Mitchell L. R. Walker

High-power, null-type, inverted pendulum thrust stand

This article presents the theory and operation of a null-type, inverted pendulum thrust stand. The thrust stand design supports thrusters having a total mass up to 250 kg and measures thrust over a range of 1 mN to 5 N. The design uses a conventional inverted pendulum to increase sensitivity, coupled with a null-type feature to eliminate thrust alignment error due to deflection of thrust. The thrust stand position serves as the input to the null-circuit feedback control system and the output is the current to an electromagnetic actuator. Mechanical oscillations are actively damped with an electromagnetic damper. A closed-loop inclination system levels the stand while an active cooling system minimizes thermal effects. The thrust stand incorporates an in situ calibration rig. The thrust of a 3.4 kW Hall thruster is measured for thrust levels up to 230 mN. The uncertainty of the thrust measurements in this experiment is +/-0.6%, determined by examination of the hysteresis, drift of the zero offset and calibration slope variation.

The Effects of Nude Faraday Probe Design and Vacuum Facility Backpressure on the Measured Ion Current Density Profile of Hall Thruster Plumes

The effects of dissimilar probe design and facility backpressure on the measured ion current densities of Hall thrusters are investigated. JPL and GRC designed nude Faraday probes are used to simultaneously measure the ion current density of a 5 kW Hall thruster in the Large Vacuum Test Facility (LVTF) at the University of Michigan. The probes are located one meter from the exit plane of the Hall thruster, which is operated over the range of 300-500 V and 5-10 mg/s. In addition, the effect of facility background pressure is evaluated by varying the nominal pumping speed from 70,000 l/s to 240,000 l/s on xenon, corresponding to backpressures of 4.3x10-6 Torr to 2.3x10-5 Torr, corrected for xenon. Detailed examination of the results has shown that the GRC probe measured a greater ion current density than the JPL probe over the range of angular positions investigated for each operating condition. Yet, both probes measure similar thruster plume profiles for all operating conditions. Because all other parameters are identical, the differences between ion current density profiles measured by the probes are contributed to material selection and probe design. Moreover, both probes measured the highest ion current density near thruster centerline at the lowest facility pumping speed. A combination of charge exchange collisions and vacuum chamber gas ingestion into the thruster is believed to be the cause of this phenomenon.

Effect of Backpressure on Ion Current Density Measurements in Hall Thruster Plumes

The effects of facility backpressure and localized electric fields on the measured ion current densities of Hall thrusters are investigated. Langmuir probe measurements are taken in the near-field plasma surrounding a nude Faraday probe, which is located 1 m from the exit plane of the University of Michigan/U.S. Air Force Research Laboratory P5 Hall thruster. The thruster is operated at an anode flow rate of 5.30 mg/s, at backpressures of 1.5 × 10 −3 Pa (1.1 × 10 −5 torr) and 4.8 × × 10 −4 Pa (3.6 × × 10 −6 torr), corrected for xenon. The effect of the facility backpressure is clearly seen in the wings of the plume. A combination of charge-exchange collisions and vacuum chamber gas ingestion into the thruster is believed to be the cause of this phenomenon. The Langmuir probe results indicate that the electric fields near the nude Faraday probe are functions of facility backpressure and the angle from the thruster centerline. The plasma potential measured within 20 mm of the probe varied by no more than 3V .Thus, the electric fields near the nude Faraday probe are not large enough to explain the increased collection of charge-exchange ions at elevated facility background pressures and large angles from the thruster centerline.

Hall Thruster Cluster Operation with a Shared Cathode

6 torr Xe). The cluster thruster elements can share a single cathode without coupling their ionization and acceleration processes over a range of centerline separation distances from 0.3 m to greater than 2 m. The discharge current of both thrusters remains constant, with the shared cathode mounted above one thruster and with the shared cathode centrally located for all thruster-to-thruster centerline separation distances. These results show that the gas flow of one thruster does not directly affect the performance of the adjacent thruster. In addition, the effects of cathode displacement from the thruster centerline on the performance of a 5-kW Hall effect thruster are investigated. The data show that the cathode-to-ground voltage remains nearly constant for thruster-centerline-to-cathode-centerline spacing greater than 0.4 m. A thruster- centerline-to-cathode separation distance up to 1.3 m does not negatively affect the performance of the thruster, and reliable restarts are possible at the maximum displacement distance.

