Magnetic topology, disruptions and electron heat transport

JET results are presented which show a ''profile consistency'' of the electron temperature under a variety of plasma conditions. This experimental fact is interpreted as due to a topology of the magnetic field lines where ordered structures such as laminar surfaces and magnetic islands coexist with ergodic domains. The development of this concept identifies different plasma regions according to the value of the safety factor and relates the observed degradation in confinement time, when Additional Heating is applied, to the plasma entering a multiphase state where the electron temperature gradient is limited by a critical value defined by the global plasma parameters. A preliminary scaling law for the electron temperature is proposed which indicates a strong favourable dependence on major radius and plasma current. Losses due to atomic processes at the plasma edge give rise to thermal instabilities with low m numbers, which induce ergodic regions. Plasma disruptions could be considered as the effect of these very low number instabilities (m=2,3). The near future JET experimental programme is briefly described and possible ways of improving the plasma confinement are presented. (author)