Development of a modular systems code to analyse the implications of physics assumptions on the design of a demonstration fusion power plant

The successful development and operation of a demonstration power plant (DEMO) is the next important step on roadmaps for fusion energy after ITER that is currently constructed in France. In the first phase of the development process for such devices, the conceptual design phase, the primary aim is to identify coherent designs that are composed of self-consistent sets of values for all key parameters like machine size, plasma current or magnetic field strength. This multidimensional parameter space can be explored with systems codes in order to identify areas that seem to be suited for more detailed investigation. Systems codes are composed of simplified models for all crucial systems of fusion devices that take into account all requirements and constraints of each component. This thesis is about the development of a new systems code called TREND (Tokamak Reactor code for the Evaluation of Next-step Devices). TREND is implemented with modular code architecture and consists of modules for geometry, core plasma physics, divertor, power flow, technology and costing. The main focus has been on the core physics module, since the development of TREND was done in parallel to work on physics design guidelines for DEMO. Moreover, the validation of TREND in terms of benchmarks with other European and Japanese systems codes is discussed. For these benchmarks, specific parameter sets were selected and the observed deviations were traced back to differences concerning the individual modellings. One of these parameter sets constitutes also the basis for parameter studies that were conducted with TREND. The general idea behind these studies is the analysis of implications that arise from specific assumptions on selected key parameters. Besides constant fusion power and constant additional heating power, the plasma density is fixed with respect to the Greenwald limit. The benchmarks helped particularly to detect shortages in the modellings of all involved systems codes. Significant improvement needs could be identified concerning the applied modellings of the divertor, the dynamical phases, the line radiation in different plasma regions, the contribution of fast plasma particles, as well as the plasma current profile. Some proposals for improved modellings are already discussed within this thesis. Furthermore, the conducted parameter studies show that due to the various boundary conditions characterising the tokamak operational space, the observed trends can be surprising when compared to simple models. In particular, it is observed that the plasma temperature plays a key role in case of fixed plasma densities. The feasible operation range for the plasma temperature is clearly restricted by the H-mode threshold power on the one side and the limits for the power exhaust on the other side.