Abstract:
In some embodiments, a NMR spectrometer includes a NMR probe circuit component (e.g. RF coil insert, capacitor, inductor) in thermal and electrical contact with a cryogenically-cooled NMR probe support through a collet assembly. The collet assembly includes a collet assembly body connected to the probe support, a collet inserted into the collet assembly body, a pin connected to the probe circuit component, and a nut threaded over a back of the collet to secure the pin to the collet. The collet assembly body is connected to the probe circuit component and the pin is connected to the probe support. A heat exchanger may be in thermal contact with the probe support. The collet assembly provides a demountable, compact, reliable, low-resistance, and strong thermal and electrical connection particularly suited for use in NMR probes, which are commonly subject to stringent spatial and other NMR-compatibility design constraints.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to nuclear magnetic resonance (NMR) spectroscopy, and in particular to systems and methods for forming demountable cryogenic NMR connections in NMR spectrometers. 
         [0002]    Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B 0 , and an NMR probe including one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B 1  perpendicular to the field B 0 , and for detecting the response of a sample to the applied magnetic fields. Each RF coil and associated circuitry can resonate at the Larmor frequency of a nucleus of interest present in the sample. The direction of the static magnetic field B 0  is commonly denoted as the z-axis or longitudinal direction, while the plane perpendicular to the z-axis is commonly termed the x-y or transverse direction. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in sample tubes or flow cells. 
         [0003]    The design of NMR probes is commonly subject to design constraints specific to NMR systems. In particular, the design of NMR probes is commonly subject to tight spatial constraints. Moreover, NMR probes include highly-sensitive RF circuits which are subject to interference from various components of the probes. 
         [0004]    Cryogenically cooled probes often allow achieving better sensitivity than conventional room-temperature probes. The increase in sensitivity of cryogenically cooled probes allows effective data acquisition from limited sample sizes and concentrations. At the same time, cryogenic probes introduce new challenges to NMR system designers. For example, system designers may need to establish durable, good-conductivity, NMR-compatible thermal and electrical connections between various NMR probe components at low temperatures. 
         [0005]    In a common approach, soldering is used to establish various thermal and/or electrical connections between NMR probe components. For example, NMR measurement circuit components such as capacitors, inductors, or NMR RF coil(s) may be soldered to one or more cryogenically-cooled NMR probe boards (e.g. the probe cold head). Soldering creates a permanent connection, and exposure to heat during the soldering process can adversely affect some system components. At the same time, soldering has remained a common connection method because of the relative difficulty of establishing durable, stable, NMR-compatible connections having good thermal and/or electrical conduction properties between cryogenically-cooled components. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one aspect, a nuclear magnetic resonance (NMR) apparatus comprises a cryogenically-cooled NMR probe support situated within an NMR probe, a NMR probe circuit component of the NMR probe, and a demountable thermal contact assembly for establishing a demountable thermal connection between the NMR probe circuit component and the NMR probe support. The thermal contact assembly comprises a contact assembly body connected to the NMR probe support, a collet positioned within the contact assembly body, the collet having a front slotted collar and a back collet threaded surface, a pin extending through the front collar of the collet and connected to the NMR probe circuit component, and a threaded collet fastener having a fastener threaded surface engaging the collet threaded surface to secure the collet to the contact assembly body and thereby establish the demountable thermal connection between the NMR probe circuit component and the NMR probe support through the pin, collet, and contact assembly body. In some embodiments, the contact assembly body is connected to the NMR probe circuit component and the pin is connected to the NMR probe support. 
         [0007]    According to another aspect, a demountable NMR probe component attachment method comprises inserting a pin into a collet, wherein the pin is connected to a first structure selected from a NMR probe circuit component of a NMR probe and a cryogenically-cooled NMR probe support situated within the NMR probe, and wherein the collet includes a collar; inserting the collet into a contact assembly body, wherein the contact assembly body is connected to a second structure selected from the cryogenically-cooled NMR probe support and the NMR probe circuit component; and clamping the collar of the collet onto the pin by connecting a threaded collet fastener to a back thread of the collet, to thermally connect the NMR probe circuit component to the cryogenically-cooled NMR probe support through the pin, collet, and contact assembly body. 
