Table of contents

The interaction of short-pulse high-intensity lasers with solid targets generates large numbers of energetic electrons. The energetic electrons can only penetrate into the solid if the solid can supply an equivalent charge-neutralizing return current. We develop a simple model which shows that in many cases the solid cannot support the required return current and the fast electrons are confined by electric fields to the surface of the target. The target response to laser irradiation depends strongly on the electrical conductivity of the solid.

Atomic (H) and molecular hydrogen densities and temperatures have been determined in a magnetized hollow cathode arc plasma burning at low pressure . Rayleigh scattering measurements are used to derive the sum of atomic and molecular densities, each weighted with its scattering cross section. Coherent anti-Stokes Raman scattering (CARS) has been used to determine the population density differences of rovibrational molecular states . The CARS intensity of many rotational states of can be detected and these levels are found to be populated according to a Boltzmann distribution. In the low-pressure plasma only the fundamental vibrational band of can be found experimentally owing to the low particle densities. In order to evaluate the density properly from the measured CARS data, the vibrational population for v0 > 0 is calculated from a spatially one-dimensional diffusion reaction model. Within the plasma centre the dissociation degree and about one third of the molecular hydrogen is found in vibrationally excited states. Here, the vibrational temperature is about , which far exceeds the gas temperature of . The dissociation degree and the vibrational distribution are mainly determined by electron-impact processes in the inner plasma region and recycling processes at the vessel walls, whereas the influence of inelastic neutral - neutral collisions is rather marginal.

We describe the design and operation of the SPHEX spheromak device and present an overview of its behaviour. The plasma is formed by ejection from a magnetized Marshall gun, and can be sustained as long as the gun is energized. The plasma is divided into the annulus comprising the closed toroidal flux, linked with the open flux forming the central column. The column current is driven directly by the central gun electrode, and the toroidal current in the annulus is driven indirectly by a mechanism associated with a coherent n = 1 oscillation of the column. The configuration exemplifies the operation of the process of relaxation to a state of minimum magnetic energy, which leads to magnetic configurations similar to those observed; to sustain these configurations requires some mechanism of toroidal current drive. Associated with this is the amplification of the poloidal flux, which is typically a factor of about five larger than the flux generated by the gun solenoid; the constancy (to a first approximation) of this factor plays a controlling role in spheromak behaviour. In standard operating conditions there is a `hard' limit, set by the solenoid flux, on the current carried by the column; any current driven by the external circuit above this apparently does not emerge from the gun. Evidence is presented that the column current is carried largely (> 50%) by accelerated ions with energy up to the gun voltage ( for a typical gun current of 60 kA). These ions are poorly magnetized and can escape across the magnetic field to the wall, a likely mechanism for the observed `loss' of current. Hydrogen is the normal operating gas: other gases ( and He) have been used, but the current drive is found to be less effective than in , with lower toroidal current maintained in the annulus.

The structure of the n = 1 mode in the SPHEX spheromak, which plays a central role in relaxation during sustainment, is investigated by analysing the measured voltage fluctuations in the central plasma column. By combining these results with a suitably defined helical magnetic flux function, the mode is found to be due to a rotating helical distortion of the open linked flux. We propose that the distortion is due to a saturated current-driven kink mode of the open flux tube. The prolongation of this `helical column' on its return around the outside of the closed flux is found to be strongly asymmetric. Previously published measurements of the Poynting flux and -profile are re-analysed in the light of these results, and implications for the mechanism of relaxation and non-inductive current drive are discussed.

Non-thermal He II spectra for discharges with helium beam fuelling are analysed. Simulated spectra are used to study the effects of plasma temperature, plasma density and on observed charge-exchange (CX) spectra. Differences in modelling the non-thermal velocity distribution function with a numerical Fokker - Planck code or alternatively using analytical expressions are investigated. The intensities and spectral shapes of both active, localized CX spectra and competing, non-localized, passive electron-impact excitation components are simulated and compared with observations. The `plume' contributions of electron-impact excited particles are found to be quite appreciable and uncertainties in the plume calculation lead to non-negligible errors in the extraction of the active signal from the total spectrum. However, for experimental conditions with magnetic field configurations minimizing the plume effect good agreement can be found between fast-particle densities derived from the numerical calculations and the experimental observations. Significant problems in deriving absolute densities are encountered when a helium beam also acts as a CX diagnostic beam. For the case of dominant passive emission components, simulated fast spectral intensities for the core lines of sight agree within a factor of two with experimental data.

