Table of contents

I would firstly like to convey my best wishes for 2006 to our readers, authors and referees. There are also some issues that I would like to communicate to you in this first issue of the new year.

Farewell to Editorial Board members

In 2005 the terms of office of half the Editorial Board came to an end. We would like to thank them for their wise advice on difficult questions, the innumerable cases where they have acted as adjudicators and for the many other forms of support they have given to the journal. While we say farewell to them as Board members, we trust that they will continue to support Nuclear Fusion.

• H. Bolt

• M.J. Fujiwara

• G.T. Hoang

• G.S. Lee

• S. Nakai

• R.R. Parker

• O.S. Pavlichenko

• S.C. Prager

• V.P. Smirnov

• M.Q. Tran

• Y. Wan

Our special thanks go to F. (Rip) Perkins who chaired the Board for many years and was instrumental in many important Board decisions. We welcome the new members of the Editorial Board which met in its new composition (see the prelim pages) during the EPS conference in Tarragona under the chairmanship of M. Kikuchi.

Refereeing

As we did last year we would like to thank our top ten most loyal referees who have helped the journal with its double-referee peer-review procedure in the last year. At the Nuclear Fusion Editorial Office we are fully aware of the load we put on the shoulders of our referees. At the end of 2004 the Editorial Board decided that a gesture of gratitude should be made to our top ten most loyal referees. We offer them a personal subscription to Nuclear Fusion with electronic access for one year, free of charge.

To select the top ten referees we have adopted the criterion that a researcher should have acted as a referee or adjudicator for at least three different manuscripts during the period autumn 2004 to autumn 2005. According to our records the following people, excluding our Board members, met this criterion. Congratulations and many, many thanks!

• D. van Eester (ERM/KMS, Belgium)

• L.R. Grisham (PPPL, USA)

• C. Hidalgo (CIEMAT, Spain)

• P. McNeely (IPP-Garching, Germany)

• V. Mukhovatov (ITER, Japan)

• T. Oikawa (JAEA, Japan)

• S. E. Sharapov (JET/UKAEA, UK)

• T. Takizuka (JAEA, Japan)

• D.G. Whyte (Wisconsin/UCSD, USA)

• S. Wukitch (MIT, USA)

In addition to this top ten there is a group of several hundred referees who have helped us in the past years to maintain the high scientific standard of Nuclear Fusion. At the end of this issue we give the full list of all referees for 2005.

Page charges and waivers

Seemingly not all members of the fusion community have noted the modification of the page charge waiver policy for Nuclear Fusion that was agreed upon and took effect from 1 Janaury 2005. We still get requests that are not in accordance with the new rules. Let us repeat what was stated last year. All IAEA Member States that are classed by the World Bank as ' developing ' can now ask for a 75 % waiver. This opens the scheme to several additional countries in South America, Eastern Europe, Asia and Africa. However the granting is not an automatic procedure: requests have to be made in writing and submitted to the Editorial Office which will advise the management of the two co-publishers (IAEA and IOP Publishing Ltd). In very exceptional cases a 100 % waiver can be granted but only after convincing evidence of hardship is given by the leader/director of the requesting institute.

Review articles

Only review articles commissioned by the Editorial Board should be submitted to the journal. In the case of unsolicited review-type articles, these can only be taken into consideration for publication with the specific consent of the Editorial Board. Authors considering submitting a review article should send a proposal to the Editor in advance for consideration by the Editorial Board.

Letters

the faster procedure for publishing letters has raised the enthusiasm for submission. In 2005 Nuclear Fusion published nine letters instead of two, which was the average for the previous years. This is good news. We would like to encourage the submission of more letters by speeding up the peer-review procedure as much as is possible without lowering the quality of the review.

Book reviews

The Editorial Board has decided to stop publishing book reviews based on books sent to us since it is difficult to find reviewers for this very time-consuming job. However, authors and/or publishers who would like to announce the publication of a book can do so by sending the Editorial Office a short text describing the contents of the book. These should be relevant for fusion research. This text will then be published as an announcement outside the editorial responsibility of Nuclear Fusion.

