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SPIDER (Source for Production of Ions of Deuterium Extracted from Radio frequency) experiment is operating at PRIMA (Padova Research on ITER Megavolt Accelerator) site since June 2018. It is a test bed to study the operation of an ITER-relevant negative ion source. Caesium (Cs) adsorption on the extraction system, in particular on the plasma grid, lowers the surface work function. This increases the probability of negative ions production and the reduction of coextracted electrons, as experimentally observed. The Cs layer in the SPIDER beam source shall be built up in a controlled and repeatable way by injecting Cs vapors by means of three Cs ovens. A SPIDER Cs oven prototype was procured and tested in the CAesium Test Stand (CATS) at PRIMA site validating the design. Following the experimental tests, few improvements were introduced in the final design of the three Cs ovens for the SPIDER source. Then, the ovens were procured and commissioned before the installation in SPIDER. This paper reports the functional tests of the SPIDER Cs ovens and of their embedded diagnostics. These tests were carried out in CATS, with the aim of characterizing the three ovens and of assessing their reproducibility, in order to validate them for use in SPIDER in the planned experimental campaigns with caesium evaporated in the source.

The SPIDER H-/D- ion source is currently in operation at the Neutral Beam Test facility (NBTF) in Consorzio RFX (Padova, Italy) to prove the possibility of generating up to 40 A of negative ions, with a maximum extracted current density of 350 A/m2 (H)/285 A/m2 (D) and a fraction of co-extracted electrons not greater than 0.5 (H)/1 (D). These performances are required for the realization of the ITER Heating Neutral Beam (HNB), which should deliver 16.7 MW to the plasma by means of negative ions accelerated up to 1 MeV and neutralized before being injected into the ITER tokamak. In order to obtain such high extracted current densities and low co-extracted electron fractions it is necessary to lower the work function of the surface of the acceleration system grid facing the source; this will be accomplished by coating the surfaces with Cs, routinely evaporated by three ovens. The functionality of the ovens has been tested at the CAesium oven Test Stand (CATS), hosted at NBTF. The test stand is equipped with several diagnostics, among which a Laser Absorption Spectroscopy (LAS) diagnostic. Using a tunable laser diode, the LAS diagnostic gets the high resolution absorption spectrum of the Cs 852 nm D2 line along a line of sight to measure Cs density at ground state. The paper describes the test stand and the LAS diagnostic, together with the characterization of the three ovens to be installed in SPIDER. The paper will also study the systematic underestimation effect on measurements caused by Cs ground state depopulation, as function of laser intensity and Cs density, in the perspective of correcting the deviation of measurements.

In hydrogen negative ion sources, co-extracted electrons are dumped on extraction grid (EG) with electron deflection magnets (EDMs). The negative ion beam trajectory is slightly perturbed by the EDM field, and the aperture displacement technique has been applied in negative ion sources for fusion in order to compensate the beam deflection [1]. Recently, it was proposed that the beam deflection can be intrinsically cancelled by the asymmetric EDM field [2]. In this concept, the asymmetric field is formed by the combination of the conventional EDMs and additional set of permanent magnets. In this study, a ferromagnetic steering grid (SG) was used as an alternative way to realize the asymmetric EDM field. The beam acceleration experiments were carried out in the NIFS R&D negative ion source (NIFS-RNIS), and the beam footprint was measured with a beamlet monitor system consisting of an infrared camera and 1D carbon fiber composite (CFC) tiles at various operational conditions. Analysis of the beam trajectory is being carried out with OPERA-3d code. In the presentation, we will discuss the beam steering characteristics of the ferromagnetic SG.

The ITER Heating Neutral Beam (HNB) injector is required to deliver 16.7 MW power into the plasma from a neutralised beam of H-/D- negative ions, produced by an ICP RF source and accelerated up to 1 MeV. To enhance the H-/D- production, the surface of the acceleration system grid facing the source (the plasma grid) will be coated with Cs because of its low work function. Cs will be routinely evaporated in the source by means of specific ovens. Monitoring the evaporation rate and the distribution of Cs inside the source is fundamental to get the desired performances on the ITER HNB. In order to proper design the source of the ITER HNB and to identify the best operation practices for it, the prototype RF negative ion source SPIDER has been developed and built in the Neutral Beam Test Facility at Consorzio RFX. In SPIDER, the dynamics of Cs will be monitored by measuring the emission intensity of the Cs 852 nm line along several lines of sight in the source. For a more quantitative estimation of Cs density a Laser Absorption Spectroscopy diagnostic will be installed; by using a wavelength tunable laser, the diagnostic will measure the absorption spectrum of the 852 nm line along 4 lines of sight, parallel to the plasma grid surface and close to it. From the absorption spectra the line-integrated density of Cs at ground state will be calculated. The paper will present the design of the diagnostic for SPIDER, with a description of the layout and of key components. The paper will also show the first experimental results from a preliminary installation of the diagnostic on the test stand for Cs ovens.

