Abstract:
In a nuclear magnetic resonance probe, the sample coil is connected to the RF excitation source via transmission lines that are arranged to generate one or more nodal points at the  1 H excitation frequency along their lengths and a balanced magnetic filed profile within the sample coil. Heat exchangers are then connected directly to the inner conductor of the transmission line at these nodal points. The transmission line inner conductors are in direct contact with the sample coil and efficiently cool the coil to cryogenic temperatures without interfering with the  1 H resonance or RF profile.

Description:
BACKGROUND 
   This invention relates to nuclear magnetic resonance probes with sample coils that are cooled by means of cryogenic materials. In conventional nuclear magnetic resonance spectroscopy apparatus, a sample to be analyzed is positioned within a static magnetic field produced within the bore of a high field strength magnet. A probe for detecting magnetic fields is positioned around the sample. The probe includes radio frequency transmitter and receiver coils (which may be the same coil) positioned near the sample for both exciting and detecting magnetic moments in the sample material. Typically, the sample coil is made of copper or other ordinary conductive materials and is arranged to be resonant at the applicable frequencies. 
   In many applications, it is advantageous to cool the sample coil. The advantages include an increased Q (quality factor) in the resonator, which in turn, results in a higher signal-to-noise ratio available from the sample. Further, thermal noise generated by copper sample coils can be reduced by cooling the coils. Reduced thermal noise associated with probe circuit improves sensitivity of the spectrometer. A higher signal-to-noise ratio means shorter experimental times and higher throughput. Another advantage is that the sample itself can be more conveniently cooled to cryogenic temperature for certain types of experiments. 
   A number of conventional approaches are used to cool the sample coil. One approach is to fabricate the sample coil from a hollow tube instead of a solid wire and to pump a cryogenic cooling fluid through the hollow tube thereby cooling the coil from the inside. This approach has the disadvantage that the production of coils of this type is difficult and the coil geometries that can be attained are limited. In addition, forcing the cryogenic cooling fluid through the tube under pressure may give rise to vibrations that detract from the probe operation. 
   In another approach, a multi-walled quartz Dewar flask is used. This flask is constructed as an annulus that is surrounded both on the inside and outside by double walls. The space between the double walls is evacuated. The sample coil fits into the annulus which is then filled with cryogenic cooling fluid. The disadvantage here is that the “filling” factor is poor resulting in poor NMR sensitivity. In addition, the Dewar flask design is difficult and costly to implement. 
   In still another prior art approach, a heat exchanger cools a substrate fabricated from a material with high thermal conductivity, but poor electrical conductivity, such as sapphire. The sample coil is in contact with the cooled substrate and is thereby cooled. A disadvantage here is that the substrate material is typically planar and the sample coil must be deposited onto the substrate for good thermal contact. Thus, the sample coil must also be planar and consequently has limited geometry. In addition, the planar coils typically do not have a power handling capability generally required for solid state NMR experiments. A further disadvantage is that the transfer of heat is inefficient due to multi-material contact. 
   In another approach, one end of the sample coil is directly in contact with, and cooled by, a cooled platform. However, in this configuration, the magnetic field within sample coil is unbalanced as the cooled platform must be grounded in order not to interfere with the  1 H resonance. 
   SUMMARY 
   In accordance with the principles of the present invention, the RF source is connected to the sample coil via transmission lines. These transmission lines are arranged to generate one or more nodal points at the  1 H frequency. Heat exchangers are then connected directly to the inner conductor of the transmission line at these nodal points. The transmission line inner conductors are in direct contact with the sample coil and efficiently cool the coil to cryogenic temperatures without interfering with the  1 H resonance or profile. 
   In one embodiment, both ends of the sample coil are connected to the inner conductors of transmission lines. The other ends of both inner conductors are terminated on a cooled plate that is grounded. The transmission line lengths are adjusted so that the cooled plate is at a nodal point at the  1 H frequency for both lines. The sample coil is driven by coupling the driving energy, either inductively or capacitively, to the inner conductor of one of the transmission lines. 
   In another embodiment, the transmission lines connected between the sample coil and the cooled plate are extended while maintaining the plate at a nodal point so that the plate can be placed outside of the probe structure and the magnet bore. This embodiment allows for a larger, more powerful heat exchanger to cool the plate. 
   In still another embodiment, both ends of the sample coil are again connected to the inner conductors of transmission lines. The other end of an inner conductor for one transmission line is terminated on a cooled plate that is grounded. The length of this transmission line is adjusted so that the cooled plate is at a nodal point at the  1 H frequency. The other end of the second transmission line is open ended and its length is adjusted so that it supports a half wavelength standing wave at the  1 H frequency with a nodal point halfway along its length. The sample coil is driven by coupling the driving energy either inductively or capacitively to the inner conductor of one of the transmission lines. This embodiment has the advantage that a second frequency can be introduced at the latter nodal point without interfering with the  1 H frequency balance and resonance. 
