Two-Photon Absorption Laser Induced Fluorescence Measurements of Neutral Density in Helicon Plasma

Neutral particles play a critical role in plasma experiments. They simultaneously act as a source of particles and a sink of momentum and energy. Also, they act as an intermediary between the plasma and surrounding material walls. However, few methods exist to make localized, direct neutral density measurements. A new diagnostic, based on two-photon absorption laser induced fluorescence (TALIF), capable of making direct ground state measurement is developed. A high intensity (5 MW/cm2), narrow bandwidth (0.1 cm -1) laser is used to directly probe the ground state of neutral hydrogen, deuterium and krypton. The diagnostic represents an improvement over traditional laser induced fluorescence in that it can be used to measure the spatial profile from a single porthole, and it can be operated in a Doppler-free configuration. The system is tested and calibrated in a low temperature helicon plasma source. Calculations show the system has sufficient performance to achieve an acceptable signal-to-noise ratio in a high temperature fusion plasma. Development of the diagnostic is presented in this work along with both steady-state and time-resolved TALIF measurements in helicon plasma