Noise reduction method

Abstract

There is provided a method of reducing noise in a cryogenic cooling system associated with a mechanical refrigerator forming part of said cooling system. The method comprises: monitoring vibrations in the cooling system during operation of the mechanical refrigerator; and modulating an operating frequency of the mechanical refrigerator based on the monitored vibrations so as to reduce the amplitude of said vibrations. This allows noise within the cooling system to be reduced.

Claims

1. A method of reducing noise in a cryogenic cooling system, the method comprising: monitoring vibrations in the cryogenic cooling system during operation of only a single mechanical refrigerator; measuring vibration amplitudes in the monitored vibrations; determining transfer functions and structural resonance coupling for the cryogenic cooling system based on the measured vibration amplitudes; and modulating an operating frequency of the mechanical refrigerator based on the determined transfer functions and structural resonance coupling so as to reduce the vibration amplitudes of the monitored vibrations.

2. The method according to claim 1, wherein modulating the operating frequency comprises adjusting the operating frequency of the mechanical refrigerator from a first frequency to a second frequency.

3. The method according to claim 1, wherein modulating the operating frequency of the mechanical refrigerator comprises modulating the operating frequency of a driving motor of the mechanical refrigerator.

4. The method according to claim 3, wherein the driving motor is a stepper motor.

5. The method according to claim 4, wherein the step rate of the stepper motor is controllable.

6. The method according to claim 3, wherein the driving motor drives a rotary valve of the mechanical refrigerator during the operating of the mechanical refrigerator.

7. The method according to claim 6, wherein the operating frequency is the frequency at which the rotary valve rotates when in use.

8. The method according to claim 1, wherein the operating frequency is between 1.20 Hertz (Hz) and 1.90 Hz, and preferably the operating frequency is between 1.30 Hz and 1.50 Hz.

9. The method according to claim 1, wherein the mechanical refrigerator is a Pulse Tube refrigerator.

10. The method according to claim 1, wherein the operating frequency is modulated by a user based on the monitored vibrations.

11. The method according to claim 1, wherein the operating frequency is modulated automatically based on the monitored vibrations.

12. The method according to claim 1, wherein the vibrations are monitored by a probe placed in contact with the cooling system.

13. The method according to claim 12, wherein the probe is placed in contact with a cryostat comprised by the cooling system.

14. The method according to any one of claim 1, wherein the vibrations are monitored by a probe placed in contact with a cooling target of the cooling system.

15. The method according to claim 12, wherein the probe is an accelerometer.

16. The method according to claim 1, wherein the operating frequency of the mechanical refrigerator is modulated to de-couple at least one harmonic of the operating frequency from a structural resonance of the cooling system.

17. The method according to claim 16, wherein the at least one harmonic of the operating frequency and the structural resonance of the cooling system are de-coupled by adjusting the operating frequency of the mechanical refrigerator, and preferably the operating frequency is adjusted by at least 0.01 Hz.

18. A frequency adjuster, comprising: a vibration detector adapted in use to: monitor vibrations associated with only a single mechanical refrigerator in a cryogenic cooling system; and measure vibration amplitudes in the monitored vibrations; and a controller adapted to: determine transfer functions and structural resonance coupling for the cryogenic cooling system based on the measured vibration amplitudes; and control an operating frequency of the mechanical refrigerator based on the determined transfer functions and structural resonance coupling so as to reduce the vibration amplitudes of the monitored vibrations.

19. The method of claim 1, wherein the transfer functions and structural resonance coupling for the cryogenic cooling system are transfer functions and structural resonance coupling for the cryogenic cooling system and components attached to the cryogenic cooling system.

20. A cryogenic cooling system comprising: a cryostat; a mechanical refrigerator coupled to said cryostat; and a frequency adjuster according to claim 18 adapted in use to monitor vibrations in the cryostat and modulate an operating frequency of the mechanical refrigerator.

