Table of contents

On 1 January 2005 I become Editor-in-Chief of Plasma Physics and Controlled Fusion. I look forward to assisting contributors, referees and the Board in maintaining the high standards of this international journal, whose bibliometric impact factor has consistently matched or exceeded that of other journals in the field.

The robust, good health of Plasma Physics and Controlled Fusion reflects that of its user communities. With a decision to proceed to the construction of ITER apparently imminent, magnetic confinement fusion research is preparing to take a major step forwards. A new generation of laser-plasma interaction facilities for inertial fusion research is also rising at key sites around the world. Technical progress in our field is underpinned by scientific excellence, and the publication of results in Plasma Physics and Controlled Fusion will, I hope, continue to play its part. The journal will continue to offer the benefits of refereeing by two experts, combined with the rapid turnaround achieved by the highly efficient editorial office at the Institute of Physics Publishing in Bristol.

Looking elsewhere, there may be opportunities for gentle incremental broadening of the scientific scope of the journal, in the medium term. One looks in particular to those branches of plasma physics that, in recent years, have become more strongly represented in the series of conferences organized by the Plasma Physics Division of the European Physical Society. The recent special issue of Plasma Physics and Controlled Fusion (Plasma Phys. Control. Fusion46 (2004) B1--592) provides an indication.

Finally, it is a pleasure to thank my predecessor, Professor Ian Hutchinson of the Massachusetts Institute of Technology, for handing on his role with the journal in such promising condition.

In this paper, the edge plasma parameters, including the electron temperature (T_e), electron density (n_e), plasma potential (ϕ_p), radial electric field (E_r) and the corresponding fluctuations in the Sino-united spherical tokamak (SUNIST) have been systematically measured using Langmuir probe arrays. The characteristics of the edge electrostatic fluctuations and the turbulence-induced transport are experimentally investigated. The results show that a naturally formed E_r × B_ϕ poloidal velocity shear layer (VSL) exists near the radial location of the limiter. Wavenumber spectrum analyses show that edge fluctuations have a radial propagation character of the drift wave turbulence, with a characteristic radial phase velocity v_phr ∼ 0.7 km s⁻¹ in the scrape-off-layer and v_phr ∼ 0.9–1.4 km s⁻¹ at plasma edge, suggesting that the edge turbulence may originate from the core and propagate to the edge in an intermittent pattern. A radial profile of the toroidal flow velocity Mach number has been observed whose maximum is just located at the VSL position, which may imply a possible relation between the poloidal shear flow and the toroidal plasma flow in SUNIST.

A neutral gas and plasma shielding model is presented that describes the interaction of a pellet with the high energy ions and electrons generated during heating or current drive experiments. The main improvements are the self-consistent calculations of the electrostatic sheath at the cloud–plasma interface and of the extra ablation due to the fast tail of the electron and ion distributions, including heating in the volume of the pellet. With regard to the comparison between the code predictions and the experimental results, realistic three-dimensional (space, energy, pitch angle) distributions have been used for both the ions and electrons. For ohmic discharges, the code has been tested on more than 40 well-documented pellets selected in the International Pellet Ablation DataBASE. For additionally heated plasmas (ion cyclotron resonance heating—minority regime—and lower hybrid current drive, 2–4 MW of injected power), Tore Supra data have been used. In these different cases, the calculations are in good agreement with the experimental penetrations and ablation profiles. A parametric study is also presented, which enlightens the control parameter that governs the pellet penetration. In what concerns the capability of pellet injection to fuel reactor grade plasmas, it is shown that no strong extra ablation due to the α-particles is expected.

The dispersion relation for symmetrical TM electromagnetic surface waves propagating in a plasma waveguide with a strongly magnetized plasma column and a dielectric rod is investigated. The growth rate for excitation of these waves is obtained and its dependence on the radius of the annular beam, current intensity and accelerating voltage are investigated.

A novel method for inverting time-resolved line-integrated interferometric plasma density measurements is described. The method uses singular value decomposition of local density profiles from Thomson scattering measurements obtained at low sampling rates in the same or equivalent plasmas to determine a set of orthogonal spatial basis functions which is well adapted to the physical processes under investigation. The sought-for density profile is expanded into a limited series of these functions and a solution is calculated by using a simple least-squares fit method. The new method overcomes the difficulties encountered with other methods, such as regularization methods, which smoothen gradients and depend on the availability of accurate measurements in the plasma edge region. The small number of computations required provides for a fast algorithm. This method, which combines the high bandwidth of interferometer systems with the spatial accuracy of Thomson scattering, is applied to invert interferometer measurements in a wide variety of operational regimes in the Tokamak à Configuration Variable and Joint European Torus (JET) tokamaks. In particular, the collisionality dependence of density peaking observed in ASDEX Upgrade is confirmed in JET H-modes using this method.

The interaction of an ion-collecting sphere at floating potential with a flowing collisionless plasma is investigated using the particle in cell code SCEPTIC. The dependence of the floating potential on the flow velocity for a conducting sphere is found to agree very well with the orbital motion limited approximation, which ignores the asymmetry in the plasma potential. But the charge, even on conducting spheres and at zero flow, is not well represented by using the standard expression for capacitance. Insulating spheres become asymmetrically charged because of ion collection asymmetry, and their total (negative) charge is considerably increased by flow. The collection flux asymmetry is documented for both conducting and insulating spheres and is not greatly different between them. The drag force upon the sphere is obtained from the code calculations. It shows reasonable agreement with appropriate analytic approximations. However, numerical discrepancies up to 20% are found, which are attributed to uncertainties in the analytical values.

