Abstract:
A cold-gas coolant nozzle which limits the amount of turbulence induced in the flow of cold gas due to interaction with ambient air by creating a intermediate temperature buffer zone between the cold gas and the ambient air.

Description:
TECHNICAL FIELD 
     The invention concerns a nozzle for delivering low temperature coolant to a position within a room-temperature atmosphere with improved flow characteristics. 
     BACKGROUND OF THE INVENTION 
     X-ray crystallography is typically performed by diffracting x-rays through a crystalline sample and determining the resultant pattern of diffracted radiation on a detector or target. In many applications, the sample must be frozen prior to and during testing to maintain the crystal structure of the sample. In such cases, the need to maintain the crystal in a frozen state is complicated by the continual influx of x-ray energy, part of which is absorbed by the sample. As the sample absorbs energy, it heats, and such heating must be offset by a cooling mechanism if the sample is to be retained in a frozen state. Further, because additional testing of samples after an initial x-ray diffraction test is often desirable, it is often necessary to be able to insure the frozen state, and thereby the integrity, of a sample throughout the x-ray crystallography process and afterward. 
     One of the mechanisms for maintaining a sample in a frozen state is to provide a steady coolant stream to the sample&#39;s location. Very cold nitrogen gas is often used as the coolant stream because nitrogen is readily available, techniques for refrigerating it are well known, and it does not introduce environmental hazards into the working area. However, a variety of other gases, including helium and other noble gases, can readily be used to provide such a coolant stream. 
     The x-ray crystallography process is normally carried out inside of an ambient, e.g. room-temperature, environment, so that when crystals undergoing crystallography must be maintained at a specialized temperature, such as approximately −180° C., a specially controlled temperature zone must be created. Further, the physical requirements of the x-ray crystallography process require open space around the sample, so that the apparatus directing the coolant stream cannot be placed in immediate proximity to the sample. Generally, the coolant stream which creates the special temperature zone for the crystals is directed by a nozzle which is preferably spaced apart from the sample so that the nozzle does not interfere with the x-rays incident to the sample or diffracted therefrom, and so that the nozzle does not interfere with the necessary movements of other apparatus. The nozzle generally comprises a vacuum jacket to insulate the coolant stream from the ambient atmosphere until the coolant stream exits the nozzle. 
     To provide maximum cooling of the sample and to avoid icing on the sample, it is desirable that the coolant stream be dry and that it flow smoothly. However, the physical circumstances described above hinder these goals. Even if the coolant stream leaves the nozzle in a laminar flow state, the outer zone of the coolant stream is rapidly heated by contact with the much warmer ambient atmosphere. As it heats, the coolant stream gas expands, and may do so unevenly, introducing turbulence into the coolant stream flow. The flow may be further disrupted by normal air currents in the room. Further, even if the coolant stream gas is dry when it exits the nozzle, the induced turbulence and mixing with moisture in the room air may create icing problems at the sample. 
     The greater the distance between the nozzle and the sample, the more time these effects have to disrupt the flow of the coolant stream. Thus, even though it is desirable to move the nozzle out of the way of the x-ray crystallography equipment and the x-rays themselves, it is sometimes impossible to achieve these goals, and the testing equipment must instead be adjusted to accommodate the cold stream nozzle. 
     To offset the disruption of the coolant stream caused by this rapid heating on contact with the ambient atmosphere, it is possible to pre-warm the outer zone of the coolant stream just after it exits the nozzle. Generally, this pre-warming is accomplished by fitting a hollow cylindrical heating element to the exit port of the nozzle. The internal diameter of the heating element is essentially the same diameter as the exit port, and thus of the coolant stream. By controlling the electrical current in the heating element, the outer zone of the coolant stream can be heated to a desired temperature as the coolant stream passes through the heating element. Generally, the outer zone of the coolant stream will be warmed to a temperature intermediate that of the ambient atmosphere and the inner zone of the coolant stream. Thus, the warmed outer zone of the coolant stream provides a buffer between the ambient atmosphere and the colder inner zone of the coolant stream. 