The linear stability of the `scrape-off-layer' (SOL) with respect to interchange-type modes is studied using the reduced MHD model and applying a ballooning approximation to the perturbations. `Line-tying' boundary conditions are used at the target plates. Employing a metric determined by the magnetic field geometry, the influence of the X-point on stability is assessed. The effect of relevant parameters like X-point height, magnetic shear and plasma shaping is qualitatively determined. It is demonstrated that the calculations for experimental configurations and those based on an analytical equilibrium model yield good qualitative agreement.

It is shown that the SOL plasma just outside the separatrix can become unstable more easily (i.e. for lower pressure gradients) than the plasma just inside the separatrix where the magnetic well is stabilizing. This finding can be important for explaining the occurrence of giant ELMs. The interchange instability in the SOL may act as a precursor and is mainly localized near the X-point, but may also have a strong signature just above the outer midplane. The scaling of the ELM repetition time with heating power and current is derived.

2 - 4 June 1997 9th National Topical Conference on High-Temperature Plasma Diagnostics St Petersburg, Russia Contact: Ms Galina Onishchuk, TRINITI, 142092 Troitsk Moscow Reg, Russia. Fax +7-095-334-5776, E-mail galina@fly.triniti.troitsk.ru. Application and abstract deadline: 1 March 1997.

7 - 18 July 1997 34th Culham Plasma Physics Summer School (UKAEA/Euratom) Oxford, UK Contact: Mrs J Bright, UKAEA Fusion, Culham, Abingdon, Oxfordshire, OX14 3DB, UK. Tel +44-1235-463773, Fax +44-1235-463474, E-mail julie.bright@ukaea.org.uk. Application deadline: 9 May 1997 (later applications may be accepted depending on accommodation constraints).

27 August - 2 September 1997 International Conference on Superstrong Fields in Plasmas Villa Monastero, Varenna (Lc), Italy Contact: Mrs D Pifferetti, Secretariat Villa Monastero, 1 Piazza Venini, 22050 Varenna, Italy. Tel +39-341-831261, Fax +39-341-831281, E-mail ccvm@ccvm.cil.lc.it (abstracts should be sent to Dr M Lontano, Istituto di Fisica del Plasma, Consiglio Nazionale delle Ricerche, Via Cozzi 53, 20125 Milano, Italy. Tel +39-2-66173260, Fax +39-2-66173239, E-mail lontano@ifp.mi.cnr.it). Application and abstract deadline: 13 June 1997

4 - 12 September 1997 International Workshop on Diagnostics for Experimental Fusion Reactors Villa Monastero, Varenna, Italy Contact: D Pifferetti, Centro di Cultura `Villa Monastero', 1 Piazza Venini, 22050 Varenna (Lc) Italy. Tel +39-341-831261, Fax +39-341-831281, E-mail ccvm@ccvm.cil.lc.it. Application and abstract deadline: 13 June 1997.

8 - 12 September 1997 12th International Conference on Gas Discharges and their Applications Greifswald, Germany Contact: Dr G Babucke, Inst. f. Niedertemperatur-Plasmaphysik, Robert-Blum-Str. 8 - 10, 17489 Greifswald, Germany. Tel +49-3834-554411, Fax +49-3834-554301, E-mail gd97@public.inp.uni-greifswald.de.

8 - 19 September 1997 3rd Carolus Magnus Summer School on Plasma Physics Spa, Belgium Contact: Dr G van Oost, c/o Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany. Tel +49-2461-61-5626, Fax +49-2461-61-3331, E-mail vanoost@fusion.rma.ac.be. Application deadline: 15 May 1997, abstract deadline: 15 July 1997.

8 - 10 October 1997 7th European Fusion Theory Conference Jülich, Germany Contact: A L Rogister, Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, EURATOM-FZJ Association, D-52425 Jülich, Germany. Tel +49-2461-61-5440, Fax +49-2461-61-2970, E-mail d.petschner@kfa-juelich.de.

13 October - 7 November 1997 Autumn College on Plasma Physics Miramare-Trieste, Italy Contact: S M Mahajan, International Centre for Theoretical Physics, Autumn College on Plasma Physics, PO Box 586, I-34100 Trieste, Italy. Tel +39-40-2240357, Fax +39-40-224163, E-mail smr1012@ictp.trieste.it. Application deadline: 30 April 1997.

10 - 14 November 1997 Topical Workshop on Plasma Physics: New Perspectives of Collective Effects Miramare-Trieste, Italy Contact: P K Shukla, International Centre for Theoretical Physics, PO Box 586, I-34100 Trieste, Italy. Tel +39-40-2240357, Fax +39-40-224163, E-mail smr1014@ictp.trieste.it. Application deadline: 31 March 1997.