中野 正勝

平成26年度宇宙輸送シンポジウム（2015年1月15日-16日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)）, 相模原市, 神奈川県資料番号: SA6000036032レポート番号: STCP-2014-032

The JIEDI (JAXA's Ion Engine Development Initiative) tool has been developed to assess the ion acceleration grid erosion of an ion thruster. The sensitivity analysis of the input parameters of the JIEDI tool is conducted in this paper. We compare the simulation results of the JIEDI tool and show that it successfully reproduces the full lifetime test of the μ10 prototype model. Through sensitivity analysis, we found that the sticking factor is the most sensitive input parameter when estimating the accelerator grid erosion and electron backstreaming time. For the worst case scenario of grid erosion (without re-deposition), the uncertainty in the neutral mass flow rate through grid holes is important when estimating the accelerator grid erosion. A 30% change in the neutral mass flow rate caused by a 6% change in the propellant utilization efficiency, corresponds to about a 20% change in the accelerator grid mass loss as well as the increasing rate of minimum potential on the axis of the ion optics. In contrast to the accelerator grid erosion, the uncertainties in the discharge voltage and grid gap (that affect the trajectory of the mainstream ions), are important when estimating decelerator grid erosion. A 40% change in the discharge voltage corresponds to about a 90% change in the decelerator grid mass loss. In addition, a 30% change in the grid gap between the screen grid and the accelerator grid corresponded to about a 40% change in the decelerator grid mass loss.

The JIEDI (JAXA's Ion Engine Development Initiative) tool has been developed as a numerical tool for the lifetime qualification of ion thruster's ion optics with high precision and accuracy. The numerical wear test from beginning-of-life to end-of-life (EOL) by the JIEDI tool was conducted for the ion optics of HAYABUSA's microwave ion thruster (μ10 engineering model (EM)). It becomes clear that the erosion profiles for high ion beam current hole are most important to estimate the EOL of μ10EM ion optics. The ion optics of μ10EM ion thruster encounters its EOL by structural failure of the decelerator grid, which is mainly caused by sputtering of ions and neutrals scattered by elastic collisions. The estimated lifetime of the ion optics is 545 khours at the longest. Through the numerical wear test for μ10EM ion optics, the process for lifetime estimation of ion optics by the JIEDI tool was demonstrated. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Some numerical wear tests are conducted for the carbon/carbon ion optics of a microwave ion thruster μ10 engineering model to evaluate the accuracy and precision of JAXA's ion optics code (JIEDI). Through comparisons with experiment, the JIEDI code showed good agreement with a real-time 18,000-hrs life test when incorporating the motion of eroded grid materials and a low-energy sputtering yield model for the energy below 300 V. Numerical error caused by the uncertainty of physical model is also studied and it is found that uncertainty in beam current and plasma parameters cause 10% or less error to estimate grid hole erosion profiles. The grid erosion profile is most sensitive to the uncertainty in sticking factor, which indicates what percentage of eroded grid material arriving at a grid surface will re-deposit onto the grid surface. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.