Table of contents

A study of turbulent particle transport and drift mode stability in deuterium-tritium plasmas is presented. The study is based on an extended implementation of the Weiland transport model for ion temperature gradient (ITG-) modes and trapped electron (TE-) modes. The dependence of the transport fluxes on the tritium density gradient and the tritium concentration is investigated. Particle pinches are demonstrated for general parameter values and an asymmetry is found in the deuterium and tritium transport fluxes.

We have investigated the effect of edge localized modes (ELMs) on the total plasma current in JET tokamak plasmas using the earth leakage current detection circuit connected to the in-vessel saddle coils. These measurements, and the geometry of the saddle coil circuit, indicate that, in typical H-mode plasmas, up to ∼300 A (corresponding to a maximum of ∼150 A per MA of plasma current) can be deposited at each ELM in regions rather far away (∼50 cm) from the strike point position and divertor target plates. These current losses decrease with increasing ELM frequency (f_ELM), being practically below or around the detection limit for f_ELM > 60 Hz, and show some saturation for increasing diamagnetic energy losses for 'slow' Type-I ELMs at f_ELM < 10 Hz. The measurements reported here are consistent with the ELM phenomenon being associated to a peeling of the edge magnetic flux surfaces.

A new oscillation with m = 1 and n = 1, where m and n are the poloidal and toroidal mode numbers, respectively, has been observed with lower hybrid current drive (LHCD) on the HT-7 tokamak. This mode appears during the sawtooth ramp, saturates, damps and finally disappears before the subsequent sawtooth collapse and before the appearance of the precursor oscillations. The necessary condition for its observation is that the sawtooth period exceeds 6–7 ms and that the LHCD power exceeds the threshold value of about 90 kW. We propose that the mode may be destabilized by the current density gradient, the evolution of which is determined by the suprathermal electrons generated by the LHCD.

The effect of toroidal rotation on the ideal magnetohydrodynamic stability of the internal kink, or quasi-interchange, mode in tokamak plasmas with the safety factor, q, close to unity within a wide region in the plasma core is analysed. For equilibria with a large aspect ratio and circular cross section it is shown that rigid, toroidal rotation stabilizes the quasi-interchange mode if the rotation frequency, Ω, exceeds a critical frequency, Ω_crit, and a simple expression for Ω_crit is derived. Unless the plasma pressure and the radial extent of the low-shear region are both rather large, rotation frequencies of the order of a few per cent of the Alfvén frequency are found to be sufficient to stabilize the quasi-interchange mode. The stabilization is caused by the nonuniform plasma density and the associated Brunt–Väisälä (BV) frequency created by the centrifugal force. When Ω exceeds Ω_crit, the quasi-interchange instability transforms into a stable oscillation with a frequency close to the lowest BV-frequency in the low-shear region of the plasma.

The influence of plasma density and edge gradients on the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. The development of the naturally occurring velocity shear layer requires a minimum plasma density. Experimental findings have shown that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increases. Electron density profiles show a broadening evolution as density increases above the critical value where sheared flow is developed, while the temperature profile remains similar, reflecting the strong impact of plasma density in the global confinement scaling. Furthermore, the shearing rate of the spontaneous sheared flow turns out to be close to the one needed to trigger a transition to improved confinement regimes. Density ramp experiments show, within the experimental uncertainty, no evidence of hysteresis during the spontaneous shear development. Power modulation, in the proximity of the critical plasma density, allows the characterization of plasma potential and electric field relaxation during the transition. The present results have a direct impact on the understanding of the physics mechanisms underlying the generation of critical sheared flow, pointing to the important role of turbulent driven flow.

Taking into account the effects of atomic processes and the observation angle, a simple code modelling the correction to the poloidal charge-exchange recombination spectroscopy (CXRS) measurement in a tokamak is developed, which is an effective tool to correct the deviation in CXRS measurements. The simulated results show that the measured poloidal velocity shift is overestimated in the ion diamagnetic drift direction and the apparent ion temperature is underestimated because of the effects of the energy-dependent emission rate, finite lifetime of excited ions and the observation direction. This deviation will be more obvious with an increase in temperature, magnetic field, or observation angle. In the core region, the pseudo-shift coming from the observation angle effect is much more significant than that from others. The contributions from gradients of density and temperature are negligible compared to that from emission rate, lifetime and observation angle effects. Using the corrected velocity, a radial electric field (E_r) well is formed near the internal transport barrier (ITB) location for a typical plasma on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U). It seems that the poloidal rotation enhances E_r shear, which is speculated to be related to the ITB formation.

The positive density dependence of energy confinement times, τ_E, as expressed in the international stellarator scaling 95 (ISS95), where ( is the line-averaged density and P is the heating power), declines in high-collisionality regimes in the large helical device experiments. In the low-collisionality regime, where parameter dependences in ISS95 agree well with the experiment, the temperature dependence of thermal diffusivity is as strong as predicted by the gyro-Bohm model and/or the neoclassical theory. As the collisionality increases to the plateau regime, the temperature dependence becomes moderate, where the thermal diffusivity is proportional to the square root of the electron temperature. Furthermore, the thermal diffusivity is inversely proportional to the magnetic field strength and the electron temperature gradient is proportional to the electron temperature in the high-collisionality regime. Compared with ISS95, the energy confinement time expected from these observations has a weaker density dependence together with mitigated power degradation: .

