Assessment of a multi-reflectometers positioning system for DEMO plasmas

Abstract

Microwave reflectometry is the prime candidate to measure the DEMO plasma position and shape. DEMO will be based on a pre-defined operation scenario, allowing the optimization of the reflectometry measurements according to the expected parameters. When evaluating the system performance for the expected baseline scenario, it is essential to take into account the effects of the plasma displacement, of the turbulence and of the MHD activity in the measurements. Therefore, the design of the DEMO plasma position reflectometer (DEMO PPR) involves the assessment of the measurement performance of different poloidal views, antenna assemblies, emitting angles and different plasma configurations. In this work we evaluate the measurement performance over 100 different poloidal positions of the DEMO PPR using a synthetic monostatic O-mode reflectometer. The antennas were initially aligned perpendicularly to the wall. The results show that most of the locations at the top of the machine and near the divertor are not adequate for measuring the plasma position. These locations are characterized by larger wall-plasma distances and by larger angle of incidence on the separatrix. In order to optimize the measurement performance of the system, the antennas were aligned perpendicularly to the expected separatrix position, reducing these parameters. With the exception of the locations near the divertor, this geometrical consideration improved the measurement performance for all the positions, placing them below the requirement for the plasma positioning (∼ 1 cm). The locations near the divertor lose the signal due to the significant plasma curvature before the separatrix. We also studied the effect of a vertical plasma displacement of Δ z = ± 5 cm in the measurements. The results show that the system is stable, fulfilling the chosen requirements. Aligning the emission perpendicularly to the separatrix proved to be a necessary condition in the optimization and stability of the DEMO PPR system.