Analytical and numerical models and first operations on the negative ion source NIO1

The development and optimization of neutral beam injectors (NBI) as heating systems of fusion plasmas are crucial in the perspective of the experimental reactor ITER under realization in Cadarache (France). In this framework the recently installed negative ion source NIO1, resulting from a collaboration between Consorzio RFX and INFN-LNL (Legnaro, Italy), can provide a test bench for source and beam optics optimization thanks to its high modularity. In this thesis fast tools, able to model the main physical phenomena taking place inside the NIO1 source and accelerating column, have been developed in order to qualitatively describe the response of the whole system to variations in the external operation parameters; comparison of the results was possible with the experimental findings obtained during the first ever NIO1 operation, showing a good agreement between the model and the experiment. In detail in this thesis, after an introduction on the basic concepts of thermonuclear fusion and neutral beam injectors, a detailed description of the negative ion source NIO1 is given, together with the commissioning tests on the source itself and some of the first experimental results. An analytic model for the RF coupling is proposed, which gives an estimate of the power coupling efficiency as a function of the plasma temperature and density and allows dimensioning the matching circuit used to couple RF power supply to the plasma. Plasma heating is due to local and non local effects, which have been introduced in the model by the use of an effective collision frequency, defined as the sum of the electron collision frequencies with ions and neutrals and a stochastic collision frequency. A model for the profile of plasma parameters, which considers the electron diffusion and energy equation and a multipole magnetic confinement, has been implemented numerically for hydrogen, nitrogen and oxygen gases: the results well agree with other numerical simulations and experimental measurements. By adopting a Monte Carlo approach, a sample of maxwellian electrons has then been evolved through the magnetic filter field in NIO1 in order to calculate the electron temperature and density as a function of the distance from the plasma grid: the simulation confirms that there is an effective electron cooling, that could be further improved by increasing the magnetic field strength. As a last step in the experiment modelization, a hydrogen and an oxygen beam have been simulated in the case of a low current density, typical of the first operations of NIO1 without caesium. The optimal electrode potentials have been studied and the results show the necessity of an appropriate scaling in the strength of the permanent magnets when operating in hydrogen.