Патент USA US3050662код для вставки
Aug. 21, 1962 G. c. BALDWIN 3,050,652 METHODS AND APPARATUS FOR DEVELOPING FORCES WITH ION BEAMS Filed Aug. 12, 1960 1C//r A.|1 s WW 6B tm wN )6 6 5. 5. 1d.vlQ?RE>NmqSwK/lk1 a1 d o ,6 n .m m r. A e0 m w .w m. WNW 5, 1 w em r nw Ox a ca C WW” .v/ 5eV. 4 00 D r O sz/ and n., ,. u.” .n m United States atet 3,050,652 Patented Aug. 21, 1952 2 1 even check all emission of ions and electrons if they are 3,050,652 METHODS AND APPARATUS FOR DEVELOPING FORCES WITH ION BEAMS George C. Baldwin, Schenectady, N.Y., assignor to Gen eral Electric Company, a corporation ‘of New York ejected from separated sources. _ For most e?icient operation of an ion propulsion sys tem, the above described neutralization process must, 5 therefore, insure that the total ion and electron currents from the vehicle be exactly equal in order that thrust Filed Aug. 12, 1960, Ser. No. 49,366 7 Claims. (Cl. 313—63) be sustained. Also, neutralization must be achieved in a manner so that the initial spatial separation of electron and ion streams ejected by the vehicle is reduced to_a My invention relates to force developing systems and, in particular, to methods and apparatus employing a beam 10 minimum, producing an electrically neutral beam, 1n order to avoid wasteful power drain. This requires that of high speed ions to develop a force for propulsion the discharge velocity of the electrons match that of the purposes. Ion propulsion is a means for generation of reaction ions. It is an object of my invention to provide new and im thrust in which electrostatic ?elds impart momentum to ions of an expellant substance. The expellant must be 15 proved apparatus and methods employing a beam of high speed ions to develop a force for propulsion purposes m ionized, its ions and electrons separately accelerated to which the total electrical charge of the beam at its exit the appropriate velocities by independent sets of electro point is substantially zero. _ static ?elds ?xed in the vehicle, remixed and exhausted It is another object of my invention to provide new from the vehicle in more or less well collimated fashion. The vector sum of the momenta of all particles per 20 and improved methods and apparatus for neutralizing the discharge of an ion propulsion system. manently leaving the vehicle per unit time is the average It is still another object of my invention to improve thrust produced in that time interval. the ef?ciency of operation of an ion propulsion system. The propulsion of a vehicle in space utilizes the re One of the features of my invention consists in pass action force mv on the vehicle due to ejection of a mass m of expellant at velocity v and mass ?ow rate m. In 25 ing a beam of high speed ions utilized for force develop ing purposes through a drift space in which the ions en order to accomplish a given mission, a de?nite increment counter a cloud of electrons and thoroughly mingle there of momentum mv must thereby be imparted to the ex with before emerging from the space and limiting the pellant. In order to conserve expellant mass, it is de number of electrons emerging from the space to that re sirable to make the exhaust velocity v as high as possible. Since v is limited for chemical propellants, electrical 30 quired to neutralize the electrical charge of the ions. In one of its forms, my ion beam force developing propulsion schemes are of considerable current interest. system employs two successive electrodes at substantially equal potentials, through which the ion beam is passed, These involve the ionization of the expellant and its acceleration by electric ?elds. It is necessary, however, that the vehicle maintain electrical neutrality. This re quires that the ejected ion beam be neutralized by the and an arrangement to introduce electrons between the electrodes. The electrons so introduced are trapped in a potential well created by positive ion space charge be tween the electrodes in which they oscillate, but do not necessarily leave the vehicle. By neutralizing space mixture of an equal current of electrons. Attempts to introduce electrons into accelerated ion beams have proved di?icult because of the high electric ?elds created by the ion space charge. These ?elds cause the electron to execute intense oscillations which preclude the match ing of electron and ion velocity and constitute a severe energy drain upon the vehicle power supply. charge in the region ‘between electrodes, they prevent the formation of strong electric ?elds in this region, allowing other electrons emitted in the space to be drawn from the vehicle by the already emitted ion beam at low uniform potentials, thereby avoiding excessive drain of When an ion beam is used as the force developing sys tem for propelling a vehicle, as a consequence of the elec power. The invention, both as to its organization and opera trical isolation and the low intrinsic capacitance of the tion, together with further objects and advantages there of, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which: system, the potential of the system will change rapidly if any net current is drawn causing it to interact with the beam expelled. This