Table of contents

Neutron time of flight signals have been observed with a high resolution neutron spectrometer using the petawatt arm of the Vulcan laser facility at Rutherford Appleton Laboratory from plastic sandwich targets containing a deuterated layer. The neutron spectra have two elements: a high-energy component generated by beam-fusion reactions and a thermal component around 2.45 MeV. The ion temperatures calculated from the neutron signal width clearly demonstrate a dependence on the front layer thickness and are significantly higher than electron temperatures measured under similar conditions. The ion heating process is intensity dependent and is not observed with laser intensities on target below 10²⁰ W cm⁻². The measurements are consistent with an ion instability driven by electron perturbations.

The evolution of core quasi-coherent modes has been investigated during the formation of electron internal transport barriers (e-ITB) in the TJ-II stellarator. These modes have been characterized using heavy ion beam probe and electron cyclotron emission diagnostics. The quasi-coherent mode evolves during formation/annihilation of the e-ITB and vanishes as the transport barrier is fully developed. These observations can be interpreted in terms of the influence of sheared flows in the stability of quasi-coherent modes.

It is well known that the classical Spitzer–Harm–Braginskii expression for the parallel plasma heat flux breaks down in the long mean free path limit, relevant to many practical applications, most crucially power exhaust via the tokamak scrape-off layer (SOL). This problem is usually addressed by limiting the heat flux to some fraction of the free streaming value, with constants of proportionality α_σ, where σ ∊ {e,i}, ranging from 0.03 to 3. The following paper presents a brief overview of the problem, compares the results of various kinetic studies, suggests the optimal values of α_σ for use in plasma–fluid codes, and examines the impact of these values on 2D SOL simulations using the EDGE2D transport code. In this context, gyro-kinetic parallel heat flux expressions for both electrons and ions are derived from the generalized transport equations—an improved version of Grad's 21-moment approach—and their implications to tokamak modelling are discussed.

In this paper, we analyse turbulent transport in the framework of the correlation approach to obtain the effective diffusion coefficient and characteristic increment in the scaling form. Such factors as anisotropy, seed diffusion mechanisms and reconstruction of flow topology significantly have an impact on the effective diffusivity. We consider the different aspects of stochastic instability such as the decorrelation mechanism to estimate characteristic correlation times and correlation scales in the framework of the scaling approach. The topics to be discussed include the Rechester–Rosenbluth correlation scalings, percolation transport in time dependent flows, anisotropic MHD spectra and a multi-scale approach to the analysis of anomalous transport. To treat long-range correlation effects, the percolation renormalization is analysed in time-dependent regimes. In the framework of the multiscale approach, a scaling for an increment of the stochastic instability in two-dimensional random flow is suggested.

One of the scenarios actively studied during the 2003 trace tritium experimental campaign performed on JET includes a gas puff of small amounts of tritium into sawtooth-unstable H-mode plasmas heated by deuterium beams. The sawtooth crashes observed in these discharges, with the electron cyclotron emission diagnostics, are frequently accompanied by the oscillations of the 14 MeV neutron emission, produced mainly due to the reaction between the deuterium beam and the thermal tritium. Such oscillations are clearly seen in low density plasmas while they are weak or absent at high density. The goal of this work is to explain the different behaviour of the oscillations of neutron emission with density. For this purpose, the dynamics of the trace tritium and beam deuterium during the sawtooth crash in discharges with different plasma density is studied numerically using the TRANSP code. In addition, a possibility of using the neutron measurements in sawtoothing low density plasmas for the test of the full and partial reconnection models is discussed and the sensitivity of the simulations to the key parameters of these models is presented.

Investigations on magnetohydrodynamics (MHD) modes using X-mode frequency modulated continuous wave reflectometry have been realized on Tore Supra. Two heterodyne reflectometers operating between 50–75 GHz and 75–110 GHz probe the low field side of the plasma, and the detection of the magnetic islands is performed through the perturbations recorded onto the phase and amplitude of the reflected signal. Fast sweep technique associated with a high repetition rate provides a high radial resolution with a high temporal resolution (up to 20 kHz). The radial localization of rational surfaces is achieved along with the measured density profile. Detection of the time of flight discontinuities in combination with equilibrium code calculations of the current profile allows for the accurate determination of core plasma MHD modes at the q = 1 and q = 2 rational surfaces. Continuous sweeping gives detailed analysis of the radial and poloidal structure of the magnetic island.

The main goal of the paper is to develop improved confinement by analysing the topography of the magnetic field strength B on nested equilibrium surfaces. Pseudosymmetry is the property of magnetic field geometry such that all isomagnetic (B = const) contours on an equilibrium surface encircle either the magnetic axis (poloidal pseudosymmetry) or the major axis of the torus (toroidal pseudosymmetry). Pseudosymmetry appears to be a necessary (but not sufficient) condition for improved confinement. The basic theorem of geometry about the coordinate scale is used to describe pseudosymmetry. The isomagnetic contours determine isomagnetic flux variables that are advantageous in describing pseudosymmetric geometry. Special types of pseudosymmetry are considered: quasi-symmetry, isometry, exact omnigeneity and quasi-isodynamicity (asymptotic omnigeneity). Particular examples of the numerical synthesis of pseudosymmetric improved-confinement configurations are addressed.

Using particle-in-cell simulations, we study the evolution of a strongly magnetized plasma slab propagating into a finite density ambient medium. Like in the previous work, we find that the slab breaks into discrete magnetic pulses. The subsequent evolution is consistent with diamagnetic relativistic pulse acceleration of (Liang E et al 2003 Phys. Rev. Lett.90 085001). Unlike in the previous work, we use the actual electron to proton mass ratio and focus on understanding trapping versus transmission of the ambient plasma by the pulses and on the particle acceleration spectra. We find that the accelerated electron distribution internal to the slab develops a double-power law. We predict that emission from reflected/trapped external electrons will peak after that of the internal electrons. We also find that the thin discrete pulses trap ambient electrons but allow protons to pass through, resulting in less drag on the pulse than in the case of trapping of both species. Poynting flux dominated scenarios have been proposed as the driver of relativistic outflows and particle acceleration in the most powerful astrophysical jets.

