[en] The electrostatic drift instability arising from the reduction of shear damping, due to toroidal effects, is assumed to be the basic source of the anomalous electron transport in tokamaks. The Maxwellian population of electrons constitutes a medium whose adiabatic nonlinear reaction to the instability (described in terms of an effective dielectric constant of the medium) determines the stationary electrostatic fluctuation level in marginally unstable situations. The existence of a random electrostatic potenial implies a fluctuating current of the Maxwellian electrons which creates a random magnetic field and a stocasticization of a magnetic configuration. The application of recent results allows the calculation of the realted radial electron transport. It is found that the confinement time under stationary ohmic conditions scales as n Tsub(i)sup(^-1/2) and is proportional roughly to the cube of the geometric dimenisions. Moreover, it is deduced that the loop voltage is approximateley the same for all tokamaks, irrespective of temperature and density and to a large extent, also of geometrical conditions. Thes results are characteristic of the ohmic stationary regime and can hardly be extrapolated to order heating regimes. (orig.)

No abstract available

[en] The control of the burn in an ignited Tokamak using a space and time dependent external vertical magnetic field is discussed. It is shown that a static field, suitably shaped in space, is able to stabilize the burn for a certain range of the plasma parameters of physical interest. An oscillating magnetic field with constant frequency and amplitude fixed by the initial plasma parameters stabilizes the burn in all situations. (orig.)

No abstract available

[en] In this note formulas are given which describe the behaviour of the plasma quantities under compression in the presence of transport and α - heating. The deviations from adiabaticity at higher orders are calculated. The formulas are applied to the calculation of the relevant quantities in the compressions - decompressions cycles around the ignition point which allow the temperature control of an ignited plasma. Explicit formulas are given for the temperature excursions, the duration of each side of the cycle and the electric energy and power directly generated in the external coils of the controlling vertical field. The application of these results to the Zephyr experiment and to INTOR is discussed. (orig./GG)

[en] The results of transport calculations are presented for a high current and particle density Tokamak, like Ignitor. The transport equations used are averaged over prescribed temperature and density profiles, with an assumed electron energy confinement time given by Alcator scaling and an ion energy confinement time slightly worse than neoclassical. The trapping, slowing down, etc., of alpha particles is included on the basis of classical theory. The results indicate that with realisable technological parameters ignition is unlikely to be achieved without the use of additional heating, even for a Z eff = 1 plasma. (author)

[en] The coupled system of the Ohm-Maxwell and transport equations is solved applying a 1-D code to the case of a one-fluid plasma in a cylindrical Tokamak. A situation is considered such that the current channel is surround by a large and cold boundary region with high thermal conductivity simulating the effect of strong cooling processes in the boundary zone. Introducing suitable boundary conditions on the interface a resistive instability exists (in addition to the thermal instability related to local overheating) which is related to marginal modes of the equation for the perturbed poloidal flux (plasma-circuit instability). The current shrinks and collapses on the axis on a time scale depending on the resistivity of the cold zone and on its width. If tausub(E) approx. na²R the nonlinear evolution of the profiles is controlled by the parameter nqPU/B (U is the loop voltage) and a very steep gradient is developed at high density. (author)

[en] A non-uniform magnetized plasma which admits a marginal mode such that in its unstable neighbourhood the real part of the dielectric constant is negative, is subject to sizeable electrostatic saturated fluctuations whose level corresponds to the existence, in this neighbourhood and in statistical sense, of a stable nonlinear Vlasov equilibrium. The fluctuation level is calculated using a statistical method and the fluctuations are large as a consequence of the importance of the nonlinear terms near the marginal point, a situation formally reminiscent of that occurring in statistical thermodynamics near the critical point of a phase transition. Two examples are discussed considering a plasma with a uniform temperature in a uniform magnetic field and in plane geometry: (1) the drift instability originated by the density gradient and (2) the flute instability originated by a gravitational field. It is found that in both cases, notwithstanding the difference in detail of the saturation mechanisms, ephi/T is proportional, for a given wave number to (T/n)sup(1/2) (where phi is the electrostatic potential, T is the temperature and n is the density), and can reach values of the order of 1%. (author)

[en] The two most successful Tokamaks currently in operation are PLT and Alcator, the former achieving the highest temperatures and the latter the highest value of ntausub(E). Transport calculations are presented for a small ignition Tokamak (SIT) which it is envisaged to operate with intense neutral injection heating, like PLT, and with a high field, high current density and high particle density, like Alcator. Besides the the physics of such a device some technological considerations are included based mainly on the design of the Frascati Tokamak. (author)

No abstract available