Table of contents

The effects of gyro-Landau drift resonances on the collisionless trapped electron mode have been studied. The drift resonances are added to a two pole fluid model of the density response. The gyro-Landau drift resonances in general give good agreement with kinetic theory. Comparison is also made with the fluid model without drift resonances. The toroidal eta _i mode is also included in the system

The evolution of the energy, momentum and particle transport through the L-H transition are determined in JET NBI heated discharges. Both normal and periodic L-H transitions are studied. It is found that all of the transport coefficients drop at the transition over a wide radial region and not just in the edge region as was previously thought to be the case. Indeed it is shown by two different modelling techniques that the conventional model in which the transport changes in a narrow region at the edge cannot explain the time behaviour of the electron temperature. Measurements of the fluctuation level by reflectometry also show a very fast drop over a wide radial region

Peaking the density profile increases the usable bootstrap current and the average fusion power density; this could reduce the current drive power and increase the net output of power producing tokamaks. The use of neutral beams and pellet injection to produce peaked density profiles is assessed. It is shown that with radially 'hollow' diffusivity profiles (and no particle pinch) moderately peaked density profiles can be produced by particle source profiles that are peaked off-axis. The fuelling penetration requirements can therefore be relaxed and this greatly improves the feasibility of generating peaked density profiles in large tokamaks. In particular, neutral beam fuelling does not require megavolt particle energies. Even with beam voltages of approximately 200 keV, however, exceptionally good particle confinement is needed to achieve net electrical power generation. The required ratio of particle to thermal diffusivities is an order of magnitude outside the range reported for tokamaks. In a system with no power production requirement (e.g., neutron sources) neutral beam fuelling should be capable of producing peaked density profiles in devices as large as ITER. Fuelling systems with low energy cost per particle-such as cryogenic pellet injection-must be used in power producing tokamaks when tau _p approximately tau _E. Simulations with pellet injection speeds of 7 km/s show that the peaking factor, n_e0/(n_e), approaches 2

The relationship between the dynamical response to perturbations of the soft X-ray emissivity (δE), the electron temperature (δT_e), the electron density (δn_e) and the impurity concentration (δn_i) for a Maxwellian plasma is analysed in detail. In particular, the so-called 'impurity function' F(Z_eff) is also strongly dependent on T_e via the direct radiative recombination (DRR) contribution to the X-ray emission, which significantly affects the relation between the perturbed quantities as derived from the popular expression E ∝ F(Z_eff)n_e²T_e^alpha even if the impurity content (or Z_eff) remains constant. In order to overcome this difficulty, a simple analytical approximation is derived for F(Z_eff,T_e) that can be used as a formula to relate the perturbed quantities δE, δT_e, δn_e and δF with ease and accuracy. This simple approximation is illustrated by studying saw-toothing discharges on the Tokamak de Varennes (TdeV) with T_e, n_e and E measured by the Thomson scattering, the FIR interferometer and the X-ray camera diagnostics, and its accuracy is tested against the predictions of a full X-ray modelling code

In previous analysis of edge localized modes (ELMs), three different types (types I, II and III) of ELM were observed in DIII-D, whereas only one type of ELM has been found in ASDEX. The type III ELMs observed on DIII-D are similar to the ELMs observed on ASDEX, whereas the type I ELMs did not occur on ASDEX. ELMs are found to be MHD events of short duration (1 to 2 ms), which, on DIII-D, may be followed by a transient L mode phase. This compound event was not observed on ASDEX. A stability analysis of DIII-D discharges indicates that in the standard DIII-D configuration, type III ELMs occur well below the ideal ballooning limit, whereas Type I ELMs generally occur close to this limit. In DIII-D discharges where the plasma parameters are close to the ASDEX values, type I ELMs are not observed, even at the beta limit. This suggests that the ELM behaviour in DIII-D is not uniquely governed by ballooning stability

