Feb. 21, 1961 S. A. JOHNSON ETAL MOLECULAR BEAM APPARATUS OF THE MASER TyPE Filed June 26, 1958 2,972,697 AEG/747OA S R NVENORS 4? AORNEY
United States Patent Office 2,972,697 Patented Feb. 21, 1961 2,972,697 MOLECULAR BEAM APPARATUS OF THE M4ASERTYPB, Stanley A. Johnson, Brooklyn, N.Y., and Ferris Eugene Alger, New Hope, Pa., assignors to PRD Electronics, Inc., a corporation of New York Filed June 26, 1958, Ser. No. 744,730 11 Claims. (Cl. 313-231) This invention relates to apparatus for producing a beam of molecules which may be used for the amplifica tion or generation of electric waves within the micro Wave range. For apparatus of the "Maser' type generally, see the articles published on pages 1253 and 1264 of volume 99, Physical Review, August 15, 1955. See also the article by J. P. Wittke published on pages 291 to 316 of the Proceedings of I.R.E., volume 45, March 1957. The present invention is concerned with apparatus in which the molecular beam is formed of molecules of a condensible gas, such as ammonia. An object of the invention is to provide an arrange ment for stabilizing the beam, that is, for maintaining the flux of the beam substantially constant. For this purpose the gas is supplied to the beam-forming nozzle through a very small (capillary) passage of extended length. A further object of the invention is to devise a system in which the molecular material forming the beam is collected and returned to the beam source without the necessity for removing the material from the system, so that the molecular material remains in a permanently closed systern and is used over and over again in re peated cycles. Due to the recycling of the gaseous material, only one out-gassing is required just before the time the apparatus is initially charged with the gaseous material and after which the system is permanently sealed. Still another object is to devise a unitary assembly of small weight and capable of being moved from place to place by hand. - A further object of the invention is to devise Maser apparatus producing only a single frequency output. For this purpose, the ammonia gas used to form the beam is compounded from a particular isotope of nitrogen, N15. The hyperfine structure of this material is symmetrical and there will be no splitting effect. Although the iso tope N is very costly, it may be used in the present invention without excessive cost due to the fact that a limited quantity is required and is used over and over again. A suitable embodiment of the invention is illustrated diagrammatically in the accompanying drawing. In the drawing, the molecular beam is formed within a closed chamber 1. The gas forming the beam is intro duced in the chamber 1 through a conduit 2 which termi nates in a beam-forming nozzle 3 provided with a plural ity of parallel perforations directed along the axis of chamber 1. The beam from nozzle 3 is directed towards and into a chamber 4 constituting a microwave cavity, one end of the cavity chamber 4 being Sealed in the opposite end of the chamber 1 from the nozzle 3. The beam enters cavity 4 through a cut-off tube 5 which pre vents loss of wave energy into chamber 1. A waveguide 6 for conducting energy away from the cavity 4 is coupled to the cavity through an iris 4a and is provided with a vacuum-tight wave-transparent window 6a. 2 Gas is supplied to the conduit 2 from reservoir 7 which contains the gaseous material in liquid form, such as liquid ammonia, the reservoir 7 being connected to con duit 2 through a beam cut-off valve 8. For the purpose of stabilizing the flow of the beam, the gas supplied to nozzle 3 from the reservoir 7 is passed through a small passage of extended length embodied in the conduit 2. One suitable construction of such capil lary passage is illustrated in the drawing as being formed O of a helical groove 9a formed in the outer surface of a plug 9 fitting tightly within the conduit 2, thus providing a helical passage of small transverse dimension between the reservoir 7 and the nozzle 3. By way of example only, the plug 9 fitting within a conduit 2 having an 5 internal bore of 58' diameter would have an outside diameter of 58', and the helical groove would be formed as a 60 groove, 0.003' deep with a pitch of 80 turns per inch. Such a groove has a transverse area of 20 0.00000503 square inch and a length of about 13 feet. The delivery of such a capillary will follow accurately Poiseuille's law. For the purpose of removing the ground state mole cules from the excited molecules forming the beam, a cage-like assembly of separator electrodes surrounds the beam between the nozzle 3 and the cavity 4. This assembly is of a known type and is formed of a plurality of conducting strips 10 supported at their ends by in sulating discs 11 and 12 carried, respectively, by the 30 nozzle assembly and the cavity assembly. Alternate bars 10 are charged positively from a suitable source of high voltage and the remaining bars are charged negatively. For the purpose of removing the beam gas molecules in chamber after they have performed their function, 35 a cooler 13 in the form of an annular chamber surround ing the separator assembly, is filled with liquefied gas such as air or nitrogen, introduced through an opening 13a at the upper end of chamber 1. The beam gas is condensed on the surface of cooler 13. 