Table of contents

Runaway production is observed to be enhanced in lower hybrid current driven (LHCD) plasmas during ion Bernstein wave (IBW) heating as compared with the LHCD only plasma in the HT-7 tokamak. The distortion of the electron distribution function is the effect of the quasilinear diffusions of two types of waves. IBWs modified the distribution function of the electrons by helping to fill the so-called lower hybrid wave (LHW) spectral gap for low parallel velocity. Thus the LHW was significantly coupled to the fast electrons produced by the IBW, and partial LHW power was absorbed on the first pass without significant n_∥-upshift. The synergy interaction of two types of waves results in high electron parallel energy and enhanced quasilinear diffusion, which is favorable for the production of runaway electrons. This is directly related to the improvement of plasma performance in the operation mode of simultaneous injection of LHW and IBW power.

Nonlinear, nonparaxial propagation of an intense, short, Gaussian laser pulse in a preformed plasma channel having a parabolic radial density profile is analyzed. The variational technique is used to obtain equations describing the evolution of laser spot size and pulse length in the channel. Relativistic nonlinearity and group velocity dispersion effects have been taken into account and numerical methods are used to study the simultaneous evolution of the laser spot and pulse length.

Electron Bernstein waves are of interest for heating and current drive in spherical tokamaks where the central region of the plasma is not accessible to the ordinary and extraordinary modes. In this paper we adapt an analytical theory of current drive in toroidal geometry developed by Lin-Liu et al (2003 Phys. Plasmas10 4064) to this system. This involves taking account of the fact that the ratio of the Larmor radius to the perpendicular wavelength is not, in general, small for the Bernstein waves and also including the effects of a non-circular plasma cross section. By comparing the results with those of a full Fokker–Planck code, we demonstrate that the analytical method can yield a good approximation to the current drive efficiency in most regimes of practical interest. Since it is much less computationally demanding than using a Fokker–Planck code we suggest that it could be a useful tool for analysing experiments on Bernstein mode current drive in spherical tokamaks.

The formation of a high-energy ion beam suitable for driving nuclear fusion reactions and producing MeV neutrons was investigated. The interaction of intense ultra short laser pulses with a double-layer thin foil for the production of MeV deuterons was studied theoretically and numerically simulated using a two-dimensional electromagnetic particle-in-cell model. The directionality and energy of the deuteron beam, specifically the conversion efficiency of laser energy into deuteron kinetic energy, and deuteron energy and angular distribution functions are studied as a function of peak laser intensity, laser pulse duration and target thickness. A range of parameters was determined for which a highly directional deuteron beam is generated.

In the presence of strong α-particle heating, active control of the formation or the properties of the edge transport barrier (ETB) might be necessary to meet the confinement and exhaust needs of burning fusion plasmas. In particular, the sensitive dependences on the magnetic configuration and divertor geometry are suitable candidates for controlling the ETB. Using both methods on the Mega Ampére Spherical Tokamak (MAST) short L-mode periods of a few milliseconds duration have been introduced into strongly heated H-modes.

In double null configuration a vertical plasma shift by ΔZ < 2.5 cm opposite to the ion ∇B-drift direction led to L-mode periods of 15 ms < Δt_L < 20 ms length. The peak power load to the targets at the H–L transition was a factor of 2 below that measured during edge localized modes (ELMs). At the H–L transition a rapid change in toroidal He II velocity indicating a fast collapse of the radial electric field within was observed spanning a wider radial region than the collapse during ELMs. In single null configuration the change in the connection length of the outer divertor leg by 2 m < ΔL_c < 4 m led to L-mode periods of 2 ms < Δt_L < 5 ms duration. In both cases fluid modelling indicates a more negative E_r and consequently an increase in the E × B shear in the L-mode just inside the last closed flux surface due to the changes to the magnetic geometry, which lead to improved H-mode access.

The active control techniques demonstrated here may help to develop suitable scenarios for future devices and improve the understanding of the physics determining the ETB.

The 3D Monte-Carlo code ERO, which calculates erosion processes, impurity transport and deposition, has been coupled to the Monte-Carlo code SDTrimSP to simulate material mixing processes in wall components more precisely. SDTrimSP calculates the transport of ions in solids by means of the binary collision approximation. It keeps track of the depth dependent material concentration caused by implantation of projectiles in the solid. Modelling with the coupled code ERO-SDTrimSP is compared with dedicated TEXTOR experiments, in which the formation of mixed surface layers has been studied. In these experiments, methane ¹³CH₄ was injected through graphite and tungsten spherical limiters during plasma exposure and the local redeposition probability was measured post mortem by surface analysis. A significant difference in the carbon ¹³C deposition efficiency, i.e. the ratio of the locally deposited to the injected amount of ¹³C, between graphite and tungsten was found, 4% for graphite and 0.3% for tungsten. Modelling of these experiments with ERO-SDTrimSP reproduces the clear substrate dependence with about 2% deposition efficiency on graphite and less than 0.5% on tungsten in good agreement with the experiment. The reason for the substrate dependence is partly explained by the higher physical sputtering yield of a thin carbon film on top of a tungsten substrate compared with a graphite substrate. Surface roughness of the materials has been identified to be another important parameter for the interpretation of the results.

Important changes for 2008

As a result of reviewing several aspects of our content, both in print and online, we have made some changes for 2008. These changes are described below:

Article numbering

Plasma Physics and Controlled Fusion has moved from sequential page numbering to an article numbering system, offering important advantages and flexibility by speeding up the publication process. Papers in different issues or sections can be published online as soon as they are ready, without having to wait for a whole issue or section to be allocated page numbers.

The bibliographic citation will change slightly. Articles should be referenced using the six-digit article number in place of a page number, and this number must include any leading zeros. For instance, from this issue:

Z Y Chen et al 2008 Plasma Phys. Control. Fusion50 015001

Articles will continue to be published on the web in advance of the print edition.

A new look and feel

We have also taken the opportunity to refresh the design of the journal cover, in order to modernise the typography and create a consistent look and feel across our range of publications. We hope you like the new cover.

If you have any questions or comments about any of these changes, please contact us at ppcf@iop.org.