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Nov 2008

Volume 79, Issue 11, Articles (11xxxx)

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

Anita Sengupta, James Kulleck, Norm Hill, and Wayne Ohlinger

Post life test (30,472 hour) condition of the interior of an impregnated-cathode showing a cascade of crystallites emanating from the cathode orifice plate, a result of tungsten transport and redeposition processes. This cathode was the electron source of the Deep Space 1 flight spare ion thruster.

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

Anita Sengupta, James Kulleck, Norm Hill, and Wayne Ohlinger

Rev. Sci. Instrum. 79, 111301 (2008); http://dx.doi.org/10.1063/1.3005784 (14 pages) | Cited 2 times

Online Publication Date: 20 November 2008

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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.
Show PACS
82.45.Fk Electrodes
52.80.-s Electric discharges
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