Ammonia formation and decomposition in the plasma of GyM linear device

Current studies on ammonia formation during nitrogen seeding experiments in tokamak divertors meet the need to find out the percentage of nitrogen converted in ammonia and study the reaction mechanism [1]. Useful information on the latter can be obtained also from the study of the inverse reaction through ammonia decomposition experiments. In this context ammonia formation and dissociation have been studied in GyM linear device and the catalytic effect of the wall on these processes has been investigated. Exploiting the versatility of GyM machine, made up of a cylindrical stainless steel (SS) vacuum vessel in which an additional tungsten (W) liner can be inserted, a comparison between the two frameworks with SS and W walls has been made. In ammonia formation experiments a mixture of nitrogen and deuterium has been injected with different gas concentrations. The amount of ammonia produced, quantified by chromatography analysis of the exhaust, was maximized up to 20% of nitrogen conversion by properly adjusting the N2:D2 ratio, the operating Te and ne, the type of wall material and the residence time of gases. The analysis of the intermediate species in plasma phase performed through the use of optical emission spectroscopy (OES) shows the progressive formation of ND species as the machine power increases and its correlation with the detected ammonia molecules by mass spectrometer (MS). In decomposition experiments ammonia was injected in mixture with helium (0,5% mol). Data from the analysis of the chemical composition of the exhaust collected during the experiments show that, under plasma conditions of Te = 7eV and ne = 6E+16 m-3, more than 30% of the ammonia fed into the reactor has been dissociated. The absence in OES spectra of spectral features due to nitrogen species suggests that no N2 is formed through ammonia decomposition. OES and MS don’t provide any direct experimental evidence about the involvement of the material surface in the reaction mechanisms. However, some inferences are reasonable based on the known reactivity of the different species involved in the process. In experiments of ammonia decomposition it is most likely that under our experimental conditions the initial rupture of the ammonia molecules mainly occurs by electron impact and, possibly, through dissociative surface adsorption. In experiments of ammonia formation excited species, such as fragments of ammonia (ND), detected by OES, might be adsorbed and react on the surface with surface-adsorbed atomic deuterium species and form ammonia molecules by the well-known Eley-Rideal mechanism [2]. These considerations about the involvement of the chemical reactions on the wall of the plasma device, complemented with the OES and MS data, indicate, under the considered GyM conditions, that ND radicals play a leading role in the synthesis of ammonia while electron impact dissociation of NH3 molecules has got a key role in ammonia dissociation processes.