Power plants – Reaction motor – Ion motor
Reexamination Certificate
2000-12-22
2003-02-25
Kim, Ted (Department: 3746)
Power plants
Reaction motor
Ion motor
C060S203100, C313S111000, C313S111000, C315S362000, C315S154000, C315S231000
Reexamination Certificate
active
06523338
ABSTRACT:
DESCRIPTION
The invention relates to a plasma accelerator arrangement. Plasma accelerators (ion thrusters, electric propulsion systems—EPS) are very important as thrusters in spacecraft both for satellites orbiting close to the earth and geo-stationary satellites, and for space missions outside of the earth orbit. The ratio of driving pulse to mass of the fuel, which is used as a measure of the degree of efficiency of the drive system is substantially more favorable for plasma accelerators than for conventional chemical drive system, with the result that the proportional weight of the fuel is reduced. Such a reduction is of particular importance for space applications. A noble gas with a high atomic weight, in particular xenon is frequently employed as the fuel.
In conjunction with grid ion thrusters, for example U.S. Pat. No. 4,838,021, a plasma is produced in an ionization chamber from neutral gas by high frequency or electron bombardment. The positively charged ions are accelerated in an applied electrical field in the direction of ejection toward a grid electrode. For the electrical neutralization, a current of free electrons has to be added to the accelerated ion current. The neutralized current of plasma exits from the drive system at high speed and accelerates the spacecraft in the opposite direction. Owing to the space charging effects, the density of the ion current is limited, and the drive systems of this type require large cross sections combined with only moderate reaction propulsion effects.
In conjunction with drive systems according to the Hall principle, for example EP 541,309 A1, a ring-shaped ionization chamber is penetrated by an electrical acceleration field parallel with the axis of the ring, and a radial magnetic field. From an external electron source, electrons are guided into the ionization chamber containing neutral xenon gas against the direction of ejection of the ions. In said ionization chamber, the electrons are forcibly guided into spiral orbits because of the magnetic field, and the running distance within the ionization chamber is multiplied in this way versus the direct distance to the anode, with ionizing interaction with the fuel gas being increased in this way as well. Secondary electrodes are affected by the magnetic deflection as well and are accelerated in the electrical field. Furthermore, the given field configuration largely prevents the development of space charging zones that might cause screening of the electrical acceleration field for the positive fuel ions. The acceleration of the positive ions therefore takes place in a largely neutral plasma. Such an arrangement permits distinctly higher current densities than a grid ion drive system arrangement; however, it exhibits an only moderate degree of efficiency due to large widening of the angle of the ejected ion current.
DE 1,222,589 B describes a device for generating a space charge-neutralized beam of charged particles in connection with which a beam of highly accelerated electrons is admitted into an ionization chamber along its longitudinal axis and guided by a magnetic field extending parallel with the longitudinal axis. An arc discharge in the ionization chamber generates from admitted gas slow electrons and positively charged ions. While the latter are accelerated in the direction of the primary electron beam by an ion acceleration electrode and exit from the ionization chamber together with decelerated electrons of the primary electron beam in the form of a neutral plasma beam, the slow electrons of the gas discharge oscillate between the electrodes on the inlet and outside sides, guided by the magnetic field extending parallel with the longitudinal axis. The accelerated ions and the decelerated electrons of the electron beam exit from the arrangement as a neutral plasma beam.
The present invention is based on the problem of proposing a plasma accelerator arrangement in particular in the form of an ion thruster in spacecraft, with an enhanced degree of efficiency.
The invention is described in patent claim 1. The dependent claims contain advantageous embodiments and further developments of the invention.
In conjunction with the arrangement as defined by the invention, the focused electron beam introduced into the ionization chamber first initiates ionization of the neutral fuel gas present in or admitted into said ionization chamber. The secondary electrons released in the course of ionization are accelerated in the opposite direction in the electrical field provided for accelerating the positive ions and themselves act again in an ionizing manner. Following initiation of the ionization process by the electron beam, the secondary electrons may assume the main part of the further ionization.
A further important effect of the admitted electron beam is that focusing of a beam of ions accelerated in the electric acceleration field is favored by compensating its positive space charge with the electron beam, so that no screening of the accelerating electric field takes place. The acceleration field for the positive ions has a decelerating effect on the electrons of the electron beam running in the same direction as the accelerated ion current, so that the space charging density of the electron beam increases in the direction of the longitudinal axis of the ionization chamber, which advantageously corresponds with the concentration of the ion beam desired in the end section of the ionization chamber. The average speed of the electrons of the electron beam and the potential gradient of the acceleration field for the ions, which corresponds with a potential increase for the electrons, are preferably coordinated with each other in such a way that at the end of the acceleration path for the ions (or decelerating path for the electrons of the electron beam), the average speeds of the electrons of the electron beam and of the ions of the accelerated ion current are approximately the same, so that an approximately neutral plasma exits at the end of the acceleration path. The average speeds differ preferably by less than the factor 10.
The electron beam acts through its negative space charge over the entire length of the ionization chamber also as a central means for attracting the positive ions and supports focusing of the accelerated ions in a focused electron current and at the same time compensates mutual repelling of the ions. A widening of the electron beam can be counteracted by a beam guiding and/or beam focusing system consisting of magnetic and/or electric fields. Advantageous is especially a magnetic beam guiding system with a field extending within the zone of the beam substantially parallel with the direction of the beam and in relation to the longitudinal axis of the ionization chamber. Electrons of the electron beam with a component of the motion acting perpendicular to the longitudinal axis are forced by the magnetic field into a spiral orbit around the axis of the beam. Magnetic beam guiding systems are known per se from electron-beam tubes in a great variety of forms and in particular in conjunction with travelling-wave valves in the form of permanently periodic magnet arrangements with field direction reversals occurring along the central axis, where the field has strong radial components as well. Reference is made to such known beam-guiding systems for the purpose of disclosure. Traveling-wave valves with such magnet arrangements are known, for example from DE 2,652,020 B2 or DE-AS-1,491,516.
A magnetic field system is also advantageously suitable for forcing the slow secondary electrons from the ionization processes that are accelerated in the electrical acceleration field for the positive ions in the opposite direction, into spiral-shaped or similarly curved orbits. In this way, the electrons are prevented from rapidly impacting an electrode following against the longitudinal axis of the ionization chamber, on the one hand, and the probability that a secondary electron triggers one or several further ionization processes is distinctly increased, so that the fuel g
Kornfeld Günter
Seidel Harald
Wegener Jürgen
Kim Ted
Thales Electron Devices GmbH
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