Figure 1. Photo showing the X2 thruster
and collection optics during a recent time-averaged LIF test.
There is a need to develop non-intrusive, time-resolved diagnostics to study short timescale dynamics in electric thrusters and improve modeling and simulation of the physical processes at work in thrusters.
Hall effect thrusters (HETs) exhibit many high-frequency plasma oscillations.
Particularly of interest is the breathing mode oscillation at tens of kilohertz, which strongly influences the ion distribution function and therefore thruster operation.
The focus of this effort is on measuring how the ions evolve during the oscillation in order to better understand how the confinement of plasma is changed by the breathing mode.
In addition, many other thruster concepts, such as the pulsed inductive thruster (PIT) and the field-reversed-configuration (FRC) thruster, have an explicitly pulsed nature and time-resolved diagnostics are necessary for understanding their plume dynamics.
To this end, PEPL is working to develop a time-resolved laser induced fluorescence (LIF) system to measure ion velocity distribution as a function of time resolved to microsecond or smaller scales.
Traditional (time-averaged) LIF is performed at PEPL by chopping a laser on the order of kilohertz and using a lock-in amplifier to remove noise and amplify the signal.
Since the signal is weak, a relatively long time constant is used and effectively averages out oscillations.
The time-resolved system will modulate the laser at high-frequency using an acousto-optical modulator and acquire data with a lock-in system that has capability to acquire data with fine time resolution and either average over individual time bins only or average in the frequency domain.
The details of this system are currently being investigated.