In the ASDEX Upgrade tokamak, the power deposition structures on the divertor target plates during type-I edge localized modes (ELMs) have been investigated by infrared thermography. In addition to the axisymmetric strike line, several poloidally displaced stripes are resolved, identifying an ELM as a composite of several subevents. This pattern is interpreted as being a signature of the helical perturbations in the low field side edge during the non-linear ELM evolution. Based on this observation, the ELM related magnetic perturbation in the midplane can be derived from the target load pattern. In the start phase of an ELM collapse, average toroidal mode numbers around n ≈ 3–5 are found evolving to values of n ≈ 12–14 during the ELM power deposition maximum. Further information about the non-linear evolution of the ELM mode structure is obtained from statistical analyses of the spatial distribution, heat flux amplitudes and number of single stripes.

In the Mega-Amp Spherical Tokamak, MAST, the formation of the edge transport barrier leading to the high-confinement (H-mode) regime is greatly facilitated by operating in a double null diverted (DND) configuration where both X-points are practically on the same flux surface. Ohmic H-modes are presently only obtained in these connected double null diverted (CDND) configurations. The ease of H-mode access is lost if the two flux surfaces passing through the X-points are radially separated by more than one ion Larmor radius (ρ_i ≈ 6 mm) at the low-field-side mid-plane. The change of the magnetic configuration from disconnected to CDND is accompanied by a change in the radial electric field of about ΔE_ψ ≈ −1 kV m⁻¹ and a reduction of the electron temperature decay length in the high-field-side scrape-off-layer. Other parameters at the plasma edge, in particular those affecting the H-mode access criteria of common L/H transition theories, are not affected by the slight changes to the magnetic configuration. It is believed that the observed change in E_ψ, which may result from differences in ion orbit losses, leads to a higher initial E × B flow shear in CDND configurations which could lead to the easier H-mode access.

In a recent series of experiments, electron cyclotron current drive (ECCD) has been successfully used, at a level of 0.75 MW, for current profile tailoring during the current ramp-up in Tore-Supra. The electron cyclotron resonance heating power deposition was varied from on-axis to off-axis and the direction of the driven current from co to counter. In these conditions, the current profile is significantly modified with respect to those typically obtained in pure ohmic scenarios. Central reversed magnetic shear conditions have been achieved with on-axis counter-ECCD, accompanied by high electron temperature gradients, exhibiting internal transport barrier features. This improved electron transport is maintained for some time on the current flat-top when combining ECCD with ion cyclotron resonance heating in a (H)D minority scheme. Integrated interpretative analysis with the CRONOS code confirms that deeply reversed magnetic shear is indeed attained by on-axis counter-ECCD in low density conditions and in combination with a relatively fast controlled current ramp. The high electron temperature gradient is found to be located inside the negative magnetic shear region.

Edge turbulence measurements from a reciprocating Langmuir probe in MAST are presented. A comparison of the range/standard deviation (R/S), growth of range, first moment and differencing and rescaling methods for calculating the Hurst exponent is made. The differencing and rescaling method is found to be the most useful for identifying scaling over long time-periods. A comparison is made between L-mode, dithering H-mode and H-mode plasma edge turbulence and evidence for self-similarity is found. Tests are performed and it is demonstrated that the results are due to properties of the data, and are not artefacts of the methods. A comparison of Hurst exponent methods with the autocorrelation function and power spectrum is used to demonstrate the presence of long-time correlation in L-mode data, and the absence of long-time correlation in the case of dithering H-mode.

Results of the simulation of the transport and radiation losses of high-Z impurities (tungsten and argon) in the ITER reference operational scenarios are presented. The impurity dynamics was modelled by the ZIMPUR impurity code, a description of which is presented. This code was integrated with the ASTRA transport code for the background plasma behaviour modelling and with the NCLASS code to take into account the neoclassical impurity fluxes. Simulations show that for flat plasma density profiles the neoclassical thermal diffusion contributes to plasma core screening from heavy impurities in all considered scenarios. Thus, a radiating layer is formed near the plasma column periphery. It is more essential for the steady-state scenario with the reversed shear region and an improved confinement in the plasma core. An appreciable effect is also found for the reference inductive ELMy H-mode scenario, where the plasma is more strongly influenced by anomalous transport. The calculated radiated power near the plasma edge exceeds by about twice the value obtained in the coronal model approximation. This can be explained as being due to the reduction of impurity ion charge states, due to their charge exchange with deuterium/tritium neutrals, fuelling the plasma. It offers the possibility of decreasing the total impurity contamination, which is necessary for re-radiation of the given power.

Imaging sensor technologies such as charge coupled devices and complementary metal-oxide-semiconductor have made remarkable progress in recent years. Fast imaging systems based on these new technologies are now being routinely employed for advanced fusion diagnostics. Since two-dimensional imaging considerably improves the investigation of three-dimensional structural physics, we have installed and operated the first high-speed two-dimensional coherence imaging camera system for the study of ion temperatures and flow velocities in the WEGA stellarator based on the Doppler broadening of 468.6 nm He II line emission. The coherence imaging camera was able to image the complete plasma poloidal cross-section over a toroidal region spanning 10°. The camera was used for basic plasma studies, including electron cyclotron resonance heating (ECRH) power step experiments. The ion temperature of helium plasmas in WEGA is found to be 1.5–2.0 eV at maximum (26 kW) ECRH power. The plasma rotates in the E × B direction with speeds between 500 and 1000 m s⁻¹, increasing at higher ECRH power. It was confirmed that the flow direction reverses with the direction of the magnetic field. The observed ion temperatures and flows were cross checked against a multi-channel Echelle spectrometer and satisfactory agreement obtained.

This report summarizes the contributions to the 10th EU–US Transport Task Force Workshop on Transport in Fusion Plasmas (Varenna, Italy, 6–9 September 2004). There was an emphasis on transport barrier physics, but also sessions on particle and heat pinches, electron and transient transport and electromagnetic and electrostatic turbulent transport, as well as some on other general issues.