   In yet another embodiment, the cooled plate is replaced with a cylindrical heat exchanger that fits into the outer conductor of the transmission line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a sample coil cascaded between two transmission lines that illustrates some of the different resonances that the cascaded structure can support. 
       FIG. 2A  is a schematic diagram that shows the cascaded structure connected to a cooled plate with RF energy at a first resonant frequency applied to the structure via inductive coupling. 
       FIG. 2B  is a partial schematic diagram that shows the cascaded structure connected to a cooled plate with RF energy at a first resonant frequency applied to the structure via capacitive coupling. 
       FIG. 3  is a schematic diagram of a cascaded structure in which one leg has been extended to allow an additional resonant frequency to be applied to the structure without disturbing the resonance of the structure at the first resonant frequency. 
       FIG. 4  is a schematic diagram showing another embodiment in which the cooled plate has been replaced by a cylindrical heat exchanger. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates one embodiment  100  of the invention in which a sample coil  102  is connected to the inner conductors  108  and  110  of two transmission lines  104  and  106 . The other ends,  112  and  116 , of the inner conductors of the transmission lines  104  and  106  are grounded to a common ground plate as indicated schematically by grounds  114  and  118 , respectively. This structure can then be driven by radio frequency energy by coupling the energy into the resonant structure  100  either capacitive or inductively. The cascaded structure  100  will be resonant at many different frequencies of the driving energy and standing waves can be supported within the structure  100 . The standing waves at three different driving frequencies are shown schematically in  FIG. 1  by the dotted lines  120 ,  122  and  124  which represent voltage values within the structure  100 . 
   At the lowest resonant frequency (n=1), the standing wave voltage  120  will have a single maximum at the center of the sample coil  102  and a minimum at the ground points  112  and  116 . At the next lowest resonant frequency (n=2), the standing wave  122  has voltage maxima of opposite phases, which occur at the two ends  108  and  110  of the sample coil  102 , while a voltage null occurs at the center of the sample coil  102 . At the next lowest resonant frequency (n=3) the standing wave  124  has three voltage maxima, which occur somewhere along the transmission lines  104  and  106  and at the center of the sample coil  102 . By selecting an appropriate driving frequency, for example n=2, a voltage null will occur at the center of the sample coil  102 . As a result, that driving frequency will produce a symmetrical and balanced magnetic field at the center of the sample coil, as desired. 
     FIG. 2A  shows a cascaded structure, such as that shown in  FIG. 1 , attached to a cooled ground plate  216  in order to cool the sample coil  202 . In this structure, sample coil  202  with terminals  201  and  203  is connected to the inner conductors  212  and  214  of two transmission lines  204  and  206 , respectively. The inner conductors  212  and  214  of the transmission lines  204  and  206  are connected to the common plate  216  which is grounded as indicated at  218 . Plate  216  may illustratively be hollow so that a cooling fluid can circulate through the plate and cool it. The cooling fluid enters the plate  216  via inlet pipe  220  as indicated by arrow  224  and exits the plate  216  via outlet pipe  222  as indicated schematically by arrow  226 . As illustrated, this structure is driven with radio frequency energy at the  1 H frequency applied to terminal  208  and inductively coupled to the inner conductor  212  of transmission line  204  via the small section  211  that extends parallel to the inner conductor  212 . Adjustable capacitor  210  matches the impedance of the resonant structure as seen by terminal  208  for efficient RF energy transfer. The lengths of transmission lines  204  and  206  are chosen to be an odd multiple of one quarter of the driving energy wavelength (n λ H /4, where n=1, 3, 5, etc.) Alternatively, the structure can driven with radio frequency energy at the  1 H frequency applied to terminal  208  and capacitively coupled to the inner conductor  212  of transmission line  204  as shown in the partial schematic view of  FIG. 2B . 
   In the structure shown in  FIGS. 2A and 2B , the cooled ground plate  216  can effectively cool the sample coil  202  down to cryogenic temperatures. Since the sample coil  202  and the inner conductors  212  and  214  of transmission lines  204  and  206  are in direct contact with the grounded and cooled platform  216 , the transfer of heat between the sample coil  202  and the plate  216  is very efficient. Although fluid cooling is shown in  FIGS. 2A and 2B  for purposes of illustration, the method for cooling the cold plate is not restricted in this structure since the plate  216  is always at ground potential and thus has no effect on the electrical length of the transmission lines  204  and  206  or resonant frequency. 
