NMR magnet system with Stirling cooler.

Abstract

The NMR magnet system uses a Stirling cooler with a cold head that cools a cold shield that extends into the system's housing and surrounds a cryogen vessel. The system may have a damper located between the cooler and the cold shield to reduce the transmission of vibrations from the cooler to a magnetic coil immersed in the cryogen. The damper may be passive or may be part of an active buffering system that uses an acceleration sensor to drive an active damper that compensates for vibrations in the cooler. The compensation device may use stored characteristics of the signal distortion caused by the vibrations and, in response to a trigger signal from the cooler, apply compensation to an excitation signal delivered by an NMR probe in the bore of the magnetic coil to a sample, or to an FID signal from the sample detected by the probe.

Description

[Technical field]

[0001]

[0001] The present invention relates generally to the field of NMR magnet systems, and more specifically to the use of active cooling with such systems. [Background technology]

[0002] Nuclear Magnetic Resonance (NMR) magnet systems are essential tools for biomolecular scientists. Over time, NMR magnet systems require regular replenishment of liquid cryogens by trained personnel. Also, NMR magnet systems cannot be operated during cryogenic transfers due to equipment instability. Furthermore, the cost of helium required to cool magnets to a suitable temperature for superconductivity has been increasing over time. To partially overcome the helium depletion in such systems, so-called two-stage pulse tube refrigerators (PTRs) have been used in recent years. The first stage is connected to the cold shield of the NMR magnet system, while the cooler second stage is heated to the NMR magnet via helium vapor. system The PTR is thermally coupled to the helium vessel of the NMR magnet system. The PTR provides constant cooling power. Electric heaters are sometimes used to heat the helium to maintain constant operating conditions for the NMR magnet system.

[0003]

[0003] In low-cost NMR magnet systems with an equivalent magnetic field strength of 400 MHz or 500 MHz, active cooling of the cold shield by a single-stage pulse tube refrigerator may be used. These NMR magnet systems do not have a container for liquid nitrogen. The PTR used in these NMR magnet systems provides a cooling power of more than 30 W at a temperature of 80 K. Due to the functional principle, the PTR requires an external compressor to provide high pressure for the operation of the cold head of the PTR. The power consumption of the compressor is high (in the range of several kW) and soundproofing is also mandatory. Moreover, the PTR requires regular component inspection by trained personnel, which significantly increases the total cost of operation. Furthermore, the operating frequency of the PTR is in the range of 1-2 Hz, which induces mechanical vibrations in the NMR magnet system at these frequencies and harmonics of said frequencies. It has been shown that NMR magnet systems are particularly sensitive to mechanical vibrations below 10 Hz (see, for example, Non-Patent Document 1). In particular, vibrations in a direction perpendicular to the z-axis (axis of the magnet bore) of an NMR magnet system induce 10 times higher noise in the NMR signal than vibrations along the z-direction.

[0004]

[0004] A Stirling cooler comprises a Stirling cooler driving an internal compressor along the compressor axis and a displacer moving asynchronously with respect to the compressor along the displacer axis. The vibrations of a Stirling cooler occur mainly in the compressor axial and displacer axial directions, but these two axes may be parallel or coaxial. Compressors with different orientations of the Stirling cooler cold head and compressor have been realized or proposed, such as in-line compressors, parallel compressors, and cross-oriented compressors. Low vibration setups also exist, but require a symmetrical arrangement of the compressor (see, for example, non-patent document 2). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Ben-Dov, et al., “NMR sensitivity to floor vibration,” Journal of the Acoustical Society of America 123.5 (2008), 3813-3813. [Non-Patent Document 2] Penswick, L., et al., “High-capacity and efficiency Stirling cycle cryocooler,” Cryocooler, Vol. 18, 2014. Summary of the Invention

[0006] The above requirements and constraints manifest a need for a cooler for the cold shield of an NMR magnet system that has a low overall cost of ownership and minimal impact on the NMR measurements.

[0007]

[0006] Shield cooling is easily achievable since Stirling coolers are commercially available that provide 30 W at 80 K with less than 500 W of motor power. Stirling coolers last a maximum of 10 years before failing. Thus, NMR magnet systems using Stirling coolers for cold shield cooling require little maintenance by trained personnel. Stirling coolers typically operate at mechanical frequencies above 10 Hz, which will have less impact on NMR measurements than PTRs, leading to better results.

