Speaker
Description
Spectroscopy in the vacuum-ultraviolet (VUV) range, i.e. below 200$\,$nm, gives access to the resonant transitions of atomic (Lyman series) and molecular hydrogen (Lyman and Werner bands). In order to get insight into the relevance of molecules and their recombination in edge plasmas, intensity calibrated emission spectroscopy is a valuable diagnostic. In the VUV spectral range, however, obtaining such spectra is a complex task, since radiation standards for intensity calibration are not readily available and the spectroscopic system needs to be directly attached to the vacuum system at hand. This contribution shows the application of two flexible and “easy-to-apply” VUV spectroscopic systems to the Upgraded Pilot-PSI (UPP) device at DIFFER.
A VUV mini-spectrometer (Resonance© VS7550) gives access to the spectral range between 117 and 420$\,$nm with a resolution (FWHM) of about 0.2$\,$nm. In addition, a self-developed VUV diode system, comprising a VUV-sensitive photo diode and interference and edge filters for spectral resolution [1], is applied to obtain access to the spectral range below 113$\,$nm. Both systems are intensity-calibrated against a 1m-VUV-spectrometer at a laboratory inductively-coupled plasma (ICP) experiment, that is in-turn calibrated within a spectral range of 46$-$300$\,$nm [2]. In combination, absolute emissivities can be obtained for the H$_2$/D$_2$ Werner and Lyman bands, the H$_2$/D$_2$ dissociation continuum as well as the H/D Lyman and Balmer series (from H$_\delta$/D$_\delta$).
The systems have been applied simultaneously to the UPP device, observing the plasma close to a tungsten dummy surface. The VUV detection cones covered the entire plasma column diameter, while Thomson scattering provides $n_\mathrm{e}$ and $T_\mathrm{e}$ profiles. Varying the source power, the gas flow rate and the confining magnetic field strength, the transition from attached to detached plasmas was observed under several conditions. The Lyman series dominates the spectrum, while molecular transitions are detected with an emissivity of about one order of magnitude less. The transition from attached to detached regime (increasing $n_\mathrm{e}$, decreasing $T_\mathrm{e}$) is clearly visible in the spectra: molecular radiation is strongly reduced, while atomic radiation via the Balmer and Lyman series increases, including emergence of the Balmer continuum arising from high quantum states of H and D close to the ionization edge. The spectra hence indicate a strong influence of recombinative processes, i.e. radiative & 3-body recombination from H$^+$/D$^+$ and dissociative recombination from molecular ions (H$_2^+$/D$_2^+$).
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[1] R.Friedl, C.Fröhler-Bachus, U.Fantz, Meas.Sci.Technol. 34 (2023) 055501.
[2] C.Fröhler-Bachus, R.Friedl, S.Briefi, U.Fantz, JQSRT 259 (2021) 107427.