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

In low-current tokamaks, in the absence of radial electric fields (Er), the widths of the drift orbits are large and the direct orbit losses can extend deep into the plasma. Furthermore, in such a plasma even a modest Er can produce rotation with a poloidal Mach number (Mp) that exceeds unity. Using the Monte Carlo code ASCOT, which follows charged particle orbits in the five-dimensional phase space, the formation of an internal transport barrier (ITB) in such a tokamak is investigated. Carrying out the simulations for the geometry corresponding to the FT-2 tokamak, it is shown that if, under these conditions, a steep density gradient is created in the plasma, the plasma responds by generating a strong (much stronger than needed to compensate the diamagnetic drift) Er in the region of the strong gradient. The generation appears to be a pure neoclassical effect, but a global solution over the entire plasma cross section is required to fully identify it. As a result, an ITB-like situation with a strongly sheared E×B flow is obtained inside the plasma. In these circumstances Mp>1, and thus the orbits of the majority of ions become strongly squeezed. The neutral fluxes observed by neutral particle analysers are also simulated to find out if the neutral spectra can be utilized to estimate the Er values across the plasma cross section in the FT-2 tokamak.