Table of contents

A description is presented of the development of H^- and D^- sources for neutral beam heating on the next generation Tokamaks such as NET or ITER. The two basic types of such ion sources are discussed, where either the ion is formed in the plasma by dissociative attachment or on the surface of a negatively biased electron of low work function. Also described are the acceleration techniques to form high energy and power beams needed for neutral beam injectors. Finally, a short discussion is presented of the future development of these sources.

Impurity transport has been studied in JET using the laser blow-off technique. Results are presented for Ohmic, L-mode and H-mode plasmas. In all cases impurity transport was found to be small inside r/a approximately 0.3 with values for D approximately 0.03-0.3 m² s^-1 close to neoclassical predictions. Outside r/a approximately 0.4, the transport was faster than neoclassical with values for D approximately 0.3-0.6, 3-5 and 0.8-1.2 m² s^-1 in H-mode, L-mode and Ohmic plasmas, respectively. These values apply only between sawtooth crashes, during the sawtooth phase itself ( approximately 100 mu s) the transport is greatly perturbed over the central region allowing the impurities to quickly leave or enter this region. In an Ohmic plasma the impurity confinement time tau _imp was between 250 and 350 ms. This value was reduced to approximately 150-200 ms in the L-mode plasma due to an increase of D in the anomalous transport region. During H-mode plasmas, on the other hand, very long impurity confinement times, of the order of several seconds, were observed. This was explained by a sharp increase of the ratio V/D in a thin region near the plasma edge and a general reduction of D in the outer half of the plasma radius.

A zone of reduced anomalous diffusivity for impurities in the central region of the discharge has been reported from several Tokamaks. In this paper the authors point out that this condition is not an occasional occurrence, but a very general feature. Indeed all JET cases analyzed, where the experimental conditions allow adequate sensitivity and space resolution for the measurement of the impurities diffusion coefficient, show unambiguously that this quantity is close to neoclassical levels within r=0.4a. Furthermore, from the analysis of the electron density profile in sawtooth-free phases of JET discharges with moderate electron temperature, the authors deduce that the electron particle diffusivity D_p is also close to neoclassical in the same region. The values obtained for D_p in that region during the early pre-sawtooth phases are the same as in Ohmic post-pellet phases.

The energy transport in the outer region of the RFP experiment ETA BETA II has been experimentally investigated by measuring the energy flux intercepted by the surface of an insertable limiter, instrumented with heat flux probes. It has been found that the energy deposited on the surface facing the electron flow is almost an order of magnitude larger than that deposited on the opposite side. The spatial dependence of the energy flux has been studied and the results are reported for different main plasma densities. The results are explained by fast electrons flowing in the outer region of the plasma, parallel to the magnetic field in agreement with the results found in other RFP experiments.

Fusion yield in He³ plasmas by 10 MW injection of 500 keV deuteron(D)-beam and by up to 30 MW injection of 120 keV D-beam is studied by taking global energy confinement into account. The global energy confinement time, tau _E, defined by the ratio of the total stored energy to the input power, is taken to be inversely proportional to the square root of the input power. Higher fusion yield is produced in the regime of lower n_e and higher T_e, unless the reduction of the He³ density due to the increase of the non-thermal deuteron population is too significant. The hot ion mode of T_i=2T_e assumed for high-power 120 keV D-beam injection is found not to be preferable to the T_i=T_e mode under the same tau _E. The effect of accumulation of thermal deuterons on the fusion yield is also discussed.

A new diagnostic method for studying the laser implosion of microshell targets based on time-of-flight and pinhole imagery of neutral atoms is described. Experiments have been conducted at the Rutherford Appleton Laboratory using the 12-beam laser facility 'Vulcan' studying the implosion of high-aspect-ratio glass shell targets. Neutral particles originate by several different mechanisms which can be distinguished by the combination of time-of-flight (TOF) and imaging techniques. Theoretical analysis and numerical simulations of neutral atom velocity spectra support the interpretation of the experimental results. One group of atoms is shown to be characteristic of the imploded microshell and estimates of the imploded core parameters are possible using a simple analytic model. Results were obtained over a wide range of conditions using both tight-focus geometry and random-phase plates to obtain uniform irradiation. Marked changes in the neutral spectra between these two cases are observed.

Motivated by the phenomena that occur at the ablation front of a laser-driven target, a theory of the convective stabilization of the Rayleigh-Taylor instability is derived which is applicable for wavelengths that are long compared with the density-gradient scale length. Thermal conduction processes are not treated. The system of linearized equations is closed by assuming that the density jump is some unspecified function of the hydrodynamic and transport variables and linearizing this relationship. The resulting growth rates are closer to those given by the Takabe formula than the results of previous analytical theories that include convection but neglect thermal conduction.

The anomalous transport and turbulence driven by a combination of the toroidal ion temperature gradient (ITG) mode and dissipative trapped electron (DTE) mode are studied for a plasma in the dissipative regime. The results from numerical mode coupling simulations show a good agreement with the results from quasilinear theory. A transition from the neo-Alcator scaling of the energy confinement time with density to a saturated state is obtained. It is shown that ion heat pinch effects (inward transport) may exist in the process of thermodynamic relaxation of the system.

