Patent Publication Number: US-7719268-B2

Title: Apparatus and method for polarizing polarizable nuclear species

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional of Ser. No. 09/904,294 filed Jul. 12, 2001 now U.S. Pat. No. 6,949,169, which claims the benefit of U.S. Provisional Application Ser. No. 60/217,569 filed on Jul. 12, 2000, both of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention is in the field of hyperpolarizing polarizable nuclear species, such as xenon. 
   BACKGROUND OF THE INVENTION 
   Nuclear magnetic resonance (NMR) is a phenomenon, which can be induced through the application of energy against an atomic nucleus being held in a magnetic field. The nucleus, if it has a magnetic moment, can be aligned within an externally applied magnetic field. This alignment can then be transiently disturbed by application of a short burst of radio frequency energy to the system. The resulting disturbance of the nucleus manifests as a measurable resonance or wobble of the nucleus relative to the external field. 
   For any nucleus to interact with an external field, however, the nucleus must have a magnetic moment, i.e., non-zero spin. Experimental nuclear magnetic resonance techniques are, therefore, limited to study of those target samples, which include a significant proportion of nuclei exhibiting non-zero spin. Certain noble gases, including xenon, are in principle suited to study via NMR. However, the low relative natural abundance of these isotopes, their small magnetic moments, and other physical factors have made NMR study of these nuclei difficult if not impossible to accomplish. 
   Existing technology for polarizing xenon, developed primarily at Princeton, is based on earlier work on nuclear polarized 3He gas targets for nuclear physics. The key component of the system is the polarizing chamber where the 3He gas is heated, saturated with rubidium, an alkali metal vapor, and illuminated with laser light. In these devices, a closed cell of 3He gas, rubidium, and nitrogen is maintained at a uniform high temperature to achieve an appropriate rubidium density. A laser illuminates the cell with circularly polarized light at the resonant absorption line of the rubidium, polarizing the rubidium electrons. Spin exchange occurs with the 3He gas nucleus, leading to an accumulation of nuclear polarization. 3He gas atoms diffuse throughout the cell. 
   Xenon polarization proceeds by a similar mechanism. Circularly polarized laser light polarizes rubidium atoms, which in turn transfer their polarization to the xenon nucleus. Xenon, however, has a large depolarization effect on rubidium. Therefore the partial pressure of xenon must be kept low. Diode lasers, which are used to illuminate the gas mixture, have a large linewidth. In order to more efficiently absorb more of this laser light, the rubidium should be in a high-pressure gas to pressure-broaden the absorption line. Princeton researchers use a high-pressure buffer gas of helium. They slowly flow a mixture of xenon, nitrogen, and helium through the polarizing cell. A sufficient quantity of rubidium is available in the polarizing cell. The unpolarized gas slowly enters this chamber and diffusively mixes with rubidium vapor and partially polarized gas already in the chamber. Rubidium condenses as the gas exits and cools down. 
   The use of a high-pressure buffer gas, such as helium, causes pressure broadening of the absorption spectrum of the rubidium, allowing greater extraction of laser power in a compact pumping cell with low rubidium density. Operation at high-pressure, however, changes the dominant mechanism for transferring polarization from the rubidium to the xenon. At high pressures the dominant mechanism is the two-body interaction. At low pressures, the mechanism mediated by three-body formation of molecules dominates which is considerably more efficient. Consequently, the improvement in polarization achieved by the gain in laser efficiency is partially offset by a reduction in rubidium-xenon polarization transfer. 
   Existing polarization techniques also use a gas mixture dominated by helium at high pressure. The high pressure of helium broadens the absorption linewidth of the rubidium, allowing it to usefully absorb more of the linewidth of the diode laser. If they reduce the pressure, they would not absorb as much light in their short polarizing cells. If they lengthened their cells using their diffusively mixed process, they would mix gas from regions with an even greater range of polarization rates. If the existing process could be performed effectively at low pressure, however, the polarization system would be capable of taking advantage of the higher efficiently molecular formation physics. 
   Existing polarization methods cannot efficiently use the full polarizing power of the laser beam. The gas mixture attenuates the laser light. Consequently, the region of the polarizing cell farthest from the laser will only achieve low rubidium polarization if the cell is long. Since the gas in the polarization cell is diffusively mixed, the xenon will achieve an average polarization that is influenced by both the high rubidium polarization and the low rubidium polarization. To minimize the region of low rubidium polarization, the laser must exit the polarizing cell after using only a portion of its polarizing power. 
