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Review of Scientific Instruments / Volume 79 / Issue 11 / INVITED ARTICLE

Rev. Sci. Instrum. 79, 111301 (2008); doi:10.1063/1.3005784 (14 pages)

Invited Article: Physical and chemical analyses of impregnated cathodes operated in a plasma environment

Anita Sengupta1, James Kulleck1, Norm Hill2, and Wayne Ohlinger3

1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA
2Georgia Institute of Technology, School of Materials Science and Engineering, 771 Ferst Dr., Atlanta, Georgia 30332, USA
31565 Pinehurst Drive, Upper St. Clair, Pennsylvania 15241, USA

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(Received 11 February 2008; accepted 21 September 2008; published online 20 November 2008)

Destructive analyses of impregnated-cathode assemblies from an ion thruster life test were performed to characterize erosion and degradation after 30 472 h of operation. Post-test inspection of each cathode included examination of the emitter (insert), orifice plate, cathode tube, heater, anode assembly, insulator, and propellant isolator. The discharge-cathode assembly experienced significant erosion due to ion sputtering from the discharge plasma. The keeper electrode plate was removed and the heater and orifice plate were heavily eroded at the conclusion of the test. Had the test continued, these processes would likely have led to cathode failure. The discharge cathode insert experienced significant tungsten transport and temperature dependent barium oxide depletion within the matrix. Using barium depletion semiempirical relations developed by Palluel and Shroff, it is estimated that 25 000 h of operation remained in the discharge insert at the conclusion of the test. In contrast, the neutralizer insert exhibited significantly less tungsten transport and barium oxide depletion consistent with its lower current operation. The neutralizer was estimated to have 140 000 h of insert life remaining at the conclusion of the test. Neither insert had evidence of tungstate or oxide layer formation, previously known to have impeded cathode ignition and operation in similar long duration hollow-cathode tests. The neutralizer cathode was in excellent condition at the conclusion of the test with the exception of keeper tube erosion from direct plume-ion impingement, a previously underappreciated life-limiting mechanism. The most critical finding from the test was a power dependent deposition process within the neutralizer-cathode orifice. The process manifested at low-power operation and led to the production of energetic ions in the neutralizer plume, a potential life-limiting process for the neutralizer. Subsequent return of the engine and neutralizer operation to full-power removed the deposits and energetic ion production ceased.

© 2008 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. BACKGROUND
  3. DISCHARGE CATHODE OPERATION AND PERFORMANCE
    1. In-test performance
  4. NEUTRALIZER CATHODE OPERATION AND PERFORMANCE
    1. In-test performance
  5. DISCHARGE CATHODE POST-TEST INSPECTION
    1. Insert analysis
    2. Cathode subassembly inspection
  6. NEUTRALIZER CATHODE POST-TEST INSPECTION
    1. Insert analysis
    2. Neutralizer cathode subassembly inspection
  7. LIFETIME IMPLICATIONS
  8. CONCLUSION

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KEYWORDS and PACS

Keywords

cathodes, discharges (electric), erosion, life testing

PACS

ARTICLE DATA

Permalink
http://link.aip.org/link/doi/10.1063/1.3005784

PUBLICATION DATA

ISSN:

0034-6748 (print)  
1089-7623 (online)

Publisher:

$abstract.society.value is a Member of CrossRef American Institute of Physics

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