A phenomenological model of Tokamaks is constructed. The model assumes the toroidal resistivity of the plasma to be Spitzer-like and the poloidal resistivity to be anomalous. The electron perpendicular thermal diffusivity is assumed to have an anomalous value typical of ohmic and beam heated experiments. The complete set of conservation equations including pressure balance and poloidal-Ohm's law are used together with experimentally relevant sources and boundary conditions. In ohmic conditions it is shown that the particle diffusivity Dperpendicular to, the electron thermal diffusivity chiperpendicular to e, and the magnetic diffusivity c2etatorare of comparable order. Under these conditions the plasma density is found to scale with plasma current like (n varies as Ip65/). The poloidal-beta is found to be limited. Scalings are also derived in the presence of beam-heating for two different laws of increase of chiperpendicular to ewith betapol. These indicate that the density-current scaling is relatively insensitive to the beam power and degradation of confinement. The model also interprets some of the evolutionary characteristics such as density-clamp in DITE and L-H transition in ASDEX.
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