   SUMMARY OF THE INVENTION 
   The present invention results from the realization that by using a longer than standard polarizing cell and flow within the cell dominated by laminar displacement, polarizing polarizable nuclear species can be accomplished at low pressure with high temperature and high velocity, thereby taking advantage of the higher efficiency molecular formation physics. 
   It is therefore an object of this invention to dominate the flow through the cell by laminar displacement. 
   It is a further object of this invention to polarize polarizable nuclear species with high velocity. 
   It is a further object of this invention to increase efficient use of resonant light. 
   It is a further object of this invention to polarize polarizable nuclear species with high temperature. 
   It is therefore an object of this invention to polarize polarizable nuclear species at low pressure. 

   
     BRIEF DESCRIPTION OF THE INVENTION 
     The novel features believed characteristic of the invention are set forth in the claims. The invention itself however, as well as other features and advantages thereof, will be best understood by reference to the description which follows, read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  shows a flow diagram with one embodiment of the inventive polarization method. 
       FIG. 2  shows a flow diagram of another embodiment of the inventive polarization method. 
       FIG. 3  shows one embodiment of the inventive polarization cell. 
       FIG. 4  shows a layout of one embodiment of the polarization apparatus. 
       FIG. 5  shows a layout of another embodiment of the polarization apparatus. 
       FIG. 6  shows a layout of another embodiment of the polarization apparatus. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is a polarizing process  10  involving a number of steps as shown in  FIGS. 1 and 4 . The first step requires moving  12  a flowing mixture of gas  52 , the flowing mixture of gas  52  at least containing a polarizable nuclear species and vapor of at least one alkali metal, with a transport velocity that is not negligible when compared with a natural velocity of diffusive transport. The second step is propagating  14  laser light  40  in a direction  58 , preferably at least partially through a polarizing cell  30 . The next step is containing  18  the flowing gas  52  mixture in the polarizing cell  30 . The final step is immersing  20  the polarizing cell  30  in a magnetic field. These steps can be initiated in any order, although moving  12  the flowing gas  52 , propagating  14  the laser  40  and immersing  20  the magnetic field must be concurrently active for the polarizing process  10  to occur. 
   Additional steps can be added to the inventive process  10  as shown in  FIG. 2 . One narrower embodiment  10   a  involves saturating  11  an original gas mixture with the alkali metal vapor to create the flowing mixture of gas before the flowing mixture of gas enters the polarizing cell; directing  16  the flowing mixture of gas along a direction generally opposite to the direction of laser light propagation, and condensing  22  the alkali metal vapor from the flowing mixture of gas in the laser light. 
   The present invention also includes an inventive polarizing cell  30  as shown on  FIG. 3 . The polarizing cell  30  is a nonferrous enclosure  32  with an interior  34  and at least two openings, an entrance  36   a  and an exit  36   b , for flowing gas to pass through the enclosure  32 . One embodiment of the polarizing cell  30  further includes a window  38  in the enclosure allowing laser light  40  to at least partially illuminate the interior  34 . Another feature of this embodiment of the polarizing cell  30  is that the window  38  is maintained at a temperature substantially lower than most of the enclosure  32 . 
   Another embodiment of the present invention also includes an inventive polarizing apparatus  50  as shown on  FIG. 4 . The inventive apparatus  50  includes a polarizing cell  30  with multiple openings an entrance  36   a  and an exit  36   b , and at least one window  38  transparent to laser light  40 . The apparatus further includes a flowing gas mixture  52 , at least containing a polarizable nuclear species, at least one alkali metal vapor, and at least one quenching gas, moving  12  through the cell  30  in a direction  54 . The apparatus  50  further includes an oven  56  at least partially containing the polarizing cell  30 . The apparatus  50  further includes a laser propagating  14  light  40 , at the absorption wavelength of the alkali metal vapor, through at least one transparent window  38  into the polarizing cell  30  in a direction  58  at least partially opposite to the direction  54  of the flowing gas mixture  52 . Finally, the apparatus  50  includes an optical arrangement  60  to cause the laser light  40  to be substantially circularly polarized. 
