A method of measuring transient plume properties
In an effort to study the transient fluctuations of plasma properties downstream of a Hall Effect Thruster (HET) on a microsecond timescale, a high-speed Langmuir probe measurement system is developed and tested. A high-voltage (±225 V), broadband (DC - 2 MHz) 30-W power amplifier is constructed to drive the Langmuir probe voltage sweep signal, and high-voltage current measurement circuitry is used to measure the plasma current drawn in the HET plume. A null probe, not in contact with plasma, is driven simultaneously with the Langmuir probe to measure and thereby correct for leakage currents, which nominally measure 5 mA. The power amplifier sources or sinks up to 200 mA, while the current measurement resolution is 3 μA (with S/N > 80 dB). Rapid successive voltage sweeping enables the acquisition of the plume's transient nature in terms of the variation of the plume plasma density, electron temperature, and floating and plasma potentials. After automated processing of the Langmuir probe I-V traces, plasma properties are recorded at intervals of 10 microseconds, thereby allowing for the examination of the highly turbulent plasma flow from the HET. While various plasma oscillations are known to exist above the frequency of 50 kHz for a HET, the largest amplitude fluctuations, caused by the thruster "breathing mode," are indeed captured and related to the measured discharge current oscillations. Data acquisition (DAQ) and control are provided by a multichannel 2 MHz 16-bit dedicated DAQ system, interface PC, and a motion control PC (all systems networked together for synchronous data collection). A grid in the plume of the HET measuring 31 cm by 50 cm is examined, collecting a total of 352 datasets. To characterize and quantify the measured turbulent plasma transient fluctuations, Fourier transforms are applied to large data sets for post processing in the frequency domain, thus constructing various Bode plots and power spectra representative of the behavior of the plasma across frequencies from DC to 50 kHz. Transfer functions are used to model the thruster-plume system from the measured plasma properties and discharge current signals. Fourier synthesis of these transfer functions is then used to create time-coherent 2D maps of the transient plume properties in the form of contour map movies with a 10 μs framerate - providing dramatic visualizations of the thruster breathing mode at 17.8 kHz. A 600-W Busek HET (BHT-600) is operated at a 200-V 2-A discharge in the Large Vacuum Test Facility to create the plume studied in these experiments. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
