Document Type thesis Author Name Byrne, Lawrence Thomas Email Address Larry at Busek.com URN etd-1219102-173938 Title Langmuir Probe Measurements in the Plume of a Pulsed Plasma Thruster Degree MS Department Mechanical Engineering Advisors Nikolaos A. Gatsonis, Advisor John Blandino, Committee Member David Olinger, Committee Member Eric Pencil, Committee Member Michael Demetriou, Graduate Committee Rep Keywords PPT Pulsed Plasma Thruster Langmuir Probe Triple Langmuir Probe Plasma Diagnostics Electric Propulsion Electron Temperature Electron Density Date of Presentation/Defense 2002-11-22 Availability unrestricted
The ablative Teflon pulsed plasma thruster (PPT) is an onboard electromagnetic propulsion enabling technology for small spacecraft missions. The integration of PPTs onboard spacecraft requires the understanding and evaluation of possible thruster/spacecraft interactions. To aid in this effort the work presented in this thesis is directed towards the development and application of Langmuir probe techniques for use in the plume of PPTs. Double and triple Langmuir probes were developed and used to measure electron temperature and density of the PPT plume. The PPT used in this thesis was a laboratory model parallel plate ablative Teflon® PPT similar in size to the Earth Observing (EO-1) PPT operating in discharge energies between 5 and 40 Joules.
The triple Langmuir probe was operated in the current-mode technique that requires biasing all three electrodes and measuring the resulting probe currents. This new implementation differs from the traditional voltage-mode technique that keeps one probe floating and requires a voltage measurement that is often susceptible to noise in the fluctuating PPT plume environment. The triple Langmuir probe theory developed in this work incorporates Laframboise’s current collection model for Debye length to probe radius ratios less than 100 in order to account for sheath expansion effects on ion collection, and incorporates the thin-sheath current collection model for Debye length to probe radius ratios greater than 100. Error analysis of the non-linear system of current collection equations that describe the operation of the current-mode triple Langmuir probe is performed as well.
Measurements were taken at three radial locations, 5, 10, and 15 cm from the Teflon® surface of the PPT and at angles of 20 and 40 degrees to either side of the thruster centerline as well as at the centerline. These measurements were taken on two orthogonal planes, parallel and perpendicular to the PPT electrodes. A data-processing software was developed and implements the current–mode triple Langmuir probe theory and associated error analysis. Results show the time evolution of the electron temperature and density. Characteristic to all the data is the presence of hot electrons of approximately 5 to 10 eV at the beginning of the pulse, occurring near the peak of the discharge current. The electron temperature quickly drops off from its peak values to 1-2 eV for the remainder of the pulse. Peak electron densities occur after the peak temperatures. The maximum electron density values on the centerline of the plume of a laboratory PPT 10 cm from the Teflon® surface are 6.6x10^19 +/- 1.3x10^19 m^-3 for the 5 J PPT, 7.2x10^20 +/- 1.4x10^20 m^-3 for the 20 J PPT, and 1.2x10^21 +/- 2.7x10^20 m^-3 for the 40 J PPT. Results from the double Langmuir probe taken at r=10 cm, theta perpendicular=70 degrees and 90 degrees of a laboratory PPT showed good agreement with the triple probe method.
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