Patent Application
20060075739
1. Field of the Invention
This invention relates to ion thruster engines in general, and in particular, to a circuit that couples to the accelerator grid of an ion engine in such a way that the voltage difference, and hence, the energy contained in a plasma arc occurring between the screen grid and the accelerator grid of the engine, is minimized, thereby reducing or eliminating the damage to the grids caused by such arcing.
2. Related Art
In accordance with well-known Newtonian principles, if electrically charged particles, or ions, are accelerated to a high velocity in a vehicle and then discharged from it, the vehicle will be propelled in a direction opposite to that of the discharged particles, thus giving rise to the development of “electrostatic,” or “ion thruster” engines for space vehicles. While such engines can produce only a very small amount of thrust compared to that produced by the larger and more familiar chemical rocket engines, they are capable of operating continuously for substantially longer periods of time than the latter, and additionally, have a favorably high “specific impulse” (“ISP”) figure when compared to the latter, where ISP is the ratio of thrust to the rate of use of propellant. Thus, while ion engines may lack the large thrust necessary to lift a heavy payload into orbit, they nevertheless have wide application in deep space missions, such as interplanetary exploration, and in orbital satellites for orbital positioning and attitude control, where engine “burns” may last for days, weeks, or even years. Exemplary ion thruster engines are discussed in, e.g., U.S. Pat. No. 4,838,021 to J. Beattie; U.S. Pat. No. 3,156,090 to H. Kaufmann; and, U.S. Pat. No. 3,052,088 to J. Davis et al.
A typical ion engine comprises an ionization chamber having a pair of separate, spaced-apart, fenestrated electrodes, or “grids” having respective, aligned apertures therein, viz., a “screen” grid and an “accelerator” grid, disposed at one end thereof. The two grids are charged with voltages of opposite polarities such that a relatively large voltage potential, and hence, a strong electric field, exists between the two grids. A “propellant,” e.g., gaseous xenon or mercury atoms, is introduced into the chamber, where it is ionized to produce a plasma. The ionized particles form a neutral plasma, which essentially fills the chamber. The propellant ions that pass through the apertures of the screen grid, and thence, into the strong electric field between the two grids, are forcefully accelerated from the chamber through the apertures in the accelerator grid, resulting in a reactive thrust being applied to the ionization chamber.
During operation of the engine, the accelerator grid is normally subject to some erosion caused by “charge-exchange” ions. This erosion is exhibited as a pattern of pits and grooves that occurs on the surfaces of the grids. In addition to this “normal” type of wear of the engine, electrical plasma arcs occasionally occur between the screen and accelerator grids. These arcs are caused by various operational anomalies occurring in the engine, and can cause additional damage to the screen and accelerator grids over and above that caused by the normal charge-exchange ion erosion described above. This additional type of damage to the grids has been shown to make the occurrence of plasma arcs between the grids more frequent by degrading the high voltage integrity of the screen-grid-to-accelerator-grid interface. Such damage can result in a substantial reduction in the reliability and operational life of the engine.
A long-felt but as yet unsatisfied need therefore exists in this field for a reliable, low-cost and lightweight mechanism for reducing the energy contained in a plasma arc occurring between the accelerator grid and the screen grid of an ion engine, thereby minimizing or eliminating the damage caused to the grids of the engine by arcing.
In accordance with the present invention, a circuit is provided that couples to the accelerator grid of an ion engine in such a way that the voltage differential, and hence, the energy contained in a plasma arc occurring between the screen grid and the accelerator grid of the engine, is minimized, thereby reducing or eliminating the damage to the grids caused by the arc.
In a first exemplary embodiment thereof, the novel grid arcing protection circuitry includes a protection circuit that comprises a fixed impedance in series between the accelerator grid and the output of the internal accelerator power source within the ion thruster. During an arc between the accelerator grid and screen grid of the ion thruster, the added impedance causes a rapid reduction in the voltage of the accelerator grid relative to that of the screen grid.
While it is desirable to have the added impedance present in the arc protection circuit during normal operation of the ion engine, it may be desirable to reduce its impedance during startup of the engine, so that the accelerator grid current can quickly achieve a relatively high startup level for a brief period of time required for the engine to start. For this purpose, the arcing protection circuit further includes a circuit for selectably coupling the impedance into and out of the accelerator grid-to-accelerator-supply output path, and in one possible embodiment, this impedance coupling circuit may comprise a simple relay.
The accelerator grid voltage is regulated at the required voltage while the fixed impedance is in series with the accelerator grid or when the fixed impedance is switched out of the accelerator-grid-to-accelerator-supply output path. This is accomplished by an error amplifier, which senses the voltage at the accelerator grid and provides a feedback signal proportional to the accelerator grid voltage that enables the magnitude of the internal accelerator power source within the ion thruster to be varied to maintain the accelerator grid at its required voltage during all modes of operation.
In an alternative exemplary embodiment, the grid arcing protection circuit can advantageously incorporate, in addition to the above accelerator grid monitoring circuit of the first embodiment, a variable impedance in place of the fixed impedance. As in the first embodiment, this impedance is coupled in series between the accelerator grid and the output of the internal accelerator power source within the ion thruster, the difference in this embodiment being that the impedance is variable. During a plasma arc between the screen and accelerator grids, the plasma arc causes the impedance of the circuit to increase rapidly. This action quickly reduces the relative voltage difference between the accelerator grid and the screen grid, thereby extinguishing or substantially limiting the energy of the arc almost instantaneously.
