Development of a novel time-resolved laser-induced fluorescence technique

Abstract

Summary form only given. We are developing a time-resolved laser-induced fluorescence (LIF) technique to interrogate the ion velocity distribution function (VDF) in Hall thruster plumes down to the microsecond time scale. Better measurements of dynamic plasma processes such as breathing and spoke modes will lead to improvements in simulation and prediction of thruster operation and erosion. We present the development of the new technique, results of validation tests using a hollow cathode, and initial tests interrogating a Hall thruster. Signal-to-noise ratio (SNR) is often a challenge for LIF studies, and it is only more challenging for time-resolved measurements since a lock-in amplifier cannot be used with a long time constant. The new system uses laser modulation on the order of MHz, which enables the use of electronic filtering and phase-sensitive detection to improve SNR while preserving time-resolved information. Statistical averaging over many cycles to further improve SNR is done in the frequency domain. This technique can have significant advantages over other averaging methods, including (1) larger spatial maps enabled by shorter data acquisition time and (2) the ability to average data without creating a phase reference by modifying the thruster operating condition with a periodic cutoff in discharge current, which can modify the ion velocity distribution1.