Performance characteristics of a cluster of 5-kW laboratory hall thrusters

5 Torr-Xe). Comparison of the cluster and the monolithic thruster performance characteristics for conditions of nearly equal operating pressure shows that for the 5:25 mg=s anode flow rate, the clusterthrustisequaltotheadditionofthetwomonolithicthrustersandtheanodeefficiencyandspecificimpulseare approximately equal to those of the monolithic thruster. An improved backpressure correction technique based on incremental current change was demonstrated to predict thrust changes within 4% to 2%, significantly better than a simple random flux calculation that uses the facility pressure and chamber wall temperature.

Vacuum chamber pressure maps of a hall thruster cold-flow expansion

TECHNICAL NOTES are short manuscripts describing new developments or important results of a preliminary nature. These Notes cannot exceed 6 manuscript pages and 3 figures; a page of text may be substituted for a figure and vice versa. After informal review by the editors, they may be published within a few months of the date of receipt. Style requirements are the same as for regular contributions (see inside back cover).

Neutral density map of Hall thruster plume expansion in a vacuum chamber

A neutral background pressure map of the large vacuum test facility (LVTF) is presented. The LVTF is mapped at cold anode flow rates of 5.25, 10.46, and 14.09mg∕s. In addition, neutral background pressure maps are created at hot anode (i.e., discharge on) flow rates of 5.25 and 10.46mg∕s for discharge voltages of 300 and 500V, corresponding to P5 Hall thruster operating conditions ranging from 1.5to5.0kW. The chamber pressure is mapped at nominal xenon pumping speeds of 140 000 and 240000l∕s. The pressure map is performed with a rake consisting of five calibrated Bayard–Alpert hot-cathode ionization gauges. The plume expansion appears to be independent of anode flow rate and facility background pressure. Analysis of axial pressure profiles on the LVTF’s centerline shows that the plume pressure decreases from a maximum at the thruster exit plane down to the facility background pressure at approximately 2m downstream of the exit plane. Comparison of axial pressure profiles on the LVTF’s centerline shows tha...

Magnetically filtered Faraday probe for measuring the ion current density profile of a Hall thruster

The ability of a magnetically filtered Faraday probe (MFFP) to obtain the ion current density profile of a Hall thruster is investigated. The MFFP is designed to eliminate the collection of low-energy, charge-exchange (CEX) ions by using a variable magnetic field as an ion filter. In this study, a MFFP, Faraday probe with a reduced acceptance angle (BFP), and nude Faraday probe are used to measure the ion current density profile of a 5kW Hall thruster operating over the range of 300–500V and 5–10mg∕s. The probes are evaluated on a xenon propellant Hall thruster in the University of Michigan Large Vacuum Test Facility at operating pressures within the range of 4.4×10−4Pa Xe (3.3×10−6Torr Xe) to 1.1×10−3Pa Xe (8.4×10−6Torr Xe) in order to study the ability of the Faraday probe designs to filter out CEX ions. Detailed examination of the results shows that the nude probe measures a greater ion current density profile than both the MFFP and BFP over the range of angular positions investigated for each operatin...

Plasma ionization by annularly bounded helicon waves

The general solution to the electrostatic and magnetic fields is derived with respect to the boundary conditions of a coaxial helicon plasma source. The electric field contours suggest that a simple antenna design can ionize the gas in a coaxial configuration. In addition, the power deposition as a function of excitation frequency is derived. The solution is validated by comparison with the standard cylindrical helicon plasma source. Further, a parametric study of source length, channel radius, channel width, and antenna excitation frequency are presented. This study suggests that it is possible to create a helicon plasma source with a coaxial configuration.

Effect of secondary electron emission on the plasma sheath

In this experiment, plasma sheath potential profiles are measured over boron nitride walls in argon plasma and the effect of secondary electron emission is observed. Results are compared to a kinetic model. Plasmas are generated with a number density of 3 × 1012 m−3 at a pressure of 10−4 Torr-Ar, with a 1%–16% fraction of energetic primary electrons. The sheath potential profile at the surface of each sample is measured with emissive probes. The electron number densities and temperatures are measured in the bulk plasma with a planar Langmuir probe. The plasma is non-Maxwellian, with isotropic and directed energetic electron populations from 50 to 200 eV and hot and cold Maxwellian populations from 3.6 to 6.4 eV and 0.3 to 1.3 eV, respectively. Plasma Debye lengths range from 4 to 7 mm and the ion-neutral mean free path is 0.8 m. Sheath thicknesses range from 20 to 50 mm, with the smaller thickness occurring near the critical secondary electron emission yield of the wall material. Measured floating potenti...