Doppler-free two-photon excitation of Lyman-α fluorescence for the diagnostics of magnetically confined fusion plasmas

Abstract

Doppler-free two-photon excitation of hydrogen Lyman-α fluorescence is investigated as a possible laser-induced fluorescence (LIF) technique for the diagnosis of magnetically confined fusion (MCF) plasmas. A formal analysis is presented of the underlying atomic and plasma physics as well as of various practical aspects, such as parameter optimization and experimental precision. The latter is analyzed with regard to the photon noise and to the sensitivity of the fluorescence signals to the plasma and laser parameters. The diagnostic potential of the LIF technique described relies on its high spectral resolution. Thus, the absorption lines of the hydrogen isotopes H, D, and T are clearly separated from each other and can serve for isotope-selective density measurements. In addition, using a tunable laser system with small bandwidth, various plasma parameters can be inferred from the spectral line shapes, such as the neutrals’ temperatures or the effective charge number Zeff. The polarization of the fluorescence can, under favorable circumstances, be exploited for magnetic field measurements. The photon statistics impose neutral densities above 1014 m−3 and thus make the diagnostic suitable primarily for the plasma edge. However, previous work has shown that it is applicable even in the plasma bulk of large machines if a neutral beam is used that generates sufficient neutral densities by charge exchange with the plasma ions. Variations or insufficient knowledge of the neutrals’ temperatures are found to seriously affect the precision of absolute isotope density measurements. They are rather uncritical, however, for the determination of the H/D/T density ratios, which are of prime importance for the burn control of large MCF devices. A notable sensitivity is also found to variations of the laser frequency detuning, which should be known and stable to better than 100 MHz. The perspectives demonstrated in the present study and the success of a first experiment on the plasma generator PSI 1 at IPP Berlin are considered to be sufficient motivation for testing the diagnostic on a tokamak or other MCF device.