The University of Michigan
Department of Aerospace Engineering
| Plasmadynamics & Electric Propulsion Laboratory |
Plasma Diagnostics

High-speed Axial Reciprocating Probe (HARP)

A linear electromagnetic rapid positioning system for <100-ms insertion and removal of probes for internal thruster measurements.

Figure 1. Photograph of High-speed Axial Positioning (HARP) system showing actuator, stainless steel and graphite shroud, and probe boom. The top cover has been removed to show the actuator. The assembly was mounted on the chamber centerline with the probe boom directly downstream of the thruster. (From ref. 1)

Figure 2. PEPL's HARP with the top cover removed.

Figure 3. HARP system alignment with respect to thruster. The HARP assembly mounted such that the probe height is coincident with the thruster centerline and its axis is aligned with the thruster axis. The probe starts from rest approximately 150 mm downstream of the thruster exit plane, accelerated to a position 10 mm downstream of the anode face, and returns. Data is collected during the entire inward and outward "sweeps" of the probe. (From ref. 1)

In plasmas with highly energetic charged particles, probe material is generally sputtered and/or ablated on very small time scales by the direct flux of these particles. The local plasma temperature and density may then be modified through emission of relatively cool probe material. These perturbations may remain localized near the probe, or may propagate further into the plasma affecting the macroscopic properties of the plasma being interrogated. As time scales increase, probe survival becomes an issue. A high-speed, reciprocating probe system can reduce the amount of ablated material, allowing for more accurate measurements of local plasma parameters and assuring probe survival.

The HARP is a linear motor assembly providing direct linear motion at very high speed and acceleration. The linear motor is an LM210 manufactured by Trilogy. It has a three-phase brushless DC servomotor consisting of a linear, "U"-shaped magnetic track and a "T"-shaped coil moving on a set of linear tracks. The motor employs Sine commutation, using a linear encoder built into the magnet track to provide position feedback for very smooth motion. The linear encoder provides positioning resolution to 5 microns. The table is covered by a stainless steel and graphite shroud to protect the HARP from excessive heating and high-energy ions. One side has a thin slit running the length of the table through which a probe boom extends.


Selected Relevant Publications

  1. Haas, J. M., "Low-perturbation Interrogation of the Internal and Near-field Plasma Structure of a Hall Thruster Using a High-Speed Probe Positioning System," Ph.D. Dissertation, University of Michigan, 2001.
  2. Haas, J. M., Gallimore, A.D., McFall, K., and Spanjers, G., "Development of a High-Speed, Reciprocating Electrostatic Probe System for Hall Thruster Interrogation," Review of Scientific Instruments, Vol. 71, No. 11, pp. 4131-4138, November 2000.
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