Preliminary Results from a Three Photon Laser Induced Fluorescence Diagnostic in a Cold Krypton Gas

Remotely situated, portable diagnostics are desired for fusion devices since electromagnetic interference (EMI) and radiation are becoming ever more intense. Specifically, diagnostics capable of measuring absolute neutral densities are critical to understanding fusion processes. Two-photon absorption laser induced fluorescence (TALIF) is one such technique that non-perturbatively measures spatially resolved neutral velocity distribution functions (NVDF) to determine absolute, ground state neutral density if the measurements are calibrated with a noble gas, commonly krypton or xenon. However, TALIF injects deep ultraviolet light (~205 nm) that is easily absorbed in air, restricting the location of the diagnostic system to regions of potentially high EMI and eliminates use of fibers and common optic materials. A three-photon laser induced fluorescence (3pLIF) technique probes the same states and species as interrogated using TALIF, while injecting a more near-visible wavelength, 300-308 nm, alleviating the restrictions for special optics, allowing the use of high-power fibers, and allows the laser system to be situated further from the intense EMI environment. In this work, a Quantel Qscan pulsed dye laser produces either ~300 nm (3pLIF) light or ~204 nm (TALIF) light over ~7 ns at a repetition of 10 Hz. Fluorescence is fiber coupled to detecting electronics. Here preliminary krypton NVDFs measured using 3pLIF are presented and compared to krypton NVDFs measured using TALIF. Integrated signals are measured across a variety of laser pulse energies to determine regions of laser saturation for each technique. A relative krypton multiphoton cross section between TALIF and 3pLIF is determined.