Table of contents

Results from an extensive database analysis of JET density profiles in stationary conditions show that the density peaking factor n_e0/⟨n_e⟩ in JET H modes increases from near 1.2 at high collisionality to around 1.5 as the plasma collisionality decreases towards the values expected for ITER. This result confirms an earlier observation on AUG. The density peaking behaviour of L modes is remarkably different from that of H modes, scaling with overall plasma shear as (n_e0/⟨n_e⟩ ∼ 1.5l_i), independently of collisionality. H-mode density profiles show no shear dependence, except at the lowest collisionalities. No evidence for L_Te, L_Ti, ρ^* or β dependences has been obtained. Carbon impurity density profiles from charge exchange spectroscopy are always less peaked than electron density profiles and usually flat in H modes. The peaking of the electron density profiles, together with the flatness of the impurity density profiles, are favourable for fusion performance if they can be extrapolated to ignited conditions.

The effect on coupling of an evanescent layer in front of ion cyclotron fast-wave antennas is commonly described by the exponential factor exp(−2k_||d), with k_|| the parallel wave vector of the peak of the launched wave spectrum and d the width of the evanescence layer. With an elementary one-dimensional model, we demonstrate that this dependence is too pessimistic, as confirmed by recent experiments.

Fully non-inductive discharges with lower hybrid (LH) current drive have been run successfully in Tore Supra, allowing for stationary plasma conditions for more than 6 min. The operational domain has been explored in terms of total plasma current (I_p), magnetic field (B_vac), and parallel refractive index (⟨n_∥⟩), which characterize the deposition profile of LH waves. MHD activity has been encountered in a number of cases, either at a low saturated level, or at a high level in the so-called MHD regime (Maget P. et al2004 Nucl. Fusion44 443). This work first addresses the linear MHD stability properties of the operational space (B_vac, I_p, ⟨n_∥⟩). Although most regions are found to be linearly unstable, the condition for triggering the deleterious MHD regime is more restrictive, and is shown to result from the full reconnection of the double-tearing mode. A simple criterion based on the helical flux ψ^*, namely ψ^* = 0 at the outer resonant surface, is used for the identification of the region where the MHD regime is triggered.

Large potential oscillations were detected in JIPPT-IIU tokamak plasmas in a wide range of plasma cross-sections in measurements using a multi-sample-volume heavy ion beam probe. These oscillations have large amplitudes reaching a few hundreds of volts and their frequencies are in the range of the geodesic acoustic mode (GAM). They are found over a wide range of plasma cross-sections and commonly have m = 0 structures. As they were Fourier analysed, it was found that the central frequency is higher in the core of the plasma and lower in the edge of the plasma. These observations agree with the properties of theoretically predicted GAM oscillations. It was also found that the frequency spectrum is peaked in the core and broad in the edge, which may have something to do with damping mechanisms of the GAM. The phase relation between the density and the electric field fluctuations was studied extensively in terms of the cross-correlation function. The level of the density fluctuation was low as it should be, and the expected 90° phase difference was found in a limited radial domain.

This paper presents a detailed analysis of the transport behaviour of the detached plasmas in W7-AS based on an extended numerical study using the EMC3-EIRENE code, aimed at understanding the underlying physics responsible for the geometry-dependent detachment stability observed in W7-AS island divertor experiments. Here, a stable detachment can only be established when the control coils are switched on to generate sufficiently large islands with relatively short connection lengths. Special attention will be paid to a discussion of the carbon radiation, location and dynamics of the radiation layer, the neutral screening efficiency specific to the island divertor geometry and its impact on the detachment stability. Based on the three-dimensional simulation results, a linear stability model is presented in order to obtain some insight into the mechanisms driving the instability. The radiation behaviour and the location and evolution of the radiation zone in the island divertor will be discussed with respect to those of tokamak-MARFEs.