A fluid model was used to study the response of a nonmagnetized plasma to a high-voltage sphere for a time interval up to 20000 plasma periods. Different applied voltages, background plasma densities and voltage rise times were considered. It was found that after the transient response to the applied voltage the system evolves to a steady state. In the steady state, the ion density satisfies the Boltzmann relation. The thicknesses of both the electron sheath and the presheath increase with the applied voltage. The sheath thickness, estimated from the electron and ion density profiles, agrees with the simple Langmuir-Blodgett (LB) theory, but the simulation current is 2-5 times higher. If the voltage is applied according to an exponential law with a rise time tau , only the electron motion at very early times is affected if tau <or= omega _pe^-1. A finite tau reduces the initial acceleration and density build-up of the electrons around the sphere. The overshoot oscillation seen at early times for small tau does not occur for tau =10 omega _pe^-1. The formation of the spherical double layer is delayed, and at a given time, it is located closer to the sphere compared to the case of tau <or= omega _pe^-1. For different tau s, the system reaches the steady state in slightly different ways.

Because of the anticipated use of high-voltage power sources on future space platforms, there is an increased interest in elucidating the phenomena associated with the interactions of a high-voltage conductor and the ionospheric plasma at low-Earth-orbit altitudes. Currently there are ongoing rocket, laboratory and modelling programs, and an electrodynamic tether will be flown on the Space Shuttle. All of these programs involve the current collection characteristics of high-voltage spheres embedded in both magnetized and nonmagnetized thermal plasmas. In support of these efforts, the authors have solved the time-dependent nonlinear fluid equations for ions and electrons and the Poisson equation in order to study the response of a nonmagnetized plasma to 'negative' high-voltage spheres. Simulations were conducted for a range of voltages (-10 to -10000 V) and for plasma densities from 10⁴ to 10⁶ cm^-3. The temporal evolution of the plasma was followed all the way to the steady state. In all simulations, there were certain qualitative similarities in the plasma response.

Three Mather-type focus devices with capacitor bank energies of 280, 250 and 12 kJ, respectively, have been studied by collective scattering of ruby laser light. In order to determine the thermal plasma parameters and detect the occurrence of suprathermal scattering from plasma turbulences, the frequency and wavevector spectra of the ion feature of scattered light have been measured. All scattering vectors were oriented in an azimuthal plane perpendicular to the focus pinch axis. The measured plasma temperature and densities are below 2 keV and 10¹⁹ cm^-3, respectively. Therefore, only a few per cent of the observed neutron emission from the studied focus plasmas can be of thermonuclear origin. During the compression phase, and after maximum compression during the rise of the neutron emission, suprathermal forward scattering has been observed. This is interpreted as scattering from enhanced density fluctuations due to plasma turbulences, which may take part in the heating of the focus plasma and the acceleration of the high-energy deuterons which are responsible for the dominant neutron generation in the focus plasma by beam-target reactions. The results are in agreement with those of former light scattering measurements at lower energy focus devices.

Experiments are performed in which a standing ion cyclotron wave of high amplitude interacts with a highly ionized plasma in cylindrical geometry. The plasma wave generates DC currents which form vortices closing within one half-wave. The magnitude of the steady currents reaches 40% of the amplitude of the oscillating currents.

An approximate semi-analytical approach is presented which uses line-of-sight measurements of D-D fusion neutrons from beam heated Tokamak plasmas to infer the behaviour of the fast ion population at sawtooth collapses. Application to recent results on JET confirms that a large fraction of the central fast beam ions are expelled from the central plasma. The approach provides an approximate but computationally fast and convenient complement to presently used inversion techniques.

Changes of the toroidal rotation velocity of up to 3*10⁴ m s^-1 have been observed during ICRH in JET. The plasma increases its toroidal rotation in the direction of the plasma current. The change in velocity is obtained from the measurement of the Doppler shifted X-ray lines emitted from He-like nickel present in the centre of the plasma. By analyzing various heating scenarios and also measuring the fast anisotropic ion energy content of the plasma the authors conclude the rotation is connected with the creation of fast ions during ion cyclotron resonance heating. Toroidal acceleration of the plasma in connection with MHD instabilities leading to similar magnitude of rotational velocity is also seen.

In the Tokamak and stellarator confinement concept, an essential role of the rotational transform is to prevent charge accumulation due to the Del B and curvature drifts. The author shows that without rotational transform, charge accumulation can still be avoided by a fast poloidal bulk plasma rotation. It is demonstrated that it is feasible to impose the necessary rotation by high power, perpendicular, off-axis neutral beam injection (NBI). The consequences of these new results are discussed.

In the paper quoted in the title (Anderson et al., 1991) it is claimed that it has been found that non-inductive current drive scenarios, as considered for Tokamak reactors with ITER-like scalings for the energy confinement time, are unstable (or at best marginally stable). The phenomenon is qualitatively transparent: for example, a positive plasma temperature excursion increases the non-inductively driven current (at fixed heating/current drive power and in the absence of a controlled external toroidal electric field) which in turn improves the global energy confinement properties so that the plasma temperature increases further. Since the time scale of the variation of the plasma current is the global skin time tau _J which is longer than any other characteristic time intervening, the growth time of the instability is of the order of tau _J, i.e. 1000 s or more in practical cases.