Thermo-mechanical design of the ITER Neutral Beam Injector grids for Radio Frequency Ion Source and SINGAP Accelerator
This paper focuses on the thermo-mechanical design of the ITER Neutral Beam Injector plasma, extraction and acceleration grids, considering the Radio Frequency option for the negative ion source and the SINGAP option for the accelerator. The SINGle Aperture -SINgle GAP (SINGAP) accelerator for ITER NBI foresees four grids for the extraction and acceleration of negative ions, instead of the seven grids of the Multi Aperture Multi Grid (MAMuG) reference configuration. The Radio Frequency ion source is a promising alternative to the Arc Driven one, showing very good performances in recent experiments at IPP Garching (D) and having the advantage of reduced maintenance requirements. A specific plasma grid was designed for RF source considering lower heat loads and different operational requirements. In particular the thermo-hydraulic and thermo-mechanical design aims at having all over the plasma grid a precisely controlled temperature to enhance the negative ions yield inside the caesium seeded source. Analytical and numerical (FE) thermo-mechanical analyses were also carried out for the extraction and acceleration grids. Structural verifications were accomplished according to the ITER Structural Design Criteria for In-Vessel Components. Design optimizations were performed in order to limit the misalignment of the grids within the tolerance specified by beam optics requirements (0.4 mm of maximum misalignment between corresponding apertures axes). The optimum cooling parameters under different operational scenarios were identified. The paper describes the design status of the grid system for the RF ion source and discusses in detail the numerical work carried out for thermo-mechanical and fluid dynamic behaviour.
Design of the RF ion source for the ITER NBI
Abstract A radio frequency (RF) driven negative ion source has been designed for the ITER neutral beam injectors, as an alternative to the traditional arc driven solution. The main advantage of this technology is to avoid the presence of the filaments, that require periodic maintenance and consequently frequent shutdowns. The requirements for the ion source of the ITER NBI are to provide a uniform flux of D−/H− to the plasma grid of the accelerator that will result in a beam current of 40 A at 1 MeV. The present specification is for a filling pressure of 0.3 Pa. The ion source needs to provide 20/28 mA/cm2 D−/H− current density across the 0.58 m × 1.54 m aperture array for 3600 s. The source, consisting of a main chamber facing the plasma grid, of eight RF drivers and the auxiliary systems for power transfer, cooling and diagnostic purposes, is housed in the same quasi-cylindrical structure that supports the arc driven solution. Specific electric and hydraulic circuits have been designed and verified. In the paper the analyses performed for the design of the components are presented in detail.
Development of large RF driven negative ion sources for neutral beam injection
Abstract The development of a large-area RF source for the negative ion beam production has been continued concentrating on investigations of volume-mode Cs-free operation. In contrast to most arc discharge sources the extracted H− current density can be significantly improved in the RF source by the addition of argon. Probe measurements close to the plasma grid show that this enhancement is due to a dramatically increased plasma density with the addition of argon. Variation of the number of extraction holes and of the effective transparency showed no dependence of the current density on the extraction area or on the transparency, however, still at a rather low level. The H− current density that has been reached at present amounts to about 5 mA/cm2 at 100 kW and 0.6 Pa without cesium as published previously. In order to investigate the role of the magnetic filter, a large electromagnet has undergone first tests. Preliminary results show no strong dependence on the magnetic field.