US3824394A - Atomic beam resonator having a confocal conics field geometry in the second state selector - Google Patents

Atomic beam resonator having a confocal conics field geometry in the second state selector Download PDF

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Publication number
US3824394A
US3824394A US00328087A US32808773A US3824394A US 3824394 A US3824394 A US 3824394A US 00328087 A US00328087 A US 00328087A US 32808773 A US32808773 A US 32808773A US 3824394 A US3824394 A US 3824394A
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state selector
atoms
field
selector means
resonator
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P Kartaschoff
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    • G—PHYSICS
    • G04—HOROLOGY
    • G04F—TIME-INTERVAL MEASURING
    • G04F5/00—Apparatus for producing preselected time intervals for use as timing standards
    • G04F5/14—Apparatus for producing preselected time intervals for use as timing standards using atomic clocks
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
    • H03L7/00—Automatic control of frequency or phase; Synchronisation
    • H03L7/26—Automatic control of frequency or phase; Synchronisation using energy levels of molecules, atoms, or subatomic particles as a frequency reference
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H3/00—Production or acceleration of neutral particle beams, e.g. molecular or atomic beams

Definitions

  • the second state selector is a dipole magnet having pole pieces of trapezoidal shaped in such a way that, in a plane perpendicular to the beam axis; the field is described by a set of confocal conics, the equipotentials being hyperbolae and the field lines ellipses.
  • Cesium beam tubes are known as being absolute frequency standards of high accuracy and are used as frequency determining elements in atomic frequency and time standards or clocks.
  • FIG. 1 is a cross-section of the second state selector described in the earlier patent (U.S. Pat. No. 3,591,800);
  • FIG. 2 shows a cross-section of a second state selector embodying this invention
  • FIG. 3 shows a cross-section of another embodiment of this invention
  • FIG. 4 shows a section along the beam axis of the state-selector according to FIG. 3.
  • FIG. 5 shows a schematic view of an atomic beam resonator comprising the second state selector 16 embodying this invention, all other parts being of prior art design.
  • the physical arrangement of the main components of an atomic beam resonator is shown schematically in FIG. 5.
  • the atomic beam is formedby a source 13 which, in the case of a cesium beam resonator, consists of an oven containing liquid cesium metal, heated to a temperature producing sufficient vapor pressure and having an aperture with collinating channels to form the beam.
  • the atomic beam passes through a first state selector 14 magnet which in this example may be a quadrupole or hexapole magnet whose symmetry axis coincides with the axis of the beam.
  • this kind of magnet has the property of focusing atoms which are in a higher hyperfine energy level by deflecting these atoms 22 towards the axis, whereas the atoms 23 of a lower hyperfine energy level are deflected away from the axis and eliminated.
  • the atoms 22 remaining in the beam pass through an interaction zone where they are exposed to an oscillating magnetic microwave field produced by means of an external generator 21. If the power' level and the frequency of the microwave generator are correctly adjusted, the atoms undergo transitions to the lower hyperfine level. The rate of transition is extremely sensitive to the frequency of the microwave field and it attains a sharp maximum at the frequency corresponding to the hyperfine energy level separation.
  • the purpose of the second state selector 16 is to collect those atoms 24 which have made the transition to the lower energy level on to a detecting device 18 and to eliminate the atoms 25 which have remained in the higher level.
  • the detecting device 18 consists of a hot wire or ribbon which ionizes the incident atoms, the latter being collected on an electrode and producing a current proportional to the rate of transitions.
  • a beam stop 17 prevents fast undeflected atoms from hitting the detector and causing an unwanted background current.
  • the second state selector 16 which in FIG. 5 is shown only very schematically, is the subject of the present invention and is therefore discussed in more detail in the following paragraphs.
  • the atomic beam apparatus is enclosed in an evacuated envelope 19.
  • the output current is fed into an automatic frequency control system 20 which acts on the microwave generator 21 to control the frequency in such a way as to keep this frequency at the value producing the maximum transition rate and hence the maximum output current.
  • the frequency 1 of the generator 21 is stabilized by means of the atomic beam reasonator and the whole system can be .used as an atomic frequency standard or clock, all as described in my earlier US. Pat. No. 3,591,800.
  • the second state selector described in the earlier patent (U.S. Pat. No. 3,591,800) and shown in FIG. 1 herein comprises a field concentrating rod 1 which is placed in the center of the gap formed by the concave pole pieces 2 and 3. .
  • the axis of the beam is perpendicular to the plane of the drawing.
  • the atoms pass through the gaps between 2, l and 1, 3 respectively.
  • the force acting on the magnetic moment of the atoms causes a deflection of those atoms which are in the desired quantum state (i.e., those having the transition to the lower energy level) towards the detector element 18 of FIG. 5 placed along the axis of the beam and located at some distancebehind the central pole piece 1.
  • the force acting on the atom is proportional to the product of its magnetic moment and the gradient of the magnetic field.
  • the direction of the force does not depend on the direction of the field itself but on the direction of its gradient.
  • the magnetic moment of the. atoms in the desired state is positive and the force acts in direction of increasing field strength.
  • This invention discloses a different and new design of the second state selector which provides a similar result and avoids the disadvantages of the prior art device shown in FIG. 1. These disadvantages are the following:
  • the central pole piece 1 obstructs a considerable fraction of the beani'cross section. Almost half of the beam intensity is lost.
  • the means of supporting and aligning the central pole piece 1 are complicated and thus relatively expensive.
  • the present invention provides a similar high efficiency but avoids the drawbacks mentioned above.
  • FIG. 2 illustrates the principle of this invention.
  • the pole pieces 4 and 5 made of a ferromagnetic alloy having a high permeability at high fields (e.g. Iron Cobalt Vanadium alloys) constitute equipotential surfaces which define the field in the region of the gap.
  • the length of this assembly in the direction of the z axis perpendicular to'the plane of the drawing, is assumed to be sufficiently large compared to the gap width so that fringe effects can be neglected.
  • the pole pieces 4 and 5 comprise flat surfaces only which are easy to machine. They constitute an approximation to a pair of surfaces having hyperbolic cross section.
  • the field in the gap is thus defined by sets ofconfocal conics.
  • the cross sections of the equipotential surfaces are hyperbolae and the field lines are ellipses.
  • the detector filament is placed in the y-z plane at some distance beyond the magnet shown in the figure. Outside the y-z plane the field gradient has a component which is directed towards this plane.
  • the atoms to be detected are deflected in the direction of the gradient and therefore in the direction of the y-z plane containing the detector element. Atoms in other states having a negative magnetic moment are deflected in the opposite direction away from the detector element and are not detected.
  • a beam stop 17 (see FIG. 5) may be placed in frontof the magnet and in the y-z plane.
  • the width (in the .r-direction) of such'a stop can be just slightly larger than that of the detector filament.
  • FIG. 3 Another embodiment of this invention is shown i FIG. 3.
  • the plane of symmetry containing the x and z axes of FIG. 2 is replaced by a flat pole piece 6.
  • the field in the remaining gap is the same as in the upper half of FIG. 2, the other pole piece 7' being identical to 4 and 5.
  • the gradient is not everywhere parallel to this plane but may be directed towards the tip of the pole piece. This causes some defocusing in the y-direction and the loss of atoms hitting the pole-piece.
  • the useful deflection angles being very small, this loss is not very important. It can be made negligible by'the modification shown in FIG. 4 which represents a section along the beam axis, i.e. in the y-z plane, of the state selector shown in FIG. 3.
  • the surface 8 of the pole tip 9 is not parallel to the plane atoms hitting the pole piece is reduced.
  • All pole pieces shown in the FIGS. 2, 3 and 4 comprise only flat surfaces of simple geometry. They are easier to manufacture and therefore less expensive than the curved pole pieces used in earlier designs. One could be tempted to see in these designs a return to the old types of state selectors designed by Stern and Gerlach and Rabi, which are described in N. F. Ramseys book Molecular Beams (Oxford 1956, pp 394-396). However, in this invention the field is well described by sets of confocal conics, and this is not the case in the tial characteristics thereof.
  • an atomic beam resonator comprising a source means for producing and projecting a beam of atoms over an elongated predetermined beam path, first state selector means adjacent to said source for deflecting atoms in said beam, a second state selector means downstream of said first state selector means for deflecting atoms in said beam, said state selector means being separated in order to provide an interaction zone, means for generating an oscillating magnetic field in said interaction zone, and a beam detector device downstream of said second state selector means for detecting only atoms in said beam having undergone tran sitions between two magnetic hyperfine energy levels bly for producing a magnetic field through which said beam path extends, at least one of said magnetic pole pieces being trapezoidal in cross section in a plane perpendicular to the axis of the beam path and having flat external surfaces for producing a magnetic field described in said plane by sets of confocal conics, the equipotentials of said field being hyperbolic and the field lines elliptic.

