Formation and detection of internal transport barriers in low-current tokamaks

Abstract

In low-current tokamaks, in the absence of radial electric fields(Er), the widths of the drift orbits are large and the directorbit losses can extend deep into the plasma. Furthermore, in sucha plasma even a modest Er can produce rotation with apoloidal Mach number (Mp) that exceeds unity. Using the MonteCarlo code ASCOT, which follows charged particle orbits in thefive-dimensional phase space, the formation of an internaltransport barrier (ITB) in such a tokamak is investigated.Carrying out the simulations for the geometry corresponding to theFT-2 tokamak, it is shown that if, under these conditions, a steepdensity gradient is created in the plasma, the plasma responds bygenerating a strong (much stronger than needed to compensate thediamagnetic drift) Er in the region of the strong gradient. Thegeneration appears to be a pure neoclassical effect, but a globalsolution over the entire plasma cross section is required to fullyidentify it. As a result, an ITB-like situation with a stronglysheared E×B flow is obtained inside the plasma. In thesecircumstances Mp>1, and thus the orbits of the majority ofions become strongly squeezed. The neutral fluxes observed byneutral particle analysers are also simulated to find out if theneutral spectra can be utilized to estimate the Er valuesacross the plasma cross section in the FT-2 tokamak.