         [0008]    According to another aspect, a NMR apparatus comprises a cryogenically-cooled NMR probe support situated within a NMR probe; a NMR probe circuit component of the NMR probe; and a thermally-conductive demountable thermal-contact collet assembly extending through the NMR probe support, and mechanically and thermally connecting the NMR probe circuit component to the NMR probe support. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where: 
           [0010]      FIG. 1  is a schematic diagram of an exemplary NMR spectrometer according to some embodiments of the present invention; 
           [0011]      FIG. 2  shows a side view of part of an NMR probe according to some embodiments of the present invention; 
           [0012]      FIG. 3-A  shows a part of an NMR probe circuit including an inductor flanked by two capacitors, according to some embodiments of the present invention; 
           [0013]      FIG. 3-B  shows the inductor of the circuit of  FIG. 3-A  according to some embodiments of the present invention; 
           [0014]      FIG. 4  shows an isometric view of a thermal contact assembly of the probe of  FIG. 2  according to some embodiments of the present invention; 
           [0015]      FIG. 5-A  shows a side view of the contact assembly of  FIG. 4  in an assembled state, according to some embodiments of the present invention; 
           [0016]      FIG. 5-B  shows an exploded side view of the contact assembly of  FIG. 5-A  according to some embodiments of the present invention; 
           [0017]      FIG. 5-C  shows a transverse view of a contact assembly of  FIGS. 5-A ,  5 -B according to some embodiments of the present invention; 
           [0018]      FIG. 6-A  shows a side view of a contact assembly of the probe of  FIG. 2  in an assembled state, according to some embodiments of the present invention; 
           [0019]      FIG. 6-B  shows an exploded side view of the contact assembly of  FIG. 6-A  according to some embodiments of the present invention; and 
           [0020]      FIG. 6-C  shows a transverse view of the contact assembly of  FIGS. 6-A ,  6 -B according to some embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    In the following description, a set of elements includes one or more elements. A plurality of elements includes two or more elements. Any reference to an element is understood to encompass one or more elements. Each recited element or structure can be formed by or be part of a monolithic structure, or be formed from multiple distinct structures. Unless otherwise stated, any recited electrical or mechanical connections can be direct connections or indirect operative connections through intermediary structures. Unless otherwise specified, the term cryogenic refers to temperatures below the liquid nitrogen temperature (77 K). 
         [0022]    The following description illustrates embodiments of the invention by way of example and not necessarily by way of limitation. 
         [0023]      FIG. 1  is a schematic diagram illustrating an exemplary nuclear magnetic resonance (NMR) spectrometer  12  according to some embodiments of the present invention. Spectrometer  12  comprises a magnet  16 , a low-temperature NMR probe  20  inserted in a cylindrical bore of magnet  16 , and a control/acquisition system  18  electrically connected to magnet  16  and probe  20 . Probe  20  includes one or more radio-frequency (RF) coils  24  and associated electrical circuit components. For simplicity, the following discussion will focus on a single RF coil  24 , although it is understood that a system may include other similar coils. RF coil  24  and the various components connected to RF coil  24  form one or more NMR measurement circuits. A sample vessel  22  is positioned within probe  20 , for holding an NMR sample of interest within RF coil  24  while measurements are performed on the sample. Sample vessel  22  may be a sample tube or a flow cell. 
         [0024]    To perform a measurement, a sample is inserted into a measurement space defined within RF coil  24 . Magnet  16  applies a static magnetic field B 0 to  the sample held within sample container  22 . Control/acquisition system  18  comprises electronic components configured to apply desired radio-frequency pulses to probe  20 , and to acquire data indicative of the nuclear magnetic resonance properties of the samples within probe  20 . RF coil  24  is used to apply radio-frequency magnetic fields B 1  to the sample, and/or to measure the response of the sample to the applied magnetic fields. The RF magnetic fields are perpendicular to the static magnetic field. The same coil may be used for both applying an RF magnetic field and for measuring the sample response to the applied magnetic field. Alternatively, one coil may be used for applying an RF magnetic field, and another coil for measuring the response of the sample to the applied magnetic field. A single coil may be used to perform measurements at multiple frequencies, by tuning the resonant frequency of the NMR measurement circuit that includes the coil. Tuning the circuit resonant frequency may be achieved by adjusting the capacitance values of one or more variable capacitors included in the circuit. Adjusting one or more capacitance values may also be used to achieve impedance matching or other desired circuit characteristics. 
         [0025]      FIG. 2  shows a side sectional view of the top part of NMR probe  20  according to some embodiments of the present invention. Probe  20  has a longitudinal measurement aperture  44  for receiving sample vessels of interest. A number of NMR probe components described below are situated within an NMR probe housing  32 . RF coil  24  includes an electrically insulative coil support (insert)  34 , and a coil conductor  36  mounted on coil support  34 . Coil conductor  36  includes a thin conductive foil patterned to define a conductive pattern of a desired shape, e.g. a saddle shape. Coil support  34  includes a cylinder made from a thermally-conductive, electrically-insulative NMR-compatible material such as sapphire. Coil support  34  includes a lip or flange  38  for securing coil  24  to a cold board  30   a  as described below. In some embodiments, other shapes or configurations for a coil support may be used, for example if the coil conductor is formed by a wire rather than a patterned foil. 
