In the last 30 days

• Open/close A review of the dense Z-pinch

  M G Haines 2011 Plasma Phys. Control. Fusion 53 093001 Tag this article

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  The Z-pinch, perhaps the oldest subject in plasma physics, has achieved a remarkable renaissance in recent years, following a few decades of neglect due to its basically unstable MHD character. Using wire arrays, a significant transition at high wire number led to a great improvement in both compression and uniformity of the Z-pinch. Resulting from this the Z-accelerator at Sandia at 20 MA in 100 ns has produced a powerful, short pulse, soft x-ray source >230 TW for 4.5 ns) at a high efficiency of ~15%. This has applications to inertial confinement fusion. Several hohlraum designs have been tested. The vacuum hohlraum has demonstrated the control of symmetry of irradiation on a capsule, while the dynamic hohlraum at a higher radiation temperature of 230 eV has compressed a capsule from 2 mm to 0.8 mm diameter with a neutron yield >3 × 10 ¹¹ thermal DD neutrons, a record for any capsule implosion. World record ion temperatures of >200 keV have recently been measured in a stainless-steel plasma designed for Kα emission at stagnation, due, it was predicted, to ion-viscous heating associated with the dissipation of fast-growing short wavelength nonlinear MHD instabilities. Direct fusion experiments using deuterium gas-puffs have yielded 3.9 × 10 ¹³ neutrons with only 5% asymmetry, suggesting for the first time a mainly thermal source. The physics of wire-array implosions is a dominant theme. It is concerned with the transformation of wires to liquid-vapour expanding cores; then the generation of a surrounding plasma corona which carries most of the current, with inward flowing low magnetic Reynolds number jets correlated with axial instabilities on each wire; later an almost constant velocity, snowplough-like implosion occurs during which gaps appear in the cores, leading to stagnation on the axis, and the production of the main soft-x-ray pulse. These studies have been pursued also with smaller facilities in other laboratories around the world. At Imperial College, conical and radial wire arrays have led to highly collimated tungsten plasma jets with a Mach number of >20, allowing laboratory astrophysics experiments to be undertaken. These highlights will be underpinned in this review with the basic physics of Z-pinches including stability, kinetic effects, and finally its applications.

• Open/close Efficient modeling of laser–plasma interactions in high energy density scenarios

  F Fiuza et al 2011 Plasma Phys. Control. Fusion 53 074004 Tag this article

  Article References Full text PDF (667 KB)

  We describe how a new framework for coupling a full-particle-in-cell (PIC) algorithm with a reduced PIC algorithm has been implemented into the OSIRIS code. We show that OSIRIS, with this new hybrid-PIC algorithm, can efficiently and accurately model high energy density scenarios such as ion acceleration in laser–solid interactions and fast ignition of fusion targets. We model, for the first time, the full-density range of a fast ignition target in a fully self-consistent hybrid-PIC simulation, illustrating the possibility of stopping the laser generated electron flux at the core region with relatively high efficiencies. Computational speedups greater than 1000 times are demonstrated, opening the way for full-scale multi-dimensional modeling of high energy density scenarios and the guiding of future experiments.

• Open/close Influence of impurities on ion temperature measurements in the tokamak scrape-off layer by retarding field analysers

  M Kočan and J P Gunn 2011 Plasma Phys. Control. Fusion 53 085016 Tag this article

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  Retarding field analysers (RFAs) are one of the only widely accepted diagnostics for measuring ion temperature T _i in the tokamak plasma boundary. Usually a pure, Maxwellian plasma composed of a single fuel ion species of charge Z is assumed, in which case the RFA would measure T _i/ Z. Incident impurity ion fluxes are neglected. We study the influence of impurity ions on T _i inferred from a RFA by simulating measurements in a plasma with several ion species. We show that for typical impurity ion fluxes in the tokamak plasma boundary and in the collisionless limit of the kinetic simulations, the assumption of plasma purity causes T _i to be underestimated by as much as ~20%.

• Open/close Characterization and interpretation of the Edge Snake in between type-I edge localized modes at ASDEX Upgrade

  F Sommer et al 2011 Plasma Phys. Control. Fusion 53 085012 Tag this article

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  A new magnetohydrodynamic instability called the 'Edge Snake', which was found in 2006 at the tokamak ASDEX Upgrade during type-I ELMy H-modes, is investigated. It is located within the separatrix in the region of high temperature and density gradients and has a toroidal mode number of n = 1. The Edge Snake consists of a radially and poloidally strongly localized current wire, in which the temperature and density profiles flatten. This significant reduction in pressure gradient leads to a reduction in the neoclassical Bootstrap current and can plausibly explain the drive of the instability. The experimental observations point towards a magnetic island with a defect current inside the O-point of the island. The Edge Snake is compared with similar instabilities at JET, DIII-D and ASDEX Upgrade.

