Table of contents

Spatial correlations and average wavenumbers of fluctuations have been measured in the edge region (0.8 < r/a < 1) of the TEXT tokamak using a heavy ion beam probe. The poloidal correlation length is substantially longer inside the limiter than it is outside. From this result it is concluded that the average wavenumber is smaller inside the limiter than it is outside. The frequency dependence of the average wavenumber also varies with radius, and a shear layer is sometimes observed. The frequency averaged wavenumbers are of the order of 1 cm^-1. The statistical phase velocities vary from 2.5 × 10⁵ to 6 × 10⁵ cm/s in this region. The sensitivity to higher wavenumbers was increased by reducing the sample volume size in one experiment, and it was found that the fluctuation level changed by a factor of two while the wavenumber at a given frequency changed by 20%. The measurements are compared with previous results from far infrared scattering and Langmuir probes. The statistical phase velocities obtained with a heavy ion beam probe disagree with those from the other diagnostics in some but not all cases. The observed low frequency statistical phase velocities are comparable to the diamagnetic drift velocity in the laboratory frame in some but not all cases

Improvement of both energy and particle confinement was observed in beam heated hydrogen plasmas in JT-60 when the heating power and the line averaged density exceeded thresholds. The radiation power from the divertor plasma increased to as much as 50% of the heating power. This regime was termed improved divertor confinement (IDC). The discharges with improvement of the energy confinement by up to 20% and enhanced divertor radiation lasted for several seconds in a quasi-steady state without significant impurity accumulation in the core. The threshold line averaged electron density for the IDC regime increased linearly with power and dropped with increasing safety factor. The improvement was observed only when the ion Del B drift was towards the X-point. The behaviour of light impurities and in-out asymmetries in particle recycling changed with reversal of the toroidal magnetic field. These findings are consistent with the theoretical model in which collisional transport near the separatrix is connected with the onset of the IDC regime

The presence of two (or more) species of ions in an inhomogeneous magnetically confined plasma can present opportunities for a new class of drift-type modes to become unstable. When the thermal velocity of a minority ion species (the impurity) is much smaller than that of the primary species, waves with parallel phase velocities intermediate between the two thermal velocities can be driven unstable by a source of the free energy (for example density or temperature gradient) and by Landau damping. Their linear stability theory and quasi-linear particle and heat transport are discussed in the context of shearless slab geometry. Also discussed is the simultaneous appearance of two unstable modes, one propagating in the ion diamagnetic direction and the other in the electron direction. When the effective impurity concentration is finite, the threshold for the ion temperature gradient instability is dramatically increased

Numerical solutions are described for three-dimensional MHD equilibria in the presence of resonant magnetic field perturbations. The effects of a realistic spectrum of resonant field errors are calculated for a range of current profiles in cylindrical geometry, and the results are benchmarked against toroidal calculations for two different current profiles. It is found that field errors of the magnitude existing in present day devices, and contemplated for future devices, can produce a set of magnetic islands occupying a significant fraction of the plasma cross-section. The calculations are applicable to those situations where flow effects are unimportant. The conditions under which this is the case are discussed, and implications for several cases of interest are considered

An efficient full domain method for plasma equilibrium construction in iron core tokamaks is described. Illustrative calculations and comparison with results obtained from a finite domain method using JET data are given. The results show that the two methods are in agreement with respect to global plasma parameters such as stored energy, internal inductance, volume, and edge safety factor. However, when there is an uncertainty in the magnetic flux loop measurement, the full domain method yields a more accurate separatrix location

The authors study the possibility of target plasma radiofrequency (RF) production in the ion cyclotron range of frequencies (ICRF) (ω ⩽ ω_ci) in large scale tokamaks before the startup of an Ohmic discharge. A number of experimental and theoretical studies on dense plasma production in the ICRF in toroidal magnetic devices are reviewed. The criteria for optimal development of the RF discharge stages are analysed, i.e., for RF breakdown of the neutral fill gas in the vicinity of the antenna (non-wave stage, n_e << n_a, where n_e and n_a are the densities of electrons and atoms), and the wave stages of initial ionization (n_e < n_a) and neutral gas burnout (n_e approximately n_a) in the whole volume of the plasma torus. A number of requirements for the design of the antenna system are formulated for all stages of dense plasma ICRF production. A scenario for plasma ICRF production with slot type antennas in the ITER tokamak is proposed. Numerical simulations with a 0-D transport code show that in ITER a target plasma with an electron density of approximately=3*10¹² cm^-3 can be produced by coupling 3-6 MW of RF power to the plasma at a frequency of approximately=3 MHz. Such a level of RF power is sufficient not only for full ionization of the neutral gas but also for heating the produced plasma to electron temperatures of approximately 80 eV