The ion source NIO1 (Negative Ion Optimization 1) is a versatile multiaperture H- source capable of continuous regime operation, with the plasma generated by a 2 MHz/2.5 kW radiofrequency (rf) power supply and nine beamlet extraction. It aims to partly reproduce the conditions of the much larger ion sources built or in construction for neutral beam injectors of fusion devices in a compact and modular ion source, where effects of individual source components can be rapidly verified and compared to simulation code results. Several modifications of the magnetic configuration (both inside the ion source and for the embedded magnets inside the accelerator grids) were investigated, with material ranging from hard ferrite to SmCo to NdFeB; evidence of the effect of the accelerator fringe field inside the ion source is discussed. Saturation of filter field beneficial effect at large (>100 G) amplitudes is discussed, as well as the advantages of crossed field operation. The radiofrequency system takes full advantage of the frequency tuning amplifier capability (+/- 150 kHz installed, 0/+20 kHz used), and it is worth noting that no adjustment of matching box capacitors was necessary in the last 3 years. Major diagnostic systems (including CCD cameras and BES measurement) have been integrated with the acquisition system along with the date measured by several power supplies (including the high voltage supplies, the rf generator and bias supplies, the pressure measurements and the beam currents).

The ITER Heating Neutral Beam (HNB) injector is required to deliver 16.7 MW power into the plasma from a neutralised beam of H-/D- negative ions, produced by an RF source and accelerated up to 1 MeV. To enhance the H-/D- production, the surface of the acceleration system grid facing the source (the plasma grid) will be coated with Cs to reduce its work function. Cs will be routinely evaporated in the source by means of specific ovens embedded in the source. Controlling and monitoring the evaporation rate of Cs inside the source will be fundamental to get the desired performances on the ITER HNB. In order to properly design the source of the ITER HNB and to identify the best operation practices for it, the prototype RF negative ion source SPIDER has been developed and built in the Neutral Beam Test Facility at Consorzio RFX. In SPIDER, liquid Cs based ovens will be used to inject Cs vapours inside the source. The CAesium Test Stand (CATS) has been specifically designed and set up for testing, commissioning, and characterizing Cs ovens in vacuum, but also to study the Cs evaporation and deposition onto surfaces. A SPIDER Cs oven prototype has been manufactured and tested in CATS in order to characterize its thermal behavior, by means of thermocouples and thermal camera, and its Cs flux, by means of Surface Ionization Detector and Laser Absorption Spectroscopy. The paper will present the CATS set up with a description of the layout and main features. The paper will also show the experimental results on the characterization of the Cs oven prototype for SPIDER.

The Neutral Beam (NB) system for ITER will deliver an ion beam with energy of 1 MeV and current of 40 A. Only NB systems based on negative ions can attain the neutralization efficiency required for the NB system in ITER. Helicon plasmas are a promising candidate for the production of negative ions. The RAID (Resonant Antenna Ion Device) at the Swiss Plasma Center represents a crucial step in understanding helicon plasma physics and negative ions production through volume processes. In this talk the last advances on RAID will be presented including the preliminary measurements of H- density by means of Cavity RingDown Spectroscopy and microwave interferometry measurements.

The NIO1 (Negative Ion Optimization 1) experiment hosts a flexible RF H- ion source, developed by Consorzio RFX and INFN-LNL to improve the present concepts for the production and acceleration of negative ions. The source is also used to benchmark the instrumentation dedicated to the ITER neutral beam test facility. Many diagnostics are installed in NIO1 to characterize the source and the extracted negative ion beam. Among them, Beam Emission Spectroscopy (BES) has been installed in NIO1 to measure the divergence and the uniformity of the beam, together with the fraction of beam ions which was neutralized inside the acceleration system. The diagnostic method is based on the analysis of the Doppler shifted H? photons emitted by the fast beam particles and collected along lines of sight. The article will present the experimental setup and the analysis algorithms of the BES diagnostic, together with a discussion of the first measurements and of their correlation with the operational parameters.

