Neutral Krypton Three-photon Laser Induced Fluorescence and 3+1 Photoionization Spectroscopy Measurements on Allowed and Forbidden Transitions*

Remotely situated diagnostics are desirable for fusion devices since electromagnetic interference (EMI) and radiation are becoming more of an issue as the field moves into an era of burning plasmas. Specifically, diagnostics capable of measuring absolute neutral densities are critical to controlling fueling rates and maintaining transport barriers in the plasma edge. Two-photon absorption laser induced fluorescence (TALIF) non-perturbatively measures spatially resolved neutral velocity distribution functions (NVDF) to determine absolute, ground state neutral densities of hydrogenic species if the measurements are calibrated with a noble gas, commonly krypton or xenon. However, TALIF injects deep ultraviolet light (<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>∼205  nm) that is easily absorbed in air and difficult to couple into vacuum chambers. These limitations restrict the location of the TALIF systems to regions of potentially high EMI and eliminate use of fibers and common optical materials. A three-photon laser induced fluorescence (3pLIF) technique has the potential to provide similar measurements, while injecting a more near-visible wavelength, <!--[if gte msEquation 12]>300-308  nm, alleviating the requirements for special optics, allowing the use of high-power fibers, and allowing the laser system to be situated further from EMI. In this work, a Quantel Qscan pulsed dye laser excites ground state krypton through three-photon excitation or photoionizes krypton through a 3+1 photon process. Light is generated over 6<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>∼6 ns pulses at 10 Hz. Fluorescence is either fiber coupled, or free space coupled to detecting electronics. Here, ground state neutral krypton spectral and photoionization current lineshapes are presented for allowed and forbidden transitions.