of turbulent transport events; gyro-Bohm behavior of turbulence correlation lengths as well as evidence for long-range transport phenomena both exist. Changes in turbulence during and after transport barrier formation are summarized and compared. The inferred turbulent particle and heat fluxes due to turbulent transport are usually consistent with global confinement, and edge plasma momentum transport appears to be linked to plasma flows at the last-closed flux surface and in the open field line region. However, inconsistencies between observed transport and turbulence have sometimes been reported and are pointed out here. Special attention is given to open issues, and suggestions for future experimental studies are given.

• Open/close Estimation of profiles of the effective ion charge at ASDEX Upgrade with Integrated Data Analysis

  S K Rathgeber et al 2010 Plasma Phys. Control. Fusion 52 095008 Tag this article

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  The knowledge of the effective ion charge Z _eff which indicates the degree of plasma pollution is of high interest, since the tolerable impurity concentration to achieve successful ignition of a fusion plasma is limited. The quantification of Z _eff at ASDEX Upgrade has been carried out by means of Integrated Data Analysis in the framework of Bayesian probability theory which allows the combined analysis of different bremsstrahlung and charge exchange diagnostics to obtain one joint Z _eff profile. With this tool it is possible to assess the quality of the data acquired by the individual diagnostics and to find a reasonable selection of data for the routine Z _eff analysis. The Z _eff results are validated by the examination of different discharge scenarios and successfully applied to simulations of the neutron rate and the loop voltage. The investigation of Z _eff for nitrogen seeded discharges reveals a correlation between the confinement improvement and the increase in Z _eff.

• Open/close Numerical simulation on edge localized mode control capability of resonant magnetic perturbation in the KSTAR tokamak

  Doohyun Kim et al 2010 Plasma Phys. Control. Fusion 52 095009 Tag this article

  Article References Full text PDF (1.52 MB)

  Numerical simulations are carried out to investigate the applicability of resonant magnetic perturbation (RMP) to KSTAR plasmas for a possible control of edge localized mode (ELM) to suppress or mitigate its damages to divertor materials. For the verification of the feasibility of RMP application, magnetic island configurations, resonant normal fields, magnetic island widths and Chirikov parameters are calculated for two types of KSTAR operation scenarios: steady state and hybrid. Field error correction (FEC) coils in KSTAR are considered to produce externally perturbed magnetic fields for RMP, and the directions of coil currents determine the toroidal mode n and the parity (even or odd). The RMP configurations are described by vacuum superposition of the equilibrium magnetic fields and the perturbed ones induced by FEC coils. The numerical simulations for n = 2 toroidal mode in both operation scenarios show that when the pitches of the equilibrium and perturbed magnetic fields are well aligned, magnetic islands are formed for a series of m poloidal modes and the adjacent islands are overlapped to generate a stochastic layer in the edge region. Even parity turns out to be more effective in making the magnetic islands overlapped to become stochastic field lines in the steady-state operation, while odd parity in the hybrid operation. The formation of the stochastic layer is verified by the calculated Chirikov parameters, which also give basic information on the current requirement of FEC coils. Additionally, lobe structures of stochastic field lines are found in the edge region extended to the divertor plate in the hybrid scenario. Based on the standard vacuum criteria for RMP, the simulation results indicate that the FEC coils will be feasible for control of ELMs and mitigation of divertor heat load by RMP in both steady-state and hybrid operation scenarios.

• Open/close Study of plasma sheath dynamics by using two magnetic probes in a low energy plasma focus device

  R A Behbahani et al 2010 Plasma Phys. Control. Fusion 52 095004 Tag this article

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  The effects of the argon gas pressure, charging voltage and anode shape on the current sheath dynamics in a low energy (4.9 kJ) Mather type plasma focus (PF) were investigated. The formation and dynamics of the current sheath were monitored by using two magnetic probes, which were inserted radially and axially to explore the evolution of the plasma sheath and to estimate the range of its velocity during the break-down and run-down phases. The radial magnetic probe measurements showed a rather constant current sheath velocity near the insulator, which was more sensitive to the variations of the gas pressure than the charging voltage, and the current sheath did not lose its uniformity by expanding away from the insulator during the break-down phase. The results found from the axial magnetic probe signals revealed a higher current sheath velocity inside the step region of the step anode than the cylindrical one. The simulated axial current sheath trajectories (Lee's model) that were obtained after the fitting process of the current signals showed good agreement both for the cylindrical anode throughout the run-down phase and the step anode before the step region. Inside the step region, the separation between the simulated and the experimental trajectories of the step anode was increased at greater axial distances.

• Open/close Impurity transport studies in RFX-mod multiple helicity and enhanced confinement QSH regimes

  S Menmuir et al 2010 Plasma Phys. Control. Fusion 52 095001 Tag this article

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  The transport of intrinsic and artificially introduced impurities is investigated in the multiple helicity (MH) and quasi-single helicity (QSH) plasma regimes of the RFX-mod reversed-field pinch. Impurities are introduced through Ni laser blow-off and Ne gas puffing in both magnetic confinement scenarios. Simulation of experimental spectroscopic measurements, including line and soft x-ray emission, electron density and radiated power is carried out with a one-dimensional collisional–radiative model. For both QSH and MH an outwardly directed pinch velocity with a large barrier in the edge and no reversal is determined, accompanied by a diffusion coefficient that is an order of magnitude greater in the core than at the edge. The established transport parameters are similar for the Ni and Ne injected impurities with the transition region and barrier less external and stronger in the QSH regime.

• Open/close Study of ion-temperature-gradient modes in RFX-mod using TRB code

  F Sattin et al 2010 Plasma Phys. Control. Fusion 52 105002 Tag this article

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  We present here a study about the stability of ion-temperature-gradient drift turbulence in the quasi-single-helicity regime of RFX-mod reversed field pinch (RFP) using the TRB fluid electrostatic turbulence code. Our results suggest that present-day RFP plasmas are marginally stable against this kind of turbulence. The onset of the instability may be envisaged for near future regimes, in the presence of hotter plasmas with sharper internal transport barriers.

• Open/close Diamagnetic measurements and plasma energy in toroidal systems

  V D Pustovitov 2010 Plasma Phys. Control. Fusion 52 085005 Tag this article

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  The sensitivity of a diamagnetic signal to several operational and geometrical factors is analysed. Among them are the flux conservation in the plasma, eddy currents induced in the outer structures in fast processes, toroidal shift and deformation of the plasma boundary due to its energy change and inhomogeneity of the confining magnetic field. It is shown that in each case, under proper experimental circumstances, the contribution, unaccounted in the traditional theory of diamagnetic measurements, can reach a level compared with β (ratio of the volume-averaged plasma pressure to the magnetic field pressure). The approach is fully analytical with all relevant dependences shown explicitly, allowing easy estimates and suggesting a resolution of the problem in order to restore the accuracy of finding β from diamagnetic measurements. This essentially extends the analysis by Yamaguchi et al ( 2006 Plasma Phys. Control. Fusion 48 L73) of possible measures to improve the separation of the useful fraction of the measured diamagnetic signal. The approach is aimed at explaining the discrepancies between model estimates and experimental results, unification of a knowledge obtained in separate numerical studies, extending a theoretical basis of magnetic diagnostics and uncovering potential dangers in interpretations. This is also an essential step from traditional cylindrical theory to analytical derivations in the toroidal geometry. The results are equally applicable to tokamaks and stellarators.