   The inventive apparatus  50  also has several narrower embodiments. One of the narrower embodiments involves having the oven  56  only partially containing the polarizing cell  30 . A narrower embodiment of this embodiment involves using the previously described inventive polarizing cell  30  having the window maintained at a temperature substantially lower than most of the enclosure. Having the polarizing cell  30  sized so that it is more than five times greater in length  62  than diameter  64 , as shown in  FIG. 5 , can further narrow this embodiment. In one embodiment, the cell can be ninety centimeters in length  62  and two centimeters in diameter  64 . 
   Another embodiment of the apparatus  50  involves the oven  56  maintaining a temperature of over 150 C. 
   Another embodiment of the apparatus  50  includes having a saturation region  66  with a quantity of liquid alkali metal exposed to an original gas mixture  68  to substantially saturate  24  the original gas mixture  68  with an alkali metal vapor to create the flowing gas mixture  52  that flows  12  through the polarizing cell  30 . 
   Another embodiment of the apparatus  50  includes having a condensation extension  70  of the polarizing cell  30 , through which the laser light  40  propagates  14 , before passing through a remainder  72  of the polarizing cell  30 , for condensing  22  the alkali metal vapor in the laser light  40 . 
   Another embodiment of the apparatus  50  includes composing the alkali metal vapor of rubidium, cesium and/or potassium. 
   Another embodiment of the apparatus  50  includes making the quenching gas nitrogen and/or hydrogen. 
   Existing practice in the polarization of polarizable nuclear species relies on the achievement of equilibrium conditions throughout the polarization chamber. The static polarization process of Rosen, Chupp et al uses very long (20 min) polarization time constants by selecting low temperature (90 C) and low rubidium density. Consequently they can use high xenon concentrations (more than one atmosphere). The quasistatic polarization process of Cates, Happer, et al uses shorter time constants (5 min) in a flowing system. By selecting higher temperature (up to 150 C) they have higher rubidium density. They must reduce their polarizable nuclear species pressure to a few percent of their working pressure of 10 atmospheres to maintain high rubidium polarization. The gases in their polarization chamber are diffusively mixed. 
   In contrast to existing practice, the present invention uses a flowing mixture of gas  52  whose flow velocity is not negligible when compared with the natural velocity of diffusive transport. The present invention is specified using this language because diffusion times are pressure dependent. Taking advantage of the transport will significantly improve performance over a wide range of pressures, both in existing operation regimes as well as in a preferred embodiment. Diffusion across a typical one-inch dimension requires one second at a pressure of one-tenth of an atmosphere and ten seconds at ten atmospheres in a typical gas mixture. The transport velocity is not negligible when compared with this velocity if it is, for example, greater than one-half this velocity. In a more favorable embodiment, the transport flow velocity will be several times this velocity, i.e. several inches per second. To achieve polarization in this regime requires polarization time constants more than one order of magnitude higher than the highest used in present practice, on order several seconds or faster. This requires higher rubidium densities to achieve faster time constants. 
   An analogy to the present polarization technique would be transferring a property such as heat from one fluid to another, which can be done with greater or lesser efficiency depending on the method employed. Placing the two fluids in thermal contact allows some of the heat to flow from one fluid to the other, until an equilibrium temperature is achieved. While a substantial amount of heat is transferred, a comparable amount remains in the original fluid. If the system is configured as a counter-flowing heat exchanger, the initially warmer fluid will transfer substantially all of its heat to the cooler fluid, exiting the exchanger at the initial temperature of the cooler fluid. The fluid that was initially cooler will exit with essentially all the heat. 
   The polarization process of the present invention relies on a similar principle. Referring again to  FIGS. 1 and 4 , the laser light  40  propagates  14  in a direction  58  generally opposite to the direction  54  of the flowing mixture of gas  52 . The mixture of gas  52  gains in polarization even as it removes intensity from the laser light  40 . Nevertheless, the highly polarized mixture of gas  52  exits the system  30  where the laser light  40  is most intense, maximizing the polarization of the mixture of gas  52 . 
   The polarization process  10  must be immersed in a magnetic field to define the angular momentum quantization axis for the alkali metal vapor quantum mechanical spin states and the polarizable nuclear species nuclear spin states. The flowing gases  52  must also be confined and isolated from then environment. In the most favorable embodiment of polarizable nuclear species polarizing processes  10 , the laser light  40  propagates  14  along the direction of a very uniform magnetic field. In the most favorable embodiment of the counter-flowing polarizable nuclear species polarization process  10 , the polarization cell  30  confining the flowing mixture of gas  52  is oriented such that the direction  54  of the mixture of gas  52  flow is exactly counter to the direction  58  of laser light  40  propagation  14 . For higher flow velocities and longer cells, diffusional mixing decreases in significance, increasing the performance of the polarization system. 
