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The momentum, energy, and velocity characteristics of plasma ablating from planar targets irradiated by long Nd-laser pulses (4 ns,<1014 W/cm2) are measured and the dependence of ablation parameters upon absorbed irradiance is determined. Large laser spots are used in these experiments so that the results are not sensitive to boundary effects.

A simple model based on non-ambipolar radial transport and planar sheath physics is used to describe the generation of radial electric fields and currents in the scape-off layer of the Tokamak de Varennes (TdeV) during divertor plate biasing. In general, the calculated predictions compare favourably with TdeV results over a variety of plasma conditions and divertor magnetic configurations. Validated by the experiment, the model is used to study the scaling laws of perpendicular ion mobility and to test existing related theories. Finally, the model is proposed as a useful tool for the design and upgrade of biased divertors through optimization of the plate and throat geometry.

Up to five arrays of soft-x-ray detectors are used with the Fourier-Bessel harmonic reconstruction method to generate 2D images of sawtooth collapses during Ohmic plasma discharges on the Tokamak de Varennes. For low-harmonic reconstructions (M< 2), a quasi-interchange behavior (Wesson model) is observed during the sawtooth crash, but a reconnectionlike behavior (Kadomtsev model) is recovered using higher harmonics {M>L 3) on the same data. This indicates that the Kadomtsev model is a better description for the sawtooth collapse, although the collapse time remains shorter than predicted.PACS numbers: 52.55. Fa, 52.30.Jb, 52.35.Py, 52.70.La Sawtooth oscillations' are internal disruptive instabilities observed on most tokamaks. This periodic phenomenon is observed on many plasma diagnostics and each cycle consists of a slow^ increase in the central temperature and density followed by a sudden collapse. As an explanation, Kadomtsev proposed the total reconnection model, ^ where an m = l mode disappears nonlinearly through resistive reconnection at the q = 1 surface, resulting in a flattening of the temperature, density, and current profile, and a release of energy. Detailed investigations using nonlinear two-dimensional codes ^"^ have shown that this model is consistent with MHD theory, and is able to explain several experimental observations on small tokamaks. ^"^ Recently though, this model has been called into question in the light of new experimental results obtained on large tokamaks.^'^ The total reconnection model would fail to explain the fast sawtooth crash, and to a lesser extent, the lack of large precursors and the presence of large slowly decaying successor oscillations. A number of recent works have attempted to explain these new observations, *^"^^ but most of the attention has been focused on the quasi-interchange (Wesson) model'^ since soft-x-ray-emission reconstructions on the Joint European Torus (JET) seem to agree with topological predictions of the sawtooth collapse phase. ^''^ In this model, a flattened q profile leads to an unstable m = l mode, which drives a cold bubble toward the center of the plasma, expelling the initial hot core through an interchange mechanism. Qualitative diff*erences between the total reconnection model and the quasi-interchange model should make clear experimental identification possible through tomographic imaging of soft-x-ray emission during the sawtooth collapse. In the total reconnection model, the hot core of the q < 1 region moves almost rigidly to one side, whereas in the quasiinterchange model, the initial hot core becomes a crescent when the cold bubble penetrates the center region due to convective flow, ^ ^ Despite such contrasting topological diff*erences, experimental identification is not straightforward since spatial inversion techniques must be used to unfold line-integrated data obtained with discrete detectors. On tokamaks, several limitations on viewing angles and the number of detectors favor the Fourier-harmonic-reconstruction technique, whic...

The most promising concept for deep fuelling a reactor is by the injection of compact toroid (CT) plasmoids. The first results showing CT fuelling of a tokamak plasma, without any adverse perturbation t o the tokamak discharge, are reported. The Compact Toroid Fueller (CTF) device was used to inject a CT-spheromak plasmoid into the TdeV tokamak. Following the CT penetration, the tokamak particle inventory increased by 16%, the loop voltage and the plasma current did not change, and there was no increase in magnetohydrodynamic (MHD) activity. The number of injected impurities was low and dominated by non-metallic elements. The plasma diamagnetic energy and the energy confinement time increased by more than 35%.

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A compound saawtooth consists of a subordinate relaxation followed by a main relaxation; it has approximately twice the period and the amplitude of a normal sawtooth. High resolution 2-D images, produced by the soft X-ray tomography system of the Tokamak de Varennes, are used to compare the different types of sawtooth relaxations. It appears that a reconnection process is involved for all collapses, although the reconnection is incomplete during the partial relaxation.