Two-photon Laser Induced Fluorescence Measurements of Neutral Density in the Prototype Material Plasma Exposure eXperiment*

The development of remotely situated diagnostics for fusion plasmas is necessary since the radiation and electromagnetic-interference (EMI) environment of such devices is becoming ever more intense. Two-photon absorption laser induced fluorescence (TALIF) measurements of neutral velocity distribution functions (NVDF) provide non-perturbative measurements of neutral deuterium temperature, absolute density, and bulk flow in fusion relevant plasmas. Recently, a TALIF system was installed at ORNL on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX). A maximum of 4 mJ of 205 nm light in 8 ns pulses is transmitted over 20 m to measure deuterium NVDFs. Signal is collected confocally and is fiber-coupled back to the laser room. The confocal design requires only one point of optical access-making this design suitable for fusion devices. However, 205 nm light is difficult to transmit through common vacuum components and fiber optics. To explore more compatible options for transmission, a three-photon absorption laser induced fluorescence (3pLIF) scheme is attempted in krypton as a proof-of-principle measurement to increase the injected wavelength. We will present preliminary TALIF deuterium NVDFs and 3pLIF krypton NVDFs.