Edge nT/nD by Two-Photon Induced Ly-α Fluorescence
Recent theoretical and experimental investigations had promising results concerning the potential of Doppler-free two-photon induced Lyman-α fluorescence (2γLy-α) as a means for selective direct measurements of atomic densities of hydrogen isotopes in magnetically confined fusion plasmas. Even under tokamak conditions, all the two-photon absorption profiles of H, D, and T exhibit sharp, nearly unshifted central components with typical widths fitting well with the bandwidth of pulsed single-mode laser spectrometers which will be able to deliver the required power and quality needed for tokamak diagnostics. A first 2γ Ly-α fluorescence signal was observed at the plasma generator PSI-1 of IPP Berlin with high spatial resolution (ø 3mm, 8 mm length) in a pure deuterium plasma beam, ø 100 mm, ne ≈ 5·1012 cm-3, Te ≈ 5 eV. We are therefore convinced, that measurements of the nT/nD ratio from 0.1 to 10 will be possible in the edge plasma of ITER, with single shot accuracy better than 20% and with high spatial and temporal resolution at a laser repetition rate of 50 Hz. The detection limit for each isotope is estimated to be 109 cm-3.
KeywordsHydrogen Isotope Laser Repetition Rate ASDEX Upgrade Relativistic Doppler Effect Motional Stark Effect
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