potential change may rapidly become su?icient to stop all net charge ?owing from the system during a time interval corresponding to a very 50 short motion of the expellant particles. Described in FIG. 1 is a schematic cross section of a portion of an ion propulsion system embodying my invention, and FIGS. 2—6 are curves illustrating certain operational another way, for each ion ejected, an electron remains characteristics of the system of FIG. 1. lowering the vehicle potential and increasing the ?eld In the structure of FIG. 1, ions are emitted normally about the vehicle which retards the ions. It is, therefore, imperative that electrons be emitted along with the ions, 55 from the surface of a curved emitter l and pass through the aperture 2 in an accelerating electrode 3 into a ?eld free region or drift space 4 de?ned by a neutralizing elec trode 5 and a drift electrode 6. Electrodes 5 and 6 are maintained at substantially the same potential, which is guarantees that steady emission is at least on the average possible. 60 intermediate the potential of emitter 1 and accelerating electrode 3. For example, electrodes 5 and 6 may be In an ion propulsion system, therefore, it is also neces at zero potential, while emitter 1 is at a potential which sary that the total electron current exactly match the is 2500 volts positive and electrode 3 at a potential 2500 total ion current if the vehicle is not to acquire a charge. volts negative with respect to electrodes 5 and 6. The So far as the vehicle potential is concerned, electrons ion beam would then receive a ?nal velocity equal to that could be emitted at some part of the vehicle other than 65 received in a direct potential fall of 2500 volts. Ob the point of emission of the ions and need not match the viously, other values of either emitter or accelerating and it has been recognized that it is desirable to mix such electrons with the ion beam in the interest of ef?ciency. The equality of ions and electrons in the current then ion density or velocity. This, however, not only is a di?‘icult feat to perform, but moreover the capacitively stored energy of the ion beam constitutes an unwanted drain on the power source. More importantly, the strong local space charge ?elds inevitably present oppose and electrode potentials may be employed as dictated by the 70 desired ?nal ion energy. Emitter 1 may be of the perfusing type and may com prise a louvered, sintered, or woven structure of a suit able metal, such as tungsten or tantalum, having a high 3,050,852 ii 4 work function. Vapors of an ionizable gas, such as cesium, are supplied to one surface of emitter 1 from a thermally insulated reservoir 7 containing metallic cesium intermediate that of the accelerator electrode 3 and ion emitter It and, moreover, is located at sui?cient distance from accelerator electrode 3. The neutralizer electrode through a duct 8. The ionizable vapor Within reservoir 7 is maintained at su?'icient pressure to produce a desired 5 is then properly positioned at the point where the potential again reaches that of electrode 6. rate of flow by a heater of a suitable material, illustrated In accordance with my invention, I provide beam neu as a resistance 9, to which heater current is supplied tralization by establishing a stationary cloud of electrons ovver leads 10. The emitter 1 is also maintained at high in the region 4 between the neutralizing electrode 5 and temperature by a similar heater, not shown. A cesium the drift electrode 6. The electrons emitted from the atom, upon striking the hot emitter material during its 10 cathode 11 are retained by positive ion space charge passage ‘through the porous structure of emitter 1, be in the drift space 4. These electrons, when introduced comes ionized so that a supply of cesium ions is provided into this region, exchange suf?cient momentum in scatter from the porous emitter surface 1. The ions formed from ing collisions with the ions to insure trapping and, there cesium which has passed through the porous emitter 1 are accelerated by the difference in potential between the emitter electrode and the accelerating electrode 3 to form a beam of high speed ions. The shapes of emitter elec fore, accumulate until they have completely neutralized the region. This region constitutes an electron reservoir from which current required to maintain the system neutrality is supplied. By regulating the electron emis trode 1 and accelerating electrode 3 are such that ions, sion from cathode H, such that it can replenish the elec which leave the emitter surface normally, move radially trons drawn from the drift region at a rate which equals to pass through the aperture in accelerating electrode 3 20 the rate at which they leave the system, this region func into the ?eld free region 4 where they ?ow in parallel tions as a plane emission limited electron source and a paths as a collimated beam until they exit through an steady state condition is achieved. In this steady state aperture in electrode 6. Any suitable structure (not condition, the potential of the ?lled electron trap region shown) may be employed to support the electrodes in remains slightly