Table of contents

An effective suppressor signal for the control of both magnetohydrodynamic and tearing modes can be obtained from the transverse energy deposition via electron cyclotron resonance in the electrons in tokamaks. Feedback modulation of a purely transverse electron cyclotron heating beam can then result in a near instantaneous suppressor signal of an extra radial non-thermal 'pressure' of appropriate amplitude and phase to stabilize the mode. Power estimates for large and medium size tokamaks appear to be modest and practical.

We investigate drift instabilities in the dust acoustic or dust lower-hybrid frequency regimes in a nonuniform, collisional, strongly magnetized dusty plasma. It is assumed that the magnetic field strength is such that the ions and electrons are magnetized, while the negatively charged dust grains are collisional and nonmagnetized. We consider a situation in which the negatively charged dust is spatially localized, creating an electron depletion or electron 'hole', with an electron density gradient in the opposite direction to a dust density gradient at the boundaries. In this case, low-frequency drift instabilities can be driven by the electron E₀ × B₀ and diamagnetic drifts, where E₀ and B₀ are the dc electric and magnetic fields, respectively. Using parameters that may be representative of possible laboratory conditions, it is found that the critical drift for such drift instabilities can be lower than that for the excitation of a low-frequency Hall current instability, which is driven primarily by the ion Hall current.

Characteristics and underlying mechanisms for plasma current spikes, which have been frequently observed during the thermal quench of JT-60U disruptions, were investigated through tokamak simulation code simulations including the passive shell effects of the vacuum vessel. Positive shear and reversed shear (PS and RS) plasmas were shown to have various current spike features in the experiments, e.g. an impulsive increase in the plasma current (positive spike) in the majority of thermal quenches, and a sudden decrease (negative spike), that has been excluded from past consideration, as an exception. It was first clarified that the shell effects, which become significant especially at a strong pressure drop due to the thermal quench of high β_p plasmas, play an important role in the current spike in accordance with the initial relation of the radial location between the plasma equilibria and the vacuum vessel. As a consequence, a negative current spike may appear at thermal quench when the plasma is positioned further out from the geometric centre of the vacuum vessel. It was also pointed out that a further lowering in the internal inductance, in contradiction to previous interpretation in the past, is a plausible candidate for the mechanism for positive current spikes observed even in RS plasmas. The new interpretation enables us to reason out the whole character of current spikes of JT-60U disruptions.

Interactions of the laser beam of iodine laser 'PALS' with low-density foam targets and acceleration of Al foils by the pressure of heated foam matter are investigated here, both experimentally and theoretically. An x-ray streak camera was used for the evaluation of the speed of energy transfer through the porous foam material. X-ray streak records show no noticeable emission near the Al foil at the rear side of a 400 µm thick foam and, thus, Al foil is most likely accelerated without significant heating and expansion. Accelerated foil velocities, measured by three-frame optical shadowgraphy, reach up to 10⁷ cm s⁻¹. Experimental results compare well with our two-dimensional hydrodynamics simulations and with an approximate analytical model.

This paper deals with the study of erosion and surface phenomena of liquid Ga exposed to a capacitively coupled rf-plasma. We have observed a very high erosion yield in Ar plasmas which cannot be explained by sputtering alone. Time resolved observations of the plasma–gallium interaction indicate that the formation of gas cavities inside the Ga and their explosive outgassing might be one of the important erosion mechanisms in our experiments.

Hydrogenic fast-ion populations are common in toroidal magnetic fusion devices, especially in devices with neutral beam injection. As the fast ions orbit around the device and pass through a neutral beam, some fast ions neutralize and emit Balmer-alpha light. The intensity of this emission is weak compared with the signals from the injected neutrals, the warm (halo) neutrals and the cold edge neutrals, but, for a favourable viewing geometry, the emission is Doppler shifted away from these bright interfering signals. Signals from fast ions are detected in the DIII-D tokamak. When the electron density exceeds ∼7 × 10¹⁹ m⁻³, visible bremsstrahlung obscures the fast-ion signal. The intrinsic spatial resolution of the diagnostic is ∼5 cm for 40 keV amu⁻¹ fast ions. The technique is well suited for diagnosis of fast-ion populations in devices with fast-ion energies (∼30 keV amu⁻¹), minor radii (∼0.6 m) and plasma densities (⪅10²⁰ m⁻³) that are similar to those of DIII-D.

Low collisionality, low particle source, ELMy H-modes (type-III) with sawteeth are produced in JET in order to address the question of density profile evolution in the reference q₉₅ = 3 ITER scenario. The paper focuses on particle transport in the core zone around the mid-radius. The pedestal region including ELMs and the region affected by sawteeth are not considered. By replacing a significant part of the neutral beam heating by RF power the beam particle flux at mid-radius has been reduced to Γ_Beam/n_e = 0.07 m s⁻¹. The additional flux due to wall neutrals is estimated as Γ_Wall/n_e ⩽ 0.12 m s⁻¹. Density profiles are found to be modestly peaked under these conditions with a relative density difference of Δn/⟨n⟩ = 0.23 across the zone not affected by sawteeth and ELMs. In a region around the mid-radius the ratio of effective particle diffusivity to electron thermal diffusivity is found to be D_e,eff/χ_e ≈ 0.2, which might indicate an anomalous pinch provided the particle diffusivity D_e is sufficiently large. The measured values of D_e,eff/χ_e are at the lower end of the range used in ITER models.

The PDF file provided contains web links to all articles in this volume.