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Description
During boronisation glow discharges, the spectral radiance on several lines-of-sight (LOS) through the vessel was measured for the complete visible spectrum in the range from 350-730 nm. The spectroscopic setup was absolutely calibrated by illuminating the respective optical heads inside the vessel with an integrating sphere of known spectral radiance. The glow discharges were run at the standard parameters, i.e. pressure of 0.5 Pa, voltage of 500 V, and current of 1.8 A per anode. Discharges with 4 and 2 anodes were compared. The gas was the standard mixture of 90% He with 10% deuterated diborane.
No signal is seen on the LOS deep in the slits between roof baffle and lower divertor tiles. On the other LOS through the main chamber and in the upper divertor region, the spectrum is dominated by the emission lines from neutral He in the singlet and triplet system with one electron going from n=3-7 to n=2. The strongest line is the 3p-2s transition in the singlet system. The n=4-3 transition of ionised He is weak. The lines of the Balmer series are well visible up to n=7-2. The BD molecule is seen on the A-X transition with the vibrational bands 0-0 and 1-1. A wide region with Fulcher band emission of the deuterium molecule is visible where the diagonal and the non-diagonal vibrational bands are emitting.
We mainly analysed the radiances of the 11 HeI-lines emitted on the transitions n=3-2 and n=4-2 with the help of a collisional radiative model which was adopted from the evaluation of the He-beam diagnostics. Even though the unknown electron energy distribution had to be approximated by a thermal distribution with an effective temperature around 30 eV, the measured radiances can be well fitted when taking the strong optical thickness on the resonance lines into account. Furthermore, we considered that the dwell time of the He atoms before colliding with the vessel walls is much shorter than the equilibration time of the exited states with the ground state. The fitted density of these fast 30eV-electrons is around 10$^{12}$m$^{-3}$ and increases by a factor of 2 when using 4 instead of 2 anodes.