         [0026]    A plurality of thermally-conductive cold boards  30   a - b  are situated within probe  20 . Cold boards  30   a - b  form a plurality of NMR probe supports configured to support NMR probe circuit components such as RF coils, capacitors, inductors. An uppermost board  30   a  forms part of a probe cold head, which is the part of probe  20  providing structural support and thermal connectivity to RF coil  24 . Cold boards  30   a - b  are in thermal contact with a heat exchanger  50 , for cooling cold boards  30   a - b.  Each cold board  30   a - b  may be soldered and bolted to heat exchanger  50 . Cold boards  30   a - b  may be formed from an electrically-conductive material, and may be electrically grounded. Heat exchanger  50  may include a volume of a metal foam having a large internal surface area, connected to a cryogenic fluid inlet  52   a  and a cryogenic fluid outlet  52   b.  Cryogenic fluid inlet  52   a  and cryogenic fluid outlets  52   b  allow the flow of a cryogenic cooling fluid such as helium or liquid nitrogen through heat exchanger  50 . 
         [0027]    A plurality of NMR circuit components  54   a - b  are mounted on cold board  30   b,  underneath cold board  30   a.  Circuit components  54   a - b  may include capacitors, inductors, and/or other circuit components electrically connected to RF coil  24  and/or other coils of probe  20 . Circuit components  54   a - b  may be electrically grounded to cold board  30   b,  and may be connected to an external electrical power (e.g. voltage/current) source through one or more leads  58 . Leads  58  pass through a feedthrough  60  extending through cold board  30   b.  Coil  24  and circuit components  54   a - b  are mechanically and thermally connected to cold boards  30   a - b  through demountable thermal contact assemblies  40   a - c,  respectively. In some embodiments, at least some thermal contact assemblies may also provide electrical conduction paths. 
         [0028]      FIG. 3-A  shows an NMR probe circuit section  64  including two capacitors  154   a - b  connected in series with and flanking an inductor  154   c,  according to some embodiments of the present invention. Capacitors  154   a - b  are connected externally (e.g. to other circuit components) through leads  158   a - b.  If cooling is achieved through the electrical connection path(s), one or both of leads  158   a - b  may also provide thermal conduction path(s) for cooling the components of circuit section  64 . In such an embodiment, capacitors  154   a - b  may be relatively poor thermal conductors, and consequently inductor  154   c  may be thermally insulated by capacitors  154   a - b  from any external heat sink. Inductor  154   c  may then be cooled through an insulative inductor support, as described below. 
         [0029]      FIG. 3-B  shows inductor  154   c  according to some embodiments of the present invention. Inductor  154   c  includes a cylindrical, thermally-conductive and electrically insulative support  164 , and an electrically-conductive winding  166  mounted on and thermally connected to support  164 . Support  164  may be a rod or tube of a thermally-conductive material such as sapphire. Support  164  is connected to a heat sink such as a cold board through a demountable thermal contact assembly. 
         [0030]      FIG. 4  shows an exploded isometric view of a demountable thermal contact assembly  40  in an unassembled state according to some embodiments of the present invention. Thermal contact assembly  40  includes a contact assembly body  80 , a collet  82  sized to be inserted into contact assembly body  80 , a pin  88  sized to be inserted into a front end of collet  82 , and a collet fastener such as a screw  90  sized to be inserted into a back end of collet  82  to secure collet  82  and pin  88  to contact assembly body  80 . Collet  82  includes a cylindrical part  92 , and a tapered collar  94  connected to and protruding transversely relative to cylindrical part  92 . A pair of co-linear slots  96   a - b  are defined through collar  94  and cylindrical part  92 , for allowing a transverse flexure of the two lateral sides of collar  94  as contact assembly body  80  pressed on the tapered surface of collar  94 , to securely grip pin  88  within collet  82 . A frontal central aperture  100  is defined through collar  94  and cylindrical part  92 , and is sized to receive pin  88 . Contact assembly body  80  includes a central aperture  104  sized to receive collet  82 . A tapered collet-contact surface  106  defined along a front side of central aperture  104  matches the size and taper angle of collar  94 . 
