Power plants – Reaction motor – Ion motor
Reexamination Certificate
2000-02-02
2001-11-20
Thorpe, Timothy S. (Department: 3746)
Power plants
Reaction motor
Ion motor
C313S360100
Reexamination Certificate
active
06318069
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to ion thrusters and, more particularly to the structure and material of construction 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 requires relatively small amounts of a consumable propellant that is ionized, and it is not necessary to lift large masses of chemical fuel to orbit. The ion thruster also has a high specific impulse, making it an efficient engine which requires very little propellant.
In an ion thruster, a plasma of ions and electrons 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. An ion thruster is often built to be small in size, so that the force produced by the ion thruster is small. The ion thruster is therefore operated for a relatively long time. Consequently, for some missions, the ion thruster must be operable and reliable for thousands of hours of operation.
The ion-optics system includes grids to which appropriate voltages are applied in order to accelerate the ions rearwardly. 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. The heating and electrostatic forces on the grids combine to cause substantial mechanical forces at elevated temperature on the grids, which distort the grids and make them susceptible to fatigue and mechanical failure. These effects are taken into account in the design of the grids, so that they remain functional for the required extended lifetimes.
At the present time, the grids are typically made of molybdenum formed into a domed shape. It has also been proposed to make flat grids of carbon-carbon composite material. Both of these materials have limitations in respect to the lifetime of the grid and its mechanical strength. Accordingly, there is a need for a better material of construction and design for the grids of 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 improved ion thruster and the grids used in its ion-optics system. The grids are made so as to allow heat produced during operation to be conducted away and also to have lower thermal expansion. The result is that the grids operate at lower temperature and experience lesser mechanical deformation than conventional grids. They have good strength, particularly when used in the preferred domed configuration.
In accordance with the invention, an ion thruster comprises a discharge cathode source of a plasma of ions and electrons, and an ion-optics system located in sufficient proximity to the source of the plasma to draw ions therefrom. The ion-optics system comprises at least two nonplanar, domed grids arranged in a facing-but-spaced-apart relationship to each other. Each grid may be described as having a local reference vector that is perpendicular to the surface of the grid and a reference plane perpendicular to the reference vector. At least one of the grids, and preferably all of the grids, is made of anisotropic pyrolytic graphite having an “ab” crystallographic plane that lies substantially in the reference plane.
Pyrolytic graphite is a highly anisotropic material. That is, its structure and properties vary greatly as a function of the direction of measurement relatively to its crystallographic directions. Its structure and properties are distinct from conventional graphite, amorphous carbon, graphite fibers, and carbon-carbon composite material, all of which are not operable in the present invention. The pyrolytic graphite has important advantages when used in the ion-optics system of an ion thruster, with the indicated orientation, based upon its different structure and properties from other forms of carbon and graphite.
In pyrolytic graphite, the “ab” crystallographic plane, also sometimes termed the “basal” direction, has a high coefficient of thermal conductivity. Heat produced by the impact of ions onto the grid is conducted rapidly away to heat sinks at the edge of the grid. Also, the thermal emittance of the pyrolytic graphite is much larger than that of metals such as molybdenum used in conventional ion-optics grids, so that a larger fraction of the heat resulting from the ion impacts is radiated away. In combination, these effects allow the pyrolytic graphite grid to operate at lower temperatures than conventional grids.
In pyrolytic graphite, the coefficient of thermal expansion in the “ab” crystallographic plane is low. For a given temperature increase, there is less distortion of the grid than for a metallic grid. The combination of low coefficient of thermal expansion and high coefficient of thermal conductivity in the “ab” crystallographic plane, and the high thermal emittance, results in less distortion of the grid and of its individual apertures than possible with other materials of construction. The apertures in the facing grids remain in a better alignment with the approach of the invention, resulting in better efficiency of the ion thruster as compared with conventional ion thrusters.
The ion etch rate for pyrolytic graphite, like other forms of carbon, is quite low. The lifetime of the pyrolytic graphite grids is therefore longer than that of metallic grids.
The pyrolytic graphite grids may be fabricated by any of several operable approaches. One approach involves machining the grid from a block of the pyrolytic graphite. In another approach, the pyrolytic graphite is deposited on a mandrel that defines the shape of the grid. 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: 4538067 (1985-08-01), Cuomo
patent: 5439191 (1995-08-01), Nichols
patent: 5924277 (1999-04-01), Beattie et al.
Reaves, et al. “Directly Heated Tungsten Dispenser Cathodes for Ion Laser Application” (1969) Laser Journal, 4 pages.
Beattie John R.
Falce Louis Raymond
Gartenberg Ehud
Gudmestad T.
Hughes Electronics Corporation
Thorpe Timothy S.
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