Arising from the myriad of electromagnetic interactions between excited propellant particles, electrons, and the imposed magnetic and electric fields, HET plasmas (as well as cathodes, ion thrusters, pulsed plasma discharges, etc.) are rich with oscillation in a wide band ranging from 1 kHz to 20 GHz.
Figure 1. High-speed electrostatic plasma measurements (using PEPL's 100-kHz HDLP
) showing the spatial and temporal evolution of plasma bursts from a 600 W HET due to a natural ionization instability termed the Hall thruster breathing mode.
Small instantaneous FASTCAM photo of thruster exit plane shows approximately half of the annular discharge channel (arb. colorscale denotes visible emission intensity).
Planar region downstream from thruster is logarithmically colorscaled to the normalized instantaneous total electron density.
(From ref. 1.)
The existence of plasma oscillations in the near- and far-field discharge of a Hall
effect thruster alters the conventionally held view of their operation as steady electrostatic
propulsion devices. Indeed, the consequences from fluctuations in ionized propellant
density, temperature, and potential may include increased thrust, exacerbated engine
erosion, and spacecraft interference. In this research project, the unsteady nature of a Hall effect
thruster discharge has been investigated via two-dimensional, time-resolved plasma
measurements. A novel dual Langmuir probe diagnostic (HDLP, see ref. 2) was developed to enable an
unprecedented temporal resolution for electrostatically acquired plasma properties near
the upper theoretical limits (1-10 MHz, see ref. 3). Observations of large amplitude transient
oscillations caused by the Hall thruster breathing mode were seen for all thruster conditions
at all spatial locations and in all measured plasma properties including: discharge current,
electron density, electron temperature, plasma potential, and visible light emission. A unique method of
spatiotemporal data fusion (see ref. 4) facilitates visualization of two-dimensional time-resolved
planar plasma density contour maps of discrete turbulent bursts of
plasma ejected as the thruster exhales breaths of ionized propellant at velocities in
excess of 12 km/s (see fig. 1). An initial time-resolved investigation of the plasma downstream from a
Hall thruster unveiled an environment rich in oscillatory behavior dominated by the Hall
thruster breathing mode. These insights emphasize the importance of time-resolved
plasma measurements and, through enhanced understanding of the discharge process,
may ultimately lead to improved thruster designs that work in concert with plasma
fluctuations to achieve enhanced performance. (From ref. 1)
The initial 100-kHz HDLP diagnostic developed for this research provided measurements of unprecedented spatial
and temporal detail within a HET plume. However, an examination of the temporal
limitations of the probe (from ref. 3) suggests that an upper probing rate of 1-10 MHz is
possible. Future development of this capability is already underway, and the finer
temporal detail (coupled with a faster 1-Mfps high-speed camera) will enable the
examination of higher-frequency plasma oscillations such as the azimuthal modes
(ionization, transit-time, drift instability, etc.) common in the very-near-field of HETs.
To that end, future measurements using this advanced HDLP to perform internal time-resolved
measurements would be tremendously insightful, and are thus part of ongoing research.
The strong magnetic fields in this
region are known to exhibit anomalous electron transport, and with detailed time-resolved
measurements, one could examine the nature of turbulent transport.
Additionally, the use of a HDLP for obtaining time-resolved electron energy distribution
functions was shown to be promising in early work and there is ongoing effort to obtain time-resolved EEDFs at a rate of 1 MHz.
Measurements of the unsteady state of electron equilibrium could help identify transient plasma phenomena that could be
tailored to enhance the efficiency of ionization and particle acceleration in Hall thrusters and other plasma devices.
3D plot (z = ne
, y = radial-position, x = time) of radial profile transient
axially travelling plasma density waves. The 31 cm wide radial profile is at a fixed
position 20 cm axially downstream from the 200 V 2 A HET discharge exit-plane. (From ref. 1.)
Selected Relevant Publications
Lobbia, R. B., "A Time-resolved Investigation of the Hall Thruster Breathing Mode,"
Ph.D. Dissertation, University of Michigan, 2010.
Robert B. Lobbia and Alec D. Gallimore,
"High-speed dual Langmuir probe,"
Rev. Sci. Instrum.,
Art. No. 073503
, Vol. 81, Issue 7, July 2010. (Published online by the American Institute of Physics, click here to view abstract.)
Robert B. Lobbia and Alec D. Gallimore,
"Temporal limits of a rapidly swept Langmuir probe,"
Physics of Plasmas,
Art. No. 073502
, Vol. 17, Issue 7, July 2010. (Published online by the American Institute of Physics, click here to view abstract.)
Lobbia, R. B. and Gallimore, A. D., "Fusing spatially and temporally separated single-point turbulent plasma flow measurements into two-dimensional time-resolved visualizations,"
12th International Conference on Information Fusion, Seattle, WA, USA, July 6-9, 2009.
Lobbia, R. B. and Gallimore, A. D., "A Method of Measuring Transient Plume Properties,"
AIAA-2008-4650, 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Hartford, CT, July 20-23, 2008.
Lobbia, R. B., Liu, T. M., and Gallimore, A. D., "Correlating Time-Resolved Optical and Langmuir Probe Measurements of Hall Thruster Dynamics,"
SPS-III-36, 6th Modeling and Simulation / 4th Liquid Propulsion / 3rd Spacecraft Propulsion Joint Subcommittee JANNAF Meeting, Orlando, FL, 8-12 December 2008.
Lobbia, R. B., Liu, T. M., and Gallimore, A. D., "Temporally and Spatially Resolved Measurements in the Plume of Clustered Hall Thrusters,"
AIAA-2009-5354, 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Denver, CO, 2-5 August 2009.