         [0031]    In some embodiments, contact assembly body  80  is connected to a cryogenically-cooled NMR probe support (e.g. a cold board, heat exchanger, or other thermally-conductive structure mounted on a heat exchanger or cold board), while pin  88  is connected to an NMR probe circuit component (e.g. a sapphire RF coil insert, capacitor, inductor). Contact assembly body  80  may be integrally formed with at least part of the probe support (e.g. may me machined into the cold board or heat exchanger). Contact assembly body  80  may also be a distinct part attached to the probe support by soldering or another thermally-conductive connection. Pin  88  may be integrally formed with at least part of the NMR probe circuit component, or may be a distinct part attached to the NMR probe circuit component through a thermally-conductive connection. In some embodiments, contact assembly body  80  may be connected to an NMR probe circuit component and pin  88  connected to a cryogenically-cooled NMR probe support. 
         [0032]    In some embodiments, a metal-to-metal connection or sapphire-to-metal connection between pin  88  and a NMR probe circuit component may be established using an adhesive and/or soldering. An adhesive may include an epoxy such as Shell EPON™ epoxy. A solder connection may be established by first metalizing the sapphire surface using a conductive paste (e.g. DuPont™ 7095 conductive paste), and soldering a conductor to the metalized sapphire. A sapphire-metal solder connection may also be established in some embodiments by direct ultrasonic soldering. 
         [0033]      FIG. 5-A  shows a side view of thermal contact assembly  40  in assembled state according to some embodiments of the present invention.  FIGS. 5-B ,  5 -C show side and top views of the components of contact assembly  40  in an unassembled state according to some embodiments of the present invention. As shown in  FIG. 5-A , contact assembly body  80  is secured to a cold board  30 , and is centered above a central aperture  120  extending through cold board  30 . Central aperture  120  accommodates screw  90 , which is threaded into the back of collet  82  to secure collet  82  to contact assembly body  80 . As shown in  FIG. 5-B , collet  82  includes an inner thread  108  sized to match an outer thread  110  of screw  90 . Pin  88  is inserted into the front of collet  82 , and is connected to an NMR probe circuit component  102 . A demountable thermal connection is established between NMR probe circuit component  102  and cold board  30  through pin  88 , collet  82 , and contact assembly body  80 . In some embodiments, a demountable electrical connection is established between NMR probe circuit component  102  and cold board  30  through pin  88 , collet  82 , and contact assembly body  80 . 
         [0034]    Tightening screw  90  pulls collet  82  longitudinally into contact assembly body  80 . The tapered surface of collar  94  is pressed by the matching tapered surface  106  of contact assembly body  80 . The transverse pressure on the tapered surface of collar  94  grips pin  88  tightly within collet  82 . To demount NMR probe circuit component  102  from cold board  30 , screw  90  is loosened and removed from collet  82 . Collet  82  is removed from contact assembly body  80  and pin  88  is removed from collet  82 . 
         [0035]    In some embodiments, the components of contact assembly  40  are made from the same electrically- and thermally-conductive material. The material may be an NMR-compatible material having suitable hardness, electrical conductivity and thermal conductivity properties. In some embodiments, all components of contact assembly  40  are formed of a conductive copper alloy such as tellurium copper or oxygen-free high-conductivity copper (OFHC). Tellurium copper may be used because of its relative hardness and good thermal and electrical conduction properties. The material hardness can facilitate the reliable and repeatable attachment of collet  82  to its corresponding fastener, particularly if a screw such as screw  90  is used. Using one or more materials with substantially identical thermal expansion coefficients for all components of contact assembly  40  allows minimizing temperature-dependent differences in the thermal expansion of the various components, thus facilitating the reliable control of interface forces along the inter-component interfaces of contact assembly  40 . If materials with substantially different thermal expansion coefficients are used for different components (e.g. stainless steel for some components and copper for others), a good thermal connection at one temperature (e.g. at room temperature) may exhibit degraded thermal conductivity characteristics at a different temperature (e.g. close to 0 K) as the different assembly components expand at different rates. Cold boards  30   a - b  may be made from an NMR-compatible, electrically- and thermally-conductive material. In some embodiments, cold boards  30   a - b  are made from oxygen-free, high-purity, high-conductivity copper. 
