Triple Langmuir Probe Measurements in the Plume and Backflow Region of a Pulsed Plasma Thruster

A current-based Triple Langmuir probe method was used to measure electron temperature and density in the plume and backflow region of a laboratory model Teflon® pulsed plasma thruster. This experiment was performed in a large vacuum facility at base pressures of 5x10~ 2x10~ Torr. Measurements were taken at a range of positions from within the thruster exit plane to 20 cm downstream of the Teflon propellant surface, and at angular locations of 0-130 degrees on both the plane perpendicular and parallel to the thruster electrodes. Preliminary data analysis shows the time evolution of the temperature and density of a 20-J PPT plume. Introduction A rectangular-geometry pulsed plasma thruster (PPT) is a type of electromagnetic propulsion shown schematically in Figure 1. The main capacitor discharge ionizes and ablates the solid Teflon® propellant. The plasma accelerates through electromagnetic and gasdynamic processes to generate thrust. As a result of the Teflon® decomposition and ionization processes in the PPT channel the plume consists of neutrals and ions, material from electrode erosion, as well as electromagnetic fields and optical emissions. Successful integration of PPTs on modern spacecraft requires the comprehensive evaluation of possible plume-spacecraft interactions. The PPT has been flown successfully several times in the past without any spacecraft interactions.' The overall goal of this WPI research program is to develop a predictive ability of the PPT plume environment and to assess potential plume/spacecraft interactions. To accomplish these goals, the program involves both plume modeling and experimental investigations carried out at NASA Glenn Research Center (GRC) facilities.'' In our previous investigation triple Langmuir probes (TLP) were used to simultaneously measure electron temperatures and electron densities in the plume of a rectangular-geometry Teflon® laboratoryGraduate Research Assistant; currently Research Engineer, Busek Co., 11 Tech Cir., Natick, MA 01760. Student Member AIAA. 1" Associate Professor. Senior Member AIAA t Aerospace Engineer; On-Board Power and Propulsion; NASA GRC; Cleveland, OH. Member AIAA Copyright © 2001 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission model PPT. The previous investigation was performed in a small bell jar (0.5 m x 1.0 m) at base pressures of ~10~ torr. Measurements were obtained at downstream location between 6 and 20 cm from the propellant face and for polar angles between 10 and 45 degrees to one side of the thruster centerline. Two planes of measurement were looked at, perpendicular and parallel to the PPT electrodes. In that investigation, the PPT was operated at discharge energy levels of 5, 20, and 40 J. Having the PPT operate in such a confined chamber could have possibly introduced facility effects on the data. Limitations in available equipment prevented the use of a rogowski coil to obtain the discharge current so there was no common time zero for the measurements. Results showed a time averaged electron temperature of 1-3 eV and an electron density of!0-10m-. This paper presents research that expanded considerably on our previous work. First, TLP measurements were conducted in a large vacuum facility in order to better accommodate the expansion of the plume and reduce facility effects. Second, measurements covered the entire plume from the thruster centerline out to the backflow regions in both directions. Triple Langmuir probes were attached to a translation stage that allowed data collection in the plume on two planes, perpendicular and parallel to thruster electrodes passing through the thruster's centerline. Measurements were obtained for radial distances of 2, 3,4, 5, 10, 15, and 20 cm with respect to the center of the Teflon® propellant face. The PPT itself was mounted upon a rotation stage that allowed for polar angles of 0-180 degrees. Third, the triple probes were operated in what shall be referred to as the current-based method. This implementation involves biasing all three probes and measuring the resulting currents. Where as the traditional triple probe 1 American Institute of Aeronautics and Astronautics c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. implementation involves biasing only two of the three probes and allows the third to float with the floating potential. The current-based method proved to be more suitable for the noisy PPT plume environment. The thruster design used in this work is similar to the one carried on the EO-1 spacecraft and the energy levels address several planned PPT applications during the course of that mission. The thruster was operated at discharge energy levels of 5, 20 and 40 J. In this paper, the current-based triple probe theory is outlined and probe construction, grounding scheme, as well as cleaning procedure are discussed. The data reduction methodology, error analysis and temperature and density measurements are presented. Thruster and Facility A laboratory-model PPT with a parallel plate electrode arrangement and rectangular geometry housing was used in this study. The PPT is similar to the one flown on the EO-1 spacecraft and was designed at NASA GRC for component life tests and plume characterization. The operational characteristics are shown in Table 1. The thruster has 2.5-cm-square electrodes as shown in a schematic of the thruster, Fig. 1. The thruster uses a 33-uF capacitor for its main discharge and is capable of operating through an energy range of 5-60 J. Experiments were conducted in a 2.13-m-diameter, 3.05-m-long vacuum facility with a volume of 6.22 m and an estimated pumping speed of 12,500 liters per second. The system used two oil diffusion pumps to achieve base pressures between 5xlO"and2xlO'Torr. A test stand was designed and built that would allow the thruster to fire along the centerline of the vacuum tank, horizontally, from one end towards the other. This arrangement is illustrated in Fig. 2. The probe assembly was mounted on a horizontal translation stage that was computer controlled. This allowed the probes to be moved horizontally downstream from the thruster exit plane remotely through a range of locations from within the exit plane to 20 cm downstream from the Teflon® fuel bar surface. For off-centerline measurements, the thruster itself was rotated on a computer-controlled rotation stage through the appropriate angle, with the probe aligned geometrically with the center of the Teflon® fuel bar surface. Triple Langmuir Probe Theory The theory of operation of a triple Langmuir probe was first outlined by Chen and Sekiguchi. A symmetric triple Langmuir probe, similar to the ones used in our experiments, consists of three identical probes placed in the plasma. In the traditional mode of operation, referred to as direct-display, one of the probes, indicated as probe-2 in Figure 3, is allowed to float in the plasma and a fixed voltage ^/3 is applied between the positive and negative probe with respect to the floating potential probe. The resulting voltage difference 0J2(t)and collected current /(/), allow for the evaluation of Tc(t) and ne(f).' Previous plume investigations of plasma density and temperature have used interferometers, single and double Langmuir probes.' -' Triple Langmuir probes offer many advantages over these methods, allowing simultaneous measurement of electron temperature and density. Triple probes have been used successfully in the steady plumes of magnetoplasmadynamic (MPD) thrusters, ' arcjets, Hall-Effect Thrusters, and PPTs. However, the PPT differs from many of these other thrusters in that it is not a steady state device and emits a considerable amount of EMI noise during the spark initiation and the discharge of its main capacitor. The voltage measurement needed to obtain 0J2(t) is a high impedance measurement that is easily susceptible to noise entering the system. Chen and Seckiguchi' outlined a technique that does not rely on a voltage measurement but rather on current measurements. This method will be referred to as the current-based TLP theory, which was used in this experiment and is illustrated in Figure 4. A fixed voltage (/>J2 is applied between probe-1 and probe-2 while another fixed voltage ^3 is applied between probe-1 and probe-3. The collected currents /,(/), I2(t), and I3(t) allow for the evaluation of Tc(t) and nc(t) as outlined below. The triple-probe theory assumes that the probes are operating in the collisionless plasma under orbitalmotion-limited conditions, A,D s) as shown in Fig. 4 the total probe current is /„='*-/*, (i) In writing the above equation, we assume that the electron current to a probe is positive and the ion current to a probe negative. A probe therefore collects electrons if Ip > 0 and ions if Ip < 0 . The electron current is Applying to all three probes assuming that Al = A2 = Az = A then American Institute of Aeronautics and Astronautics c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.