   The gas pressure and mixture of gas  52  significantly affect the two stages of the polarization process  10 , that is the transfer of polarization from the laser light  40  to the rubidium atoms, and the transfer of polarization from the rubidium atoms to the polarizable nuclear species nuclei. Existing practice uses four components in various amounts: the gas receiving the nuclear polarization, the alkali metal vapor, the quenching gas, and a buffer gas. The unpolarized fraction of the alkali metal vapor absorbs the laser light  40 . Maximum polarization of the alkali metal vapor allows transmission of the laser light  40  to deeper regions of the polarizing cell  30 . Higher concentration of alkali metal vapor (higher temperature) increases the rate of polarization transfer to the gas receiving the nuclear polarization, but absorbs more laser light  40 . Higher concentrations of the gas receiving the nuclear polarization can also depolarize the alkali metal vapor, increasing the absorption of laser light  40 . To effectively quench the atomic re-radiation, the quenching gas must have a pressure of several tens of torr, typically more than 60 torr. The pressure of all gases combined broadens the absorption spectrum of the alkali metal vapor, improving the utilization of the laser light for polarizing the alkali metal vapor. Higher pressures, however, reduce the alkali-xenon molecular formation, decreasing the rate of transfer of polarization from the alkali to the xenon. 
   The static, or batch-mode, process of Rosen, Chupp, et al, uses low temperatures, low rubidium density for approximately 20 minute time constant, high xenon concentration, intermediate pressure, and no buffer gas to increase pressure broadening. The quasi-static system of Cates, Happer, et al, uses intermediate temperatures and rubidium densities for time constants around a minute. 
   In one embodiment of the inventive process  10 , the temperature is considerably higher than either of these methods. Alkali metal density is higher and the polarization time constant is shortened. Furthermore, if the overall pressure is low, polarization transfer from alkali to the species being polarized is further improved by the increase in transfer by molecular formation. To minimize the depolarization of the alkali vapor and maintain laser transmission, the density of the species being polarized should be low. 
   The short polarization time constant allows continuous rapid replacement of the gas mixture of gas  52 , hence high velocity flow. 
   The opportunity to exploit this highly efficient polarization process  10  can be more favorably realized in an embodiment that causes the alkali metal vapor to become substantially condensed on the cell  30  walls while the mixture of gas  52  is still in the presence of the polarizing laser light  40 . In one embodiment this can be accomplished by maintaining at least some portion of the polarizing cell  30 , that portion which includes the gas exit opening  36   b  and laser entrance window  38 , at a temperature substantially below that which would maintain an alkali vapor density constant in time. Note that when the mixed gas  52  leaves the polarizing cell  30 , the alkali vapor density becomes quickly depolarized. That depolarization is transferred to the nuclear species being polarized at a rate that corresponds to the alkali metal vapor density at that point. Condensing the alkali metal vapor minimizes this depolarizing effect. 
   The polarization process  10  benefits from higher alkali metal densities and the associated shorter polarization time constant. In one embodiment of this process  10 , the alkali metal saturates  11  into the original gas mixture  68  before entering the polarizing cell  30 . Transport of the mixture of gas  52  from the alkali vapor saturation region (“presaturator”)  66  to the polarizing cell  30  is maintained at an adequate temperature to deliver the mixture of gas  52  to the polarizing cell  30 . In various embodiments, this alkali vapor presaturator  66  may or may not be collinear with the polarizing cell  30 . In various embodiments, this alkali vapor presaturator  66  may or may not have a means of increasing the liquid-vapor surface area, such as a copper mesh insert. 
   High alkali metal vapor densities and high flow velocities will result in a substantial quantity of alkali metal transferred from the entrance opening  36   a  to the exit opening  36   b  of the polarizing cell  30 . In embodiments where there is an alkali metal presaturator  66 , the alkali metal source will diminish. In embodiments where there is an alkali metal condensation region  70 , alkali metal will accumulate in this region  70 . Excess accumulation could reduce the efficiency of operation. In one embodiment, the alkali metal accumulated in the condensation region  70  can be removed from the condensation region  70  without disassembling the polarization cell  30 . In a more favorable embodiment, the condensed alkali metal can be returned to the polarizing cell  30  or the alkali metal presaturator  66  either by the force of the gravitational weight of the liquid droplets, or by the force of its weight aided by mechanical motion (shaking). This restoration may occur either with the cell  30  installed at least partially in a vertical orientation, or by reorienting the cell  30  to an at least partially vertical orientation. 