Characteristics of toroidal plasma rotation have been experimentally investigated using charge exchange recombination spectroscopy in the ASDEX Upgrade tokamak. Ion cyclotron resonance frequency (ICRF) heating is found to cause a reduction of the toroidal rotation velocity, V_ϕ, driven by neutral beam injection (NBI) in the co- and counter-current directions. The reduction of plasma rotation is attributed to an increasing momentum diffusivity connected with the confinement degradation by the additional ICRF power flux, and not to an ICRF induced toroidal force related to radial non-ambipolar transport of resonant particles. Toroidal momentum transport is found to be anomalous in various plasma regimes including standard and improved H-modes and ion-internal transport barrier (ITB) plasmas. In the inner half of those plasmas, except for high density H-modes with on-axis NBI only, the momentum diffusivity, χ_ϕ, is found to be similar to the ion and electron heat diffusivities, χ_i and χ_e. In the outer half region, χ_ϕ becomes smaller than χ_i, while χ_ϕ is still comparable with χ_e except for ITB plasmas. It is found that the normalized gradient length of the toroidal rotation velocity, is smaller than that of the ion temperature, , in H-modes and ITB plasmas. The magnitude of in an ITB region exceeds that in H-modes, as seen for the T_i profile. In H-modes, the T_i profile is stiff ( ), while the V_ϕ profile is not stiff, with ranging from 0 to 7. The V_ϕ profile tends to become flat at high densities with on-axis NBI only. Additional ICRF heating can lead to a small decrease in both and , while it sometimes causes a flattening of the V_ϕ profile in the inner region. It is shown that the neoclassical correction of V_ϕ does not affect strongly the results obtained with the measured V_ϕ.

Eddy currents generated during the fastest disruption current decays represent the most severe design condition for medium and small size in-vessel components of most tokamaks. Best-fit linear and instantaneous plasma current quench rates have been extracted for a set of recent JET disruptions. Contrary to expectations, the current quench rate spectrum of high and low thermal energy disruptions is not substantially different. For most of the disruptions with the highest instantaneous current quench rate an exponential fit of the early phase of the current decay provides a more accurate estimate of the maximum current decay velocity. However, this fit is only suitable to model the fastest events, for which the current quench is dominated by radiation losses rather than the plasma motion.

We consider the nonlinear interaction of magnetized electrons with an oblique narrow-band electromagnetic wave-packet. The interaction is analysed over and near the local threshold to chaos. The statistical character of the forcing that controls the trajectories of the particles is also studied. We focus our analysis on issues related to energy and spatial diffusion across the magnetic field by following the evolution of the ensemble mean squares ⟨(γ − γ₀)²⟩ and ⟨(r_⊥−r_⊥0)²⟩ for various values of the wave amplitude and angle of wave propagation. We study, in particular, the interaction of magnetized electrons with waves having strong and moderate amplitudes, near the transition to chaos, where the dynamics is complex and a mixture of periodic and stochastic orbits coexist. The electron diffusions in real and energy spaces are found to obey simple power laws in time, and the scaling exponents are indicative of sub-diffusion. This is a direct consequence of the effect of the resonant phase-space islands in the particle motion.

A theory of coupling between electromagnetic and electron Bernstein waves in a plasma slab is presented. The theory uses an approach that associates the linear mode conversion with the singularity of the cold plasma wave equation at the upper hybrid resonance (UHR). The singularity results in linear interaction of cold plasma (electromagnetic) and hot plasma (Bernstein) modes. Applicability of the WKB theory to interacting modes is not required. In this method the full solution of the mode conversion problem including calculation of the excited Bernstein wave complex amplitude is reduced to finding a solution to the cold plasma wave equation, which describes dissipative wave power absorption at the UHR. This method is applicable to a variety of plasma configurations practically without limitations on the inhomogeneity scale-length. It permits one to consider in the framework of a single procedure particular cases like direct tunnelling of the incident wave, O–X–B conversion and transformation of the X-mode launched from the high-field side of a tokamak and having free access to the UHR.

This paper describes a new technique which has been implemented on the JET tokamak to investigate integrated real-time control of several plasma profiles simultaneously (such as current, temperature and pressure) and reports the results of the first experimental tests. The profiles are handled through their projection on a suitable basis of functions according to the Galerkin scheme. Their response to three actuators (heating and current drive powers injected in the plasma) is linearized in an experimentally deduced multi-input multi-output model. The singular value decomposition of this model operator allows us to design a distributed-parameter real-time controller which maximizes the steady state decoupling of the multiple feedback loops. It enables us to control several coupled profiles simultaneously, with some degree of fuzziness to let the plasma evolve towards an accessible non-linear state which is the closest to the requested one, despite a limited number of actuators.

The first experiments using these techniques show that different current and electron temperature profiles can be obtained and sustained by the controller during a closed-loop operation time window. Future improvements and perspectives are briefly mentioned.

It is shown by calculating the wave transmission coefficient T directly from the WKB absorption coefficient α, for the fundamental ordinary and the second harmonic extraordinary electron cyclotron wave modes propagating perpendicular to the magnetic field in a weakly relativistic magnetoplasma, that the thermal corrections present in α do not affect the functional dependence of T (which in fact, as has been suggested earlier, is also true for non-adiabatic effects like wave reflection, which cannot be treated by a WKB approach). This result can readily be generalized to all harmonic modes except for the fundamental extraordinary wave mode in the so called finite-density regime. For the latter case, general analytical expressions are obtained in the WKB approximation for both α and T.