Numerical Modelling of Marfe Phenomena in TEXTOR Tokamak

The 2D boundary layer code TECXY has been used to study the evolution of the so-called Marfes, thermal instabilities which arise locally in a zone near the high-field side of the TEXTOR tokamak boundary layer, as a result of strong plasma cooling by the interaction with the deuterons recycled at the bumper limiter, and by the subsequent highly increased radiation which is limited by the heat transport along the magnetic field and radially from the core. The bumper limiter is introduced into the scrape-off layer in a region between -45° and +45° below and above the high-field side, leaving a width of about 2 cm between the bumper surface and the last closed flux surface. We model the bumper region by assuming high additional recombination losses inside this region and a suitably chosen profile for the recycled neutrals penetrating from the bumper surface into the adjacent plasma. A parameter study has been carried out which shows that for sufficiently high particle influx into the plasma edge layer and/or sufficiently low energy influx a stability boundary is crossed and the Marfe develops during about 1 msec. The stability boundary between the Marfe and the no-Marfe region is not sharp because bifurcated solutions are possible which depend on the history (initial conditions) of the plasma. Critical values (taken at the separatrix and the low-field side) which always produce Marfes are densities n_e ≳ 8 × 10¹⁸/m³ or temperatures (T_e+T_i)/2 ≲ 50 eV. Within the Marfe n_e increases up to about 10²⁰/m³ and T_e, T_i decrease down to about 4 eV. The effect of drifts as found by TECXY is a shift of the Marfe position towards the upper bumper edge and a weakening of the Marfe strength, but the stability boundary is not affected. The influence of additional radiation losses by carbon impurities is also investigated.