US 3613370 A
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Oct. 19, 1971 T. O. PAINE ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION ION 'IHRUSTER Filed Nov. 26, 1969 FIG,
INVENTORS WOLFGANG KNAUER ROBERT 1.. POESCHEL Q A"? 01.
ATTORNEYS United States Patent O 3,613,370 ION THRUSTER T. O. Paine, Administrator of the National Aeronautics and Space Administration, with respect to an invention of Wolfgang Knauer, Malibu, and Robert L. Poeschel,
Los Angeles, Calif.
Filed Nov. 26, 1969, Ser. No. 880,246 Int. Cl. F03h 1/00; F04b 37/02; F05h 1/04 U.S. Cl. 60202 11 Claims ABSTRACT OF THE DISCLOSURE Improving the efiiciency of an ion thruster by utilizing a radial magnetic field to achieve uniformity in electron density and energy.
ORIGIN OF THE INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
BACKGROUND OF THE INVENTION This invention is concerned with an improved ion thruster. The invention is particularly directed to an ion thruster having a uniform ion beam profile.
A typical electron bombardment ion thruster of the type shown in U.S. Pat. No. 3,156,090 utilizes a centrally disposed cathode in an ionization chamber. Electrons emitted from this cathode diffuse across the axial magnetic field to an anode cylinder while ionizing a propellant gas. As the electrons approach the anode their kinetic energies become depleted by the ionizing collisions. Also, the radial diffusion across the field results in a decrease in electron density near the anode. The combination of low electron temperature and low electron density results in a greatly reduced rate of ionization of the gas near the anode. Consequently, propellant gas can escape in significant quantities near the anode.
In an attempt to solve these problems and reduce losses to a tolerable level the electron injection rate has been increased. This enhances the electron density throughout, but the central density becomes unnecessarily high. The additional injection current not only increases the power consumption, but also the centrally peaked electron density produces a centrally peaked exhaust rate of ions.
OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to improve the electrical effciency of the ion generation mechanism in an ion thruster.
Another object of the invention is to provide a more uniform ion exhaust across the face of an ion thruster.
A further object of the invention is todecrease the length of the discharge chamber of an ion thruster.
These and other objects of the inventioen will become apparent from the specification which follows and from the drawing wherein like numbers are used throughout to identify like parts.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of an electron bombardment ion engine constructed in accordance with the present invention.
FIGS. 2, 3 and 4 are schematic views of alternate embodiments of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT The electrostatic ion thrusters shown in the drawings 3,5133? Patented Oct. 19, 1971 utilize electron bombardment ion sources. In each of these thrusters, neutral particles in the form of a gaseous propellant enter an ionization chamber 10 where high velocity electrons ionize the particles to form a plasma. A screen grid 12 and an accelerator grid 14 at the downstream end of the ionization chamber focus and accelerate ions that reach that end. The thrust producing mechanism is the momentum change of the ions as they are accelerated by the electrostatic field which is applied between the screen and accelerator grids.
The particles densities of the plasma are sufficiently small that the mean free path for ionization is quite long thereby necessitating the containment of the high velocity ionizing electrons within the ionization chamber 10 as explained in U.S. Pat. No. 3,238,715. A magnet field is utilized to lengthen the path of thesehigh velocity electrons.
In the embodiment shown in FIG. 1 a propellant vapor, such as mercury, is introduced into the ionization chamber 10 about its periphery adjacent the screen grid 12 through an annular passage 16. This propellant feed system is similar to that disclosed in U.S. Pat. No. 3,262,262. Particles of propellant vapor are bombarded by electrons from a hollow cathode 18 supported in the center of the chamber 10.
According to the present invention a disk-shaped electrode 20 forms an anode that is positively charged relative to the cathode 18 for collecting the electrons. The 'anode 20 extends radially outward from the cathode 18 at the opposite end of the chamber 10 from the screen grid 12. An electromagnet 22 provides a magnetic rfield that contains the ionizing electrons emitted from the cathode 18. The electromagnet 22 encircles a tubular housing 24 which forms the outer wall of the ionization chamber 10.
A central post 26 extends along the axis of the ionization chamber 10. Also a ring 28 attached to the housing 24 extends about the periphery of the ionization chamber 10. Both the post 26 and the ring 28 are of a magnetic material, such as soft iron. These elements provide permeable paths for the magnetic lines of flux illustrated by the dotted lines in FIG. 1. The magnetic field extends radially outward from the central post 26 and is substantially parallel to both the disk-shaped anode 20 and screen grid 12.
The central post 26 is hollow, and its interior is placed in communication with the chamber 10 by a plurality of apertures 30. The cathode 18 extends into the post 26 as shown in FIG. 1. Electrons from the cathode 18 pass through the apertures 30 in the post 26 toward the anode 20.
