Tungsten in the ITER Baseline Scenario in DIII-D

The impact of Tungsten (W) in burning plasmas can be studied in present machines using W-equivalent radiators, i.e. gases with the same radiative loss rate index (Lz) at low temperature, as W at the higher reactor-relevant Te. Using Kr and Xe, whose Lz index at 1-4 keV matches the Lz of W at 10-15 keV, we studied the ITER Baseline Scenario with the radiated fraction (frad) from W predicted for ITER (<=30%) and a range of torque=0-4 Nm. The scenario remains robustly stable up to frad~30%, where the lost power can overtake the input power and transport losses, decreasing the core Te. Lz being a function of Te in this range, a non-linear process ensues, that reduces the radiated power and can trigger a predator-prey behaviour, increasing the temperature again. A Lotka-Volterra model, modified to include two-region diffusion (core and pedestal) and noise, can account for variable signs in the better known non-linear terms of the equations, representing the experimental conditions of dLz/dT>0 and <0 as in the IBS Te range. The limit of this cycle is reached when Te is low enough to significantly reduce the core current density, raise qmin and eliminate the sawteeth: without the ST flushing out impurities, accumulation increases and the scenario experiences a radiative collapse