ITER is the first reactor-scale scientific experiment that aims to demonstrate the scientific and the technological feasibility of fusion energy. It is based on the tokamak concept of magnetic confinement, in which the fuel, a mixture of deuterium and tritium heated to temperatures in excess of 150 million degrees Celsius, is contained in a toroidal vacuum chamber. Among the systems used to reach such high temperature range, a fundamental role is played by the injection of intense beams of neutral particles into the plasma, which is consequently heated by collisions. This process is realized by means of two Neutral Beam Injectors (NBIs), capable of delivering to the plasma a power of 16.7 MW each. These devices are mainly composed of a negative deuterium ion source, an electrostatic accelerator where a 40 A beam of negative deuterons will be accelerated to 1 MV and a neutralizer which converts part of the beam into high energy neutrals able to penetrate the high magnetic field confining the ITER plasma. The ITER requirements for these devices have never been simultaneously achieved so far in a full scale, full performance device and therefore a neutral beam test facility is being constructed at Consorzio RFX in Padova.
The research activity presented in this thesis work is in the framework of the development of the negative ion source (SPIDER) and full injector (MITICA) prototypes for the ITER neutral beam. In particular, it is focused on two main topics: particle transport studies inside the MITICA accelerator and the development of a tomographic beam diagnostic.
A proper modeling of the particle transport inside the MITICA accelerator, considering the main processes that generate secondary particles relevant for the evaluation of the heat loads on the accelerator grids is essential for the thermo-mechanical analysis and the mechanical design of the accelerator. For this reason an upgrade of the relativistic particle tracking code called EAMCC has been undertaken and the simulations performed for evaluating the thermal power deposited on the MITICA accelerator grids are presented in the first part of the present thesis work. For the first time, an entire source called NIO1 installed at RFX and made of nine beamlets has been simulated in EAMCC considering multi-beamlet effects which were neglected earlier and discarding the axis-symmetry hypothesis of the electric fields imposed by the original version of the code. Results obtained, also presented in the first part, will be used for benchmarking the modifications introduced in the code.
The second part of the thesis is dedicated to beam tomography, an important diagnostic for the assessment of the density profile of the beam. A tomography code based on algebraic reconstruction techniques has been developed and numerically tested. Beam emissivity profiles considered for testing the code are calculated by the upgraded version of EAMCC. The tomography code has been developed with the aim of realizing a versatile instrument, applicable to linear accelerators as well as to a tokamak and without adding any hypotheses about the beam characteristics or the emissivity in a particular region of the tomography plane, not to limit the capability of the code of detecting irregularities in the beam profiles. The effects of the instrumental noise on tomography reconstructions have also been studied and, in order to reduce its impact, different filtering techniques have been considered both in the frequency and in the spatial domain, demonstrating the feasibility to filter out the effect of the noise by post-processing the reconstructed image of the bea
