Miniature ion thrusters are well suited for future space missions that require high efficiency, precision thrust, and low contamination in the mN to sub-mN range. JPL's miniature xenon Ion (MiXI) thruster has demonstrated an efficient discharge and ion extraction grid assembly using filament cathodes and the internal conduction (IC) cathode. JPL is currently preparing to incorporate a miniature hollow cathode for the MiXI discharge. Computational analyses anticipate that an axially upstream hollow cathode location provides the most favorable performance and beam profile; however, the hot surfaces of the hollow cathode must be sufficiently downstream to avoid demagnetization of the cathode magnet at the back of the chamber, which can significantly reduce discharge performance. MiXI's ion extraction grids are designed to provide>3 mN of thrust; however, previous to this effort, the low-thrust characteristics had not been investigated. Experimental results obtained with the MiXI-II thruster (a near replica or the original MiXI thruster) show that sparse average discharge plasma densities of∼5×1015–5×1016 m-3allow the use of very low beamlet focusing extraction voltages of only∼250–500 V, thus providing thrust levels as low as 0.03 mN for focused beamlet conditions. Consequently, the thrust range thus far demonstrated by MiXI in this and other tests is 0.03–1.54 mN.
Miniature ion thrusters are well-suited future space missions such as Terrestrial Planet Finder-Interferometer (TPF-I), where high efficiency thrusters using non-contaminating noble gas propellant are desirable. Transient dynamic and orbital analyses have shown that the low-noise, continuous thrust of the Miniature Xenon Ion (MiXI) thruster is desirable for TPF-I formation rotation maneuvers when compared with other thruster options [1], [2]. The 3cm diameter MiXI thruster, Figure 1, was originally designed using experimental methods and is capable of high Isp (> 3,000 sec), propellant efficiency > 80%, and thrust from <0.1 mN to >1.5 mN [3]. The MiXI thruster must demonstrate high levels of thrust resolution and a low minimum impulse bit to ensure it meets the precision formation flying needs of missions such as TPF-I. A novel concept for controlling the ion extraction voltages yields the necessary thrust characteristics for the MiXI thruster. Experiments verify these techniques and twodimensional computational models show that such techniques should have minimal effect on the lifetime of the thruster. During this effort, the MiXI thruster incorporates, for the first time, flight like hollow cathodes for both the discharge chamber and beam neutralization.
The hollow cathode discharge plasma in ion engines is highly non-uniform, geometrically complex, and confined by a magnetic field; elucidating the mechanisms responsible for producing ions with anomalously high energies observed in the downstream regions of hollow cathodes will contribute to understanding the behavior of complex plasmas. Ions with energies in excess of ten times greater than the energy associated with electrostatic acceleration from the largest steady-state potential difference in the plasma discharge of ion engines have been detected. Several ion acceleration mechanisms have been proposed. By process of elimination, experimental evidence appears to support wave-based acceleration; however, the details of the wave and of the acceleration process are unknown. Ion energy distributions collected with an electrostatic energy analyzer reveal multiple ion populations when the analyzer is aligned along the cathode axis, including a high-energy tail. The high-energy population and tail are present in the distributions when the detector is rotated off of the cathode axis up to 90 o , thereby eliminating the primarily axial acceleration mechanisms. A newly designed instrument that consists of a velocity filter and an energy-per-charge filter in series allows determination of the charge-state of the high-energy ions, thus revealing the importance of multiple charge-exchange collisions. In addition, this instrument is used to examine whether ions of different masses are preferentially accelerated. Recent experiments demonstrate a relationship between ion energies and fluctuations in the discharge plasma. Preliminary investigations that recorded fluctuations in the applied voltage difference between the cathode and anode reveal that when the average energy of ions which have an energy-to-charge ratio greater than the steady-state discharge voltage is high, the power of oscillations with frequencies up to 2MHz increases. In addition, the average energy of ions is a shifted-exponential function of the peak-to-peak fluctuations in the discharge voltage. High sample-rate measurements of ion energies correlated with measurements of plasma potential recorded with a Langmuir probe provide further insight into the relationship between ion energies and oscillations in the plasma.978-1-4244-2636-2/09/$25.00 ©2009 IEEE
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