Power plants – Reaction motor – Ion motor
Reexamination Certificate
1999-12-29
2001-08-28
Thorpe, Timothy S. (Department: 3746)
Power plants
Reaction motor
Ion motor
Reexamination Certificate
active
06279314
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a closed electron drift plasma thruster having a steerable thrust vector, the thruster comprising at least one main annular ionization and acceleration channel fitted with an anode and ionizable gas feed means, a magnetic circuit for creating a magnetic field in said main annular channel, and a hollow cathode associated with the ionizable gas feed means.
PRIOR ART
By steering the thrust vector of ion thrusters or of closed electron drift thrusters, it is possible to perform attitude control operations by offsetting the thrust vector from the center of gravity of the satellite, or on the contrary, it is possible to counteract parasitic torques by aligning the thrust vector in such a manner as to track the displacements of the center of gravity of the satellite as induced by thermal deformation and by consumption of propellant.
This need has been recognized since the 1970s. Since mechanisms for controlling the thrust vector are naturally rather complex, numerous attempts have been made to replace mechanical thrust control by control that is electrostatic or electromagnetic.
With bombardment ion thrusters, electrostatic deflection has appeared to be the most suitable. The technique most commonly used consists in subdividing each hole of the accelerator grid into four sectors of potential that can be controlled independently, making it possible to achieve an angle of deflection of as much as 3°. Nevertheless, no industrial embodiment has yet been implemented using that type of technique.
Thus, bombardment ion thrusters generally use a mechanical thrust steering device.
By way of example, mention can be made of the Hughes XIPS 13 thrusters on the HS 601 HP satellite and the RIT 10 and UK 10 thrusters on the experimental ARTEMIS satellite.
With closed electron drift thrusters, electro-magnetic deflection has appeared to be the most suitable.
The electric field in a plasma thruster is determined by the radial magnetic field in the magnetic gap. If it is desired to vary the azimuth of the radial magnetic field, the electric field is also varied. The deformation of the equipotential surfaces then causes the angle of the thrust vector to be deflected.
That solution is described, for example, in document U.S. Pat. No. 5 359 258.
Under such circumstances, the external polepiece is subdivided into four sectors, each sector being mounted on a magnetic core with a coaxial coil. Differential feed to the coils serves to modify the azimuth distribution of the magnetic field.
Nevertheless, that disposition has never been used on an operational thruster.
Also known, from document EP 0 800 196 A1, is a thrust steering system in which four coils mounted on four magnetic cores in the form of circular arcs serve to vary the radial magnetic field in azimuth.
Although the various techniques for electro-magnetically controlling the thrust vector of a closed electron drift thruster make it possible to obtain deflection angles of up to 30, they present a series of drawbacks due specifically to the physics of such thrusters. In particular, the fact of locally increasing the electric field changes the position of the erosion zone. Instead of being axially symmetrical, the wear profile then becomes more pronounced on one particular side (since the direction in which the center of gravity of a satellite moves is deterministic). Insofar as it is necessary to change the reference direction in which the beam is pointed, the interface between the plasma and the worn channel wall is no longer symmetrical. This gives rise to wear that is more marked on the side that was previously subjected to moderate wear, but in particular it gives rise to the wear threshold being displaced, and that can be highly disturbing to operation.
It should also be observed that a lifetime test is difficult to specify with an electromagnetically controlled device. As soon as lifetime runs the risk of being a function of the way in which the thrust vector is steered, it becomes practically impossible to demonstrate that the way in which the thrust vector is steered during a lifetime test is more severe than some random law that might be encountered in real operation.
Another drawback is associated with the large drop in efficiency when the ion beam (the thrust vector) is deflected.
In an axially symmetrical thruster, there is nothing opposing the drift motion of the electrons in the annular channel under the effect of the crossed electric and magnetic fields (whence the term “closed electron drift” thrusters).
If the walls of the channels are offset relative to the polepieces, then efficiency is observed to decrease because of the increase in collisions between the electrons and the walls.
The same effect occurs if the magnetic field is increased locally. It will be made worse by asymmetrical wear.
A simple means for controlling the thrust vector can consist in using a plurality of thrusters with the thrust from each being under individual control.
It is then very easy to fix the direction and the amplitude of the resultant thrust vector, and lifetime becomes independent of the way in which thrust is steered. Unfortunately, such a method suffers from the drawback of being expensive when at least three thrusters and at least three electricity power supplies are required.
OBJECT AND BRIEF SUMMARY OF THE INVENTION
The invention seeks to remedy the above-specified drawbacks, and in particular to steer the thrust vector by means of a system that does not excessively increase cost or overall on-board mass, and consequently does not comprise a full set of multiple thrusters, while nevertheless making it possible to achieve control over the steering of the thrust vector that is easy and effective, with deflection angles of sufficient magnitude, and without creating uncontrollable asymmetries.
These objects are achieved by a closed electron drift plasma thruster having a steerable thrust vector, the thruster comprising at least one main annular ionization and acceleration channel fitted with an anode and ionizable gas feed means, a magnetic circuit for creating a magnetic field in said main annular channel, and a hollow cathode associated with the ionizable gas feed means, wherein the thruster comprises a plurality of main annular ionization and acceleration channels having axes that are not parallel and that converge downstream from the outlets of said main annular channels, wherein the magnetic circuit for creating a magnetic field comprises a first external polepiece that is downstream and common to all of the annular channels, a second external polepiece common to all of annular channels and that is disposed upstream from the downstream first external polepiece, a plurality of internal polepieces in number equal to the number of main annular channels and mounted on first cores disposed about the axes of the main annular channels, a plurality of first coils disposed respectively around the plurality of first cores, and a plurality of second coils mounted on second cores disposed in empty spaces left between the main annular channels, said second cores of the second coils being interconnected via their upstream portions by ferromagnetic bars and being connected via their downstream portions to said downstream first external polepiece, and wherein the thruster comprises means for regulating the ionizable gas feed flow rate to each of the main annular channels and means for controlling the ion discharge and acceleration current in the main annular channels.
The axes of the main annular ionization and acceleration channels converge on the geometrical axis of the thruster and may form angles lying in the range 5° to 20° relative to the geometrical axis of the thruster.
Each main annular ionization and acceleration channel comprises an anode associated with a manifold fed with ionizable gas by means of a pipe connected via an isolator to a flow rate regulator.
The hollow cathode is fed by a pipe connected via an isolator to a head loss member.
The flow rate regulators and the head loss member are fed f
Klinger Eric
Lyszyk Michel
Valentian Dominique
Gartenberg Ehud
Societe Nationale d'Etude et de Construction de Moteurs d&a
Thorpe Timothy S.
Weingarten, Schurgin Gagnebin & Hayes LLP
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