Simulations of density profiles, pellet fuelling and density control in ITER

The paper presents the results of 1.5-dimensional simulations of density profiles and pellet fuelling for the ITER baseline scenario performed with the ASTRA and JETTO transport codes by the ITER Scenario Modelling working group within the European Task Force on Integrated Tokamak Modelling. The first part of the paper describes the physics of the problem and how it is implemented in the different codes available to the working group. The second part presents the results of the simulations. Results obtained with the GLF23 physics based transport model and a simplified description of the pellet particle source are described alongside results obtained with the simpler Bohm/gyro-Bohm semi-empirical transport model and a more sophisticated pellet ablation/deposition code providing a completely self-consistent description of the pellet source. A parametric study has been performed to assess the effect of varying parameters independently, the values of which in ITER are either uncertain or not easily controllable (such as particle diffusivity, edge stability, wall recycling and boundary conditions), on the target plasma density, temperature, Q and pellet frequency required to achieve a certain degree of density control. To this end the edge particle diffusivity was increased by a factor of three, the pedestal normalized critical pressure gradient for ballooning stability was decreased by 20%, the boundary conditions on density and temperature were modified by 30-40% and the wall recycling particle source was increased from zero to 20% of the particle outflux. The results show that variations in the order of 15% for density and temperature, 40% for Q and 100% for the pellet frequency can be expected. Open problems and modelling needs are also discussed in the paper.