Observations and modelling of diatomic molecular spectra from JET

Spectra in the visible range may be obtained along multiple lines of sight directed at the axisymmetric poloidal divertor of the JET tokamak. In X-point magnetic field configurations, the scrape-off-layer plasma contacts the divertor target plates. We have conducted observations and analysis of diatomic spectra in the divertor plasma along lines of sight in the strike zone vicinity. We have identified unambiguously CD, C2 and BeD. Synthetic spectra have been calculated. In addition to identification of the molecular features and separation of overlapping atomic lines, the comparison of simulations and observations allow quite sensitive determination of rotational and vibrational temperatures for the above diatomics in a divertor fusion plasma. Systematic studies show that Tr and Tv are comparable for BeD but Tv is significantly less than Tr for CD and C2. The BeD spectrum time history during a pulse (modulated by sweeping) appears as localized emission from the strike points similar to that of the atomic BeI visible spectrum lines. On the other hand, the CD bands during a swept pulse indicate a more distributed source. Thus both the BeD spectrum and time history suggest physical sputtering of BeD at the strike points. In contrast, the CD spectrum and time history indicate chemical release of higher deuterides followed by catabolism to CD. This work constitutes the first detailed study of diatomic species in the JET tokamak. (author)