The plasma–sheath matching problem has attracted new interest during the last few years. It is complicated by the singular structure of the asymptotic (λ_D/L → 0) plasma and sheath solutions and by a coupling with the eigenvalue problem originating from the plasma balance of bounded plasmas. Due to these difficulties the existence of a matched asymptotic expression uniformly valid from the plasma core to the wall is widely questioned. The issue is clarified both analytically and numerically by the explicit construction of a matched asymptotic expression and comparison with exact solutions for the hydrodynamic plane Tonks–Langmuir problem. The approximations obtained by consistent matching show excellent agreement with numerical potential curves. The singularities of the asymptotic components are reflected by small discontinuities in the derivatives that vanish in the limit λ_D/L → 0.

Electron heat transport experiments are performed in L-mode discharges at various plasma triangularities, using radially localized electron cyclotron heating to vary independently both the electron temperature T_e and the normalized electron temperature gradient over a large range. Local gyro-fluid (GLF23) and global collisionless gyro-kinetic (LORB5) linear simulations show that, in the present experiments, trapped electron mode (TEM) is the most unstable mode. Experimentally, the electron heat diffusivity χ_e is shown to decrease with increasing collisionality, and no dependence of χ_e on is observed at high values. These two observations are consistent with the predictions of TEM simulations, which supports the fact that TEM plays a crucial role in electron heat transport. In addition, over the broad range of positive and negative triangularities investigated, the electron heat diffusivity is observed to decrease with decreasing plasma triangularity, leading to a strong increase of plasma confinement at negative triangularity.

The mechanisms leading to ionization of neutral gas by means of microwaves in the electron cyclotron range of frequencies in a simple toroidal plasma configuration are investigated in the TORPEX device (Fasoli A et al 2003 Bull. Am. Phys. Soc.48 119). The role of the upper hybrid resonance, where most of the ionization events occur once the plasma is formed, is clearly demonstrated. The fast phenomena related to ionization are discriminated from the slower mechanisms leading to the stationary background profiles by measuring the density perturbation caused by a modulation of the injected microwave power. The same method permits reconstruction of the spatial profile of the particle source. The ionization processes are then related to experimental control parameters such as the injected microwave power.

A new approximate electrostatic relativistic dispersion relation for electron Bernstein waves (EBWs) in a Maxwellian plasma has been derived. The relation is reasonably simple for analytical analysis and accurate enough for numerical applications. Comparisons with corresponding exact formulae are presented. Formulae for absorption of the EBWs based on this dispersion relation have been implemented into a ray-tracing code routinely used for the EBW heating and current drive simulations on MAST at Culham Science Centre, UK. Typical simulation run time of the EBW package takes a couple of minutes on a moderate workstation with a single processor.

Asymmetric cold pulse perturbation has been observed by means of electron–cyclotron emission (ECE) and soft-x-ray array during pulse-modulated molecular beam injection (MBI) experiments on HL-2A. The perturbation depth is about 30 cm from the low field side and only about 10 cm from the high field side. The cold pulses cannot propagate to the plasma centre from either the low or high field side. The electron temperature in the plasma centre does not change during MBI, but electron density pulse perturbations can be observed in the plasma centre from the ECE 3rd harmonic measurements, which corresponds to the results of the far-infra-red laser interferometer. The experimental results indirectly provide evidence of the shielding mechanisms of the MBI physics.

Basic features of electron Bernstein waves propagating in the magnetic well inherent to a spherical tokamak magnetic configuration are investigated in a slab model.

Waves at a frequency close to the lower hybrid (LH) resonance are widely used in tokamaks for non-inductive current drive. Modelling of LH waves is usually carried out by combining a Ray-tracing (RT) code for computing the LH waves propagation to a solver of the Fokker–Planck (FP) equation which calculates an electron distribution function self-consistently with the waves absorption. The DELPHINE code has been developed along this approach with accurate treatment of the magnetic equilibrium and the fast electrons dynamics in momentum space. Using this code, the influence of the plasma current on the LH waves propagation and absorption is investigated in detail. High plasma current is found to broaden the absorbed LH spectrum towards high phase velocities, thus increasing the current drive efficiency of the waves. The shape of the current density profile also has an impact on the propagation of the waves and the resulting power deposition. In discharges where the current profile is dominated by LH current drive (LHCD), this dependence leads to the auto-regulation of the LHCD via the current density profile. The RT/FP technique reproduces at least qualitatively some of the experimental trends, though inconsistencies still remain. Perspectives for improving the relevance of the modelling are discussed.

Experimental evidence of parallel flows dynamically coupled to radial turbulent transport is reported in a linear plasma machine (SLPM). Probability density functions (PDFs) for parallel flows and transport are different. PDFs for transport show clear non-gaussian features with large and sporadic bursts and PDFs of parallel flows are rather gaussian. Plasma profiles of parallel Mach number (in the range 0.1–0.3) show gradients dM_||/dr comparable to 1/L_n. Experimental results show that the dynamical coupling between transport and flows shows differences at different plasma radii. The degree of coupling also shows variations with plasma heating conditions. These findings suggest that, although turbulence can be considered as quasi-bidimensional in magnetized plasmas, parallel dynamics still play an important role in the regulation of radial transport.

The data in figures 6 and 13(a) were incorrectly plotted. Please see the pdf for the corrected versions.