A demonstration tokamak fusion power plant Demo-CREST is proposed as the device for early realization of net electric power generation by fusion energy. The plasma configuration for Demo-CREST is optimized to satisfy the electric breakeven condition (the condition for net electric power, ) with the plasma performance of the ITER reference operation mode. This optimization method is considered to be suitable for the design of a demonstration power plant for early realization of net electric power generation, because the demonstration power plant has to ensure the net electric generation. Plasma performance should also be more reliably achieved than in past design studies. For the plasma performance planned in the present ITER programme, net electric power from 0 to 500 MW is possible with Demo-CREST under the following engineering conditions: maximum magnetic field 16 T, thermal efficiency 30%, NBI system efficiency 50% and NBI current drive power restricted to 200 MW. By replacing the blanket system with one of higher thermal efficiency, a net electric power of about 1000 MW is also possible so that the performance of the commercial plant with Demo-CREST can also be studied from the economic point of view. The development path from the experimental reactor 'ITER' to the commercial plant 'CREST' through the demonstration power plant 'Demo-CREST' is proposed as an example of the fast track concept.

In plasmas exhibiting an internal transport barrier (ITB), locally very high pressure gradient (∇P) is obtained. It induces high values of the magnetohydrodynamic α parameter (α = −q² βR∇P/P, with R the major radius, q the safety factor, P the pressure, ∇ the radial gradient and β the ratio between kinetic and magnetic pressure). Similarly to low or negative magnetic shear (s), high α reduces the curvature and ∇ B drifts driving curvature-type microinstabilities. Therefore, high values of α can stabilize part of the microturbulence, which leads to higher pressure gradient and to even higher α. This possibility for entering a positive feedback loop is very attractive to sustain ITBs in high performance plasmas. Indeed, α scales favourably with higher pressure and does not require any external momentum input. In this paper, after having discussed the α stabilization mechanism in detail, we report the experimental microstability analyses of ITBs from an international multi-machine database—the International Tokamak Physics Activity database, accessible on the webhttp://tokamak-profiledb.ukaea.org.uk/ . We show that α is indeed a relevant parameter of ITB physics that should be taken into account in interpretative and predictive one-dimensional transport codes.

Significant experimental and theoretical progress has been made in the US heavy ion fusion programme on high-current sources, injectors, transport, final focusing, chambers and targets for high-energy density physics and inertial fusion energy (IFE) driven by induction linac accelerators. One focus of the present research is the beam physics associated with quadrupole focusing of intense, space-charge-dominated heavy ion beams, including gas and electron cloud effects at high currents, and the study of long-distance-propagation effects such as emittance growth due to field errors in scaled experiments. A second area of emphasis in the present research is the introduction of background plasma to neutralize the space-charge of intense heavy ion beams and assist in focusing the beams to a small spot size. In the near future, research will continue in the above areas, and a new area of emphasis will be to explore the physics of neutralized beam compression and focusing to high intensities required to heat targets to high-energy density conditions as well as for IFE.

A database of the confinement properties of the toroidal pinch experiment (TPE) series reversed-field pinch (RFP) devices was established, and an empirical scaling law for the energy confinement time, τ_E, was deduced for a consistently selected set of the database (Yagi Y. et al2003 Nucl. Fusion43 1787). The scaling for τ_E [TPE-scaling; ], in particular, is a power law similar to that predicted on the basis of the tearing modes, as a function of the plasma minor radius (a), plasma current (I_p), and I_p/N, where N is the column density. On the other hand, the TPE-scaling has a strong dependence on the pinch parameter, Θ, namely τ_E ∼ Θ³. Recently, we have investigated the database of the improved confinement in the pulsed poloidal current drive (PPCD). We show that the TPE-scaling agrees well with the improved τ_E in the PPCD database, because of its strong Θ dependence. Namely, Θ³ is a factor of merit for RFP plasmas. We discuss why this agreement is obtained in spite of the general difference between the underlying transport mechanisms of the standard and PPCD discharges. We also show that this improvement, represented by Θ³, is related to the increase in magnetic shear with Θ.

Neoclassical tearing modes (NTMs) are examined with a reduced MHD model including the bootstrap current as a simple model, J_BS ∝ − dP/dr, in cylindrical tokamaks. It is found that linear instabilities due to the perturbation of bootstrap current appear for large J_BS cases with large parallel thermal diffusivity, χ_∥, under small perpendicular thermal diffusivity, χ_⊥, in the pressure evolution equation. Here, other dissipations such as resistivity and viscosity are also finite. Although the linear growth rate of this unstable mode is small, the growth of magnetic energy of this unstable mode is usually accelerated after the saturation of kinetic energy. Its nonlinear behaviour shown by the evolution of magnetic islands is consistent with the standard nonlinear NTM theory.