The use of fully toroidal co-ordinates permits the two-dimensional problem of the axisymmetric plasma toroidal equilibrium to be reduced to the one-dimensional problem of determining a limited number of its toroidal multipolar moments. This has allowed the creation of a fast semi-analytic predictive equilibrium code that can be used in both free and fixed boundary conditions for plasmas with circular or mildly noncircular cross-section. The concept of toroidal multipoles is also particularly suitable for the analysis of experimental data from magnetic probe measurements and clarifies the conditions under which the plasma thermal and electrical self-inductances βp and ℓi can be estimated separately. Finally, the interpretation of the magnetic equilibrium measurements in terms of toroidal multipoles can directly provide the boundary conditions for a fast equilibrium reconstruction code. Examples of the application of such a code to the JET magnetic measurements are reported.

In this work we describe the heating results in the LH frequency range (f = 2.45 GHz, Pw 5 250 kW, P < 6 k_W/cmz) both in the electron and ion regimes. Efficient electron heating was observes f o r n 5 5 ~1 0 ' ~ c w 3 . Increases of the peak electron temperature up to 700 eV were measured. At higher densities the interaction with electrons disappears. In the ion regime fast neutral tails and neutron enhancement were observed. The influence of plasma boundary conditions on the penetration of the wave is demonstrated. The principal physical problems are pointed out and some possible conclusions are given.

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The main subject of the discussions at the 4th European Tokamak Programme Workshop was plasma behaviour approaching operational limits in Tokamak discharges. Such limits are encountered for increasing plasma density, pressure or current. The pressure limit effectively manifests itself as a limit to the plasma "beta", the plasma kinetic pressure normalized to the pressure of the confining magnetic field. Similarly, the limit in plasma current. in dimensionless terms, actually implies that there is a lower bound to the "safety factor" q. All three limits are of great practical importance because they impose constraints on the plasma parameter range that is accessible in a Tokamak reactor. A short session was dedicated to the question of what the experimental evidence is for the appearance of "profile consistency" effects in European Tokamaks. This issue is essential for understanding and making progress in the difficult field of transport properties of Tokamak plasmas. Furthermore, a summary of recent results from JET was presented and discussed. Finally. an account of the status of NET was given.The Workshop was sponsored by the Risa National Laboratory. Roskilde, and by the Commission of the European Communities.The following is a summary of the Workshop proceedings.P:R, with 2 2 0.5. Moreover. these ASDEX resultssuggest that in strongly NBI heated discharges the Hugill-Murakami parameter i R q B is meaningless, since the density limit becomes a fucction of input power rather than current density. This is an important point that requires further attention.

The design study of PROTO-SPHERA, a novel compact torus configuration, has been completed. It is composed of a spherical torus (ST) (with closed flux surfaces) and a force-free screw pinch (SP) (with open flux surfaces and fed by electrodes). PROTO-SPHERA is formed at spherical-tokamak-like densities (∼10 19 m −3) with low voltage (∼200 V) between the electrodes. The idea of replacing the metal centrepost current (I tf) of the spherical tokamaks with the SP plasma electrode current (I e) is aimed mainly at getting rid of the rod at the centre of the plasma configuration, which is the most critical component of spherical tokamak design. As a consequence it should be possible to decrease the aspect ratio A = R/a (R = ST major radius, a = ST minor radius) in the course of experiment and to increase the ratio between the toroidal plasma current (I ST) and the plasma electrode current, I ST /I e 1. Matching two plasma configurations, i.e. an open flux-surface SP and a closed flux-surface ST, brings to life several radically new issues. The purpose of this paper is to analyse the equilibrium, the ideal MHD stability and the formations and modelling issues of such a combined magnetic confinement system. The MULTI-PINCH experimental setup, which is being assembled inside the START vacuum vessel (now in Frascati), will represent the first phase of PROTO-SPHERA: its goal is to prove the feasibility of a stable disc-shaped SP around the electrodes.