[0008] In accordance with the present invention, a nuclear magnetic resonance (NMR) system is provided having an enclosure having a neck tube, and a helium vessel located within the housing and containing a magnetic coil. A Stirling cooler is mounted on the neck tube such that a cold shield surrounds the helium vessel and a cold head of the cooler extends into the enclosure. The cold head is thermally connected to the cold shield via a thermal bond.

[0009] In an exemplary embodiment, the magnetic coil generates an equivalent magnetic field strength of 400-500 MHz. The Stirling cooler may provide, for example, at least 30 W of cooling power at a temperature of 80 K. In a preferred embodiment, the cooler operates at a mechanical frequency of greater than 10 Hz. The thermal coupling may include a flexible thermally conductive means, in particular a set of stranded copper wires that mechanically isolate the Stirling cooler from the cold shield. In one version of this embodiment, the wires have a stiffness and orientation that provides a resilient connection between the cooler and the cold shield that minimizes vibration transmission between the cooler and the cold head. In an exemplary embodiment, the NMR magnet system is a vertical NMR magnet system with a z-axis centered on the room temperature bore, and the compressor axis of the Stirling cooler is parallel to the z-axis.

[0010]

[0009] In another embodiment, the Stirling cooler is mounted to the NMR magnet system with a damper that further reduces the transmission of mechanical vibrations from the Stirling cooler to the NMR magnet system. The damper may comprise a deformable elastic element to passively reduce the mechanical vibrations. In another embodiment, an acceleration sensor is attached to the Stirling cooler to measure the motion of the Stirling cooler. In this configuration, the damper is actively driven by the signal received from the acceleration sensor to perform an opposing motion.

[0011] In yet another embodiment, an NMR magnet system is provided having a Stirling cooler that generates a periodic trigger signal indicative of a state of motion of the Stirling cooler. For example, the trigger signal is generated once per operating cycle of the cooler. The system may be used in an arrangement with an NMR probe inserted into a room temperature bore of the NMR magnet system, the probe providing an RF excitation signal to a sample in the magnet bore and detecting a free induction decay (FID) signal from the sample. In this embodiment, a compensation device is also provided that receives the trigger signal from the Stirling cooler and applies compensation to the excitation signal and / or the FID signal to cancel distortions.

[0012]

[0011] The compensation device may include a data storage element in which a representation of the distortion is stored. The excitation signal may be generated using a numerically controlled oscillator (NCO), and applying compensation to the excitation signal may involve providing the stored representation of the distortion as an input to the NCO, which provides an offset that counteracts the distortion induced in the excitation signal. Applying compensation to the FID signal may include inputting the FID signal to a frequency mixer along with an oscillator signal incorporating the representation of the stored distortion. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram illustrating an NMR magnet system that uses a Stirling cooler to cool the cold shield of the magnet system in accordance with the present invention. [Diagram 2] FIG. 2 is a diagram showing details of the installation of a Stirling cooler in an NMR magnet system. [Figure 3A]

[0014] FIG. 3A is a graph of the distortion-free free induction decay (FID) signal from an NMR probe. [Figure 3B]

[0015] FIG. 3B, like FIG. 3A, is a graph showing constant frequency of an undistorted FID signal. [Figure 4A]

[0016] FIG. 4A is a graph of an FID signal distorted by vibrations from the operation of a Stirling cooler. [Figure 4B]

[0017] FIG. 4B is a graph showing the distorted frequency characteristics of the FID signal, similar to FIG. 4A. [Diagram 5]

[0018] 1 is a schematic graph illustrating the generation of an analog RF signal in which distortion characteristics due to cooler operation are incorporated into the RF signal from a look-up table. [Figure 6]

[0019] FIG. 6 is a schematic diagram of a distortion compensation apparatus that can be used to compensate for the presence of distortions in the NMR probe signal and the detected FID signal due to cooler operation. [Figure 7A]

[0020] FIG. 7A is a graph showing the frequency characteristics of a signal having distortion caused by cooler operation, and the result of mixing that signal with a mixing signal of a constant frequency. [Figure 7B]

[0021] FIG. 7B is a graph showing the frequency characteristics of a signal having distortion caused by a cooler operation, and the result of mixing that signal with a mixing signal into which a distortion compensation characteristic has been introduced. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014]