The attractiveness of future commercial tokamak reactors is sensitive to the attainable plasma performance, notably plasma energy confinement and allowable beta. The impact of varying levels of confinement and beta on the size and cost of the resulting tokamak reactor is systematically quantified. Several different classes of tokamak reactors are considered, and designs are optimized in terms of cost of electricity (COE) via a coupled physics/engineering/costing systems code. Surprisingly narrow ranges of plasma confinement and beta are found to be simultaneously useful in minimizing the reactor COE, i.e. improvement in only one of these quantities is not useful beyond some point without accompanying improvements in the other. For steady state, current driven reactors characterized by H mode confinement (where τ_E=Hτ_E,L; τ_E,L being the confinement time predicted by the ITER.89 L mode scaling, and H ~ 2), the maximum useful Troyon β coefficient (βN) is only ~ 4.3%.mT/MA. These confinement levels are similar to those observed in present day experiments. If slightly better confinement is achievable (i.e. an enhancement factor over L mode of H ~ 2.5), the maximum useful Troyon coefficient increases to βN ~ 6 and the reactor COE decreases by 20%. Inductively driven, pulsed reactors have somewhat increased useful ranges of confinement relative to the steady state cases. In general, increasing the allowable beta over presently accepted limits offers the single biggest improvement in reactor attractiveness of the tokamak concept

The electrostatic quasi-linear particle and thermal diffusivities, due to the toroidal ion temperature gradient (ITG) driven modes and the dissipative trapped electron (DTE) mode, are considered in the dissipative limit, ω << v_ef/. Impurities are included by assuming a plasma with three particle species (electrons, main ions and impurity ions). The diffusivities are studied by computing interpretative radial transport profiles based on a TEXTOR background variation, using a saturation level approximation. The transport is found to be driven also by impurity modes, in particular when a light impurity species, such as helium, is assumed. The existence of inward fluxes of particles and heat is discussed

The propagation of waves in the ion cyclotron range of frequencies has been investigated experimentally in the Tokamak Fusion Test Reactor (TFTR). A small, broadband, radiofrequency (RF) magnetic probe located outside the plasma limiter, at a major radius near that of the plasma centre, was excited with a low power, frequency swept source (1-200 MHz). Waves propagating to a distant location were detected with a second, identical, probe. The RF transmission spectrum revealed a region of attenuation over a band of frequencies for which the minority fundamental resonance was located between the outer plasma edge and tile major radius of the probe location. Distinct, non-overlapping attenuation bands were observed from hydrogen and ³He minority species; a distinct tritium band should be observed in future DT experiments. Rapid spectrum acquisition during a ³He gas puff experiment showed that the wave attenuation involved the plasma core and was not a surface effect. A model in which the received power varied exponentially with the minority density, averaged over the resonance region, fitted the time evolution of the probe signal relatively well. An estimation of the one dimensional tunnelling parameter from the experimental observations is discussed. Minority concentrations of less than 0.5% can be resolved with this measurement

The orbits of ions and electrons in toroidal edge plasmas in the presence of low frequency turbulence are investigated. It is found that at low fluctuation levels, ion banana orbits are washed out by turbulent waves and that transport becomes ambipolar. The effective radial step sizes of ions and electrons are comparable. Thus the ion orbit loss mechanism may become ineffective in driving poloidal rotation at the L-H transition in tokamaks

The launching conditions for an electron cyclotron wave in a plasma with a sheared magnetic field are studied. One dimensional second order coupled equations are solved for the ordinary and extraordinary modes. It is shown that, for wave absorption, the rotation angle and ellipticity of the wave should be determined by taking into account the effect of the sheared magnetic field, particularly in heliotron/torsatron configurations. Launching conditions for effective heating are obtained for second harmonic extraordinary mode heating in the Heliotron-E, CHS and LHD helical devices

The effective collision frequency relevant to the problem of current drive (CD) is evaluated taking into account the radiative slowing down due to (spontaneous) electron cyclotron (EC) emission as well as an ion charge state Z ⩾ 1. The efficiency of CD by EC radiation from high temperature plasmas is obtained straight-forwardly on the basis of the salient properties of the single electron emissivity at mildly relativistic energies. The CD degradation due to radiative slowing down is evaluated and the relevant scaling with respect to Z is discussed

Report on the IAEA Technical Committee Meeting, held at Naka-machi, Ibaraki-ken, Japan, 10-12 November 1993