A TALIF Diagnostic for the DIII-D Tokamak

The density profile of hydrogenic neutrals in the edge of DIII-D plays an important role in the problems of momentum transport, pedestal formation, and plasma-wall interaction, but an accurate measurement has proven difficult. A two-photon absorption laser induced fluorescence (TALIF) diagnostic is under construction and is intended to provide temporally and spatially resolved neutral density measurements in the pedestal region. This three-level TALIF scheme offers the advantages of direct excitation of ground state atoms, emission in the visible portion of the spectrum, a high degree of spatial localization, and the potential for a Doppler-free measurement. The large background of D$_\alpha$ emission, the principal challenge of the measurement, can be overcome by the focusing of a high power (1~MW) UV laser. Calculations of the SNR show that densities of 10$^{15}$~m$^{-3}$ or lower can be measured with a spatial resolution of 0.3~mm. We present design details of the proposed laser system, calculations of the expected performance in DIII-D and in a helicon source plasma, and measurements of the HI profile in the helicon plasma.