Gridded ion propulsion technology holds great promise for enabling future robotic exploration missions with large delta-v requirements. Such missions are made possible by the high-specific-impulse capability of gridded ion thrusters. However, such missions will require that the engines operate continuously for up to 10 years! Thruster lifetime definition, improvement, and validation are, therefore, very important. Microwave plasma production, in contrast to conventional hollow-cathode-based ion-thruster-discharge plasma production, can literally eliminate the discharge cathode failure mechanism. Because microwave electron cyclotron resonance (ECR) plasma production is electrode-less, there is nothing to wear out. In addition, the microwave power source itself has a lifetime measured in hundreds of thousands of hours.
Microwave ECR ion thruster microwave thruster. Left: Discharge-only testing. Right: Beam-extraction testing.
High-power ECR plasma production research conducted at the NASA Glenn Research Center has culminated in the design and testing of a large-area(90- by 40-cm) plasma source. The source has been operated up to 2000 W at a frequency of 2.45 GHz and 2500 W at a microwave frequency of 5.85 GHz. Beam extraction at 2.45 GHz at powers up to 16 kW also has been demonstrated with this flexible device.
Recently, a series of plasma measurements were conducted on the ECR plasma source. Plasma uniformity at the exit plane of the source along the thruster center was measured to be greater than 90 percent. Transverse uniformity was also greater than 90 percent. Current densities measured at the exit plane suggest that the source can satisfy the beam current requirements originally laid out in the 2002 NASA announcement calling for the development of a high-power, high-specific-impulse electric propulsion system. Low measured plasma potentials and the absence of doubly charged xenon, as indicated by emission spectra, suggest a long-life plasma source. The ECR plasma source effort establishes a credible path for the resolution of a key ion thruster failure mode, cathode life. The effort, therefore, establishes a firm base for continued growth and technology readiness level advancement.
ECR cathode operating at 100 W.
In addition, Glenn won a NASA Research Announcement award to develop an electrodeless cathode for charge neutralization under the support of the Advanced Systems and Technology Program of the Prometheus Nuclear Systems and Technologies project.In an ECR cathode, whenever the microwave frequency is tuned to the electron-cyclotron-frequency (by applying suitable magnetic field strength), electrons can be resonantly excited and, thereby, given sufficient energy to cause ionization within a low-pressure gas. During phase I of the three-phase effort, a number of ECR cathode configurations were designed and fabricated and their performance was evaluated. Tests were performed using different microwave-injection schemes (i.e., different antenna types), different ECR cathode chamber configurations, various magnetic field circuits, and various propellants.
At the conclusion of the phase I effort, two competing ECR cathode configurations were successfully operated. An ECR cathode with a longitudinal antenna was operated at 2.45 GHz and generated an electron current of 2.6 A with a xenon flow rate of 3 standard cubic centimeters per minute (SCCM) while consuming only 100 W of microwave power. In addition, a multislot ECR cathode operated at 5.85 GHz generated an electron current of 1.8 A with a xenon flow rate of 4 SCCM while consuming only 100 W of microwave power.
Glenn contacts: Dr. Hani Kamhawi, 216-977-7435, Hani.Kamhawi-1@nasa.gov; and Dr. John E. Foster, 216-433-6131, John.E.Foster@nasa.govLast updated: October 7, 2006
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