Convective particle transport arising from poloidal inhomogeneity in tokamak H mode

Abstract

In tokamak high-confinement modes ͑H modes͒, a large poloidal flow exists within an edge transport barrier, and the electrostatic potential and density profiles can be steep both in the radial and poloidal directions. The two-dimensional structures of the electrostatic potential, density, and flow velocity near the edge of a tokamak plasma are investigated. The analysis is carried out with the momentum conservation law using the shock ordering. For the case with a strong radial electric field ͑H-mode case͒, a particle flux is induced from asymmetry of the poloidal electric field in the transport barrier. This convective transport is found to depend weakly on collisionality, and changes its direction in accordance with the direction of the radial electric field, the toroidal magnetic field, and the plasma current. The divergence of a particle flux is a source of temporal variation of the density, and there are negative divergence regions both in the inward and outward flux cases. Thus this convective particle flux is a new candidate for the cause of the rapid establishment of the density pedestal after the onset of low to high confinement mode ͑L / H͒ transition.

Two dimensionally steep structure of the electric field in tokamak H-mode

“…The radial electron flux is zero with the Boltzmann relation, so it does not affect directly to the density pedestal formation as described in Ref. [9]. If there is a mechanism to violate the Boltzmann relation of electrons, such as electron-ion collisions, the 2-D structure contributes to the radial electron flux.…”

“…The poloidal electric field generates an E × B convective flow in the radial direction, so the flux-surface-averaged radial flux is calculated to estimate its effect on particle transport. The E × B drifts of ions and electrons direct to the same direction, depending on the sign of the radial electric field, the toroidal magnetic field and the plasma current [9]. Adding the diamagnetic flow with electrons which satisfied the Boltzmann relation on the same flux surface, the electron radial flux is canceled to be zero.…”

“…τ p is the time normalized by t p , which is of the order of the duration for which the particle with the thermal velocity completes one poloidal rotation, so the time scale of 2D structural formation is given by this poloidal transit time t p . The form of I ps depends on M p and the collision frequency [19].…”

“…(10) of Ref. [5], and depends on M p and the collision frequency [9]. Time evolution of the 2-D structure is calculated from Eq.…”

Two-Dimensional Model Including the Mechanism of a Poloidal Shock Structure and Geodesic Acoustic Mode in Toroidal Plasmas

“…(10) of Ref. [4], which depends on M p and the collision frequency [16]. The variables n 1 and Φ 1 are replaced by χ and E 1 in this set, respectively.…”