40 Beam chamber 1 should be maintained at a low gas pressure, of the order of 105 mm. of mercury or lower. Initially, this vacuum is established by the usual pumping 45 50 55 60 70 (operation after which the system is sealed. The low vacuum as far as the beam gas is concerned is main tained by the cooler 13. However, a certain amount of extraneous gases are given off by the walls of chamber 1 and by the elements enclosed therein, which extrane ous gases are not condensed on the cooler 13. For the purpose of removing the extraneous gases and maintain ing the proper vacuum within chamber 1, a getter cham ber 14 is connected to the chamber 1. This getter cham ber may be formed of a glass bottle containing a heater filament 15 for evaporating a getter material, such as titanium, from a crucible 16. The titanium is deposited upon the inner walls of the chamber 14 as a metallic film 14a and serves as a getter for removing extraneous gases which may be present in the chamber 1. Filament 15 need not operate continuously in the chamber 1 but only at intervals for the purpose of renewing the getter film 14a. The chamber 14 also functions as an ion trap by pro viding a separate filament 15a which is energized con tinuously and operates to ionize gaseous. molecules enter ing the chamber. These ionized molecules are trapped on the metallic film 4a by maintaining the film at a high negative voltage with respect to filament 15a through a connection 14b leading to a suitable high voltage source. The chamber 1 is connected to reservoir 7 through a conduit 7 which by-passes the nozzle conduit 2 and the beam valve 8 and is controlled by by-pass valve 17a. In order to prevent variations in output of the wave
3 cavity 4 by reason of changes in the dimensions of this cavity due to changes in temperature, the cavity is main taimed ata substaatially constant temperature by a system formed of a pipe coil 18a surrounding the cavity in good heat transfer relation and having water pumped there through by a pump 19, the water heated by heater 20 and being maintained at a constant temperature by a thermal regulator 21 by regulating either the rate of flow of water or the rate of heating. During operation, the cooler 13 is filled with liquid gas and maintains the vacuum within chamber 1 by con densing the beam gas after it leaves the beam. During this time the beam valve 8 is open and the by-pass valve 7a is closed. The liquid gas in reservoir 7 is allowed to evaporate by subjecting the reservoir to atmospheric temperature, or to a higher temperature. At this time, the pressure within reservoir 7 will range between 10 atmospheres and 20 atmospheres, depending upon the temperature, and it will remain at a fixed pressure as long as any liquefied gas remains in the reservoir and So long as the external temperature of the reservoir re mains constant. Under these conditions, the beam is formed in chamber by reason of a constant pressure differential acting to force gas through the high resist ance passage 9a from the reservoir 7 to chamber 1. The high resistance of capflary passage 9a prevents any sub stantial change in the flow of gas to nozzle 3 due to changes in ambient temperature with the result that a highly stable beam is formed and is maintained as long as any liquid gas remains in reservoir 7. Before the liquid gas is completely exhausted in reser voir 7, beam valve 8 is closed and the by-pass valve 17a is opened. Liquid air is renoved from cooler 13, and reservoir 7 is immersed in or surrounded with liquid air or liquid nitrogen, so that beam gas evaporated from the cooler 13 passes into the reservoir 7 and is liquefied. For this purpose the reservoir 7 may be placed within a container 18 containing liquid gas. Container 18 may be in the form of a jacket around reservoir 7. After this process continues for a time sufficient to remove sub stantially all of the ammonia gas from chamber 1, the beam may be re-established in chamber 1 by opening valve 8, closing valve 17a, introducing liquid refrigerant in cooler 13 and removing liquid refrigerant from around reservoir 7. This cycle of operation may be repeated over and over again without loss of beam gas from the system. It will be seen that the apparatus is of a unitary as sembly capable of being moved from place to place. The weight of the assembly may be no more than 25 pounds. We claim: 1. Molecular-beam apparatus comprising - a beam containing liquefied gas, said reservoir being exposed to a temperature causing evaporation of said liquefied gas within said reservoir thereby establishing a pressure of several atmospheres within said reservoir, a conduit con necting said nozzle with said reservoir and including a flow-regulating restriction comprising a single capillary passage of relatively small transverse section and long length, and means for maintaining a low vapor pressure within said beam chamber comprising a cooler for con densing upon the surface thereof the gas molecules leav ing said beam. 2. Molecular-beam apparatus comprising a beam sage of relatively small transverse section and long length, a cut-off valve in said conduit between said capillary passage and said reservoir, a by-pass conduit connecting said beam chamber to said reservoir around said capillary passage and said cut-off valve, and a by-pass valve in said by-pass conduit, 3,972,69? 5 O 5 20 30 35 40 45 50 55 60 35 70 75 4. 