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HPEPL employs a wide-range of plasma probe diagnostics and facility measurement techniques. Graduate students design, manufacture, and maintain many of the probes used during actual test operations. HPEPL also employs a rigorous probe calibration and check-out procedure to accurately define the uncertainty of each measurement. For increased precision, probes have also been manufactured by professional vendors employing techniques such as laser spot welding and nanoscale processes.

Inverted Pendulum Thrust Stand

A null-type stand differs from other inverted pendulum thrust stands in that it actively holds the thrust stand in a nearly stationary position at all thrust levels. This eliminates changes in the elevation angle of the thrust vector and reduces error in the thrust measurement. Xu, K., Walker, M. L. R., “High-Power, Null-Type, Inverted Pendulum Thrust Stand,” Review of Scientific Instruments, Volume 80, Number 5, May 2009, pp. 055103 1-6.

Thrust Stand for Pulsed-mode Devices

In order to test high-power thrusters such as magnetoplasmadynamic thrusters, infrastructure often necessitates operation in short pulses. With this thrust stand, accurate measurements of thrust can be obtained for devices operating in such a mode. Gilland, J. H., Walker, M. L. R., Pencil, E. J., “Thrust Stand for Applied Field MPD Thrusters“, IEPC-2005-215, 29th International Electric Propulsion Conference, Princeton, NJ, Oct. 31-Nov. 4, 2005.

Langmuir Probe

The venerable Langmuir Probe is the original plasma diagnostic. A comparison of the methods of determining electron densities in afterglow plasmas from langmuir probe characteristics C V Goodall and D Smith

RF-compensated Langmuir Probe

Taking Langmuir probe measurements in an RF plasma (such as a helicon discharge) is non-trivial due to fluctuations induced in the plasma potential. This probe employs both a floating electrode immediately adjacent to the probe tip and multiple RF chokes to facilitate measurement. RF compensated probes for high-density discharges by Isaac D Sudit, Francis F Chen

Faraday Probe – JPL

Faraday probes provide a measurement of plasma ion current, and are often used in angular sweeps to characterize the divergence of thruster plumes. The probe pictured is modeled on the JPL nude Faraday probe design. Walker, M. L. R., Hofer, R. R., Gallimore, A. D., “The Effects of Nude Faraday Probe Design and Vacuum Facility Backpressure on the Measured Ion Current Density Profile of Hall Thruster Plumes” AIAA-2002-4253, 38th Joint Propulsion Conference, Indianapolis, IN, July 7-10, 2002.

Emissive Probe

True to its name, the emissive probe obtains its measurement (plasma potential) by emitting electrons to neutralize the Debye sheath surrounding the probe. A comparison of emissive probe techniques for electric potential measurements in a complex plasma J. P. Sheehan, Y. Raitses, N. Hershkowitz, I. Kaganovich, and N. J. Fisch

High-Speed Axial Reciprocating Probe

The HARP was designed to address the challenge of obtaining probe measurements in high-energy plasma environments, such as the discharge channel of a hall thruster.

Harp in use in VTF2 (Video)

Harp Test 4 m/s (Video)

http://pepl.engin.umich.edu/diagnostics/HARP.html

Retarding Potential Analyzer – HPEPL

A Retarding Potential Analyzer directly measures ion energy distribution by quantifying their ability to migrate across a biased grid. Three other grids provide shielding from the plasma and repulsion/suppression of electrons.

Xu, K. G., Walker, M. L. R., “Plume Characterization of an Ion Focusing Hall Thruster,” AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, San Diego CA, August 2011.

Retarding Potential Analyzer – AFRL

A Retarding Potential Analyzer directly measures ion energy distribution by quantifying their ability to migrate across a biased grid. Three other grids provide shielding from the plasma and repulsion/suppression of electrons.

Xu, K. G., Walker, M. L. R., “Plume Characterization of an Ion Focusing Hall Thruster,” AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, San Diego CA, August 2011.

Hiden EQP 300

The EQP is a high-sensitivity energy analyzer and mass analyzer combined, enabling accurate measurements of ionization and energy distribution of arbitrary species.

Hiden EQP Information

ExB Probe

An E x B probe exploits the fact that when aimed into uniform perpindicular electric and magnetic fields, ions will feel opposing forces from the two fields. In addition, ions of different charge states will feel different forces. This allows for measurement of the charge populations of ions.

Casey C. Farnell (Plasma Controls); Daniel L. Brown (AFRL/RZSS); Garret Willis & Richard Branam (AFIT); John D. Williams (CSU) “Remote Diagnostic Measurements of Hall Thruster Plumes” Aug 14, 2009.

Anode Flame Test Stand

HPEPL has developed a novel method for propellant flow characterization employing a premixed flame stabilized on the exit plane of the distributor. This technique yields immediate visual feedback on the flow through the distributor, making visible any nonuniformities, blockages or manufacturing defects in the unit under test. A quantitative measure of the uniformity can be obtained by examination of the flame luminosity and its variability in circumferential and radial directions.

Testing of a Single-Inlet Propellant Distributor (Video)

Testing of T-140 Propellant Distributor (Video)