Beam emission spectroscopy studies in a H-/D- beam injector

Abstract

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