     Although such a heating element improves the distance over which laminar flow of the coolant stream can be achieved, there remain undesirable effects which limit its utility. The addition of an extension to the nozzle introduces a discontinuity in the flow containment which can disrupt the laminar nature of the coolant stream flow. Additionally, the heating elements used are omnidirectional, that is, they radiate heat outward into the ambient atmosphere as well as inward into the coolant stream. Because warming the outer zone of the coolant stream requires a significant amount of heat, the heating element presents a safety hazard to personnel working around the nozzle. 
     It is an object of this invention to provide a coolant stream nozzle with improved flow characteristics for the coolant stream after it exits the nozzle. 
     It is a further object of this invention to provide a coolant stream nozzle which allows greater separation distance between the nozzle and the sample being maintained in the coolant stream. 
     It is another object of this invention to provide a coolant stream nozzle which provides a coolant stream which limits icing of the sample being maintained in the coolant stream. 
     BRIEF DISCLOSURE OF THE INVENTION 
     A coolant stream nozzle is provided which allows the outer zone of the coolant stream to be warmed inside the nozzle. The nozzle comprises a tubular cavity which guides the coolant stream within the nozzle, and an outlet from which the coolant stream exits the nozzle. The tubular cavity is enclosed in a vacuum jacket which insulates the tubular cavity, and thus the coolant stream, from the ambient atmosphere. Inside the vacuum jacket, an electrical heater is wound around the portion of the tubular cavity which is essentially adjacent the outlet. In the preferred embodiment, the outer surface of the tubular cavity comprises a scalloped heater seat which allows maximizes the physical contact between the electrical heater and the tubular cavity. The scalloped heater seat provides a continuous, threaded groove into which the heater is seated. 
     Also in the preferred embodiment, the electrical heater comprises an active heater wire and a heater lead wire, wherein the heater lead wire provides an electrical connection to the active heater wire, but does not itself produce significant heat. As those of skill in the art will recognize, it is desirable to have the heater as close to the end of the tubular cavity adjacent the orifice as possible, and also to have the heating effect on the coolant stream restricted to the linear length of the heater. Because the heater lead wire does not produce significant heat, it may be extended within the vacuum jacket to a convenient feed-through or connection point without producing undesired heating effects on the coolant stream. 
     An additional aspect of the preferred embodiment is to provide a flared portion of the tubular cavity essentially adjacent the outlet. It is preferred that the active heater wire is in contact with the tubular cavity for the length of the flared portion of the tubular cavity. Thus, the heater will provide heat to the coolant stream in a zone where the gas in the outer zone of the coolant stream will have room to expand as it is heated. Allowing this expansion to occur in the same area in which the heat is applied to the coolant stream and while the coolant stream is still enclosed by the flared portion of the tubular cavity helps to limit turbulence and maintains the laminar nature of the coolant stream flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of one embodiment of a coolant stream nozzle. 
     FIG. 1A is a cross-sectional view of the coolant stream nozzle of FIG. 1, sectioned along plane A—A of FIG.  1 . 
     FIG. 1B is a cross-sectional view of the coolant stream nozzle of FIG. 1, sectioned along plane A—A of FIG. 1, with the heater elements omitted for clarity. 
     FIG. 2 is a schematic representation of one embodiment of an x-ray crystallography system. 
     FIG. 3 is a cross-sectional view of an alternative embodiment of the coolant stream nozzle of FIG. 1, sectioned corresponding to plane A—A of FIG.  1 . 
     FIG. 4 is a side view of a prior-art coolant stream nozzle. 
     FIG. 4A is a cross-sectional view of the coolant stream nozzle of FIG. 4, sectioned along plane A—A of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, one embodiment of an x-ray crystallography system is shown to illustrate the use of the coolant stream. In x-ray crystallography, an x-ray source  210  produces an x-ray beam  212  which impinges on a sample  214  mounted on a sample holder  216 . Diffraction of the x-rays beam  212  by the sample  214  results in a diffracted x-ray beam  218  which impinges on a detector  220  for measurement and analysis. Because data must generally be collected over a large angular swath, one or more of the x-ray source  210 , the sample holder  216 , or the detector  220  is usually repositionable, and such repositioning is often done under automatic control. During the course of performing x-ray crystallography, it is also important that the x-ray beam  212  and the diffracted x-ray beam  218  be unobstructed. Further, it is also necessary to be able to remove or replace the sample  214  when an x-ray crystallography test is complete. 
