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Coaxial microwave electrothermal thruster performance in hydrogenThe microwave electro thermal thruster (MET) is an electric propulsion concept that offers the promise of high performance combined with a long lifetime. A unique feature of this electric propulsion concept is its ability to create a microwave plasma discharge separated or floating away from any electrodes or enclosing walls. This allows propellant temperatures that are higher than those in resistojets and reduces electrode and wall erosion. It has been demonstrated that microwave energy is coupled into discharges very efficiently at high input power levels. As a result of these advantages, the MET concept has been identified as a future high power electric propulsion possibility. Recently, two additional improvements have been made to the coaxial MET. The first was concerned with improving the microwave matching. Previous experiments were conducted with 10-30 percent reflected power when incident power was in excess of 600 W(exp 6). Power was reflected back to the generator because the impedance of the MET did not match the 50 ohm impedance of the microwave circuit. To solve this problem, a double stub tuning system has been inserted between the MET and the microwave power supply. The addition of the double stub tuners reduces the reflected power below 1 percent. The other improvement has prepared the coaxial MET for hydrogen experiments. To operate with hydrogen, the vacuum window which separates the coaxial line from the discharge chamber has been changed from teflon to boron nitride. All the microwave energy delivered to the plasma discharge passes through this vacuum window. This material change had caused problems in the past because of the increased microwave reflection coefficients associated with the electrical properties of boron nitride. However, by making the boron nitride window electrically one-half of a wavelength long, power reflection in the window has been eliminated. This technical note summarizes the experimental performance of the improved coaxial MET when operating in nitrogen, helium, and hydrogen gases.
Document ID
19950005171
Document Type
Contractor Report (CR)
Authors
Richardson, W. (Michigan State Univ. East Lansing, MI, United States)
Asmussen, J. (Michigan State Univ. East Lansing, MI, United States)
Hawley, M. (Michigan State Univ. East Lansing, MI, United States)
Date Acquired
September 6, 2013
Publication Date
September 15, 1994
Subject Category
SPACECRAFT PROPULSION AND POWER
Report/Patent Number
NASA-CR-196839
NAS 1.26:196839
Funding Number(s)
CONTRACT_GRANT: NAG3-760
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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