Founding of an annual Nuclear Fusion Award

The Editorial Board and the IAEA accepted a proposal by the Chairman of the Editorial Board to establish an annual award for the best article published in Nuclear Fusion during a given period. This award will be presented for the first time during the upcoming IAEA Fusion Energy Conference 2006. The Nuclear Fusion Award paper will be selected every year from among original papers published in Nuclear Fusion 2--3 years before. The 2006 award will therefore be selected from the 2003 and 2004 volumes and we will usually consider the ten most cited papers and the ten most downloaded papers. Other exceptional papers may also be proposed and considered. Selection of the winning article will be based on confidential voting by all members of the Editorial Board.

The stability of proposed ITER scenarios with respect to low-medium mode number Alfvénic instabilities driven by fusion-produced alpha particles is investigated by hybrid MHD-particle simulations. Three cases are considered: the monotonic safety factor scenario (SC2), the reversed-shear one (SC4) and a flat safety factor profile 'hybrid' scenario (SCH). All the three cases are found unstable, even though the mode growth rates are quite small. The investigation is extended to cases with larger drive, which could be realistically obtained because of a moderate increase in electron and bulk-ion temperatures as well as by inclusion of energetic particles produced by additional heating methods. It is shown that, in the SCH case, if the drive exceeds the reference value by a factor ∼1.6, fast growing energetic-particle modes (EPMs) are driven unstable by the resonance with trapped alpha particles. The effects of nonlinear mode dynamics on the alpha-particle confinement at the reference drive are negligible for the SC2 and the SCH cases. For the SC4 scenario, some broadening of the alpha-particle pressure profile is observed, indicating inconsistency problems of the scenario itself. Simulations performed with increasing drive intensity show that a strong flattening of the alpha-particle pressure profile can occur in the inner plasma region for the SC2 case, while global confinement is not significantly affected. In the SC4 and SCH scenarios nonlinear effects are more pronounced in the outer portion of the discharge, where the modes are localized, with less impact on the on-axis pressure value with respect to the SC2 case. While for SCH such effects are observed only above the threshold for EPM destabilization, in the SC4 case significant alpha particle losses can occur even at moderately increased drive.

Plasma shape control using real-time equilibrium reconstruction has been implemented on the National Spherical Torus Experiment (NSTX). The rtEFIT code originally developed for use on DIII-D was adapted for use on NSTX. The real-time equilibria provide calculations of the flux at points on the plasma boundary, which are used as input to a shape control algorithm known as isoflux control. The flux at the desired boundary location is compared with a reference flux value, and this flux error is used as the basic feedback quantity for the poloidal field coils on NSTX. The hardware that comprises the control system is described, as well as the software infrastructure. Examples of precise boundary control are also presented.

Velocity–space structures of ion distribution function associated with the ion temperature gradient (ITG) turbulence and the collisionless damping of the zonal flow are investigated by means of a newly developed toroidal gyrokinetic-Vlasov simulation code with high velocity–space resolution. The present simulation on the zonal flow and the geodesic acoustic mode (GAM) successfully reproduces the neoclassical polarization of trapped ions as well as ballistic mode structures produced by collisionless particle motions. During the collisionless damping of GAM, the finer-scale structures of the ion distribution function in the velocity–space continue to develop while preserving an invariant defined by a sum of an entropy variable and the potential energy. The simulation results of the toroidal ITG turbulent transport clearly show generation of the fine velocity–space structures of the distribution function and their collisional dissipation. Detailed calculation of the entropy balance confirms the statistically steady state of turbulence, where the anomalous transport balances with the dissipation are given by the weak collisionality. The above results obtained by simulations with high velocity–space resolution are also understood in terms of generation, transfer and dissipation processes of the entropy variable in the phase–space.

We present a method of control which is able to create barriers to magnetic field line diffusion by a small modification of the magnetic perturbation. This method of control is based on a localized control of chaos in Hamiltonian systems. The aim is to modify the perturbation (of order ε) locally by a small control term (of order ε²) which creates invariant tori acting as barriers to diffusion for Hamiltonian systems with two degrees of freedom. The location of the invariant torus is enforced in the vicinity of the chosen target (at a distance of order ε due to the angle dependence). Given the importance of confinement in magnetic fusion devices, the method is applied to two examples with a loss of magnetic confinement. In the case of locked tearing modes, an invariant torus can be restored that aims at showing the current quench and therefore the generation of runaway electrons. In the second case, the method is applied to the control of stochastic boundaries allowing one to define a transport barrier within the stochastic boundary and therefore to monitor the volume of closed field lines.

