Modelling and optimization of neutral beam injectors for fusion neutron source "DEMO-FNS"
Neutral beam injection (NBI) system of the thermonuclear neutron source DEMO-FNS is suggested for a steady-state plasma heating and current drive. The injector scheme for DEMO-FNS incorporates a negative ions source coupled with accelerator delivering D- beam accelerated to 500keV with the current 40A. The subsequent neutralization and ions removal from the beam, as well as its further path to plasma lead to a significant distance between beam source and injection point (~20 m). An effective beam transmission to such a long distance with minimum losses is a real challenge, requiring a detailed 3D-modelling of the beam and a beamline structure optimization. The simulation includes minimization of beam losses due to direct interception and reasonable reduction of beam particles loss due to reionization. The optimization problem has proved to include a large amount of parameters, constraints, and it is rather sensitive to small deviations of input data, therefore the models should be fine-tuned and allow for high accuracy estimations. The most important conditions affecting the injector performance, constraints and operation scenarios are discussed in this report, as well as the performance criteria for the baseline (assumed) configuration. The NBI system for DEMO-FNS is simulated by beam tracing codes – PDP and BTR, both developed earlier and verified for the ITER project. Optimal geometry searching methodology and the investigation results of various NBI operation modes are presented. An optimal self-consistent injector configuration and the initial ion beam are chosen. Brief the evolution of the beam power profile and total losses, detailed load distributions for all elements of the NBI are calculated.