Table of contents

At ASDEX Upgrade the role of the collisionality for the excitation of neoclassical tearing modes has been investigated. In addition to the (3/2)-mode, which is always observed limiting the maximum at high enough -values, the excitation of a neoclassical (2/1)-mode at very low collisionalities has been observed. Schemes for discharges at ASDEX Upgrade with high -values at high heating power are presented.

In this review of plasma polarimetry, first the theory is discussed in general, then exact analytic solutions of the evolution equation for polarization are presented, as well as approximate analytic solutions. Numerical integration of the evolution equation is also discussed.

The design of experiments is then considered taking into account the maximum amount of information which can be obtained from polarimetric measurements. Special attention is devoted to the techniques of polarization modulation (including both progressive and alternating modulation). Different alternative configurations are described which are of special interest because they can be realized in the far infrared and because they allow a measurement of phases rather than amplitudes. The effects of refraction are then briefly considered. Finally, the combination of polarimetry and interferometry on the same instrument is discussed, including the effects of polarization modulation.

Ideal MHD stability, the low-n free-boundary kink mode and the infinite-n ballooning mode of a high-elongated JT-60SU plasma are investigated. Equilibria are obtained for the plasma with steady-state neutral-beam-driven and bootstrap currents. It is shown that without the shell stabilizing effect the maximum value of normalized beta is mainly limited by the localized external mode at the plasma surface, when the equilibrium surface current by the bootstrap current is increased. The triangularity is shown to have a stabilizing effect on the localized external modes, while the external modes, which have a relatively broad structure, and the internal modes are not affected as much by the triangularity. It is also shown that the localized external modes can be unstable for a wide range of values. Even infinite-n ballooning modes driven at the plasma surface are destabilized if the triangularity is increased for JT-60SU equilibrium.

A plasma double layer is created by an anode grid in the target chamber of the double plasma device. When the separation grid of the double plasma device is floating, very coherent plasma oscillations, triggered by the unstable double layer, are observed. The oscillations have a frequency close to 1 kHz and very high amplitude. The amplitude and frequency of these oscillations depend on the anode bias and on the discharge voltage of the plasma source. The anode characteristics exhibit hysteresis. When plasma oscillations are coupled to the external oscillator, by modulating the floating potential of the separation grid, synchronization and periodic pulling are observed. When external oscillations are tuned to the second, third or fourth harmonics, period doubling bifurcations are observed, when the external amplitude is increased. At synchronization with the third and fourth harmonics, oscillations become chaotic after the first period doubling bifurcation. The correlation dimension of the underlying attractor is found to be fractal between 2.3 and 2.8, with a positive largest Lyapunov exponent.

Time-resolved high-resolution soft x-ray spectra from gas-puff injected Ar impurity ions have been investigated for neutral beam heated and ohmically heated discharges in the TEXTOR tokamak. The experimental spectra show systematic deviations from corona model calculations for the line intensities of the forbidden He-like lines x, y, z and the Li- and Be-like dielectronic satellite spectra: theoretical corona model calculations predict intensities significantly too low. High-intensity Li-like inner-shell excited satellites correlate with the neutral beam injection. The discrepancies could also be observed in the stationary phase of an inductively heated discharge. In the heating phase the discrepancies are even larger. We propose charge-exchange processes between the neutral atoms and the impurity ions as an explanation of the experimental findings. Good agreement with the experimental observations can then be obtained without the need for invoking large (anomalous) diffusion coefficients. A self-consistent coupling of the population kinetics of the neutrals and the impurity ions, also taking into account charge-exchange processes from excited states of hydrogen/deuterium permit the determination of the neutral fraction and of the electron lifetime on the sole basis of impurity spectra analysis. Independent Monte Carlo simulations of neutral gas transport also provides the ionization degree in the centre and the electron lifetime. These calculations are also in good agreement with the spectroscopic results.

Ideal magnetohydrodynamic stability limits for various profiles of pressure and current density in reversed magnetic shear toroidal plasmas are numerically investigated. The plasma beta limits are calculated for n = 1 and ballooning modes, and the bootstrap current contribution is also computed. The results in the high-value regime of the plasma internal inductance contrast well with those in the low- regime and are very dependent on the choice of the pressure profile. In the high- regime the stability limit set by the n = 1 external mode without any external stabilizing wall can be significantly improved compared to that in the low- regime. This is more significant for a broader pressure profile. However, the bootstrap current profile in the high- regime is generally not well aligned with the total current profile. This is worse for broader pressure profiles. Also, if the pressure profile is not suitably chosen, the ballooning mode rather than the n = 1 mode can become the beta limiting mode.

The `drift island motion concept' is studied for a helical plasma for the purpose of ash removal and the injection of high-energy ions with large parallel velocity. The possibility of ash removal and the injection of the high energy ions in the helical device is shown. The control in time of the helical field amplitude and the magnetic field perturbations are used. Externally induced magnetic perturbations produced by the additional coil system for the `island divertor', for example, are mentioned here. The island structures with = 1/1 and = 1/2 are used, where m and n are the `wave' numbers of the magnetic perturbation. The variation of the helical field in time means that the magnetic configuration can be changed slowly during a plasma shot.

