Power plants – Reaction motor – Ion motor
Reexamination Certificate
2000-05-09
2001-06-26
Kim, Ted (Department: 3746)
Power plants
Reaction motor
Ion motor
C313S360100
Reexamination Certificate
active
06250070
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to ion thrusters and, more particularly, to the shapes of the grids used in the ion-optics system of the ion thruster.
Ion thrusters are used in spacecraft such as communications satellites for stationkeeping and other functions. An important advantage of the ion thruster over an engine using chemical propellants is that it utilizes the electrical power generated by the solar cells of the satellite to achieve the propulsion. The ion thruster has a high specific impulse, making it an efficient engine which requires very little propellant. Since the ion thruster requires relatively small amounts of the consumable propellant that is ionized, it is therefore not necessary to lift large masses of chemical fuel to orbit.
In an ion thruster, a plasma is created and confined within the body of the thruster. Ions from the plasma are electrostatically accelerated rearwardly by an ion-optics system. The reaction with the spacecraft drives it forwardly, in the opposite direction. The force produced by the ion thruster is relatively small. The ion thruster is therefore operated for a relatively long period of time to impart the required momentum to the heavy spacecraft. For some missions the ion thruster must be operable and reliable for thousands of hours of operation, and with multiple starts and stops.
The ion-optics system includes grids to which appropriate voltages are applied in order to accelerate the ions rearwardly. The grids are in a facing orientation to each other, spaced apart by relatively small clearances such as about 0.035 inches at room temperature. The grids include aligned apertures therethrough. Some of the ions accelerated by the applied voltages pass through the apertures, providing the propulsion. Others of the ions impact the grids, heating them and etching away material from the grids by physical sputtering. The heating and electrostatic forces on the grids combine to cause substantial mechanical forces at elevated temperature on the grids, which distort the grids unevenly. The uneven distortion of the grids causes adjacent grids to physically approach each other, rendering them less efficient and prone to shorting against each other. These effects are taken into account in the design of the grids and the operation of the ion thruster, so that the thruster remains functional for the required extended lifetimes. However, limitations may be placed on the operation of the ion thruster because of grid distortion, such as a relatively slow ramp-up in power during startup and operation, so that the adjacent grids do not expand so differently that they come into contact.
At the present time, the grids are usually made of molybdenum formed into a domed shape. The molybdenum resists material removal by physical sputtering. The domed shape establishes the direction of change due to thermal expansion and aids in preventing a too-close approach of the adjacent grids as a result of differences in temperatures of the adjacent grids. While the available grids are operable in current engines, it is expected that uneven expansion of the grids may limit the extension of ion thrusters to larger sizes and higher power ranges, as well as to certain desired operating ranges such as rapid start-up and acceleration.
Accordingly, there is a need for a better approach to the grids used in the ion-optics systems of ion thrusters. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an ion thruster whose grids have an improved structure. The grids are shaped so that they maintain a desired clearance between the adjacent grids during both transient and steady-state conditions. An overly close approach of the grids and failure by shorting due to contact between adjacent grids are avoided. With these grids, the ion thrusters may be built to produce larger power outputs in smaller volumes than previously possible. They may also be started and adjusted in power output more quickly.
In accordance with the invention, an ion thruster comprises a source of a plasma, and an ion-optics system located in sufficient proximity to the source of the plasma to extract ions therefrom along a thrust axis. The ion-optics system comprises at least two grids arranged in a facing-but-spaced-apart relationship to each other. In some designs of particular interest, there are exactly two grids and in other designs there are exactly three grids. Each grid comprises a peripheral region defining a grid plane perpendicular to the thrust axis, a first region of curvature adjacent to the peripheral region, and a second region of curvature along the thrust axis such that the first region of curvature lies between the second region of curvature and the peripheral region. The first region of curvature is convexly curved relative to the grid plane, and the second region of curvature is concavely curved relative to the grid plane. There may be additional regions of curvature, such as an intermediate region joining the first region and the second region.
Desirably, the first region of curvature is a segment of a first sphere, and/or the second region of curvature is a segment of a second sphere. In this embodiment, the first sphere has a first-sphere radius of curvature and a first-sphere center lying along the thrust axis at a first-sphere distance from the grid plane, and the second sphere has a second-sphere radius of curvature and a second-sphere center lying along the thrust axis at a second-sphere distance from the grid plane. In this embodiment, the first-sphere radius of curvature and the second-sphere radius of curvature are the same, and the second-sphere distance is greater than the first-sphere distance. Each grid is preferably made of molybdenum, but it may be made of other operable grid materials as well.
When an ion thruster is started, stopped, or otherwise changed in output power, the grid nearest the plasma changes temperature first, then the second grid changes temperature, then the third grid (if any) changes temperature. As each grid changes temperature, it distorts due to its coefficient of thermal expansion. The axial temperature gradient and consequent varying distortions of the grids potentially lead to a loss in efficiency and even to catastrophic failure by shorting if two grids come into sufficiently close proximity to allow shorting.
A key consideration in the design of the multi-grid structure of the ion-optics system of the ion thruster of the present invention is maintaining the gap clearance dimensions between adjacent grids to a high degree of accuracy, during steady state operations and during transients, regardless of temperature changes, temperature differences between the adjacent grids, thermal gradients, and thermal transients. Loss of efficiency due to changes in the gap between the adjacent grids is minimized, and catastrophic failure resulting from contact shorting of the adjacent grids is avoided. The compound curvature of each grid of the present approach permits the gap between the adjacent grids to be maintained without substantial variation, over a wide range of conditions. This result is achieved because the expansions in the two regions of curvature tend to offset each other. The grid structure is therefore usable in larger sizes, under higher power loads and power densities, and with more demanding operating conditions than heretofore possible such as rapid startup and rampup procedures.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 3702416 (1972-11-01), Bex et al.
patent: 3793550 (1974-02-01), Thompson, Jr.
patent: 4731558 (1988-03-01), Haisma et al.
patent: 4879518 (1989-11-01), Broadhurst
patent: 5689950 (1997-11-01),
Beattie John R.
Kreiner Kurt B.
Pilcher Lewis S.
Wittmann Alois
Gudmestad T.
Hughes Electronics Corporation
Kim Ted
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