   Because the transmission lines  204  and  206  can be extended in length by odd multiples of λ H /4, the cooled plate  216  can be placed outside the probe structure and magnet bore (not shown in  FIGS. 2A and 2B ) to allow a larger, more powerful heat exchanger to be used to cool the plate  216 . 
   In another embodiment of the invention illustrated in  FIG. 3 , a modified structure allows the introduction of another driving frequency. In this structure, sample coil  302  with terminals  301  and  303  is connected to the inner conductors  312  and  314  of two transmission lines  304  and  306 , respectively. The inner conductor  312  of the transmission line  304  is connected to a cooled plate  316  which is grounded as indicated at  318 . Plate  316  may illustratively be hollow so that a cooling fluid can circulate through the plate and cool it. The cooling fluid enters the plate  316  via inlet pipe  320  as indicated by arrow  324  and exits the plate  316  via outlet pipe  322  as indicated schematically by arrow  326 . As illustrated, this structure is driven with radio frequency energy at the  1 H frequency applied to terminal  308  and inductively coupled to the inner conductor  312  of transmission line  304 . Adjustable capacitor  310  matches the impedance of the resonant structure as seen by terminal  308  for efficient RF energy transfer. The length of transmission line  304  is chosen to be an odd multiple of one quarter of the driving energy wavelength (n λ H /4, where n=1, 3, 5, etc.). Alternatively, the structure can driven with radio frequency energy at the  1 H frequency applied to terminal  308  and capacitively coupled to the inner conductor  312  of transmission line  304  along the lines of the structure illustrated in  FIG. 2B . 
   Transmission line  306  is extended to a length of λ H /2 and is open-ended. The outer conductor  328  is grounded as indicated at  330 . As extended, transmission line  306  supports a half wavelength standing wave for  1 H frequency with a voltage null half way along its length. Voltage maxima of opposite phases still occur on both ends of the sample coil  302  and, thus, the structure remains balanced at the  1 H frequency. 
   The voltage null at the center of transmission line  306  allows RF energy with a second resonant frequency, such as the  13 C frequency, to be coupled into the structure without any effect on the  1 H resonance. For example, RF energy at the  13 C frequency at terminal  336  can be coupled to the nodal point  342  on the inner conductor  314  of transmission line  306  by means of adjustable capacitor  334 . Adjustable capacitor  338  adjusts the frequency of the resonant structure to the  13 C frequency as seen by terminal  336 . As nodal point  342  is at ground potential at the  1 H frequency, adjustment of adjustable capacitor  338  has no impact on the  1 H resonance. 
   Additional RF energy at other resonance frequencies, such as the  15 N resonance, can be added to the structure either through the same nodal point  342  that the  13 C resonant frequency is added, or by extending the open-ended transmission line  306  by another λ H /2 length to generate another voltage null at the  1 H frequency farther along the line  306  and then coupling the additional RF energy to the structure at the position of the second voltage null. Either method has no impact on  1 H balance or frequency as additions are made at voltage nulls of the  1 H frequency. 
   In this embodiment, one terminal  301  of the sample coil  302  is connected directly to the inner conductor  312  of one of the transmission lines  304 , which, in turn, is in direct contact with the grounded cold platform  316 ; hence the transfer of heat between the sample coil  302  and the cold platform  316  will also be very efficient.  FIG. 4  shows another embodiment which uses a cylindrical “cold-finger” heat exchanger. Elements in  FIG. 4  that correspond to elements in  FIG. 3  have been given corresponding numeral designations. For example, sample coil  402  in  FIG. 4  corresponds to sample coil  302  in  FIG. 3 . To shorten the description, elements in  FIG. 4  that correspond to elements in  FIG. 3  will not be described further herein. In the  FIG. 4  embodiment, the outer conductor  415  of transmission line  404  has been extended and a cylindrical heat exchanger  450  is placed within the extended outer conductor  415 . The heat exchanger  450  is connected directly to the inner conductor  412  of transmission line  404  in order to cool sample coil  402 . This compact and efficient cooling arrangement can be implemented in standard bore probes where use of components with large radial dimensions are limited. 
   The present invention does not require special materials or construction for the sample coil. Since the sample coil is cooled via direct contact at its terminals rather than through sapphire substrates on the surface of the coil or surrounded by dewars for cryogenic fluids, this method can be used in areas where space is severely restricted, such as within the probe body of a Magic Angle Spinning (MAS) system. Use of transmission lines also provides greater power handling typically required by solid state NMR experiments, a requirement that is particularly challenging at high fields. 
   While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.