Abstract
Heating and current drive in the next generation tokamak ITER requires the use of large and powerful neutral beams, generated by a precursor ion beam from an ion source around 1 m × 2 m in cross-section. To avoid energy losses and component damage, strict requirements are placed on the divergence and uniformity of this ion beam, which is comprised of many individual beamlets. Understanding, controlling, and predicting the behaviour of these large ion beams requires knowledge of these individual beamlets and their interactions with one another. This is hindered by available experimental diagnostics on these large beams typically only having access to volume averaged information. A forward simulation of beam diagnostics would allow the connection of experimental results with otherwise unobtainable individual beamlet properties. The particle tracking and ray tracing code Bavarian Beam Code for Negative Ions was developed for this reason, and takes into account the interaction of individual component beamlets with whole-beam diagnostics to produce synthetic data that can be compared with experimental results. In this work a significantly reworked and upgraded version of the code is presented and example results are given and analysed for the ITER relevant test facility BATMAN Upgrade. It is shown how the simulation can recreate experimental results, and that one must consider the whole beam in order to do so. The impact of beamlet mixing on beam emission spectroscopy results is shown, as is the importance of long range magnetic fields on the beam transport. The capabilities and limitations of the code are discussed with a view toward application to ITER size ion sources.
Highlights
The operation of large tokamaks, such as the ITER facility currently under construction, calls for correspondingly large heating and current drive capabilities
The particle tracking and ray tracing code Bavarian Beam Code for Negative Ions (BBCNI) was developed for this reason, and takes into account the interaction of individual component beamlets with whole-beam diagnostics to produce synthetic data that can be compared with experimental results
The accurate recreation of diagnostic data relies not just on what happens during beamlet formation, and on the transport of the beamlets to the location of the diagnostics, and the consequent beamlet merging that occurs. The consideration of these processes is shown by BBCNI to give good recreations of experimental diagnostic data, which can be used to further understand the generation and transport of ITER-relevant large negative ion beams
Summary
The operation of large tokamaks, such as the ITER facility currently under construction, calls for correspondingly large heating and current drive capabilities. The plasma inhomogeneities mentioned above have a second order effect of causing nonuniform distribution of caesium, which affects the surface production of H− or D− from the atomic species, and subsequently the extracted current and divergence of each aperture Despite these issues, low divergences have been measured at a number of facilities. In order to accurately interpret the experimental data available for large, high power negative ion beams, and to link it with the formation of beamlets, a simulation is required that can forward calculate whole-beam diagnostic results from the generation of individual beamlets. The improved understanding of the beam divergence, uniformity, and transport will allow for the suggestion of specific improvements to the ion sources, and help to bring large negative ion sources towards their targets for ITER and future projects
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