Table of contents

Electron energy transport experiments conducted on the Vulcan 100 TW laser facility with large area foil targets are described. For plastic targets it is shown, by the plasma expansion observed in shadowgrams taken after the interaction, that there is a transition between the collimated electron flow previously reported at the 10 TW power level to an annular electron flow pattern with a 20° divergence angle for peak powers of 68 TW. Intermediate powers show that both the central collimated flow pattern and the surrounding annular-shaped heated region can co-exist. The measurements are consistent with the Davies rigid beam model for fast electron flow (Davies 2003 Phys. Rev. E 68 056404) and LSP modelling provides additional insight into the observed results.

Over the last several years, rapid progress has been made evaluating the double-z-pinch indirect-drive, inertial confinement fusion (ICF) high-yield target concept (Hammer et al 1999 Phys. Plasmas6 2129). We have demonstrated efficient coupling of radiation from two wire-array-driven primary hohlraums to a secondary hohlraum that is large enough to drive a high yield ICF capsule. The secondary hohlraum is irradiated from two sides by z-pinches to produce low odd-mode radiation asymmetry. This double-pinch source is driven from a single electrical power feed (Cuneo et al 2002 Phys. Rev. Lett.88 215004) on the 20 MA Z accelerator. The double z-pinch has imploded ICF capsules with even-mode radiation symmetry of 3.1 ± 1.4% and to high capsule radial convergence ratios of 14–21 (Bennett et al 2002 Phys. Rev. Lett.89 245002; Bennett et al 2003 Phys. Plasmas10 3717; Vesey et al 2003 Phys. Plasmas10 1854). Advances in wire-array physics at 20 MA are improving our understanding of z-pinch power scaling with increasing drive current. Techniques for shaping the z-pinch radiation pulse necessary for low adiabat capsule compression have also been demonstrated.

The error-field penetration process of the dynamic ergodic divertor (DED) on the TEXTOR tokamak has been investigated analytically in terms of a single fluid MHD model with a finite plasma resistivity and viscosity in a cylindrical geometry. The linear model produces a localization of the induced current at the resonance surface and predicts a vortex structure of the velocity field near the resonance layer. Moreover, effects of the Alfvén resonance for the error-field penetration are identified by two peaks in the radial profiles of the perturbed toroidal current and the perturbed magnetic flux when the relative rotation velocity between the DED and the rotating tokamak plasma is set to large. Fine structures of the vorticity induced by the DED in the vicinity of the rational surface disappear by introducing a finite plasma perpendicular viscosity. In addition, it is shown that the two peaks of the perturbed toroidal current overlap by an anomalous plasma perpendicular viscosity. Likewise, a bifurcation of the penetration process from the suppressed to the excited state is obtained by a quasi-linear approach taking into account modifications of the radial profiles of the equilibrium current and the plasma rotation due to the DED. A comparison with real experimental results of the DED on the TEXTOR tokamak is shown.

The ion drag force on a spherical grain situated in a flowing collisionless plasma is obtained from the specialized coordinate electrostatic particle and thermals in cell simulation code (SCEPTIC) (Hutchinson 2002 Plasma Phys. Control. Fusion44 1953, Hutchinson 2003 Plasma Phys. Control. Fusion45 1477, Hutchinson 2005 Plasma Phys. Control. Fusion47 71) and compared with recent analytic approximate treatments in the interesting and relevant case when the Debye length is only moderately larger than the sphere radius. There is a substantial complex structure in the results for transonic flows, which is explained in terms of the details of ion orbits. Naturally the prior analytic approximations miss this structure, and as a result they seriously underestimate the drag for speeds near the sound speed. An easy-to-evaluate expression for force is provided that fits the comprehensive results of the code. This expression, with minor modification, also fits the results even for Debye length much smaller than the sphere radius.

Energetic particles found in planetary magnetospheres and other plasmas, where mirror geometries occur, often exhibit two typical characteristics: a pronounced high energy tail and an anisotropy. A relativistic kappa-loss-cone (KLC) distribution function f^κL is initially developed which incorporates features of the well-known kappa type and loss-cone type, i.e. the anisotropy behaves as a loss-cone distribution; the energy satisfies ∝ [1/v²]^(κ+1) for a relatively large velocity v as a kappa distribution f^κ does and spreads ∝ [1/p]^κ+1 at the relativistic energies (where κ and p are the energy spectral index and the particle momentum, respectively). This indicates that the new distribution f^κL obeys the power-law not only at the lower energies but also at the relativistic energies since the relativistic energy ∝ p. Numerical calculations are performed for a direct comparison between the new KLC distribution and the current kappa distribution, respectively. It is found that the regular kappa distribution generally decreases faster than the KLC distribution with the kinetic energy E_k especially when θ² increases (where θ² is the energy weight parameter), e.g. f^κ/f^κL ≲ 10⁻² for E_k ≳ 2.0 MeV and θ² ≳ 0.25. However, no big difference occurs between both distributions through energies up to ∼500 keV for θ² ≲ 0.025. Furthermore, the regular kappa distribution containing either the temperature anisotropy or both the loss cone and temperature anisotropy is quite different from the KLC distribution. The new KLC distribution may be applicable to the outer radiation belts of the Earth, the inner Jovian magnetosphere and other plasmas (including the laboratory machine) where relativistic particles are present.