The limited availability of the present energetic resources and the climate changes induced by the increase of the level of CO2 in the atmosphere are pushing humanity to completely rethink the ways to produce and consume energy. In the long term, a significant contribution to the solution of the world energy issue may come from nuclear fusion: the raw materials, deuterium and lithium, are worldwide accessible and in sufficient quantity to feed the future fusion reactors for several centuries. The largest fusion experiment is ITER, in phase of construction in France. The ITER machine is designed to produce a fusion power of 500 MW, 10 times the input power. Moreover ITER will be the first experiment to continuously operate with deuterium-tritium plasmas, and will have to stand the neutron irradiation produced by nuclear fusion. To reach the desired performances various additional heating systems are foreseen to heat the plasma core up to 10-15 keV. One of the most important heating systems will be composed by 2 neutral beam injectors (NBIs), which produce 2x16.7 MW beams, composed by H/D atoms. To obtain the beams H-/D- ions produced by an ion source will be accelerated at high energy (870 keV for hydrogen, 1 MeV for deuterium) and then neutralized thanks to the charge exchange interaction with the molecules of a H2/D2 gas. The construction of such a system is very challenging and 2 prototypes will be built and tested in Padua at Consorzio RFX. The first one, SPIDER (Source for the Production of Ions o Deuterium Extracted from an Rf plasma), operative in 2016, aims at studying the source part of the NBI, accelerating ions at lower energies, up to 100 keV. The prototype of the full ITER negative NBI system is MITICA (Megavolt ITer Injector Concept and Advancement), it will start operating in 2019. Both SPIDER and MITICA will be equipped with a large number of diagnostics: in particular, the Beam Emission Spectroscopy (BES) will measure the spectrum of the radiation produced by the beam when interacting with the background gas in order to measure the divergence and the uniformity of the beam produced by the ion source. The divergence is a key parameter for the safety of the test facilities, since it is related to the power deposition of the beam on the beam line component. The beam uniformity is crucial for the heating effectiveness of the beam itself. The PhD activity was mostly devoted to develop the BES diagnostics of SPIDER and MITICA, with the target to measure divergence in the range 3- 7 mrad (e-folding) with a relative error below 10%, and values of uniformity in the range 90-100%. In particular, the work consisted in: • developing a set of codes (dBES) for the simulation of the behavior of the BES diagnostics; • studying and improving the methods of analysis for the data that will be collected by BES. • completing the design of the BES diagnostics, also selecting and testing its hardware components; Experimental data of the test facilities BATMAN (BAvarian Test MAchine for Negative ions), MANITU (Multi Ampere Negative Ion Test Unit) and ELISE (Extraction from a Large Ion Source Experiment) at the Max Planck Institut für Plasmaphysik (IPP) in Garching, have been used to validate the numerical model at the base of dBES and to improve the method of analysis of the BES spectra. During my stays at the ITED group of IPP Garching the results of dBES were also benchmarked against those of the BBC-NI code, developed at IPP for similar purposes. Besides SPIDER and MITICA the studies were also addressed to NIO1 (Negative Ion Optimization 1), hosting an RF ion source which produces a 130 mA beam composed by H- ions. The test facility was jointly built by Consorzio RFX and INFN-LNL, with the aim of studying and improving the production of negative ions, as well as testing concepts and instrumentation for SPIDER and MITICA. For NIO1 the BES diagnostic was entirely designed; the tests on NIO1 of this diagnostic will allow a first experimental validation of the design of those for SPIDER and MITICA. The PhD research is described in this thesis as follows: • Chapter [chap:Introduction] gives a briefly describes the ITER reactor. The principle of operation of the neutral beam injectors is then explained, together with the characteristics of the considered experiments: SPIDER and MITICA, BATMAN, MANITU, ELISE and NIO1. The main diagnostics installed in the negative ion sources and more generally in the NBIs are presented, too. • Chapter [chap:Beam-Emission-Spectroscopy] describes the work done on the simulation codes dedicated to Beam Emission Spectroscopy. The physical phenomena at the base of the BES diagnostic operation are explained. The code dBES, developed for the simulation of the BES diagnostic and for the analysis of its measurements, is described. dBES is then benchmarked against the BBC-NI code and with the experimental data of the BES diagnostics in the BATMAN, MANITU and ELISE test facilities. • Chapter [chap:Designtotal] presents the design of the BES diagnostics in the experiments NIO1, SPIDER and MITICA, carried out by means of the dBES code. • Chapter [chap:Experimental-work] presents the experimental activities carried out to test the available instrumentation for the BES and the optical emission spectroscopy diagnostics in NIO1, SPIDER and MITICA; the design activity of some components is also illustrated. • The summary is reported in chapter [chap:Conclusions]. There follows the list of the articles published and of the technical notes written during the PhD.