The post 26 and housing 24 are maintained at the potential of the cathode 18 so that the electrons are reflected from both. In this arrangement the electrons are trapped and must diffuse a considerable distance across the magnetic lines of flux. This provides an opportunity for the electrons to strike the particles of propellant gas for the ionization process.
An ion thruster embodying the features shown in FIG. 1 having a diameter of 15 cm. was tested. This thruster yielded an ion beam of 260 ma. at an energy expenditure of 190 ev. per ion at a mass utilization of DESCRIPTION OF THE ALTERNATE EMBODIMENTS of the central post 36 in the ionization chamber provides support for the screen grid 12 and the accelerator grid 14. It will be appreciated that an electron source may be mounted on the post 36 to serve as an ion beam neutralizer.
A liquid mercury cathode 38 extends into the post 36. Electrons from the cathode 38 pass through apertures 40 in the post 36. Both the center post 36 and the housing 34 are maintained at the potential of the cathode 38 so that the electrons are reflected from them. The center post 36 and the housing 34 are both made of a magnetically permeable material. There is a predominantly radial magnetic field between these elements.
In the embodiment shown in FIG. 3 a disk-shaped anode 24 again forms the end of an ionization chamber 10 opposite the screen grid 12. Permanent magnets 42 are mounted adjacent the anode 20. A bifurcated conduit 44 directs propellant into the ionization chamber 10 through a central post 46. An oxide cathode 48 extends about the periphery of a central portion of the post 46.
Both the central post 46 and the tubular housing 34 are of magnetic material. These elements form pole pieces, and with the permanent magnets 42 they provide a radial field for the magnetic lines of flux from the permanent magnets 42.
In the embodiment shown in FIG. 4 a curved housing 50 extends between the screen grid 12 and the disk-shaped anode 20 to form an ionization chamber 10. Permanent magnets 52 are mounted adjacent the anode 20. Propellant feed tubes 54 provide propellant to the chamber 10. A hollow cathode 56 extends into a central post 58 mounted along the axis of the chamber 10. Both the curved housing 50 and the post 58 are of a magnetic material to provide a generally radially extending magnetic field. Electrons pass through apertures 60 in the center post 48 and move toward the anode 20.
What is claimed is:
1. In an electron bombardment ion thruster of the type having a chamber containing an ionizable propellant, a
cathode for emitting electrons to ionize said propellant,
and an apertured grid system extending across one end of said chamber for focusing and accelerating propellant ions, the improvement comprising an anode mounted at the opposite end of the chamber from the aperture grid system for collecting electrons from the cathode, magnetically permeable material extending about said ionizable propellant between said anode and said grid system, a centrally disposed post extending into said chamber,
and a magnet adjacent said magnetically permeable material for forming a magnetic field in said chamber for lengthening the path of the electrons, said magnetic field being substantially parallel to said grid system.
2. Apparatus as claimed in claim 1 wherein the anode forms a wall at the opposite end of the chamber from the apertured grid system.
3. Apparatus as claimed in claim 2 wherein the anode comprises a disk-shaped electrode, and
the magnetic field extends across the chamber parallel to said electrode.
4. Apparatus as claimed in claim 1 wherein the central post and the cathode are mounted substantially at the longitudinal axis of the chamber normal to the grid system, and
the magnetic field extends radially outward from said central post and said cathode.
5. Apparatus as claimed in claim 4 wherein the central post comprises a hollow member of a magnetic material extending along said longitudinal axis of the chamber to provide a permeable path for magnetic lines of flux.
6. Apparatus as claimed in claim 5 including a magnet extending radially outward from said hollow member, said magnet being mounted adjacent the anode.
7. Apparatus as claimed in claim 5 including an annular cathode extending about the hollow member.
8. Apparatus as claimed in claim 5 wherein the hollow member has a plurality of apertures, and
the cathode extends into said hollow member whereby electrons pass through said apertures to the chamber.
9. Apparatus as claimed in claim 8 including a magnet extending about the periphery of the chamber, and
an annular member of magnetic material extending about the periphery of said chamber adjacent said magnet for providing a permeable path for magnetic lines of flux.
10. Apparatus as claimed in claim 8 including a hollow cathode extending into the hollow member of magnetic material.
11. Apparatus as claimed in claim 8 including a liquid mercury cathode extending into the hollow member of magnetic material.
References Cited UNITED STATES PATENTS 3,238,715 3/1966 Reader et al 202 3,262,262 7/1966 Reader et al. 60202 3,279,175 10/1966 Hendel et al. 60202 3,412,559 11/1968 Sohl 60202 FOREIGN PATENTS 1,366,930 6/1964 France 60202 CARLTON R. CROYLE, Primary Examiner J. J. VRABLIK, Assistant Examiner US. Cl. X.R.