Advantageously, the accelerator grid voltage is regulated at the required voltage by the accelerator grid monitoring circuit, while the variable impedance is in series with the accelerator grid, by an error amplifier, which senses the voltage at the accelerator grid and provides a feedback signal proportional to the accelerator grid voltage that enables the magnitude of the internal accelerator power source within the ion thruster to be varied so as to maintain the accelerator grid at its required voltage during normal operation.
In one advantageous embodiment thereof, the variable impedance comprises an electrically actuated switch. The active switch may comprise a plurality of first transistors, which may be MOSFETs or Bipolar Junction Transistors, (“BJTs”), coupled in series between the accelerator grid and the output of the internal accelerator power source within the ion thruster. These transistors have their respective bases coupled in parallel and biased such that they operate in the fully saturated mode during normal thruster operation. A second transistor is coupled between the bases of the first transistor and the output of the internal accelerator power source within the ion thruster. A pulse transformer is also included, which has a primary winding coupled in series between the accelerator grid and the collector of the first transistor in the variable impedance string of transistors. The excess current during a plasma arc is sensed by the pulse transformer, with the secondary of the pulse transformer connected directly to the second transistor and a base circuit of the second transistor. The second transistor is coupled between the bases of the first transistors in the variable impedance string, which is connected to the internal accelerator power source within the ion thruster; the action of the pulse transformer being used to turn off all of the transistors in the variable impedance string rapidly. This action quickly reduces the voltage difference between the accelerator grid relative to the screen grid, thereby almost instantaneously extinguishing or substantially limiting the energy of the arc.
A better understanding of the above and many other features and advantages of the grid arc protection circuit of the present invention may be obtained from a consideration of the detailed description thereof below, particularly if such consideration is made in conjunction with the several views of the appended drawings.
An ion thruster engine 100 incorporating a grid arcing protection circuit 102 and 104 in accordance with a first exemplary embodiment of the present invention is illustrated in the schematic diagram of
Electrically neutral atoms of a propellant 124, e.g., gaseous xenon or mercury, are stored under pressure in a storage container 126 and introduced through a control valve 128 into the ionization chamber 108 adjacent to the discharge cathode 106, where they collide with electrons generated by the discharge cathode to produce a plasma containing ions 130, i.e., positively ionized atoms of the propellant. The positively charged ions form a neutral plasma, which essentially fills the discharge chamber. The propellant ions that pass through the apertures of the screen grid, and thence, into the strong electric field between the two grids, are forcefully accelerated from the discharge chamber through the apertures in the accelerator grid, resulting in a reactive thrust. As the propellant ions exit the accelerator grid, they are neutralized, i.e., rendered electrically neutral again, by electrons produced by the neutralizer cathode 114, so that that the ion engine 100, and the spacecraft to which it is attached, are maintained at a neutral electrostatic potential.
While the particular parameters may vary widely, depending on the particular ion engine 100 under consideration, a typical value for the voltage supplied by the screen power supply 120 is +1800 Volts DC (VDC). A typical discharge power supply 116 voltage is +25 VDC. This results in a net screen grid 110 voltage of approximately +1775 VDC with respect to a neutralizer supply 118 current return path, or “neutralizer common” return 132. A typical value for the voltage supplied by the accelerator supply 122 to the accelerator grid 112 is −250 VDC with respect to the neutralizer common. This results in a relative screen-grid-to-accelerator-grid voltage differential of approximately 2000 VDC.
During operation of the ion engine 100, plasma arcs can occur between the screen and accelerator grids 110, 112. These arcs are caused by various operational anomalies occurring in the engine, and can cause additional damage to the screen and accelerator grids over and above that caused by the normal charge-exchange ion erosion described above. This additional type of damage to the grids has been shown to make the occurrence of plasma arcs between the grids more frequent by degrading the high voltage integrity of the screen-grid-to-accelerator-grid interface. Such damage can result in a substantial reduction in the reliability and operational life of the engine. A first embodiment of a grid arcing protection circuit, comprising two parts 102 and 104, for preventing or reducing this type of damage to the grids is outlined by the dashed lines of
As illustrated in
Referring again to
An alternative embodiment of a grid arcing protection circuit 204 for an ion thruster engine is illustrated in
The variable impedance comprises an electrically actuated switch 248 described in more detail below. The switch comprises a plurality of first transistors 253 coupled in series between the capacitor 242, which is at the output 250 of the accelerator power supply, and the accelerator grid 212, as illustrated in
As in the first embodiment above, during normal operation of the engine 200, the power dissipated in the protection circuitry 204 is very low in comparison to the fixed impedance approach illustrated in
As will by now be evident to persons of skill in this art, many modifications, substitutions and variations can be made in and to the materials, configurations and methods of implementation of the grid arcing protection circuit of the present invention without departing from its spirit and scope. For example, the first transistors 253 of the switch 248 of the second embodiment may be implemented either as BJTs or as MOSFETs, depending on the speed, current and voltage requirements of the particular problem at hand. Accordingly, the scope of the present invention should not be limited to the particular embodiments illustrated and described herein, as they are merely exemplary in nature, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.