     During x-ray crystallography, the sample  214  will be subject to heating by the incident x-ray beam  212 . If the sample  214  is frozen to maintain its crystal structure, this heating must be offset to prevent the sample from thawing. Further, the sample is subject to heating from the ambient atmosphere in the room. One means of offsetting the heating is to provide a coolant source  230  such as a source of extremely cold nitrogen gas, although those of skill in the art will recognize that other coolants than nitrogen gas are usable in this situation. The coolant source  230  is connected via a coolant transport tube  232  to a nozzle  234 , which then directs a coolant stream  236  over the sample. 
     However, the coolant stream  236  must maintain essentially laminar flow over the distance between the nozzle  234  outlet and the sample  214 . Once turbulence is introduced, the coolant stream  236  begins mixing with the ambient atmosphere, the coolant stream  236  heats rapidly, losing its cooling capacity. Further, the introduction of additional moisture from the ambient atmosphere can result in icing on the sample or the sample holder and disruption of the x-ray crystallography measurement. Because the ambient atmosphere is inherently in motion and non-laminar, such mixing between the coolant stream  236  and the ambient atmosphere will always occur. 
     Such problems might be avoidable if the nozzle  234  could always be positioned so that the coolant stream  236  exited the nozzle  234  almost immediately adjacent the sample  214 , and there were essentially no spatial transit required before the coolant stream  236  reached the sample  214 . However, such a placement of the nozzle  234  would interfere with the need to reposition the x-ray source  210 , sample  214 , or detector  220 , and would also obstruct the x-ray beam  218  or the diffracted x-ray beam  218 . 
     Referring to FIGS. 4 and 4A, one prior art nozzle  410  for extending the distance over which laminar flow can be maintained in the coolant stream  436  is shown. The nozzle  410  comprises a tubular cavity  414 , which guides the coolant stream  436  through the nozzle  410  and insures that the coolant stream  436  is in a laminar flow state which it exits the nozzle  410 . The tubular cavity  414  is enclosed in a vacuum jacket  412  which insulates the tubular cavity  414  from the ambient atmosphere (not shown). The coolant stream enters the tubular cavity through an inlet  416 . As the coolant stream  436  exits the nozzle  410 , it flows though an extender  418 . The extender  418  comprises and inner tubular section  420  of essentially the same diameter as the tubular cavity  414 , and a scalloped heater seat  428 , which allows a heater  430  to be threaded around and seated onto the extender  418 . The scalloped heater seat  428  allows maximum thermal contact between the heater  430  and the extender  418 . The heater  430  is electrically connected to a power source (not shown) by means of a connecting lead  432 . 
     In use, sufficient current is applied through the heater  430  so that, as the coolant stream  436  exits the extender  418 , the coolant stream  436  comprises a relatively warm outer zone  440  and a very cold inner zone  438 . The relatively warm outer zone  440  of the coolant stream has a temperature intermediate that of the ambient atmosphere and the very cold inner zone  438 , and thus serves as a buffer zone between the ambient atmosphere and the very cold inner zone  438 . The existence of the relatively warm outer zone  440  extends the distance over which the coolant stream  436  can maintain its laminar characteristics. However, the heated gas in the relatively warm outer zone  440  of the coolant stream  436  quickly expands, which it is unable to do in the extender  418 . Thus, some of the benefits of the relatively warm outer zone  440  are lost as its expansion induces mixing with the ambient atmosphere. Further loses in laminar characteristics in the coolant stream  436  can result from discontinuities between the tubular cavity  414  and the extender  418 . An additional disadvantage to this device results from the high heat created at the heater  430 , which presents a safety hazard to personnel working around the device. 
     Referring to FIGS. 1,  1 A, and  1 B, an embodiment of the present invention is shown. A nozzle  10  comprises an essentially cylindrical vacuum jacket  12  and a tubular cavity  14 . Although not required, the tubular cavity may extend longitudinally down the entire nozzle  10  to provide an orifice  20 , into which a measuring device, such as a thermocouple  22  may be inserted. In such cases, a seal  24  is provided to close the orifice  20  and prevent loss of material from the coolant stream  36  through the orifice  20 . 