The role of the current profile and ion and electron temperature ratio in Tokamak à Configuration Variable (TCV) electron internal transport barrier (eITB) non-inductive discharges is inspected by means of the linear gyrokinetic global code LORB5 (Bottino et al 2004 Phys. Plasmas11 198) and the Gyro–Landau fluid transport model GLF23 (Waltz et al 1997 Phys. Plasmas4 2482). In this work we have compared two different MHD equilibria for a TCV discharge with an eITB, corresponding to a monotonic and a reversed safety factor profile. The latter has been obtained using the calculated electron–cyclotron driven density profile, the bootstrap current and the experimental pressure. Global simulations show that the most unstable electrostatic instabilities in the spectrum are trapped electron modes (TEM) for both equilibria. In the vicinity of the reverse shear region, where the magnetic shear s is positive and small (s ≪ 1), modes tend to preserve the ballooning structure but their mixing length estimate for the thermal diffusion coefficient is significantly reduced as compared with the monotonic q equilibrium. This effect is further increased in the reverse shear region. Simulations performed with GLF23 show that the inclusion of the Shafranov shift in the model has an important stabilizing effect in all the plasma core and, in particular, in the barrier region. However, the Shafranov shift and reverse shear stabilization are not the only mechanisms through which the q profile affects the stability of the system. The local value of q in the barrier region also contributes to the stabilization. Global gyrokinetic and local Gyro–Landau fluid results suggest that the current profile modification alone, with the consequent negative shear and Shafranov shift stabilization on TEM modes, can explain the presence of the ITB in these TCV discharges.

The effects of magnetic perturbation on the quality of confinement in the sustained spheromak plasma experiment (SSPX) (Hooper E B et al1999 Nucl. Fusion39 863) are studied. The magnetic field is taken as a sum of an unperturbed axisymmetric magnetic field, B₀, calculated from a magneto-hydrodynamic equilibrium, and a non-axisymmetric part, δB, representing the effects of kinking perturbation of the central plasma column. Poincaré maps show that flux surfaces are strongly affected by perturbations with δB/B₀ ≳ 5%, with islands and stochastic regions covering a substantial part of plasma volume. The effective heat transport in the perturbed magnetic field, calculated by a Monte-Carlo procedure, is found to be consistent with the experimentally measured level of confinement.

In this paper transient phenomena related to the electron heat transport when electron cyclotron resonance heating (ECRH) power is switched on or off are investigated by predictive and interpretative transport analysis. In the latter two numerical approaches—one based on the transport code ASTRA and the other by COBRA code—are compared. The transport simulations were done in the ASTRA code based on a critical ∇T_e/T_e model. The results for the evolution of the electron temperature in steady state and transient phases are compared with the experiment. The changes of the heat diffusivity with the ECRH power are compared with the results of the transport calculations from the interpretative analysis. The critical ∇T_e/T_e model reproduces well both the heat diffusivity and the electron temperature evolution in the region outside the ECRH deposition, and the results show good agreement with the experimental data there. The model cannot recover the electron temperature changes inside the ECRH deposition region with accuracy due to the simple assumption made in transport below the threshold.

The paper presents an integrated view of the population structure and its role in establishing the ionization state of light elements in dynamic, finite density, laboratory and astrophysical plasmas. There are four main issues, the generalized collisional–radiative picture for metastables in dynamic plasmas with Maxwellian free electrons and its particularizing to light elements, the methods of bundling and projection for manipulating the population equations, the systematic production/use of state selective fundamental collision data in the metastable resolved picture to all levels for collisonal–radiative modelling and the delivery of appropriate derived coefficients for experiment analysis. The ions of carbon, oxygen and neon are used in illustration. The practical implementation of the methods described here is part of the ADAS Project.

We examine the fast-ion driven Alfvén eigenmodes (AEs) in the frequency range 0–500 kHz during the current ramp-up phase of advanced JET plasma before the main heating starts (the preheating phase). Modelling of AEs using equilibrium reconstruction and the ideal MHD spectral code MISHKA shows that three types of mode commonly found during the preheating phase correspond to: global toroidal Alfvén eigenmodes (TAEs) found in regions of moderate to high magnetic shear; core-localized TAEs found in regions of low magnetic shear; and Alfvén cascades found in regions of zero shear. MHD spectroscopy diagnosis of the time evolution of the plasma current within the central region of the plasma and of the minimum safety factor value based upon the frequency evolution of these unstable AEs is discussed.

This paper presents a study of the hollow bulk electron temperature profiles measured in the low-temperature (2–15 eV) plasmas of the WEGA stellarator. For this the global power balance equation was solved for the bulk electrons. The observed hollow temperature profiles could be reproduced satisfactorily assuming that the bulk electrons are heated through energy transfer from fast electrons and that the parallel heat conduction in the scrape-off layer is determined by the potential drop in the sheath in the vicinity of the wall. These results imply that the cause of the hollow shape of bulk electron temperature profiles is the heating of them through collisions with fast electrons produced by the heating method.

Density and power scan experiments have been carried out at various magnetic field strengths on the Large Helical Device (LHD), to investigate the temperature and magnetic field dependences of the thermal diffusivity. In the moderate density regime, the thermal diffusivity shows gyro-Bohm-like parameter dependences. In the high-density and low-temperature regime, on the other hand, the thermal diffusivity increases with the square root of the local electron temperature and decreases with a 1.2 power of the magnetic field strength. The turning point temperature where the weak temperature dependence changes to the gyro-Bohm type is also found to increase with a 0.8 power of the magnetic field strength. Based on the experimental results, dimensionally correct scalings for the energy confinement time, τ_E, and the plasma stored energy for the high-density LHD plasmas have been derived. Compared with the gyro-Bohm model (τ_E ∝ (n/P)^3/5, where n and P denote the density and the heating power, respectively) and the international stellarator scaling 1995 (τ_E ∝ n^0.51P^−0.59), our scaling has a weaker positive dependence on the density together with the mitigated power degradation as τ_E ∝ (n/P)^1/3, owing to the weak temperature dependence of the thermal diffusivity.