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From January 2007, Plasma Physics and Controlled Fusion (PPCF) is introducing a new category of paper: Brief Communications. Limited to four published pages, Brief Communications will enable the publication of significant new results for which a full-length article may not be necessary or appropriate. They should be of interest to the journal's specialist readership and meet the usual high scientific standard for Papers in the journal, but do not have to be of general interest. Standard review procedures, involving the use of two referees, will apply. Brief Communications will be free to read online.

In a parallel development, the Letters category is being phased out. While some of us retain an attachment to this category (including myself, for example, partly because my first paper in this journal was a Letter published in 1984), its continued existence is becoming problematic. Importantly, the average time to publication of Letters in PPCF is no longer significantly faster than that of standard Papers. Technological advances have played a role in this. Additionally, some of the special criteria for publication as a Letter are sufficiently subjective that challenges on these credentials, as distinct from scientific content, sometimes create undue delay.

I hope that the new category will meet a clear need and enhance the standing of the journal. The Editorial Board will review the working of the system as it develops, and we welcome comments and suggestions.

Toroidally-asymmetric biasing of the divertor plates may increase convective cross-field transport in SOL and thereby reduce the divertor heat load. Experiments performed with the MAST spherical tokamak generally agree with a simple theory of non-axisymmetric biasing. However, some of the experimental results have not yet received a theoretical explanation. In particular, existing theory seems to overestimate the asymmetry between the positive and the negative biasing. Also lacking a theoretical explanation is the experimentally observed increase of the average floating potential in the main SOL in the presence of biasing. In this paper we attempt to solve these problems by accounting for the closing of the currents (driven by the biasing) in a strong-shear region near the X-point. We come up with a picture which, at least qualitatively, agrees with these experimental results.

On the basis of the electromagnetic energy balance equation, a quasi-exact analytical evaluation of the electron-cyclotron (EC) absorption coefficient is performed for arbitrary propagation (with respect to the magnetic field) in a (Maxwellian) magneto-plasma for the temperature range of interest for fusion reactors (in which EC radiation losses tend to be important in the plasma power balance). The calculation makes use of Bateman's expansion for the product of two Bessel functions, retaining the lowest-order contribution. The integration over electron momentum can then be carried out analytically, fully accounting for finite Larmor radius effects in this approximation. On the basis of the analytical expressions for the EC absorption coefficients of both the extraordinary and ordinary modes thus obtained, (i) for the case of perpendicular propagation simple formulae are derived for both modes and (ii) a numerical analysis of the angular distribution of EC absorption is carried out. An assessment of the accuracy of asymptotic expressions that have been given earlier is also performed, showing that these approximations can be usefully applied for calculating EC power losses from reactor-grade plasmas.

In this paper, edge stability analyses of the MAST tokamak plasmas are presented. The analyses show that the experimental equilibrium prior to an edge localized mode (ELM) is unstable against very narrow peeling modes with low growth rate. When the edge pressure gradient becomes steeper, wider peeling–ballooning modes with larger growth rate become unstable. These modes are the likely triggers of ELMs. In the analyses the required pressure increase for destabilization is sensitive to how the X-point is modelled in the equilibrium reconstruction. A 'sharp' X-point approximation is more stable against the peeling–ballooning modes than a 'round' one. An experimental ELM-free single null plasma is significantly more stable against the peeling–ballooning modes than the double null plasma, but this is unlikely to be directly due to the single null geometry but rather due to the different plasma profiles. Sheared toroidal rotation is able to stabilize the peeling–ballooning modes. This suggests the following model for the ELM triggering: the rotation shear keeps the edge stable until the pressure gradient has sufficiently exceeded the stability boundary for the static plasma. When the mode becomes unstable, it starts to grow, ties the flux surfaces together and flattens the rotation profile. This further destabilizes the edge plasma leading to an ELM crash.

Ray optics is used to model the propagation of short electromagnetic plasma waves in toroidal geometry. The new RAYCON code evolves each ray independently in phase space, together with its amplitude, phase and focusing tensor to describe the transport of power along the ray. Particular emphasis is laid on caustics and mode conversion layers, where a linear phenomenon splits a single incoming ray into two. The complete mode conversion algorithm is described and tested for the first time, using the two space dimensions that are relevant in a tokamak. Applications are shown, using a cold plasma model to account for mode conversion at the ion-hybrid resonance in the Joint European Torus.

A theoretical description of a wiggler pumped ion-channel free electron laser (WPIC-FEL) in high-gain regime is presented. The analysis involves a perturbation of the Vlasov–Maxwell equations about the constant-velocity helical trajectories, and the general deriving currents are derived for this configuration. The complete dispersion equation is then obtained for a mono-energetic beam. An extensive numerical analysis is presented for a wide range of operating parameters.

We have studied anomalous line intensities in space resolved helium plasmas in different configurations of the MISTRAL-B plasma divertor simulator experiment. Spectral simulations show that the different experimental configurations differ from one another by the hot electron fraction which drives the observed anomalous large emission of the singlet line 1s2s ¹S–1s5p ¹P compared with the triplet lines 1s2p ³P–1snd ³D. Theoretical spectra analyses carried out with the recently developed non-Maxwellian SOPHIA code are in good agreement with the data and allowed one to identify non-Maxwellian signatures. Independently performed Langmuir probe measurements are in good agreement with the spectroscopic analysis.

To clarify the effect of the plasma-wall separation on the stability of the resistive wall modes (RWM), we have performed current-driven RWM experiments in the JT-60U ohmic plasmas. In the experiment, the plasma was placed near the outer wall, and the plasma current was ramped up so as to destabilize the current-driven RWM. An instability with a clear m/n = 3/1 mode structure was observed just before a thermal collapse. The growth time of this instability is about 10 ms, which is of the order of the skin time of the JT-60U wall. Therefore, the instability has been identified as a current-driven RWM. The plasma-wall separation was systematically changed, and it is found that the growth rates have strong dependence on the wall position. Thus, the growth rates became smaller with decreasing the plasma-wall separation. Additionally, the observed growth rates have been compared with the AEOLUS-FT code, which is based on the resistive MHD equations with a resistive wall, and the model of the RWM dispersion relation. The dependence of the observed growth rate on the wall position is in qualitative agreement with these numerical results.