     The tubular cavity also comprises an inlet  16  and an outlet  18 , through which the coolant stream  36  enters and exits, respectively, the tubular cavity  14 . In the preferred embodiment, the tubular cavity also comprises a flared portion  15  in the section of the tubular cavity  14  adjacent the orifice  18 . The outer surface  17  of the tubular cavity  14  comprises a scalloped heater seat  28 , which provides a continuous groove into which a heater  30  may be threaded and seated. The scalloped form of the scalloped heater seat  28  allows the heater  30  to be placed in maximum physical contact with the tubular cavity  14  to maximize the heat transfer from the heater  30  to the tubular cavity  14 . 
     The heater  30  comprises an active heater element  34  and a heater lead wire  32 . The heater lead wire  32  provides an electrical connection to the active heater element  34 , but does not itself produce significant heat while carrying current. This feature allows the heater lead wire  32  to be extended within the vacuum jacket to a convenient location where it can be connected to an external power source (not shown) via a vacuum feed through  26 . Because the heater lead wire  32  does not produce significant heat, it does not adversely affect the conditions of the coolant stream  36  within the tubular cavity  14 . 
     With the heater  30  turned on, the coolant stream  36  is heated in the region near the outlet  18 , resulting in the coolant stream having an essentially hollow cylindrical, relatively warm outer zone  40  and an essentially cylindrical very cold inner zone  38 . The flare  15  in the tubular cavity  14  allows the relatively warm outer zone  40  to expand as it is being heated, thereby maintaining the laminar characteristics of the coolant stream  36  during the heating process. By heating the outer zone of the coolant stream  36  within the nozzle  10 , the linear distance over which the coolant stream  36  maintains its laminar flow characteristics once it exits the nozzle  10  is greatly enhanced. 
     Referring to FIG. 3, an alternative section along plane A—A of FIG. 1 is shown. In this embodiment, A nozzle  310  comprises an essentially cylindrical vacuum jacket  312  and a tubular cavity  314 . Although not required, the tubular cavity may extend longitudinally down the entire nozzle  310  to provide an orifice  320 , into which a measuring device, such as a thermocouple  322  may be inserted. In such cases, a seal  324  is provided to close the orifice  320  and prevent loss of material from the coolant stream  336  through the orifice  320 . 
     The tubular cavity also comprises an inlet  316  and an outlet  318 , through which the coolant stream  336  enters and exits, respectively, the tubular cavity  314 . The outer surface  317  of the tubular cavity  314  comprises a scalloped heater seat  328 , which provides a continuous groove into which a heater  330  may be threaded and seated. The scalloped form of the scalloped heater seat  328  allows the heater  330  to be placed in maximum physical contact with the tubular cavity  314  to maximize the heat transfer from the heater  330  to the tubular cavity  314 . 
     The heater  330  comprises an active heater element  334  and a heater lead wire  332 . The heater lead wire  332  provides an electrical connection to the active heater element  334 , but does not itself produce significant heat while carrying current. This feature allows the heater lead wire  332  to be extended within the vacuum jacket to a convenient location where it can be connected to an external power source (not shown) via a vacuum feed through  326 . Because the heater lead wire  332  does not produce significant heat, it does not adversely affect the conditions of the coolant stream  336  within the tubular cavity  314 . 
     With the heater  330  turned on, the coolant stream  336  is heated in the region near the outlet  318 , resulting in the coolant stream having an essentially hollow cylindrical, relatively warm outer zone  340  and an essentially cylindrical very cold inner zone  338 . This embodiment differs from that of FIG. 1 by the absence of a flared zone in the tubular cavity  314 , so that expansion of the relatively warm outer zone  340  of the coolant stream  336  would be expected to occur at a faster rate once the coolant stream exits the outlet  318  than would be the case in the preferred embodiment of FIGS. 1,  1 A, and  1 B. 
     Those of skill in the art will recognize that variations of the above description may be made without departing from the scope and spirit of this invention, and this invention shall not be unduly limited to these illustrative embodiments.