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Nonambipolar Electron Source (NES)
NES is a device that produces electron beams from plasma created with RF fields in a magnetized plasma combined with electron extraction by electron sheaths. With the hollow cathode sources currently employed to provide neutralizing electrons on ion thrusters, operation is limited in time and/or current density by cathode deterioration. RF e-beam sources provide an alternative approach that does not consume electrode material. The extracted current exceeds the current normally extracted from conventional rf plasma sources by a factor of (m_i /m_e )^1/2 .where m_i and m_e are the ion and electron mass because electrons are extracted through an electron sheath that contains no ions. Ions are lost to a negatively biased conducting cylinder with area A_i chosen to be A_i = A_e*(m_i /m_e )^1/2 where A_e is the electron extraction area. Slots in the conducting cylinder allow it to serve as a Faraday shield to reduce capacitive coupling. Video: publications: Presentations and Posters:
NES background: Radio frequency (RF) plasmas are attractive as electron beam sources because they allow for a design where the cathode dose not participate in electron production while providing high efficiency and long life operation. Traditionally, dispenser hollow cathodes have been used as electron sources because of their high electron current density and relatively low power requirements. However, their operational lifetime is limited by cathode deterioration, contamination, and barium diffusion rates, thus rendering them less suitable use in corrosive environments and long duration sustained use (>3-4 years). Longer duration spacecraft missions that use electric propulsion, such as the proposed Jupiter Icy Moons Mission (JIMO), will take 6-10 years for the total orbital transfer time. While using electric propulsion for longer duration missions is very beneficial for fuel, mass, and time savings (as opposed to an impulsive chemical rocket burn), the lifetime of some operating components for electric propulsion, such as the hollow cathode, may be limited to 3-4 years. The hollow cathode neutralizer and plasma sources that were used for the highly successful Deep Space 1 and SMART-1 missions may be limited to 3-4 years of operational lifetime due to significant erosion, sputtering, and re-deposition of material within the keeper region and surrounding areas. There exists a need for these types of missions for an electron source that is able to function reliably for much longer lifetiems. Ion and Hall thrusters that are currently used onboard communications, NASA, and DOD satellites use hollow cathodes as the primary plasma source with an additional hollow cathode as an electron source for neutralizing the positive ion beams. Here, the neutralizing hollow cathode uses a significant fraction of the total neutral propellant onboard the spacecraft and takes approximately 5 to 10 minutes to heat the thermionic material surface. These inefficiencies in propellant usage and startup time have stimulated interest in innovative electron sources. RF plasma sources provide an alternative neutralizing approach that does not consume electrode material while providing electrons, thereby allowing for a longer operational lifetime. A variety of RF sources exist including capacitive and inductive sources, which can operate without magnetic fields, and both electron cyclotron resonance (ECR) and helicon sources, which require axial magnetic fields. Helicon sources appear to be the best choice of RF plasma sources for use in ion propulsion because they can produce the highest plasma densities, up to 10^13 cm-3 is common, for a given RF power but they also require larger magnetic field strengths and/or larger RF powers than inductively coupled plasma sources. If insufficient power is available, helicon sources will operate as inductive sources. At much lower RF powers, the plasma is capacitively coupled and results in lower plasma densities. Inductively coupled plasmas can achieve significant plasma densities, 10^10 cm-3 to 10^12 cm-3 and allow for a large total electron extraction current. The current proof of principle device at the University of Wisconsin - Madison produces an inductively coupled plasma with a plasma density of 10^10 cm-3 to 5x10^11 cm-3 . 15 A of electron neutralizing current was extracted at an electron sheath (sheath where ion density is neglected) near a grounded disc/anode located at the plasma source boundary.
Future Research goals with NES: � Development of electron source for use onboard small electric propulsion spacecraft with technology transfer to Phoenix Nuclear Labs LLC for creation of a flight ready prototype. � Operate with xenon at similar conditions to those of argon operation � Reduce propellant usage to < 2-5 sccm xenon for 5 Amps, < 1-2 for 2 Amps � Reduce RF power to 100W for 4-5 Amp source, 50W for 1-2 Amp source � Reduce overall weight and dimension for use on spacecraft � Evaluate low sputter materials for use with electron source � Magnetic field optimization
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Copyright © 2006 Ben Longmier