negative and electrons ?ow smoothly ?xed spaced relation. out with the ion beam. As has been pointed out before, in utilizing an ion The condition described above is illustrated in FIG. 6, beam as a force developing device, particularly for pro in which line 1-1 illustrates the position of the emitter pulsion purposes, it is essential that the beam be electri surface, line 3—3 the position of the accelerating elec cally neutral when it emerges from. the system to prevent trode 3, line 5—5 the position of neutralizer electrode rapid charging of the system and also to eliminate unde~ 30 5, line 6—-6 the position of the drift electrode 6, and the sirable space charge ?elds. In order to produce such beam potential at each point is given by the solid curve. Before neutralization within the ?eld free region de?ned by the introduction of electrons, the potential between elec neutralizing electrode 5 and drift electrode 6, I provide trodes 5 and 6 has a maximum, as shown in FIG. 4, and means to introduce or inject into the region a controllable amount of electrons, illustrated in FIG. 1 as a cathode is shown as a dashed line in FIG. 6. In the steady state neutralized condition, this region is ?eld free because 11, supplied with heating current from a suitable poten it contains a stationary cloud of electrons. tial source through a variable resistance 12. that the drift space may retain electrons without the use FIGS. 2-5 illustrate possible potential distributions in the region between accelerator electrode 3 and drift electrode 6 before electrons are introduced into this region. A detailed theory of these types of potential dis tributions has been presented by C. E. Pay, A. L. Samuel and W. Shockley in “Bell System Technical Journal 17,” page 49 (1938), for the analogous case of electron beams in a vacuum tube. Thus, FIG. 2 represents the condi tion where at point 13 the potential is su?iciently positive that all ions are re?ected and return to the accelerating electrode. FIG. 3 illustrates a condition where the potential at point 14 is of such a value that a virtual anode is estab lished. Some ions will be re?ected and some will pass beyond this point. Obviously, if electrons can be intro duced into this region, they will lower the potential to a value such that all ions will pass beyond the point and In order of grids at its entrance and exit, the potential variation to the left of line 5--5 must be such that a potential bar~ rier is established which extends completely across the aperture in electrode 3 of FIG. 1, and electrons are not drawn into the ion acceleration region between electrodes I and 3 of FIG. 1 since such action would remove elec trons from the trap region and, bombard the ion emitter. Similarly, the potential at electrode 6 must be such that an electron does not pass this point unless it accompanies an ion which it electrically neutralizes. It can be demon strated mathematically that the scattering action of the electric ?eld of each ion on the electrons is such as to make their motions while in region 4 become entirely random and insure their retention between electrodes 5 and 6 so that the electrons constitute a cloud of nega tive charges. It can also be demonstrated mathematicah ly that the attraction of the positively charged ions be 55 yond electrode 6 will extract electrons from this cloud FIG. 4 illustrates a potential distribution in which all at Such a rate that a neutral ion beam is maintained be none will be re?ected. ions proceed through the region and electrons can be retained Within the region. yond the electrode 6, provided that electrons so drawn from region 4 are exactly replenished by emission from From an inspection of FIGS. 3 and 4, it is apparent cathode ill. Since the region within the electron reser that the potential distribution curve must pass through voir, in which the stationary cloud of electrons exists, a maximum point, 14 or 15, of value not greater than is substantially a field free region, ions entering the the emitter potential, within the drift space de?ned by region retain the velocities with which they enter this electrodes 5, 6 in order that electrons can be trapped in neutralized region. When they leave the region to a this drift space. Furthermore, the electrons emitted point downstream of the ion beam, each ion is accom from a source at the potential of electrodes 5, 6 will be 65 panied, on the average, by an electron so that the beam unable to move through aperture 2, so that they will not is electrically neutral. In addition, the electron achieves enter the region of ion acceleration and will not strike a velocity equal, on the average, to that of the ions, there the ion emitter 1. by minimizing the power required for a given thrust to be FIG. 5 illustrates the ?nal condition, in which the developed. potential in this region does not possess a maximum, so 70 While in the foregoing a particular embodiment of this that electrons cannot be retained in the region 4 to invention has been shown, it will, of course, be under— neutralize the ion beam, but will, instead, pass through stood that it is not limited thereto since many other modi? aperture 