Langmuir probe diagnostics on the LEAP electrothermal thruster
As microsatellite technology progresses, the need for improved on-board propulsion systems has grown. The Purdue Laboratory for Electric and Advanced Propulsion (LEAP) electrothermal thruster is a device that attempts to fill this niche, by providing a low-power, non-complex electrothermal thruster that shows vast improvements over conventional cold gas thrusters. Preliminary tests have shown that the proposed RF thruster can improve upon cold gas thrust by up to 75%. To further investigate the viability of this concept, a sophisticated diagnostics system is necessary; in addition to thrust, mass flow, and power/frequency, a detailed analysis of the feasibility of a low-power RF thruster also requires an investigation of its plasma and plume characteristics in order to determine the thruster’s performance. Experimental plume diagnostics of RF thrusters are inherently difficult due to the high-frequency oscillations that occur in the thruster chamber, as well as the electrical interference produced by the RF signal itself. In this thesis, a double Langmuir probe apparatus was designed and used to obtain electron temperature and electron number density measurements in the plume of the LEAP electrothermal thruster. To simplify the diagnostics system and provide a baseline for future comparison, the thruster plume that was investigated was of the LEAP thruster operating in DC-mode, producing a glow discharge. This experimental investigation provides further information about the plume, and about the relationship between the plume characteristics and the variables of interest of the RF thruster – voltage, power, and mass flow rate. The thruster used in this study is a cylindrical geometry prototype with a ¾” outer electrode bore, and a ¼” inner electrode diameter, arranged concentrically and separated by an alumina dielectric sleeve. The experimental setup includes a Langmuir double probe in a fixed location with respect to the thruster. It was found to be necessary to reduce flow velocity as much as possible, which indicates a very low degree of ionization, necessitating the removal of the nozzle plate. Electron temperatures were found to be within 2-3eV, depending heavily on mass flow, but not on the applied DC voltage.
Hrbud, Purdue University.
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