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Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks to their multi-beamlet configuration. As the single-beamlet optics defines the transmission losses along the beamline, the extraction of a similar current for all beamlets is extremely desirable, in order to facilitate the beam source operation (i.e., around perveance match). This Review investigates the correlation between the vertical profile of beam intensity and the vertical profiles of plasma properties at the extraction region of the source, focusing on the influence of increasing cesium injection. Only by the combined use of all available source diagnostics, described in this Review, can beam features on the scale of the non-uniformities be investigated with a sufficient space resolution. At RF power of 50 kW/driver, with intermediate bias currents and a filter field of 2.4 mT, it is found that the central part of the four vertical beam segments exhibits comparable plasma density and beamlet currents; at the edges of the central segments, both the beam and electron density appear to decrease (probably maintaining fixed electron-to-ion ratio); at the bottom of the source, an increase of cesium injection can compensate for the vertical drifts that cause a much higher presence of electrons and a lower amount of negative ions.

The RF-driven negative ion source testbed BATMAN upgrade is being developed at IPP Garching in the framework of the ion source development for ITER and DEMO neutral beam injection systems. The testbed has recently been enhanced to allow for steady state operation with a focus on beam optics studies. The previous titanium sublimation pumps and inertial calorimeter limited the beam pulse length to about 6 s every 3 min. The upgrade comprises a long-pulse compatible, actively cooled diagnostic calorimeter. This has been designed and is currently being manufactured to substitute the inertially cooled calorimeter that has limited diagnostic capabilities. The new diagnostic calorimeter consists of a copper plate with dimensions of 910 × 660 × 25 mm 3 placed about 2 m from the ion source extraction grids, and through a novel solution, it will provide a 2D profile of beam power density with a 20 mm spatial resolution. Water flowing through cooling channels embedded in the copper plate will actively cool the calorimeter, which is loaded with about 160 kW beam power at ITER-relevant current density, but 45 kV acceleration. A fraction of the beam will pass through many small apertures (ø2 mm) positioned in the calorimeter plate and will be collected by thin (0.2 mm) copper foils attached to the calorimeter back side. Evaluation of power density will be performed by measuring the temperature of the heat flux foils with a high-resolution infrared camera observing the calorimeter from the back side and calibrated by thermocouples.

Negative ion sources for neutral beam injection (NBI) in fusion experiments are based on the surface production of H - or D - on cesiated low work function surfaces. In the recent years, it was demonstrated at the large RF driven ion source of the ELISE (Extraction from a Large Ion Source Experiment) test facility that the requirements for the ITER NBI systems can be fulfilled by hydrogen. This is a big step toward the first operational period of ITER, planned for up to 2035. However, for the following operational period, neutral beam systems working in deuterium are needed. Operation of negative hydrogen ion sources in deuterium is significantly more demanding than in hydrogen: the amount of coextracted electrons is much higher and their increase during pulses is much more pronounced, limiting the achievable performance. This paper presents the results of investigations aimed to improve the insight into the physics related to this isotope effect. Due to the higher atomic mass of deuterium, cesium is removed much more effectively from reservoirs at the walls, resulting in a depletion of these reservoirs and a strongly increased cesium density in the plasma. Additionally, a correlation between the fluxes of charged particles toward the inner ion source surfaces and the coextracted electrons is identified.

Along the route to the development of a neutral beam injector for ITER, the Padua based Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER) and megavolt ITER injector and concept advancement facilities will make use of neutron diagnostics to quantify the homogeneity of the neutral beam profile by measuring the map of the neutron emission from the beam dump with the close-contact neutron emission surface mapping (CNESM) system. Neutrons are here produced from beam-target reactions between the deuterium beam and the deuterons previously adsorbed in the calorimeter. In order to aid the interpretation of the diagnostic data, a dedicated experiment on neutron emission from beam-target reactions with beam parameters approaching those expected at SPIDER has been performed at the Extraction from a Large Ion Source Experiment (ELISE) neutral beam test facility. The time trace of neutron emission has been measured using a calibrated liquid scintillator detector at increasing power densities on the target. Compared to calculations based on the local mixing model, a systematic discrepancy was observed exceeding the statistical accuracy of the measurements and increasing as a linear function of the power density. The data are used to derive an empirical temperature dependent correction for applications to neutron measurements at SPIDER.