2 and strike the ion emitter 1. It can be shown cations in the circuit arrangement and in the instrumen that the desired condition for electron retention can always be achieved if the drift electrode 6 is at a potential 75 talities employed may be made. It is contemplated by the 5 3,050,652 appended claims to cover any such modi?cations as fall within the true spirit and scope of this invention. What I claim as new and desire to secure by Letters 6 5. In an ion propulsion system comprising means for forming a collimated beam of high speed ions, a pair of electrodes having apertures therein through which said Patent of the United States is: 1. In a force developing system, the combination com prising means providing a beam of high speed ions, means region through which said beam passes, and means for establishing a ?eld free region through which said beam injecting electrons into said ?eld free region substantially beam passes, said electrodes being maintained at substan tially the same potential whereby they de?ne a ?eld free ~ is projected, and means including a source of electrons to neutralize the electrical charge of the beam, the rate for introducing electrons into said beam as it passes of injecting electrons into said beam being such that the through said region at a rate such that the total electrical 10 total electrical charge of the beam is substantially zero charge of the beam is substantially zero as it emerges as it emerges from the region. from said region. 6. In an ion propulsion system employing a collimated 2. In a force developing system, the combination com beam of high speed ions, the method of neutralizing the prising means providing a beam of high speed ions, a electrical charge of the beam which comprises passing the pair of electrodes de?ning a space through which said beam through a region free of external ?elds, injecting beam is projected, means for injecting electrons into said electrons into the beam as it passes through said region, space to establish therein a cloud of electrons of a density establishing a potential barrier across the entrance of the sufficient to neutralize the charge of the ions and form region so that electrons cannot freely leave the region, a ?eld free region, the potential of said electrodes being the same and of a value relative to the potential of the beam that ions are decelerated as they enter the space and substantially all electrons are trapped in said space, the rate of injection of electrons into said space being such that the total electrical charge of the beam is sub stantially zero as it emerges from said space. 3. In a force developing system of the type employing a collimated beam of high speed ions, the method of neu tralizing the electrical charge of the beam which com prises passing the beam through a region free of external ?elds, forming a cloud of electrons in the region through which the ions pass, injecting electrons into the region at a rate suf?cient to substantially completely neutralize the electrical charge of the beam, and permitting only elec and controlling the rate of injecting electrons into the region so that only enough electrons enter the region to substantially completely neutralize the electrical charge of the beam. 7. In a force developing system, a ?rst electrode, a second electrode having an aperture therein and arranged to be maintained at a negative potential with respect to the ?rst electrode, means supplying an ionizable vapor to said ?rst electrode whereby a high speed beam of ions is directed through said aperture, a pair of electrodes having apertures therein aligned with the aperture in said second electrode, means for introducing electrons into the region between said pair of electrodes, ‘the potentials of said pair of electrodes relative to the potentials of said 4. In an ion propulsion system, the combination com prising an emitter electrode, an accelerating electrode, a ?rst and second electrodes being such that electrons in troduced into said region are prevented from reaching said ?rst and second electrodes and are trapped in said region to neutralize said beam, said beam as it leaves said neutralizing electrode, and a drift electrode supported in ?xed spaced relation, said neutralizing electrode and drift ion beam is maintained beyond said pair of electrodes, trons utilized for neutralizing the beam to emerge from the region with the beam. region attracting electrons at a rate such that a neutral and means to control the rate of introducing electrons electrode being maintained at substantially the same po into said region to equal the rate at which electrons are tential whereby they de?ne a drift space, means providing attracted by the beam leaving said region. ions to said emitter electrode, said accelerating electrode being at a high negative potential relative to said emitter References Cited in the ?le of this patent electrode whereby ions are accelerated to enter into said drift space, and means for introducing electrons into said 45 UNITED STATES PATENTS drift space to neutralize the electrical charge of the ions 2,880,337 ' Langmuir et al. _______ __ Mar. 31, 1959 within the space.