A setup for femtosecond two-photon absorption laser-induced fluorescence in fusion-relevant hydrogen plasmas.

Two-photon absorption laser-induced fluorescence (TALIF) is a well-established diagnostic technique for measuring the density of neutral atoms in plasmas. We demonstrate a new setup for a femtosecond-TALIF (fs-TALIF) diagnostic in fusion-relevant hydrogen plasmas in the Resonant Antenna Ion Device (RAID). The system is designed to produce high-energy laser pulses with a broad spectral width, suitable for exciting both atomic hydrogen and krypton in known TALIF schemes without the need to retune the laser wavelength. The setup consists of a commercial fs laser, an in-house-developed optical harmonic generator, and the fluorescence detection system on the plasma device. The system produces laser pulses at the atomic hydrogen two-photon absorption line at 205 nm, with pulse energies up to 145 μJ, spectral widths of 1.2 nm (1/e2 width), and pulse durations of 250 fs (FWHM). The pulse parameters are stable on a pulse-to-pulse basis and over longer time periods. We characterize the effect of the transport of the pulses to the plasma device through 19 m of atmospheric air and a thin MgF2 window. We show a one-dimensional fs-TALIF profile in a krypton gas in RAID, in which the laser is collimated (3 mm diameter) and tuned to the hydrogen resonance. This result shows the potential of the system to be used as a fs-TALIF diagnostic for calibrated atomic hydrogen density measurements.