3. Molecular beam apparatus comprising a beam sage of relatively small transverse section and long length, a cooler surrounding the beam-path within said beam chamber to condense, upon the surface thereof gas mole cules leaving the beam, and an ion-trap chamber con nected with said beam-chamber for removing gases which are not condensed by said cooler. 4. Apparatus according to claim 1 and including means within said ion-trap chamber for forming a film of get ter material on the inner wall thereof. 5. Apparatus according to claim 4 wherein said means for forming a film of getter material comprises a heater filament for evaporating titanium within said ion-trap chamber. 6. Molecular-beam apparatus comprising a beam sage of relatively Small transverse section and long length, a cooler Surrounding the beam-path within said beam chamber to condense upon the surface thereof gas mole cules leaving the beam, and a getter chamber connected with said beam-chamber and containing getter material for removing gases which are not condensed by said cooler. 7. Apparatus according to claim 6 wherein said get ter material is a film of getter material carried on the inner walls of said getter chamber, and means sealed within said getter chamber for renewing said film from time to time. 8. Apparatus according to claim 7 wherein said means for renewing the film of getter material comprises a bulk Supply of getter material sealed within said getter cham ber and heater filament for evaporating getter material from said bulk supply, 9. Molecular beam apparatus comprising a gas-tight system formed of a beam-chamber containing a beam forming nozzle, a reservoir containing gas under pres sure, a conduit connecting said reservoir to said nozzle and including a capillary passage for restricting the flow of gas to said nozzle, a by-pass conduit connecting said beam-chamber with said reservoir around said capillary passage, a by-pass valve in said by-pass conduit, means for maintaining a low vapor pressure within said beam chamber comprising a cooler for condensing upon the Surface thereof the beam gas molecules leaving said beam, said cooler being controllable to effect evaporation of condensed beam gas from the surface thereof when gas is to be returned from said beam-chamber to said reservoir, and means for cooling said reservoir to effect condensation of beam-gas therein and to effect removal of beam-gas from said chamber through said by-pass conduit when said by-pass valve is open. 10. Molecular-beam apparatus comprising a beam sage of relatively small transverse section and long length, a by-pass conduit connecting said beam chamber to said reservoir around said capillary passage and a by-pass valve in said by-pass conduit, said by-pass conduit, when said valve is open, providing for the unrestricted flow of gas from said chamber to said reservoir. 11. Molecular-beam apparatus comprising an evacu ated beam-chamber containing a beam forming nozzle, a reservoir containing liquified gas and being exposed to a temperature below the vaporizing temperature of said liquid gas, whereby the gas exists within said reser voir in gaseous and liquid forms in equilibrium at a fixed pressure equal to its characteristic vapor pressure at the temperature of said reservoir, the said fixed pres sure remaining constant as long as gas in liquid-phase
2,972,697 remains in the reservoir even if gas is withdrawn from the reservoir, a channel connecting said reservoir to said nozzle to conduct gas from said reservoir to said beam chamber, said channel including a capillary passage of relatively narrow cross-section and great length, the proportions of said capillary passage being such as to reduce the said fixed pressure of the reservoir to a rel atively much lower fixed pressure suitable for beam-for mation in the said evacuated chamber, whereby a con stant and stabilized molecular beam is maintained with in said chamber, a separator structure of cage-like form surrounding the beam from said nozzle and operating when electrically charged to separate certain gas mole cules from the beam, and a cooler having a condensing Surface surrounding said cage-like separator to intercept and condense thereon gas molecules leaving said beam and passing through said separator. References Cited in the file of this patent UNITED STATES PATENTS 1,162,982 Crocker ------------------------- Dec. 7, 1915 O 2,037,425 Martin ---------------- Apr. 14, 1936 2,285,622 Slepian ---------------- June 9, 1942 2,393,650 Metcalf ---------------- Jan. 29, 1946 2,583,898 Smith ----------------- Jan. 29, 1952
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,972,697 February 21, 196l Stanley A. Johnson et al. It is hereby certified that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as corrected below. Column 4 line ll for the claim reference numeral 'l" read -- 3 - - Signed and sealed this 7th day of November 196l. (SEAL) Attest: ERNEST W. SWIDER Attesting Officer DAWLFDL. LADD Commissioner of Patents USCOMM.DC
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,972, 697 February 21, 196l Stanley A. Johnson et al. It is hereby certified that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as corrected below. Column 4 line ll for the claim reference numeral 'l" read -- 3 -- Signed and sealed this 7th day of November 196l. (SEAL) Attest: ERNEST W. SWIDER Attesting Officer DAWID L. LADD Commissioner of Patents USCOMMDC