Scaling laws for the divertor conditions to be expected in a next step device, ITER, are derived from 2D modelling. Model calculations are carried out for varying density, power, transport coefficients, pitch, size and particle throughput. In the low temperature, high recycling domain, the peak power, the electron temperature, and the particle density at the plate are found to be extremely sensitive to upstream parameters, especially to upstream density. Low density, high power operation therefore remains very demanding for the divertor. For operation at a critical density, for which the electron temperature at the point of peak power load is a fixed low value, the peak power load scales more gently, proportional to the power density flowing into the scrape-off layer. Precise control of the density is required to maintain this temperature

Toroidal Alfven eigenmode (TAE) and elliptical Alfven eigenmode (EAE) instabilities in plasmas with high energy ions are considered in the context of local theory. The instability growth rate is found for cases when waves are excited by alpha particles or by ions produced as a result of neutral injection or RF heating. Electron and ion Landau damping due to the toroidal sideband wave-particle interaction is also calculated. The electron damping rate is shown to be much lower than the generally accepted value. The TAE instability observed in the experiment with neutral beam injection on TFTR is analysed and the principal experimental features of TAE instability are explained

Measurements of toroidal and poloidal rotation of the TCA plasma with Alfven wave heating and different levels of gas feed are reported. The temporal evolution of the rotation was inferred from intrinsic spectral lines of CV and C III, and, using injected helium gas, from He II. The light collection optics and line intensity permitted the evolution of the plasma rotation to be measured with a time resolution of 2 ms. The rotation velocities were used to deduce the radial electric field. With Alfven wave heating there was no observable change of the electric field that could have been responsible for the density rise which was characteristic of the RF experiments on TCA. The behaviour of the plasma rotation with different plasma density ramp rates was investigated. The toroidal rotation was observed to decrease with increasing plasma density. The poloidal rotation was observed to follow the value of the plasma density. With strong gas puffing, changes in the deduced radial electric field were found to coincide with changes in the peaking of the plasma density profile. Finally, with frozen pellet injection, the expected increase in the radial electric field due to the increased plasma density was not observed, which may explain the poorer confinement of the injected particles

The usual argument for automatic ambipolarity of neoclassical particle fluxes in a tokamak is based on the averaged toroidal momentum equation. It does not apply when fluctuations are present, because they also contribute to the momentum. The finite Larmor radius pressure tensor produced by electrostatic fluctuations makes a small contribution to the averaged momentum balance and also gives rise to a small nonambipolar radial flux. The (j × B)_ϕ force from stochastic magnetic fluctuations can be much larger, as is the associated nonambipolar particle flux. The ambipolar electric field is determined by the combined neoclassical and anomalous fluxes. Since the transport produced by electrostatic fluctuations is nearly ambipolar, the resulting change in the radial electric field from its neoclassical value is small. However, electron loss along a stochastic magnetic field is strongly nonambipolar and can reduce or reverse the inward electric field produced by neoclassical transport alone. Impurity neoclassical transport is most sensitive to the electric field because of its high atomic charge. This can explain the observed pump-out of impurities during intense MHD activity