A possibility of applying a compact radiation dosimetry system to burning plasma machines, such as detecting high energy neutrons and gamma-rays with a wide dynamic range at elevated temperatures, has been studied, utilizing radiation resistant optical fibres and radioluminescence (radiation-induced luminescence) materials. Appropriate radioluminescence materials were found for dosimetry of each radiation field, namely, gamma-rays and neutrons. Especially, a high-energy neutron detection system utilizing optical fibres and radioluminescence materials will have several advantages, such as being compact and robust, and can be a strong candidate for diagnostics of fusion neutrons in a burning plasma machine.

Code validation activities have been promoted inside the European fusion development agreement (EFDA) to test the capability of codes in simulating accident phenomena in fusion facilities and, specifically, in the International thermonuclear experimental reactor (ITER). This work includes a comparison between three different computer codes (CONSEN, MAGS and MELCOR) and one analytical model (ITER Model) in simulating cryogenic helium releases into the vacuum vessel (VV) which contains hot structures. The scope was the evaluation of the transient pressure inside the VV. The results will be used to design a vent duct (equivalent diameter, length and roughness) to allow pressure relief for the protection of the VV, which has a maximum design pressure of 200 kPa. The model geometry is a simplified scheme preserving the main features of the ITER design. Based on the results of the simulations, a matrix of experiments was developed to validate the calculated results and to design the vent duct for the ITER VV. The experiments are planned to be performed in the EVITA test facility, located in the CEA Cadarache research centre (France).

Recent experiments on DIII-D point to the importance of two factors in determining how effectively the deuterium particle inventory in a tokamak plasma can be controlled through pumping at the divertor target(s): (1) the divertor magnetic balance, i.e. the degree to which the divertor topology is single-null or double-null (DN) and (2) the direction of the of B × ∇B ion drift with respect to the X-point(s). Changes in divertor magnetic balance near the DN shape have a much stronger effect on the particle exhaust rate at the inner divertor target(s) than on the particle exhaust rate at the outer divertor target(s). The particle exhaust rate for the DN shape is strongest at the outer strike point opposite the B × ∇B ion particle drift direction. Our data suggests that the presence of B × ∇B and E × B ion particle drifts in the scrape-off layer and divertor(s) play an important role in the particle exhaust rates of DN and near-DN plasmas. Particle exhaust rates are shown to depend strongly on the edge (pedestal) density. These results have implications for particle control in ITER and other future tokamaks.

High triangularity regimes, approaching double null, have become more important recently. H-mode access and edge stability are affected by the close proximity of the second X-point to the last closed flux surface, but more experimental evidence is needed. Inter-machine comparisons of MAST with ASDEX Upgrade (AUG) and NSTX confirm the reduction of the H-mode power threshold in the true double null configuration (C-DN), first reported on MAST. A more negative radial edge electric field in L-mode observed on MAST and ASDEX Upgrade may be connected to the improved H-mode access. About twice the power density is needed on MAST to achieve edge pressures similar to AUG in L- and H-modes. However, on MAST the pressure is dominated by the edge density in H-mode. So far, in single null (SN) ELMs have not been observed on MAST with neutral beam heating powers P_NBI ⩽ 2 MW, due to the different edge profiles in SN H-mode. This is supported by the modelling of the stability of the measured edge profiles against ideal peeling–ballooning.

Experiments were performed on JET where high-density plasmas with an internal transport barrier (ITB) were created by means of combined use of lower hybrid current drive (LHCD) and pellet injection before the barrier formation. Attempts were also made to use pellets to fuel the plasma and to sustain the density during the ITB phase. It was found that shallow pellets ablating in the region r/a ⩾ 0.8 and far from the foot of the barrier did not destroy the ITB, whereas deeper pellets penetrating up to 0.6 ⩽ r/a ⩽ 0.7 affected the barrier and led to its disappearance. Modelling of these experimental scenarios has been performed with transport and fluid turbulence codes. The codes used in the analysis were: JETTO, a 1.5 dimensional transport code, TRB, a global electrostatic fluid turbulence code and CUTIE, a global electromagnetic fluid turbulence code. The results show that for the shallow pellet case all codes reproduce the general features of the experiment, whereas for the deep pellet case, there are differences in the degree of agreement between the different codes and the experiment. Runs performed varying the pellet penetration depth indicate that not only the pellet penetration, but also the barrier strength plays a key role in the dynamics of the pellet-ITB interaction.

