Investigation of Keeper Erosion in the NSTAR Ion Thruster

The goal of the present investigation was to determine the cause for the difference in the observed discharge keeper erosion between the 8200 hr wear test of a NASA Solar Electric Propulsion Technology Applications Readiness (NSTAR) engineering model thruster and the ongoing extended life test (ELT) of the NSTAR flight spare thruster. During the ELT, the NSTAR flight spare ion thruster experienced unanticipated erosion of the discharge cathode keeper. Photographs of the discharge keeper show that the orifice has enlarged to slightly more than twice the original diameter. Several differences between the ELT and the 8200 hr wear test were initially identified to determine any effects which could lead to the erosion in the ELT. In order to identify the cause of the ELT erosion, emission spectra from an engineering model thruster were collected to assess the dependence of keeper erosion on operating conditions. Keeper ion current was measured to estimate wear. Additionally, post-test inspection of both a copper keeper-cap was conducted, and the results are presented. The analysis indicated that the bulk of the ion current was collected within 2-mm radially of the orifice. The estimated volumetric wear in the ELT was comparable to previous wear tests. Redistribution of the ion current on the discharge keeper was determined to be the most likely cause of the ELT erosion. The change in ion current distribution was hypothesized to caused by the modified magnetic field of the flight assemblies.

[1]  J. Beattie,et al.  Mercury ion thruster technology , 1989 .

[2]  Michael J. Patterson,et al.  The 2.3 kW Ion Thruster Wear Test , 1995 .

[3]  James E. Polk,et al.  Performance characteristics of the NSTAR ion thruster during an on-going long duration ground test , 2000, 2000 IEEE Aerospace Conference. Proceedings (Cat. No.00TH8484).

[4]  M. Mantenieks.  Sputtering Threshold Energies of Heavy Ions , 1999 .

[5]  G. K. Wehner,et al.  Sputtering Yields for Low Energy He+‐, Kr+‐, and Xe+‐Ion Bombardment , 1962 .

[6]  George J. Williams,et al.  FMT-2 discharge cathode erosion rate measurements via laser- induced fluorescence , 2000 .

[7]  Matthew T. Domonkos,et al.  Absolute Density Calibration Cell for Laser Induced Fluorescence Erosion Rate Measurements , 2001 .

[8]  Noriaki Itoh,et al.  Energy dependence of the ion-induced sputtering yields of monatomic solids , 1984 .

[9]  John R. Anderson,et al.  An overview of the results from an 8200 hour wear test of the NSTAR ion thruster , 1999 .

[10]  George J. Williams,et al.  CHARACTERIZATION OF THE FMT-2 DISCHARGE CATHODE PLUME , 2000 .

[11]  I. Kameyama.  Characteristics of ions emitted from high-current hollow cathodes , 1994 .

[12]  M. L. Parsons,et al.  Rapid evaluation of ion thruster lifetime using optical emission spectroscopy , 1985 .

[13]  A. Striganov,et al.  Tables of spectral lines of neutral and ionized atoms , 1968 .

[14]  H. E. Wilhelm.  Theoretical investigations of plasma processes in the ion bombardment thruster , 1975 .

[15]  Michael J. Patterson,et al.  Development status of a 5/10-kW class ion engine , 2001 .

[16]  G. Wehner,et al.  Sputtering Yields at Very Low Bombarding Ion Energies , 1962 .

[17]  O. B. Mantenieks,et al.  A Review of Low Energy Sputtering Theory and Experiments , 1997 .

[18]  Michael J. Patterson,et al.  Derated ion thruster design issues , 1991 .

[19]  Roger M. Myers,et al.  A 100 Hour Wear Test of the NASA NSTAR Ion Thruster , 1996 .