[0022] FIG. 1 shows an NMR magnet system 1 with a Stirling cooler 7. The NMR magnet system comprises a housing 2 and a magnetic coil 3 in a helium vessel 4. The helium vessel is surrounded by a cold shield 5, which is fixed to the housing by a retaining rod 11. The helium vessel is connected to a retaining tube 10, which may be one of several, which provides a passageway through which the helium gas in the helium vessel 4 may pass to an external collection chamber (not shown). The retaining tubes 10 are also used to connect the magnetic coil 3 to an external power source that charges the magnet, and to refill the helium vessel 4 with helium. Additionally, they help to structurally support the helium vessel 4 and hold it in the proper position relative to the housing 2. The Stirling cooler 7 is attached to a neck tube 6 of the NMR magnet system. Thermal contact between the cold shield 5 and the neck tube 6 is provided by a thermal coupling 8, which also provides mechanical isolation of the Stirling cooler from the cold shield. A room temperature bore 9 along the z-axis provides access to the magnetic center of the NMR magnet system. An NMR probe with a sample (not shown) may be inserted into the room temperature bore to measure the NMR spectrum of the sample.

[0015]

[0023] In operation, the housing 2 as well as the internal space surrounded by the cold shield are evacuated. The helium vessel 4 is filled with liquid helium at a temperature of 4.2 K. By evaporation of the liquid helium, cooling of the magnetic coils is realized so that superconductivity is maintained. The evaporated helium is collected in an external container at atmospheric pressure via a pipe (not shown) passing through the holding tube 10. The cold shield 5 surrounding the helium vessel 4 is cooled by a Stirling cooler and is kept at a temperature below 80 K to absorb the thermal radiation incident on the NMR magnet system. This configuration with a cooling power of a Stirling cooler of 30 W at 80 K is suitable for NMR magnet systems of 400-500 MHz. NMR magnet systems with high magnetic field strengths, due to their large thermal mass and large size, experience more thermal radiation that cannot be adequately compensated by a 30 W Stirling cooler. The Stirling cooler 7 operates at a mechanical frequency of about 50 Hz, i.e. 50 mechanical cycles per second. The transmission of mechanical vibrations from the Stirling cooler to the NMR coil is reduced by appropriate design of the thermal coupling 8, e.g., stranded copper wires.

[0016]

[0024] The NMR magnet system 1 is sensitive to mechanical vibrations since any magnetic field fluctuations at the location of the sample in the room temperature bore 9 result in fluctuations in the measured NMR signal. Despite mechanical isolation by the thermal coupling 8, 50 Hz vibrations can still be transmitted to the magnetic coils, resulting in 50 Hz fluctuations in the magnetic field at the location of the sample in the room temperature bore 9. Harmonics of the NMR signal line can occur at multiple frequency intervals of 50 Hz relative to the original signal line, so artifacts can distort the measured spectrum. A major advantage of using a 50 Hz Stirling cooler is that the NMR magnet system is insensitive to external mechanical vibrations above 10 Hz. The PTR generates harmonics in the range of multiples of 1 Hz, which has a large impact on the NMR signal, making it more difficult to disconnect the PTR from the magnetic coils and to disconnect the NMR probe inserted in the room temperature bore 9.

[0017]

[0025] Due to the low vibration sensitivity of a vertical NMR magnet system along the z-axis, it is advantageous to place the Stirling cooler axis 12 parallel to the z-axis of the NMR magnet system, as shown in Figure 1. In this embodiment, the Stirling cooler axis 12 corresponds to the axis about which the cooler compressor reciprocates and may also correspond to the axis about which the cooler displacer moves. In the case of a Stirling cooler with multiple compressors, this alignment is less critical, provided that the compressors are symmetrically placed with respect to the coldhead and operate synchronously.

[0018]

[0026] A Stirling cooler allows fine tuning of its operating frequency without significant changes in cooling capacity. In this way, vibration resonances in an NMR magnet system can be avoided by changing the operating frequency of the cooler away from the magnet system's resonant frequency or its harmonics, e.g., from 50 Hz to 48 Hz. The systems described herein can be used for NMR magnet systems with horizontal room temperature bores as well as NMR magnet systems with vertical room temperature bores (as shown in the drawings).

[0019]

[0027] Figure 2 shows details of the mounting of the Stirling cooler in a particular embodiment of the invention. A damper 15 is inserted between the NMR magnet system and the Stirling cooler 7. In a first variant, the damper comprises a deformable elastic element to passively reduce mechanical vibrations. In a second variant, an acceleration sensor 16 is attached to the Stirling cooler 7 to measure its movement relative to the magnet system. In this variant, the damper 15 is actively driven by the signal received from the acceleration sensor and performs a counter movement that compensates for the initial relative movement.