There is an increasing body of evidence that the energy lost from diverted tokamak plasmas due to edge localized mode (ELM) activity may not be confined solely to deposition on divertor components. Plasma-facing surfaces in the main confinement chamber also appear to intercept significant fluxes. Whilst this is of no practical consequence for the operation of present day facilities, concerns are being raised over the possible impact on future devices, in which ELMs carrying higher energies are expected. A key parameter required in this assessment is the energy transported by ions in the ELM as it moves through the scrape-off layer (SOL). This contribution presents the first known direct experimental demonstration that ELM events can convect ions with considerable energies to regions in the far SOL. These measurements, obtained on the JET tokamak with an ion energy analyser probe, are combined with a recently developed SOL transient model to show that the ions can, indeed, reach first limiting surfaces with energies that are a considerable fraction (∼50%) of those found in the H-mode edge pedestal region. This experiment–theory comparison supports a picture of the ELM in which filaments of hot plasma originating in the pedestal region dissipate energy primarily through parallel losses to the divertor targets during their radial propagation across the SOL.

A new ignition scheme is proposed, in which the compressed DT main fuel is ignited by impact collision of another fraction of separately imploded DT fuel, which is accelerated in the hollow conical target to super high velocities of about (1–2) × 10⁸ cm s⁻¹. Its kinetic energy is directly converted into thermal energy corresponding to temperatures >5 keV on the collision with the main fuel, and this self-heated portion plays the role of ignitor. The ignitor shell is irradiated typically by nanosecond pulses at intensities well beyond 10¹⁵ W cm⁻² at such a short laser wavelength as 0.25 µm to exert ablation pressures of 150–300 Mbar. A preliminary two-dimensional hydrodynamic simulation demonstrates substantial heating of 3–5 keV on the impact. Simple physics, potential for high gain designs and low cost—these are the crucial advantages of the present scheme.

The University of Saskatchewan compact torus injector has been modified to allow vertical injection of the compact torus (CT) into the STOR-M tokamak. The injector stayed at the horizontal position and the CT trajectory was bent by 90° using a curved conducting drift tube. The curved drift tube did not significantly slow the CT velocity down or change the CT magnetic field topology. Preliminary vertical CT injection experiments have revealed a prompt increase in the line averaged electron density and in the soft x-ray radiation level in the tokamak discharge immediately following vertical CT injection. Suppression of the m = 2 Mirnov oscillation amplitude has also been observed after CT injection.

A variety of electron density (n_e) profiles have been observed in the Large Helical Device (LHD). The density profiles change dramatically with heating power and toroidal magnetic field (B_t). The particle transport coefficients, i.e. diffusion coefficient (D) and convection velocity (V) are experimentally obtained in the standard configuration from density modulation experiments. The values of D and V are estimated separately in the core and edge. The diffusion coefficients are found to be a function of electron temperature (T_e), and vary with B_t. Edge diffusion coefficients are proportional to . Non-zero V is observed, and it is found that the electron temperature gradient can drive particle convection, particularly in the core region. The convection velocity both in the core and edge reverses direction from inward to outward as the T_e gradient increases. However, the toroidal magnetic field also significantly affects the value and direction of V. The density fluctuation profiles are measured by a two-dimensional phase contrast interferometer. It was found that fluctuations which are localized in the edge propagate towards the ion diamagnetic direction in the laboratory frame, while the phase velocity of fluctuations around mid-radius is close to the plasma poloidal E_r × B_t rotation velocity. The fluctuation level becomes larger as particle flux becomes larger in the edge region.

