The negative-ion based neutral beam injector for heating and current drive of the ITER plasma (ITER HNB) is under development, at present focusing on the optimization of the full-scale plasma source in the SPIDER test stand. The production of H- or D- ions in the ion source is based on the low work function surfaces obtained by caesium evaporation. This paper describes the caesium conditioning procedure and the corresponding beam performances during the first operation of SPIDER with caesium. Technical solutions to overcome present limitations of the test stand are described. The influence of source parameters on the caesium effectiveness was investigated in short beam pulse operation; with total RF power of 400 kW and filling pressure below 0.4 Pa, and a limited number of extraction apertures, a negative ion current density of about 200 A/m2 was extracted in hydrogen, with beam energy lower than 60 keV. Beam optics and beam uniformity were assessed thanks to the acceleration of isolated ion beamlets. A possible procedure to accelerate a uniform beam was demonstrated at low RF power. The results obtained in this first investigation provided key indications on the operation of one of the largest existing sources of accelerated negative hydrogen-like ions.

Deuterium plasma discharges of the Divertor Tokamak Test facility (DTT) in different operational scenarios have been predicted by a comprehensive first-principle based integrated modelling activity using state-of-art quasi-linear transport models. The results of this work refer to the updated DTT configuration, which includes a device size optimisation (enlargement to R0 = 2.19 m and a = 0.70 m) and upgrades in the heating systems. The focus of this paper is on the core modelling, but special attention was paid to the consistency with the SOL parameters required to achieve divertor plasma detachment. The compatibility of these physics-based predicted scenarios with the electromagnetic coil system capabilities was then verified. In addition, first estimates of DTT sawteeth and of DTT ELMs were achieved.

Neutral beam injection is one of the most important methods of plasma heating in thermonuclear fusion experiments, allowing the attainment of fusion conditions as well as driving the plasma current. Neutral beams are generally produced by electrostatically accelerating ions, which are neutralised before injection into the magnetised plasma. At the particle energy required for the most advanced thermonuclear devices and particularly for ITER, neutralisation of positive ions is very inefficient so that negative ions are used. The present paper is devoted to the description of the phenomena occurring when a high-power multi-ampere negative ion beam travels from the beam source towards the plasma. Simulation of the trajectory of the beam and of its features requires various numerical codes, which must take into account all relevant phenomena. The leitmotiv is represented by the interaction of the beam with the background gas. The main outcome is the partial neutralisation of the beam particles, but ionisation of the background gas also occurs, with several physical and technological consequences. Diagnostic methods capable of investigating the beam properties and of assessing the relevance of the various phenomena will be discussed. Examples will be given regarding the measurements collected in the small flexible NIO1 source and regarding the expected results of the prototype of the neutral beam injectors for ITER. The tight connection between measurements and simulations in view of the operation of the beam is highlighted. operating in the existing tokamaks. Consequently, PRIMA, the ITER Neutral Beam Test Facility [2], was set up to constitute a test-bed, where the solutions to all the issues related to the achievement of full performances in the heating NBI system for ITER are going to be addressed and optimised, particularly regarding critical aspects like density and uniformity of the extracted negative ion current, high voltage holding and heat loads on the components [3]. Megavolt ITER Injector Concept Advancement (MITICA) is the full-scale prototype of the ITER NBIs [4]; it includes an RF-driven plasma source for the production of negative ions and should operate at a pressure as low as 0.3 Pa in hydrogen or deuterium gasses. The negative ions are produced on the surface of the grid (Plasma Grid, PG) that closes the plasma region; their production is enhanced thanks to a thin caesium layer continuously deposited over the PG by evaporation. Negative ions are extracted through the 1280 apertures in the PG by application of a suitable positive voltage to the extraction grid (EG), located just downstream of the PG. The RF plasma source operates at an applied electric potential of about −1 MV. Five additional acceleration grids (AG1, AG2, AG3, AG4, GG), at intermediate electric potential increasing by 200 kV steps, are located downstream with respect to the EG, thus constituting a 5-stage electrostatic accelerator. The resulting negative ion beam at 1 MeV, after passing through a gas cell neutraliser and an electro...