A highly radiating zone (MARFE) just above the divertor X-point has been used to access the marginal transition regime to study the existence of a critical point for the L to H transition. Phase transition models predict that at the critical point the transition duration increases and the plasma parameters vary continuously between the L- and H-mode. In these experiments, the L to H transition duration increased 50-100 times over fast transitions. However, the evolution of shear, the edge density gradient, the H-mode pedestal, and fluctuations is essentially unchanged from that in fast transitions. The only difference is in the speed with which and the degree to which the fluctuation amplitudes are transiently reduced. This difference is understandable in terms of the time scales for fluctuation amplitude reduction and edge pressure gradient increase (several ms), provided the edge fluctuations are pressure-gradient driven.

A physically motivated, theoretical basis for defining scrape-off layer (SOL) widths is proposed. Applying it to theoretical models proposed for transport in the SOL shows how the width is affected by the form of the cross-field transport and the parallel transport to target plates. For a cross-field diffusivity and a parallel loss time , the estimate , with C typically in the range 0.6-1.0 and and evaluated at the SOL-core interface, is found to provide a physically meaningful and accurate expression for the power SOL width .

The first experimental results of a large reversed-field pinch machine, TPE-RX, are reported. A reversed-field pinch configuration in TPE-RX was successfully obtained in March 1998. The highest plasma current, I_p, of 480 kA and the longest pulse duration time of 70 ms have so far been obtained separately. A minimum loop voltage of about 15 V is obtained at I_p=150-250 kA. A locked mode has been found to exist in TPE-RX from the magnetic and vessel-temperature measurements, while the C⁺⁺ Doppler spectrum shows a finite toroidal rotation.

A two-dimensional nonlinear Fokker-Planck analysis is performed for collisional relaxation of a velocity distribution function of electrons heated in an ultra-short pulse laser-irradiated fully ionized plasma. The effect of electron-electron (e-e) collisions on inverse bremsstrahlung heating due to the electron quiver following tunnelling and/or above-threshold ionization (ATI) and electron-ion collisions is studied. It is found that the difference in very low-energy population between the cases with and without e-e collisions can reach up to 20-30% over the laser pulse length. This indicates that the electron self-collision may not be neglected for evaluation of the recombination rate due to slow electrons. Also discussed is the two-dimensional structure of the velocity distribution and the one-dimensional energy spectrum.

The motional Stark effect measurement of magnetic field pitch angle in tokamaks is a mature and powerful technique for estimating plasma current density in tokamaks. However, its range of applicability is limited by the requirement that and manifolds are spectrally sufficiently well separated (high magnetic fields, high beam energies) to ensure adequate net polarization for a successful measurement. This paper proposes alternative schemes based on the optical coherence properties of the Stark multiplet that are somewhat more versatile than the standard method and better suited to measurements on low-field toroidal confinement devices. An interference filter is used to transmit the Stark multiplet to a polarimeter (which uses a single photoelastic plate) that modulates the light temporal coherence and/or its first spectral moment. This light is subsequently processed using a novel electro-optically modulated solid-state interferometer that is sensitive to low-order spectral moments. The modulation of these quantities conveys information about the orientation of the light polarization and hence the magnetic field pitch angle.

Fast electron transport in a dense plasma is investigated with one-dimensional particle-in-cell simulations. We show the importance of collisions in the interaction of an ultra-intense ultrashort laser pulse with a solid target. We demonstrate that the fast electron penetration depth and distribution depend on target resistivity. Collisions reinforce the fast electron confinement near the irradiated surface.

A study of a plasma gun (modified Bostic type) with titanium hydride electrodes has been carried out. The total number of released hydrogen atoms was in the range 10¹⁶-10¹⁸ and the maximum plasma flow velocity was 2.5×10⁵ m s^-1. The ion density near the gun edge reached 1.8×10²⁰ m^-3 and the electron temperature was around 40 eV as estimated from probe measurements. No species other than hydrogen or titanium were seen in the plasma line radiation. The plasma injector was successfully used for gas pre-ionization in the Extrap T2 reversed-field pinch device (ohmic heating toroidal experiment (OHTE)).

The expansion of a collisionless plasma with negative ions into a vacuum is considered. The flow pattern involves a rarefaction shock or a wave with an expanding front, two plateau-like regions with a discontinuity between them and a rarefaction wave. The ranges of the mass and density ratios of negative to positive ions for the existence of the rarefaction shock are found. A simple model is in a good agreement with simulation data.

Active charge exchange spectra representing the local interaction of injected neutral beams and fully stripped impurity ions are hard to analyse due to strong blending with passive emission from the plasma edge. As a result, the deduced plasma parameters (e.g. ion temperature, rotation velocity, impurity density) cannot always be determined unambiguously. Also, the speed of the analysis is limited by the time consuming nonlinear least-squares minimization procedure. In practice, semi-manual analysis is necessary and fast, automatic analysis, based on currently used techniques, does not seem feasible. In this paper the development of a robust and accurate analysis procedure based on multi-layer perceptron (MLP) neural networks is described. This procedure is fully automatic and fast, thus enabling a real-time analysis of charge exchange spectra. Accuracy has been increased in several ways as compared to earlier straightforward neural network implementations and is comparable to a standard least-squares based analysis. Robustness is achieved by using a combination of different confidence measures. A novel technique for the creation of training data, suitable for high-dimensional inverse problems has been developed and used extensively. A new method for fast calculation of error bars directly from the hidden neurons in a MLP network is also described, and used as part of the confidence calculations.

For demonstration purposes, a real-time ion temperature profile diagnostic based on this work has been implemented.