         [0036]    Contact assembly  40  may be on the order of several mm to several cm in length (longitudinal extent), for example about 0.5-5 cm when assembled. Screw  90  has a length sufficient to pass through its corresponding NMR probe fixed support (e.g. cold board  30  in  FIG. 5-A ) and mate with the inner thread of collet  82 . In some embodiments, screw  90  has a length on the order of several mm to several cm, for example about 0.3-3 cm. For example, screw  90  may be a 0-80 screw having an outer diameter of 1.5 mm. Collet assembly body  80  may have an outer diameter on the order of several mm to cm, for example 0.5-2 cm, an inner diameter smaller than the outer diameter by an extent on the order of mm, for example 1-3 mm, and a length on the order of cm, for example 1-2 cm. Collet  82  may have an inner diameter on the order of mm, for example 2-10 mm, and length substantially equal to that of collet assembly body  80 . Pin  88  may have a diameter on the order of mm, for example 2-10 mm, and a length on the order of mm to cm, for example 2-15 mm. The taper angle of collar  94  and tapered surface  106  may have a value between 15 and 60°, for example about 45°. 
         [0037]      FIG. 6-A  shows a side view of a thermal contact assembly  240  in an assembled state according to some embodiments of the present invention.  FIGS. 6-B ,  6 -C show side and top views of the components of contact assembly  240  in an unassembled state according to some embodiments of the present invention. As shown in  FIG. 6-A , contact assembly body  80  is secured to cold board  230 , within a central aperture  220  extending through cold board  230 . The inner diameter of central aperture  220  matches the outer diameter of contact assembly body  80 . A collet  282  is mounted within the central aperture  104  extending through contact assembly body  80 . Collet  282  includes a cylindrical part  292 , and a tapered collar  294  connected to and protruding transversely relative to cylindrical part  292 . Cylindrical part  292  protrudes outside the back surface of cold board  230  when collet  282  is mounted within central aperture  220 , exposing an external threaded surface  208 . The diameter of threaded surface  208  may be substantially equal to the diameter of cylindrical part  292 . For example, threaded surface  208  may have a 4-40 size, which corresponds to a diameter of about 3 mm. A nut  290  having a matching internal threaded surface  210  engages collet  282  to secure collet assembly  240  to cold board  230 . Pin  88  is inserted into the front of collet  282 , and is connected to an NMR probe circuit component  102 . A demountable thermal connection is established between NMR probe circuit component  102  and cold board  30  through pin  88 , collet  82 , and contact assembly body  80 . In some embodiments, a demountable electrical connection is established between NMR probe circuit component  102  and cold board  30  through pin  88 , collet  82 , and contact assembly body  80 . 
         [0038]    Tightening nut  290  pulls collet  282  longitudinally with respect to contact assembly body  80 . The tapered surface of collar  294  is pressed by the matching tapered surface  106  of contact assembly body  80 . The transverse pressure on the tapered surface of collar  294  grips pin  88  tightly within collet  282 . To demount NMR probe circuit component  102  from cold board  230 , nut  290  is loosened and removed from collet  282 . Collet  282  is removed from contact assembly body  80  and pin  88  is removed from collet  282 . 
         [0039]    Exemplary demountable connection systems and methods as described above allow achieving good thermal conductivity properties in NMR probe connections having limited surface areas and subject to tight spatial constraints, such as connections between a cold board or other NMR probe support and NMR probe circuit components such as RF coil inserts, capacitors and/or inductors. Exemplary collet assemblies as described above allow achieving relatively high contact forces and good thermal conduction properties for inter-component interfaces, while allowing demounting the connections. An external collet fastener such as a nut enclosing the back side of the collet allows using a larger thread size than an internal collet fastener such as screw inserted within the collet. A larger thread size allows achieving improved connection durability while maintaining good connection thermal conduction properties, while minimally affecting the spatial extent of the connection assembly. A number of metals with good thermal conduction properties, such as OFHC, may be relatively soft. The durability of threaded connections made from such metals may be particularly sensitive to thread size. 
         [0040]    The common cryogenic attachment approach of soldering various components generally allows achieving compact and reliable connections with good thermal conduction properties, but creates permanent attachments and may require undesirable heating of sensitive NMR probe components during assembly. Other mechanical attachment approaches, such as bolting or using a three-jaw chuck, may not allow achieving sufficiently-good thermal conduction properties for common cryogenic NMR probe applications. 
         [0041]    It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. For example, thermal contact assemblies as described above may be used to mount NMR probe circuit components to heat sinks/supports other than cold boards or heat exchangers, such as for example thermally conductive structures mounted on cold boards, heat exchangers, or other cryogenically cooled NMR probe structures. A collet collar may include multiple slots or other flexure apertures allowing the collet collar to tighten transversely in response to applied longitudinal forces. A collet fastener such as a nut or screw may form part of a larger structure. Collet fasteners other than nuts and screws may be used in some embodiments, particularly if spatial constraints permit the use of such fasteners. A contact assembly body may be machined into a cold board, heat exchanger, or other NMR probe support, or be a separate piece connected to the NMR probe support by soldering or other thermally-conductive attachment. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.