[0020]

[0028] In an alternative embodiment of the invention, the effect of a cooler such as a Stirling cooler 7 on the NMR spectrometer is directly compensated for in the signal used in the spectrometer. FIG. 3A shows an undistorted free induction decay (FID) output signal from an NMR spectrometer. The phase change at a single frequency is given by a linear slope over time corresponding to a frequency whose derivative is constant, as shown in FIG. 3B. This indicates a stable magnetic field for the sample. However, the operation of the Stirling cooler 7 can have a distorting effect on the NMR signal, as shown in FIG. 4A. In this figure, the operating cycle of the cooler is shown by a dashed line and is superimposed on the FID signal. The operation of the cooler causes distortions that appear as a deviation of the phase change of the FID signal from the linear slope of the undistorted signal. The phase change over time can be represented as a change in frequency with respect to time, as shown in FIG. 4B.

[0021]

[0029] To address this error, the measured frequency distortion characteristic may be used to adjust the frequency of the NMR excitation signal to compensate for the distortion effect. The frequency distortion characteristic may be measured preferably using a narrowband NMR sample, i.e., a sample with one sharp resonance line (equivalent to a long FID) and a high signal-to-noise ratio (e.g., H2O, typically mixed with D2O). In particular, the distortion characteristic may be stored in a look-up table and used to effect corresponding changes in the frequency of the NMR pulse train being used and in the downmixing of the detected FID signal.

[0022]

[0030] Illustrated in FIG. 5 is a schematic diagram showing the operating cycle of a Stirling cooler along the bottom of the figure. At the start of each cooling cycle, the cooler sends a trigger signal to the NMR spectrometer, which is used to output a digital representation of the frequency correction from a lookup table 50 in which it is stored. This value (called the "frequency control word") is input to a numerically controlled oscillator (NCO) 52 of a digital direct synthesizer (DDS) 54, along with a frequency control word representing the desired pulse train for spectroscopic analysis of the sample. From the combined input of the two control words, the NCO 52 generates a data stream of digital information representing the desired RF pulse train, but with phase variations due to frequency distortion. The DDS 54 converts the data from the NCO 52 to an analog RF signal by using a sine wave lookup table (LUT) 56 and a digital-to-analog converter (DAC) 58, along with appropriate filters (not shown).

[0023]

[0031] A schematic of the entire system is shown in Figure 6. The top of the figure shows an NCO 60, which operates in a similar manner to NCO 52 in Figure 5. A sine wave lookup table 62 and a DAC 64 output an analog frequency signal representing the desired RF pulse train, modified by frequency distortion. However, in modern NMR spectrometers, the detection frequencies are too high to be generated directly by a DAC. Therefore, the output of the DAC 64 is input to a mixer 66 along with the output frequency of a local oscillator (LO) 68. This mixing step upconverts the low frequency analog signal to an analog RF signal, which is amplified in amplifier 70 and used to excite a sample located in the magnet bore 69 of the spectrometer.

[0024]

[0032] The FID signal from the sample is detected and amplified by amplifier 67. This high frequency signal is 68The FID signal is input to mixer 65 along with the output of the NCO 70 and downmixed to a much lower frequency that can be used for ADC conversion. This low frequency FID signal is digitized by analog-to-digital converter (ADC) 63 and directed to digital mixer 61, which receives as its other input the digital low frequency signal generated by NCO 70 and sine wave LUT 72. This digital mixing step reduces the low frequency digital FID signal to zero frequency. However, the FID signal also contains frequency distortion, so the downmixing is performed using the output of NCO 70, which, like NCO 60, has as one of its inputs the offset signal generated by the frequency distortion. Because the low frequency signal from the NCO has this offset, the output of the mixer is a zero frequency FID signal with the effects of frequency distortion removed.

[0025]

[0033] The effect of mixing both on the excitation signal and the FID signal is demonstrated by Fig. 7A and Fig. 7B, each showing the result of frequency vs. time characteristics after mixing two different signals. In Fig. 7A, the signal of interest (either the excitation signal or the FID signal) is shown in dotted line and has a frequency distortion caused by the cooler operation. In this example, the mixing signal is depicted in dashed line and is of constant frequency. As a result, the output signal, shown in solid line, also shows frequency distortion. A similar representation is shown in Fig. 7B, where the signal of interest (dotted line) also has frequency distortion, but in this example, the mixing signal (dashed line) also has the same frequency distortion. As a result, the output signal (solid line) does not show frequency distortion.

