The effect of the radial electric field on the L-H mode transition

Consider the plasma in a tokamak, which is not initially in a state of equilibrium. Quasi-electrostatic electric fields usually emerge in a tokamak plasma due to varying departure from the exact local charge neutrality. In the standard neoclassical theory the particle fluxes across magnetic surfaces to order (ρ/r)2 result from the toroidal component of the momentum balance. The contribution to the radial flux emerge due to the inertial, viscous and frictions force. The temporal and spatial discharge ontogeny consists of two phases. The first phase governed by the balance of the parallel inertial and viscous forces is not intrinsically ambipolar owing to the sifnificant difference of the ion and electron inertia and viscosity. Thence the parallel viscous force associated with the magnetic field modulation in a tokamak rapidly damp the poloidal flow, producing a radial current caused by the force of inertia and proportional to dEr/dt. The convective derivitive of the electric field ur∂Er/∂r arises. If ur is of the order of the Pfirsch-Schluter particle flow then the poloidal rotation resulting from the radial electric field is quickly damped. However, the transport in a tokamak is notoriously anomalous and exceeds the value of the Pfirsch-Schluter particle flow by orders of magnitude. Therefore the role of this phase of the discharge is very important in reality. The intrinsically non-ambipolar phase of the discharge terminates when the mean parallel viscous force compensates exactly the inertial force. Then the radial flux is primarily governed by the friction force owing to Coulomb collisions between ions and electrons. This results in the intrinsically ambipolar fluxes across magnetic surfaces. Thus more subtle phenomena such as the shear viscosity invoke at this phase. The well-known conservation of the toroidal angular momentum by Coulomb collisions result for an arbitrary value of the electric field Er. Therefore the ambipolar electric field at this stage is determined as a linear function of the toroidal angular momentum and the pressure-gradient-driven diamagnetic fluxes. (author) 6 refs., 1 fig