He3 solenoid magnetometer

Abstract: A magnetometer having an enclosed cell (19) containing He³ therein for receiving light from a light source (5), a light source (3, 5, 11, 15) for directing light to the enclosed cell (19) along a predetermined path, a single pick-up coil containing therein the light source and the enclosed cell (1a), s solenoid containing the pick-up coil therein and a data collection system coupled to the pick-up coil. The light source is a source of circularly polarized light and can comprise a He⁴ cell and an optical system and a circular polarizer disposed serially between He⁴ cell and enclosed cell containing He³. The predetermined path of the light source, the enclosed cell and the pick-up coil are disposed along the axis of the solenoid. The light source can also be a laser.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a magnetometer for use in detecting magnetic signals. BRIEF DESCRIPTION OF THE PRIOR ART High sensitivity He 3 magnetometers are particularly useful in detecting quasistatic magnetic signals occurring over periods of many minutes, such as the signal produced by a slowly moving magnetic object passing a stationary magnetometer. In the use of such systems, the sensor, as configured in accordance with the prior art, must have a significant component of its axis, as defined by the light beam, aligned with the ambient (earth's) magnetic field during the brief preparation cycle in order for the sensor to operate properly. In fact, the signal size is maximum when aligned with the ambient magnetic field and is zero when at 90 degrees to the ambient magnetic field. He 3 magnetometers of the above general type are described in Patent No. 3, 206, 671 with improvements thereof in Patent Nos. 4, 567, 439, 4, 780, 672, 4, 814, 707 and 4,697,519. In the conventionally configured He 3 magnetometer, polarization is produced in a cell by a beam of circularly polarized resonant radiation from a He 4 or He 3 lamp or a 10830 Angstrom laser directed along the ambient magnetic field direction for several minutes. The lamp and discharge in the cell are then turned off and the nuclear spins are caused to rotate to a plane perpendicular to the magnetic field. This rotation is accomplished by applying a varying voltage to a pair of Helmholtz coils as described in Patent No. 4,567,439. The nuclear precession frequency is measured by the AC current induced in a pair of coils near the cell. Two coils are used because, in this geometry, the light beam must be directed perpendicular to the coil axes instead of aligned with the axes. Coupling of energy from precessing He 3 moments into the coils is not as efficient as in the case of a single coil. When the signal decays, the process must be repeated. It is desirable to remove this orientation requirement and also to improve device sensitivity. The document IEEE Trans. Magnetics. Vol 10. 1974, pages 528-531 R. E. SLOCUM discloses a He 3 nuclear magetometer. A He 3 sample is contained at a pressure of 10 Torr in a cylindrical Pyrex cell with a diameter of 38 mm and a length of 64 mm. A receiver coil consists of 1300 turns wound directly on the cylindrical cell. Pumping light is obtained from a He 4 capillary lamp driven by 50 MHz rf excitaion. An adiabatic field rotation is produced by a sixth-order end-corrected solenoid which encloses the optical pumping apparatus. The sensor is positioned to place the pumping beam in approximate alignment with the ambient field. The present invention provides a magnetometer sensor comprising, in combination: (a) an enclosed cell containing He 3 therein for receiving light from a light source; (b) a light source comprising an He lamp for directing light to said cell along a predetermined path; (c) a single pick-up coil containing therein said enclosed cell; and (d) a solenoid containing the pick-up coil therein, characterized in that the single pick-up coil also contains therein said light source. Briefly, in a practical example of the present invention, the light source, preferably an He 4 light source, optical polarizers and He 3 cell are disposed within a single pick-up coil, all of the elements being aligned within and along the axis of an end corrected solenoid. An end corrected solenoid is a solenoid wherein extra turns are wound on each solenoid end to produce a more uniform field inside the solenoid. During the pumping cycle, a direct current is passed through the solenoid to produce a field somewhat larger than the ambient field to be measured. The direct current serves the sole purpose of producing a steady magnetic field inside the solenoid and must be large enough (depending upon the number of windings and other characteristics of the solenoid) so that the resulting vector sum of the solenoid field and the external field is directed substantially along the solenoid axis. After the pumping cycle, the weak discharge in the cell, the light source and the current in the solenoid are turned off. The spin rotation is then accomplished by the technique described in Patent No. 4,567,439 using the solenoid instead of a Helmholtz pair which is no longer required. The pick-up coil with its axis colinear with the light beam is more efficiently coupled to the cell than in the prior art, resulting in a larger signal and therefore a more sensitive magnetometer. The voltage induced in the pickup coils by the precessing He 3 nuclei has a frequency: f = GH, where G is a constant and H is the ambient magnetic field. The purpose of the device is to measure the magnitude of the magnetic field. It does so by measuring the precession frequency of the He 3 nuclei which is directly related to the field by the equation f = GH. A magnetometer sensor according to the invention is characterized in that the single pick-up coil also contains therein the light source. An example of elements for such a magnetometer sensor according to the invention will now be given with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is an exploded view of a portion of an He 3 magnetometer in accordance with the present invention; FIGURE 2 is a schematic diagram of the elements of FIGURE 1 in assembled state; and FIGURE 3 is an elevational view of a solenoid and housing containing the magnetometer portion of figures 1 and 2 therein. Referring first to FIGURE 1, there is shown an exploded view of a portion of the magnetometer sensor 1 in accordance with the present invention. The magnetometer sensor portion 1 includes on each side of the sphere 19, a mirror 3, an He 4 lamp 5 having a pair of electrodes 7 and 9 thereon to which is connected a source of high frequency