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • General Physics & Mathematics (AREA)
  • Stabilization Of Oscillater, Synchronisation, Frequency Synthesizers (AREA)
US00328087A 1972-02-15 1973-01-30 Atomic beam resonator having a confocal conics field geometry in the second state selector Expired - Lifetime US3824394A (en)

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CH211072A CH552244A (enrdf_load_stackoverflow) 1972-02-15 1972-02-15

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4199679A (en) * 1975-11-27 1980-04-22 Ami Rav Aviv Method and apparatus for the separation of isotopes
CN105045959A (zh) * 2015-06-23 2015-11-11 兰州空间技术物理研究所 一种二极型磁选态单束铯束管束光学结构的建立方法

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3420996A (en) * 1966-08-08 1969-01-07 Hewlett Packard Co Atomic beam maser having improved energy state selection to provide increased short term stability
US3591800A (en) * 1968-04-19 1971-07-06 Suisse De Rech Horlogeras Lab Atomic or molecular beam resonator having field concentrating means for the second state selector

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3420996A (en) * 1966-08-08 1969-01-07 Hewlett Packard Co Atomic beam maser having improved energy state selection to provide increased short term stability
US3591800A (en) * 1968-04-19 1971-07-06 Suisse De Rech Horlogeras Lab Atomic or molecular beam resonator having field concentrating means for the second state selector

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4199679A (en) * 1975-11-27 1980-04-22 Ami Rav Aviv Method and apparatus for the separation of isotopes
CN105045959A (zh) * 2015-06-23 2015-11-11 兰州空间技术物理研究所 一种二极型磁选态单束铯束管束光学结构的建立方法
CN105045959B (zh) * 2015-06-23 2018-01-09 兰州空间技术物理研究所 一种二极型磁选态单束铯束管束光学结构的建立方法

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CH211072A4 (enrdf_load_stackoverflow) 1974-02-28
CH552244A (enrdf_load_stackoverflow) 1974-07-31

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