PHAse Space MApping (PHASMA) Experiment – Upgrade Status and Frequency-Modulated Laser Induced Fluorescence*

The PHAse Space MApping (PHASMA) experiment employs non-perturbative, optical diagnostics for ion velocity distribution, electron velocity distribution, magnetic field, density, and turbulence measurements. A facility upgrade in 2024 has increased access for laser-based particle velocity distribution function measurements, tomography, and probe arrays. The plasma guns have upgraded to employ 3D printed gas feeds to reduce collisionality and increase the magnetic Reynolds number of the merging flux rope plasma. A new optical method for measurement of the reconnecting magnetic field (quantum beat spectroscopy) is implemented in the upgrade. We have also demonstrated a new method for laser induced fluorescence (LIF) with a continuous laser. The small signal-to-noise (SNR) of conventional LIF measurements is typically overcome by amplitude modulating the laser at a specific frequency and then using lock-in amplification to extract the signal from measurement noise. Here we present LIF measurements of argon ions using frequency modulated laser injection. A pulse train of 100% amplitude modulation is generated synthetically with a random sequence of pulse lengths. The frequency modulated signal drives an acoustic optic modulator placed in the path of the injection beam. The signal from a fast photomultiplier tube is then digitized and cross correlated with the known pulse width modulated driver signal. The SNR of the resultant LIF signal is compared to the SNR of another LIF measurement in the same system obtained with conventional signal frequency modulation and a lock-in amplifier. The improvements in SNR are investigated as a function of the length of the pulse width modulated driver signal and the frequency range of the pulse with modulations.