In order to investigate the separatrix structure inside the plasma, the magnetohydrodynamic (MHD) equilibrium and the high-energy particle orbit are studied for the Large Helical Device plasmas with two magnetic axes located within the two lobes of a figure-eight magnetic separatrix. Magnetic axes are split for strongly elongated cross section by the external quadrupole field. MHD equilibria without assuming existence of nested flux surfaces are obtained from the HINT code. For the vertical elongation, the figure-eight structure evolves due to the finite beta effect and the structure of magnetic field lines outside the separatrix is ergodized. For the horizontal elongation, the Shafranov shift is very large Δ/a > 0.5. To see MHD equilibrium beta limit, high beta equilibrium (∼5%) is studied. The magnetic axis shifts extremely to the outside of the torus and a new figure-eight separatrix appears. The particle orbit on the separatrix is studied by solving the guiding-centre drift equation on the rectangular grid. If the reflecting point of the trapped particle is close to the separatrix, the banana width is very large implying large, nonlocal neoclassical transport.

Transient transport experiments are performed in plasmas with and without internal transport barriers (ITB) on LHD and JT-60U. The dependence of χ_e on the electron temperature, T_e, and on the electron temperature gradient, ∇T_e, is analysed with an empirical non-linear heat transport model. In plasmas without an ITB, two different types of non-linearity of the electron heat transport are observed from cold/heat pulse propagation: the χ_e depends on T_e and ∇T_e in JT-60U, while the ∇T_e dependence is weak in LHD. Inside the ITB region, there is none or weak ∇T_e dependence both in LHD and JT-60U. Growth of the cold pulse driven by the negative T_e dependence of χ_e is observed inside the ITB region (LHD) and near the boundary of the ITB region (JT-60U).

An inertial electrostatic confinement (IEC) fusion device is studied for a compact fusion neutron/proton source using a built-in magnetron ion source. The addition of an ion source to the IEC fusion device enhances fusion reactions by allowing a lower operating gas pressure and by providing a beam-like ion energy distribution. Under lower gas pressures, charge exchange collisions are reduced, resulting in longer ion lifetime and thus enhanced ion re-circulation. The performance characteristics of this IEC fusion device found in the experiments were compared with the numerical calculations and found qualitatively in good agreement. An improvement in normalized neutron yield (defined as neutron yield divided by the product of grid current and operating gas pressure), more than a factor of two, has been observed compared with the conventional glow-discharge driven IEC fusion device.

A study of the influence of the dynamic ergodic divertor (DED) on transport and radiation of intrinsic carbon in TEXTOR has been done, based on calculations with the 2D multifluid code TECXY and experimental data from spectroscopic diagnostics involving the brilliances of lines emitted by C⁺² and C⁺⁴ lines in the UV range. This allows the investigation of the radiation properties of carbon and the contamination of the main plasma for a variety of experimental conditions. The physical model in the TECXY code was extended to incorporate the increased radial transport in the stochastic magnetic field applying a model of 'optimal paths'. Two DED operational modes (m/n = 12/4 and 3/1), both resonant at the q = 3 surface, have been experimentally studied so far. In the present paper we have concentrated on the 12/4 mode of operation. The radial decay length of the 12/4 mode is very short. In order to maximize the field penetration, the plasma column has to be shifted horizontally towards the high field side. Both the simulation and the experimental evidence indicate that the main changes are related to the horizontal shift of the plasma column rather than to the stochastic transport itself. A high recycling regime with good impurity screening is predicted for an un-shifted position of the plasma column as foreseen for the future 6/2 mode operation.

Results of three-dimensional (3D) numerical simulations of field-reversed configurations (FRCs) are presented. The emphasis of this work is on the nonlinear evolution of magnetohydrodynamic (MHD) instabilities in kinetic FRCs and the new FRC formation method by counter-helicity spheromak merging. Kinetic simulations show nonlinear saturation of the n = 1 tilt mode, where n is the toroidal mode number. The n = 2 and n = 3 rotational modes are observed to grow during the nonlinear phase of the tilt instability due to the ion spin-up in the toroidal direction. The ion toroidal spin-up is shown to be related to the resistive decay of the internal flux and the resulting loss of particle confinement. Three-dimensional MHD simulations of counter-helicity spheromak merging and FRC formation show good qualitative agreement with the results from the SSX–FRC experiment. The simulations show the formation of an FRC in about 20–30 Alfvén times for typical experimental parameters. The growth rate of the n = 1 tilt mode is shown to be significantly reduced compared with the MHD growth rate due to the large plasma viscosity and field-line-tying effects.

See the PDF file for the list of referees.