Open Access

Benjamin Ruf

• For the experimental fusion reactor ITER, which should show the feasibility of sustaining a fusion plasma with a positive power balance, some technology still has to be developed, amongst others also the plasma heating system. One heating technique is the neutral beam injection (NBI). A beam of fast deuterium atoms is injected into the fusion plasma. By heavy particle collisions the beam particles give their energy to the plasma. A NBI system consists of three major components. First, deuterium ions are generated in a low temperature, low pressure plasma of an ion source. At ITER, the requirements on the beam energy of 1 MeV cause the necessity of negative charged deuterium ions. Secondly, the ions are accelerated within an acceleration system with several grids, where the plasma grid is the first grid. The grids are on different descending high voltage potentials. The source itself is on the highest negative potential. Thirdly, the fast deuterium ions have to be neutralised. ThisFor the experimental fusion reactor ITER, which should show the feasibility of sustaining a fusion plasma with a positive power balance, some technology still has to be developed, amongst others also the plasma heating system. One heating technique is the neutral beam injection (NBI). A beam of fast deuterium atoms is injected into the fusion plasma. By heavy particle collisions the beam particles give their energy to the plasma. A NBI system consists of three major components. First, deuterium ions are generated in a low temperature, low pressure plasma of an ion source. At ITER, the requirements on the beam energy of 1 MeV cause the necessity of negative charged deuterium ions. Secondly, the ions are accelerated within an acceleration system with several grids, where the plasma grid is the first grid. The grids are on different descending high voltage potentials. The source itself is on the highest negative potential. Thirdly, the fast deuterium ions have to be neutralised. This thesis deals with the second step in the mentioned beam system, the ion acceleration and beam formation. The underlying experiments and measurements were carried out at the testbeds BATMAN (BAvarianTest MAchine for Negative ions) and ELISE (Extraction from a Large Ion Source Experiment) at the Max-Planck-Institut für Plasmaphysik Garching (IPP Garching). The main goal of this thesis is to provide a tool which allows the determination of the beam properties. These are beam divergence, stripping losses and beam inhomogeneity. For this purpose a particle trajectory code has been developed from scratch, namely BBC-NI (Bavarian Beam Code for NegativemIons). The code is able to simulate the whole beam and the outcome of several beam diagnostic tools. The data obtained from the code together with the measurements of the beam diagnostic tools should allow the reconstruction of the beam properties. The major beam diagnostic tool, which is used in this thesis, is the beam emission spectroscopy (BES). BES measures the beam divergence and beam power losses from heavy particles collisions by evaluating the spectrum of the Balmer H_alpha light of the beam. The light is emitted since beam particles are excited by collisions with the hydrogen background gas. For ITER, BES will be the main beam diagnostic tool for beam quality measurements. The main results are, that first of all, the evaluation of the beam divergence from a BES spectrum was improved with the parametrisation method. Furthermore it turned out that the evaluation of stripping losses and beam inhomogeneity in large negative hydrogen ion sources cannot be performed by backward calculations from a BES spectra, i.e. by the analysis of the spectra. This means forward modeling has to be done, which does also include the simulation of other beam diagnostic tools, like the power density profile measured by the calorimeter. Combining all beam diagnostic tools and reconstructing their outcome with a BBC-NI Advanced simulation, gives the possibility to determine the beam parameters by extracting them from the BBC-NI code protocols. This requires a lot of effort and is not well suited for a routine analysis. For ITER this means that solely a BES system for the determination of the beam parameters (i.e. stripping losses and beam inhomogeneity), as it is presently foreseen, is not sufficient. Several beam diagnostic tools, e.g. the calorimeter which can determine the power density profile of the beam, and a code like BBC-NI are necessary. Additionally for the application of BBC-NI Advanced, a beam optic code is needed, which is able to generate a realistic electric field map in the extraction system. Such an optic code is not available so far.…