Neutral Atomic-Hydrogen Measurements in a Mirror/FRC Plasma Device using fs-TALIF

We report on temporally- and spatially-resolved neutral atomic H density measurements carried out in a mirror/field-reversed configuration plasma device. The facility uses multiple RF-heating techniques with powers exceeding 100 kW to create 5-100ms duration quasi- and fully-steady-state magnetized (50-500G and mirror ratio \textasciitilde 10) plasmas from various gases, achieving densities approaching 10$^{\mathrm{14}}$ cm$^{\mathrm{--3}}$ and electron temperatures in excess of 100 eV. The density of neutral H atoms 50cm off-midplane is imaged and time-resolved via fs two-photon absorption laser-induced fluorescence using a fs laser system at 205nm with 200$\mu $J/pulse at 1kHz rep rate. A fast CMOS camera with a 5ns gated image intensifier records the H-$\alpha $ fluorescence at 656nm for every laser shot. By varying the delay between the RF trigger and laser pulses, the temporal dynamics of the H atoms is measured across multiple discharges to better than 15$\mu $s resolution. For high-power RF, the measured H atom density rises to 10$^{\mathrm{12}}$ cm$^{\mathrm{-3}}$ in 10's of $\mu $s, resolving axial and temporal dynamics of the neutrals near the device's central region. The steady-state ``seed'' plasma generated using low-power (10-500W) RF exhibits unexpectedly slow production and depletion of neutrals. Comparing the dynamics of the H neutrals and plasma-excited H atoms yields a measured lifetime of 160$\mu $s for the neutrals. Calibration is performed using 1mTorr of Kr.