   Existing polarizing processes have not been able to exploit the highest alkali metal densities, and the associated highest polarization rates. The present process  10  exploits the motion  12  of the mixture of gas  52  to accomplish the stages of the polarization process  10  in sequential stages, allowing shorter time constants and higher polarization rates. In the most favorable embodiment of the inventive process  10 , operating temperatures as high as 190 C can be exploited to achieve a polarization time constant of one-third of a second. 
   For a macroscopic sample of gas nuclei to become substantially polarized, the polarization rate must exceed the depolarization rate for that species. Noble gas spin one-half nuclei have two features that enable their polarization: they have closed electron shells thereby isolating the nucleus from asymmetric binding effects, and they have no electric quadrupole moment to allow the external surroundings to exert a torque on the nucleus. Consequently the depolarization times range from minutes for xenon-129, to hours for helium-3. A favorable application of the present polarization method  10  is the polarization of macroscopic samples of xenon-129 nuclei. The substantially reduced polarization time achievable by the present process  10 , however, allows polarization of macroscopic samples of nuclei with much shorter depolarization times. Such species could include, but are not limited to xenon-131, which does not have a spin one-half nucleus, atomic hydrogen or deuterium, or even nuclei within molecules. 
   The novel elements of the embodiment of the present apparatus  50 , as shown in  FIG. 5 , include a polarizing cell  30  immersed in a magnetic field. To allow for the mixture of gas  52  to flow through the cell  30 , the cell  30  has at least one opening  36   a  for gases to enter, at least one opening  36   b  for gases to exit, and at least one transparent window  38  or provision for a source of laser light  40 . 
   The openings  36   a  and  36   b  and shape of the cell  30  are optimized to allow for the mixture of gas  52  to flow at a velocity that can be greater than the velocity of diffusive transport. In a favorable embodiment, the polarizing cell  308  will have a length  62  that is greater than its transverse dimension  64 . In a favorable embodiment the transparent window  30  will allow illumination of the full cross section of the cell  30  for a large fraction of its length  62 . In a favorable embodiment the gas entrance opening  36   a  will be at the farthest end from the transparent window  38  and the exit opening  36   b  will be close to the transparent window  38 . In a favorable embodiment, a substantial portion of length  62  of the cell  30  including the transparent window  38  and the gas exit opening  36   b  will be maintained at a temperature below the temperature used to obtain the operating density of alkali vapor, thereby causing the alkali to condense. 
   Again referring to  FIGS. 1 and 5 , the present polarization apparatus  50  also includes a saturation region  66  with a sufficient quantity of liquid alkali metal exposed over a sufficient surface area to substantially saturate  11  the original polarizable gas mixture  68  with alkali metal vapor to create the flowing mixture of gas  52  that enters the polarizing cell  30 . This alkali vapor presaturator  66  is independently novel. In one embodiment it may consist of a sufficient length of tubing, either straight, wound in a helix, or some other configuration, containing several grams of alkali metal exposed over a large surface area. In another embodiment it may include a mesh of some material such as copper, to provide a large surface area. In another embodiment, it may consist of an extension of the same material, glass tubing for example, as comprises the polarization cell  30 , with or without a copper mesh. 
   The present polarization apparatus  50  also includes an extension  70  of the polarizing cell  30  that is collinear with the polarizing cell  30 , and through which the laser light  40  propagates  14  before entering the unextended portion  72  of polarizing cell  30  for condensing  22  the alkali vapor in the presence of the polarizing laser light  40 . This extension  70  is closest to the exit opening  36   b  of the polarizing cell  30 . This extension  70  acts as an alkali vapor condensation region. In one embodiment it is the same diameter  64  as the unextended portion  72  of polarizing cell  30 . It projects out from the oven  56  into a region of lower temperature. In one embodiment this lower temperature is room temperature. In another embodiment, the extension  70  has a larger cross section than the unextended portion  72  of the polarizing cell  30 . In still another embodiment the unextended portion  72  of the polarizing cell  30  can be at reduced temperature. This situation may be optimal for very high velocity flow when high alkali densities are prepared in the alkali presaturator  66 . 
   In an embodiment optimized for very high velocities and high alkali densities, the entire polarizing cell may also be treated as the extension  70 , acting as the condensing region  70 . 