RF discharge (e.g. 25 MHz) which glows brightly when excited by the RF transmitter and a lens system for focussing or collimating light from the lamp 5 onto the sphere 19, the lens system including a first lens 11 and a second lens 15 spaced from the first lens 11 by a spacer 13. The system further includes a linear polarizer 17 with a quarter wave plate thereon whereby the linear polarizer receives unpolarized light from the light source with an electric field in all directions perpendicular to the direction of propagation and passes only light with an electric field in one direction, the quarter wave plate placing the electric field in two out-of-phase directions to provide a rotation (i.e, an electric vector rotating in a plane perpendicular to the direction of propagation, which is circularly polarized light), which light passes into the hollow sphere filled with He 3 . There is nothing critical about either the frequency or voltage of the RF energy, it merely being required that a discharge be created in the lamp. A typical frequency used for this purpose is 25 KHz. which ignites the discharge in the lamp and maintains the electrical discharge in the He 3 cell during the pumping cycle. The sphere 19 includes a pair of electrodes 21 and 23 on the surface thereof connected to a source of high frequency RF energy (e.g. 25 KHz.) to excite a weak electrical discharge within the cell. It should be understood that the mirror 3, lamp 5 and lens system 11, 13 and 15 can be replaced by a laser which emits light at the absorption wavelength of He 3 metastable atoms (approximately 1083 nanometers). Referring now to FIGURE 2, there is shown the elements described with respect to FIGURE 1 as those elements would be arranged in an actual embodiment. It can be seen that the elements on opposite sides of the sphere 19 are identical and arranged in a mirror image of each other. The magnetometer structure of FIGURES 1 and 2 is contained within a solenoid 41 as shown in FIGURE 3 wherein only the solenoid is shown. The solenoid 41 includes wires thereto (not shown) for applying a signal thereto. In operation, a high frequency, (e.g. 50 MHz.), RF voltage sufficient to produce an electrodeless electrical discharge is applied to the lamp electrodes 7 and 9 of lamp 5. A direct current is passed through the solenoid 41 to produce an axial magnetic field along the solenoid axis which is approximately twice the earth's magnetic field or larger. This provides good alignment of the magnetic field with the optical axis for more efficient pumping and improved polarization. The helium nuclei in the sphere 19 are excited to the 2 3 S 1 metastable level by application of a high frequency voltage (e.g. 25 KHz.) across the electrodes 21 and 23 of cell 19. The light from the lamps 5 is passed both directly and by reflection from the mirror 3 through the lens system 11, 13, 15 and the circular polarizer 17 through the sphere 19 to provide circularly polarized light within the sphere. This circularly polarized light causes atomic transitions in the He 3 disposed within the sphere 19 from the 2 3 S 1 metastable levels to higher atomic levels from which they automatically return to the metastable level causing an alignment or polarization of the He 3 atoms along the magnetic field by a well known optical pumping process. The scientific theory is well known and will not be discussed herein. The system is allowed to remain in this state for about 3 to 4 minutes. The lamps 5 and the discharge in the cell 19 are then turned off and the field produced by the solenoid is then turned off slowly (about 0.1 second or longer) to allow the moments to gradually move and align with the then existent magnetic field. At this time, an external RF field is applied across the sphere 19 by applying a signal in the form of a frequency ramp as described in Patent No. 4,567,439 to the terminals 35 of the solenoid 41 for the spin flip since the nuclei are now lined up with the earth's magnetic field. This causes the polarization to be shifted by ninety degrees with respect to the direction of the earth's magnetic field. This is accomplished in all cases except when the axis of the solenoid 41 is lined up with the earth's magnetic field. Since the nuclei are precessing and act in the manner of a bar magnet, the magnetic signals therefrom are picked up by the pick-up coil (not shown in FIGURE 1) which surrounds the structure of FIGURE 1. The signal from the pick-up coil is then sent back to a data collection system (not shown) for evaluation or other use. The frequency of the signal from pick-up coil is a direct and precise measure of the magnetic field surrounding the sensor. If a magnetic object, such as, for example, a submarine, moves within the detection range of the sensor (or conversely if the sensor is moved when near the magnetic object), the magnitude of the magnetic field changes, causing the frequency detected in coil to change. The changes in frequency are recorded by an external device and the passage of the submarine or other object is detected. Though the invention has been described with respect to a specific preferred embodiment thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

Claims: A magnetometor sensor comprising, in combination: (a) an enclosed cell (19) containing He 3 therein for receiving light from a light source (5); (b) a light source (5) comprising an He lamp for directing light to said cell (19) along a predetermined path; (c) a single pick-up coil containing therein said enclosed cell (19); and (d) a solenoid (41) containing said pick-up coil (19) therein, characterized in that the single pick-up coil also contains therein said light source (5). A sensor as set forth in claim 1, wherein said light source (5) is a source of circularly polarized light. A sensor as set forth in claim 2, wherein said source of circularly polarized light comprises a He 4 cell (5) and an optical system (11, 13, 15) and a circular polarizer (17) disposed serially between said He 4 cell (5) and said enclosed cell (19) containing He 3 . A sensor as set forth in any preceding claim 1, wherein said predetermined path of said light source (5), said enclosed cell (19) and said pick-up coil are disposed along the axis of said solenoid. A sensor as set forth in any preceding claim, further including data collection means coupled to said pick-up coil.

Inventors: COLEGROVE FORREST D, MARTON BELA I, MCGREGOR DOUGLAS D, COLEGROVE, FORREST D., MARTON, BELA I-, MCGREGOR, DOUGLAS D.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1991
Main classification: G01R33
IPC codes: G01R33