• Open/close Modification of particle distributions by magnetohydrodynamic instabilities II

  R B White 2011 Plasma Phys. Control. Fusion 53 085018 Tag this article

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  A new method of determining domains of phase space in which good KAM surfaces do not exist is further developed and investigated. The technique permits the rapid calculation of the modification of a particle distribution in a toroidal confinement device due to instabilities. We use the method to examine a well documented case of profile modification by Alfvén modes in a tokamak.

• Open/close A unified treatment of kinetic effects in a tokamak pedestal

   2011 Plasma Phys. Control. Fusion 53 089501 Tag this article

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• Open/close Understanding complex magnetohydrodynamic activities associated with a relaxation in the HT-7 tokamak

  Erzhong Li et al 2011 Plasma Phys. Control. Fusion 53 085019 Tag this article

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  A new relaxation instability with complex magnetohydrodynamics (MHD) activities is found in the HT-7 tokamak operational region, which manifests itself in bursts of hydrogen alpha-ray radiations, electron cyclotron emission and soft x-ray (SX) radiations on outer channels, as well as complex MHD perturbations, but without hard disruptions. The MHD modes are identified as m/ n = 2/1, m/ n = 3/2 and m/ n = 5/3 just before the relaxation ( m and n are poloidal and toroidal mode numbers, respectively). The Poincaré mapping, according to the measured perturbations of the MHD modes at the edge, is used to show the topology of magnetic field lines. It is found that a stochastic annular belt resulted just before the relaxation due to the m/ n = 5/3 island overlapping with m/ n = 2/1 and m/ n = 3/2 islands. This qualitatively coincides with the results of SX tomography.

• Open/close Controlling sawtooth oscillations in tokamak plasmas

  I T Chapman 2011 Plasma Phys. Control. Fusion 53 013001 Tag this article

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  The sawtooth instability in tokamak plasmas results in a periodic reorganization of the core plasma. A typical sawtooth cycle consists of a quiescent period, during which the plasma density and temperature increase, followed by the growth of a helical magnetic perturbation, which in turn is followed by a rapid collapse of the central pressure. The stabilizing effects of fusion-born α particles are likely to lead to long sawtooth periods in burning plasmas. However, sawteeth with long quiescent periods have been observed to result in the early triggering of neo-classical tearing modes (NTMs) at low plasma pressure, which can, in turn, significantly degrade confinement. Consequently, recent experiments have identified various methods to deliberately control sawtooth oscillations in an attempt to avoid seeding NTMs whilst retaining the benefits of small, frequent sawteeth, such as the prevention of core impurity accumulation. Sawtooth control actuators include current drive schemes, such as electron cyclotron current drive, and tailoring the fast ion population in the plasma using neutral beam injection or ion cyclotron resonance heating.

• Open/close Characterization of edge profiles and fluctuations in discharges with type-II and nitrogen-mitigated edge localized modes in ASDEX Upgrade

  E Wolfrum et al 2011 Plasma Phys. Control. Fusion 53 085026 Tag this article

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  Edge localized modes (ELMs) with high frequency and low power loss (type-II ELMs) occur in high triangularity, near double null configurations in ASDEX Upgrade with full tungsten plasma facing components. The transition from type-I to type-II ELMs is shown to occur above a collisionality threshold. For the first time the characteristic MHD fluctuations around 40 kHz have been localized. The fluctuations are observed in a wide region extending from the pedestal inward to normalized poloidal radius ρ _pol = 0.7. Their amplitudes on the low-field side of the plasma exhibit maxima above and below the mid-plane. The fluctuations move in the electron drift direction and lead to a reduced edge electron temperature gradient. The reduction in the edge pressure gradient is connected with these MHD fluctuations, which affect the electron temperature but not the electron density profiles. A comparison with nitrogen-mitigated type-I ELMs in the same plasma shape shows that core profiles are also affected. The electron temperature profile is self-similar for type-I and nitrogen-mitigated type-I ELMs but is not self-similar in the case of type-II ELMs.

• Open/close Plasma edge density and lower hybrid current drive in JET (Joint European Torus)

  R Cesario et al 2011 Plasma Phys. Control. Fusion 53 085011 Tag this article

  Article References Full text PDF (185 KB)

  Externally launched lower hybrid (LH) waves do not propagate into the plasma core during operations of JET with radial profiles with relatively high density even at the periphery, approaching the condition necessary for ITER. Modelling results indicate that this problem would be caused by parametric instability (PI)-induced LH spectral broadening, which is expected to occur in the plasma edge and prevents the coupled LH power penetrating the plasma core. However, operation with relatively high electron temperature at the edge is expected to diminish the PI effect and extend the LH current drive effectiveness to reactor-grade high density plasmas, consistent with results obtained in other experiments.