   Another narrower embodiment involves the polarizing cell  30  having a length  62  substantially greater than the laser light  40  attenuation length, thereby causing efficient transfer of polarization from the laser light  40  to the alkali metal vapor, even at low operating pressure where the most efficient alkali-polarizable nuclear species polarization transfer mechanism dominates. 
   Another narrower embodiment involves the transport velocity of the flowing mixture of gas  52  being substantially greater than the natural velocity of diffusive transport. 
   Another narrower embodiment of the inventive method  10  involves the polarizing cell  30  having an operating gas pressure that is less than two atmospheres but greater than a pressure required to efficiently quench an alkali optical pumping using a combination of at least 2 torr of a polarizable nuclear species and a minimum pressure of a quenching gas, typically 60 torr of nitrogen. 
   Another narrower embodiment involves the magnetic field being uniform and substantially aligned with the direction  58  of laser light  40  propagation  14 . 
   Referring again to  FIGS. 2 and 5 , another narrower embodiment of the inventive method  10  includes the additional step of condensing  22  the alkali metal vapor from the gas mixture  52  in the propagating  14  laser light  40 . A narrower embodiment of this embodiment involves the condensation  22  occurring in an extension  70  of the polarizing cell  30  that is collinear with the polarizing cell  30 , and through which the laser light  40  propagates  14 , thereby providing continuous polarization of the alkali metal vapor up to and during condensation  22 . Another narrower embodiment of this embodiment involves the resulting condensed rubidium droplets coming to rest in either a saturating region  66 , a region of the polarizing cell  30  heated by the oven  56 , or both. 
   Another narrower embodiment involves the laser light  40  entering the polarizing cell  30  by passing through a window  38  of the polarizing cell  30  which is at a temperature substantially lower than that of the polarizing cell  30 , thereby reducing attenuation of the laser light  40  in an unpolarized alkali metal vapor layer in contact with the window  38 . 
   In a very favorable embodiment, the polarizing cell  30 , the extension  70 , and the alkali vapor presaturator  66  are fabricated from sections of tubing arranged coaxially. The diameter  64  of the tubing is 2.5 cm and the length is 2 meters. The tubing is oriented vertically in a uniform, vertical magnetic field. An opening  36  at the bottom allows the original gas  68  to enter the alkali vapor presaturator  66 . The alkali vapor presaturator  66  is maintained at a temperature of 200 C, and contains a copper mesh saturated with liquid alkali metal. The alkali vapor presaturator  66  occupies the lower 70 cm of the vertical tubing. The central 80 cm of the tubing comprise the unextended portion  72  of polarizing cell  30  in the oven  56 . This region is maintained at 180 C. This region is fully illuminated with laser light  40  from above. The uppermost 50 cm of tubing comprise the extension  70  of the polarizing cell  30 . This region is outside the oven  56 , and maintained at room temperature. In this region, the alkali metal vapor diffuses to the walls and condenses  22 . The length of this extension  70  must allow for several diffusion time constants to elapse while the mixture of gas  52  is passing through. A length of 50 cm and diameter of 2.5 cm allows gas velocity of approximately 15-20 cm/sec at pressure of 0.1 atmosphere. Higher flow velocities require a longer presaturation region  66  and extension  70 . 
   In another embodiment, as shown in  FIG. 6 , optimized for higher flow velocities, higher temperatures, and shorter polarization time constants, the alkali metal presaturator  66  is not an extension of the polarizing cell  30 . Rather, the alkali metal presaturator  66  consists of thirty turns of 2.5 cm diameter glass tubing, wound in a helix with 10 cm inner diameter. The tubing has been prepared with ridges to prevent the alkali metal liquid from flowing to the bottom. The exit of the alkali vapor presaturator  66  is connected at the bottom to the entrance opening  36   a  of the polarizing cell  30 . The polarizing cell  30  is 2.5 cm diameter  64  glass tubing, oriented vertically. It is illuminated with laser light  40  through a window  38  from above. The unextended portion  72  of the polarizing cell that is in the oven  56  is 70 cm. The extension  70  of the polarizing cell  30  is a slightly larger diameter  64 , 3.0 cm, and 130 cm in length. This embodiment allows production of polarizable gas with even higher temperatures and shorter polarization time constants. This system could operate optimally at 195 C and 40 cm/s flow rate achieving polarization time constants approaching one-fifth of a second.