Laser Absorption Spectroscopy Studies on the Caesium Ovens Test Stand

ITER, which is under construction in France, will be the largest and most ambitious experiment on thermonuclear controlled fusion. To reach the desired performances, additional heating systems will be needed: one of the most important will be the neutral beam injection (NBI). In particular, the technique consist in a negative ions source, followed by an acceleration system and a neutraliser, to produce a high-energy neutral beam; the beam collides with the ITER plasma particles, heating them. In the Neutral Beam Test Facility (NBTF) at Consorzio RFX in Padova SPIDER, a prototype of ITER NBIs, is under development. One of the key points of the system is the production of negative ions: to enhance the amount of ions produced inside the source, the inner walls will be covered with caesium. For this purpose, caesium will be vaporised inside the source. The ovens for caesium vaporisation and the diagnostics for caesium density monitoring are nowadays subjects of study. In particular, a Laser Absorption Spectroscopy diagnostic (LAS) will be installed: it will permit, using the D2 caesium optical absorption line, the estimation of the average Cs density along different lines of sight. This is fundamental to control the distribution of the caesium vapours inside the source: peaks or lack of caesium can result in high efficiency drops, non uniformity in beam intensity and beam divergence, and operational problems, that can cause lack of supply in ITER. To test the SPIDER caesium ovens, a test stand has been built: the CAesium ovens Test Stand, CATS. In the same test stand, a Laser Absorption Spectroscopy diagnostic is tested: during the work reported in this thesis, the optical system has been set up and calibrated, the absorption phenomena has been studied and the analysis algorithm has been improved. A particular attention has been payed to the source of systematic errors, such as saturation and depopulation: their entity has been detected, an experimental guideline to minimise them has been produced and an analysis method to correct them a posteriori has been partially successfully tested. As final result, LAS has been used to characterise the first two SPIDER ovens: in particular, the caesium density around the oven nozzle has been proved to be dependent on the oven reservoir temperature, and measurements from the two tested ovens has been found compatible within about 25%.