A two-dimensional coupled plasma-neutral fluid transport computational model is used to examine divertor recycling. Realistic magnetic flux surface geometries and recycling between the edge and core plasma regions are included. Results are compared for several divertor cases to minimize the influence of uncertain anomalous radial transport. Time dependent studies show that a characteristic time to establish recycling is typically tens of milliseconds, determined by particle recycling across the separatrix. The results show that achieving high recycling solutions (high density and low divertor plasma temperature) requires two conditions: first, that x_d > λ₀, where x_d is the target to separatrix distance and λ₀ is the neutral mean free path; and second, that the ratio of the heat flux to particle flux across the separatrix (core boundary) Q_bc/Γ_bc be below a critical value. The latter condition makes extrapolation to the fusion reactor regime appear difficult. High recycling solutions can drive parallel flow away from the target in large regions of the edge plasma, which means flow reversal. Poloidal flow is dominated by a large drift flux near the separatrix. Poloidal gradients in electrostatic potential are significant. It is speculated that the electric fields and currents combined with the large poloidal flow contribute to fluctuations observed in the edge plasma

A model based on the linearized Vlasov-Maxwell equations, taking into account the nonlocal interactions of particles due to their finite Larmor radii has been developed. Assuming an inhomogeneous 1D slab plasma, Maxwellian equilibrium distribution functions and k_y = 0, this model leads to a system of one first-order and two second-order integro-differential equations for E_x and E_y, E_z, respectively. These equations are valid for arbitrary values of k_⊥ρ_σ, where k_⊥ is the perpendicular wave number and ρ_σ the Larmor radius of species sigma . Therefore, the code SEMAL, which solves these equations, is appropriate for studying the effects of alpha particles on heating in the ion cyclotron range of frequencies. These effects are shown to be much less significant for heating at the second harmonic of deuterium than is expected from local models. Also investigated are other heating scenarii of deuterium, as well as the influence of k_z, T_α, non-Maxwellian distribution functions and n_α/n_e. The results indicate under which conditions the power absorption by alpha particles begins to dominate and therefore to degrade the heating efficiency

Lower hybrid current drive experiments were carried out in the HT-2 tokamak (major radius 0.44 m) with four sets of an eight-waveguide multijunction launcher and a 2.46 GHz magnetron. The four independent radiofrequency (RF) systems, operating at frequencies differing by 2 MHz each, generated incoherent waves in the plasma. A plasma current of 8.6 kA was sustained during 20 ms by an RF power of 16 kW in a low density plasma. The current drive efficiency was 0.5 × 10¹⁸ A.W^-1.m ^-2, which did not decrease with the number of active RF sources. This showed that current drive with incoherent waves is not inferior to that using coherent waves

High poloidal beta (β_p approximately 5) discharges produced with nearly balanced tangential neutral beam injection into low current discharges in the TFTR tokamak are described. As β_p exceeds approximately 1.2 times the aspect ratio, a separatrix with an inside poloidal field null is observed to limit the outer boundary of the plasma. Since the curvature of TFTR's applied vertical field is constant, the appearance of the poloidal field null corresponds to the equilibrium poloidal beta limit

The dependence of the H-mode energy confinement time τ_E on the plasma elongation and the divertor configuration is studied. A regression analysis of the H-mode database is presented in which the dependence on the closed/open divertor configuration, estimated from the ASDEX experiments, is included. The dependence of τ_E on the elongation factor k is found to be positive (approximately k^0.6) for ELM-free H-mode discharges, in accordance with experimental results for ELM-free limiter H-mode plasmas in JFT-2M. In addition, the influence of the X-point configuration (single null or double null) on the confinement time is briefly addressed

Because of the presence of much fewer untrapped electrons that can resonate with drift-type, low frequency modes in toroidal geometry, the anomalous transport coefficients based on the slab geometry are significantly reduced for typical tokamak parameters in the banana regime. In drift-type modes, anomalous electron transport may be dominated by trapped electrons

Kammash and Galbraith (see ibid., vol.29, p.1079 (1989)) calculated the energy gain factor Q for the MICF pellet by using a simplified set of particle and energy balance equations and reported that, for a typical example, a Q value of approximately 300 is obtainable with an input energy of approximately 4 MJ (Q is the ratio of the fusion energy to the input energy). The input energy is equal to the initial thermal energy. They proposed to apply the concept to space propulsion. On the other hand, Hasegawa et al (1988) have performed numerical simulations by using the hydrodynamic code HISHO together with theoretical estimation and found a Q value of approximately 15 with a laser input energy of 25 MJ for a typical MICF target. The authors have tried to explain the differences in the input energy requirement and gain values between these works