Claims

1. 1. A nuclear magnetic resonance (NMR) magnet system comprising: a housing having a neck tube; a helium vessel located within an enclosure and including a magnetic coil immersed in liquid helium, the magnetic coil being cooled by evaporation of the liquid helium so as to maintain superconductivity; a cold shield surrounding the helium vessel; a single stage Stirling cooler mounted on the neck tube such that a cooler cold head extends into the housing and is thermally connected to the cold shield via a thermal bond; an NMR probe inserted into the room temperature bore of the housing, the NMR probe providing an RF excitation signal to a sample in the room temperature bore and detecting a free induction decay (FID) signal from the sample, wherein mechanical vibrations transmitted from the Stirling cooler induce distortions in either or both of the excitation and FID signals; a compensation device that applies compensation to said excitation signal and / or FID signal to counter said distortion.

2. NMR magnet system according to claim 1 , wherein said magnetic coil produces an equivalent magnetic field strength of 400 to 500 MHz.

3. 3. An NMR magnet system according to claim 1, wherein said Stirling cooler provides a cooling power of at least 30 W at 80 K.

4. NMR magnet system according to any one of claims 1 to 3, wherein said Stirling cooler operates at a frequency above 10 Hz.

5. NMR magnet system according to any one of claims 1 to 4, wherein the thermal coupling comprises a flexible, thermally conductive element that mechanically isolates the Stirling cooler from the cold shield.

6. 6. The NMR magnet system of claim 1, wherein the NMR magnet system is a vertical NMR magnet system having a z-axis centered around a room temperature bore, and a compressor axis of the Stirling cooler is oriented parallel to the z-axis.

7. 7. The NMR magnet system according to claim 1, further comprising a damper located between said Stirling cooler and said cold shield, for reducing vibration transmission from said Stirling cooler to said magnetic coil.

8. 8. The NMR magnet system of claim 7, wherein said damper is part of an active damping system and is actively driven to compensate for movement of said cooler.

9. 9. The NMR magnet system of claim 8, further comprising an acceleration sensor attached to said Stirling cooler that detects movement of said Stirling cooler relative to said cold shield and provides a signal to said active damping system that is used to actively drive said damper.

10. 2. The NMR magnet system of claim 1, wherein said compensation device includes a data storage element in which a representation of said distortion is stored.

11. 11. The NMR magnet system of claim 10, wherein the excitation signal is generated using a numerically controlled oscillator (NCO), and applying the compensation to the excitation signal includes providing a stored representation of the distortion as an input to the NCO.

12. 11. The NMR magnet system of claim 10, wherein applying said compensation to said FID signal comprises inputting said FID signal to a frequency mixer along with an oscillator signal incorporating said stored representation of said distortion.

13. 2. The NMR magnet system of claim 1, wherein the Stirling cooler generates a periodic trigger signal indicative of a state of operation of the Stirling cooler, the trigger signal being received by the compensation device and used in applying the compensation to the excitation signal and / or the FID signal.

14. 1. A nuclear magnetic resonance (NMR) spectrometer comprising: a housing for the magnet system having a neck tube; A refrigerant container located within the housing; a magnetic coil immersed in a cryogen located within the cryogen vessel, the magnetic coil being cooled by evaporation of the cryogen such that the magnetic coil maintains superconductivity; a cold shield surrounding the refrigerant vessel; a single stage Stirling cooler mounted on the neck tube such that a cooler cold head extends into the housing and is thermally connected to the cold shield via a thermal bond; an NMR probe inserted into a room temperature bore of the magnet system housing, the NMR probe providing an RF excitation signal to a sample in the room temperature bore and detecting a free induction decay (FID) signal from the sample, wherein mechanical vibrations transmitted from the Stirling cooler induce distortions in either or both of the excitation and FID signals; a compensation unit that applies compensation to one or both of the excitation signal and the FID signal to counteract the distortion.

15. 15. The NMR spectrometer of claim 14, wherein the compensation device includes a data storage element in which a representation of the distortion is stored.

16. 16. The NMR spectrometer of claim 15, wherein the excitation signal is generated using a numerically controlled oscillator (NCO), and applying the compensation to the excitation signal includes providing a stored representation of the distortion as an input to the NCO.

17. 16. The NMR spectrometer of claim 15, wherein applying the compensation to the FID signal comprises inputting the FID signal into a frequency mixer along with an oscillator signal incorporating the stored representation of the distortion.

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