Description

BRIEF DESCRIPTION OF FIGURES

(1) Examples of a noise reduction method and a corresponding frequency adjuster and cryogenic cooling system are described in detail below, with reference to the accompanying figures, in which:

(2) FIG. 1 shows a flow diagram of an example noise reduction method;

(3) FIG. 2 shows a schematic view of an example cryogenic cooling system;

(4) FIG. 3 shows a plot of operational temperature of an example pulse tube refrigerator against frequency of the pulse tube refrigerator rotary valve;

(5) FIG. 4 shows a comparative plot of vibrations in an example cryogenic cooling system across a frequency spectrum when a pulse tube refrigerator is operating and when the pulse tube refrigerator is not operating;

(6) FIG. 5 shows a plot comparing vibration amplitudes across a frequency spectrum at different pulse tube refrigerator operating frequencies; and

(7) FIG. 6 shows a comparative plot of vibrations in an example cooling system across a frequency spectrum when the mass of the cryogenic cooling system is altered.

DETAILED DESCRIPTION

(8) We now describe an example of a noise reduction method, along with a description of an example cryogenic cooling system including an example frequency adjuster.

(9) Referring now to FIG. 1 and FIG. 2, a process of a first example noise reduction method is illustrated generally at 1 in FIG. 1 and an example cryogenic cooling system is illustrated generally at 10 in FIG. 2.

(10) In the cryogenic cooling system 10, a pulse tube refrigerator (PTR) 12 is coupled to a cryostat 14. The cryostat is typically mounted in a support frame (not shown). An accelerometer 16 is in contact with the cryostat and is connected to a controller 18 to which the accelerometer outputs data. The accelerometer and the controller make up the frequency adjuster.

(11) At step 101, the PTR 12 is operated at a first operating frequency. This is achieved by operating a rotary valve (not shown) in the PTR at the first operating frequency. Additionally, the PTR typically has external components coupled to it. An example of such a component is an external compressor used to oscillate high and low pressures to promote motion of the .sup.4He working fluid within the PTR. A further example of an external component is a pump or pumping system. External components coupled to the PTR (or to any other mechanical refrigerator of other examples) typically vibrate while operating and therefore, since they are coupled to the PTR, contribute to the operating frequency of the PTR.

(12) The PTR 12 is operated to cool a cooling target (not shown) in the cryostat 14 to an operational temperature of about 3.5 K to 4.0 K. Once the cooling target has reached the operational temperature, in step 102, vibrations within the cryostat are monitored. This is achieved using the accelerometer 16 in contact with the cryostat. This allows vibrations that cause displacement within the cryostat to be observed across a frequency spectrum.

(13) The cooling target may be further cooling stages (not shown), such as a dilution refrigerator, a .sup.3He circuit or a .sup.4He circuit. These provide further cooling to lower temperatures, such as to about 0.01 K. Vibrations caused by these further cooling stages are significantly less than the vibrations caused by the PTR 12 or another mechanical refrigerator. It would of course be possible for any contribution to vibrations within the system of such further cooling stages to be monitored and taken into account.

(14) As noted above, the PTR 12 has a first operating frequency. Due to the coupling of the PTR to the cryostat 14, the operation of the PTR at this frequency causes a primary vibration within the cryostat at this frequency due to mechanical motion of the PTR caused by the operation of the rotary valve. In addition to the primary vibration caused by the PTR directly due to the first operating frequency, secondary vibrations are caused in the cryostat. The secondary vibrations are each vibrations at higher frequencies than the first operating frequency caused by harmonics of the first operating frequency. The harmonics are generated in part because the mechanical oscillations of the PTR generated by the operation of the rotary valve are not sinusoidal.

(15) Additionally, the cryostat 14 has its own structural resonances due to the natural frequency vibration of the cryostat. This is at least in part due to normal modes of oscillation of the cryogenic cooling system and its various components including the cryostat. The vibrations are able to be output to a display (not shown). When a structural resonance coincides or is close to a harmonic of the PTR's operating frequency, the resonance and the harmonic couple. The coupling causes a vibration within the cryostat of a greater amplitude than the amplitude of the respective independent vibrations that would have been caused by each of the resonance or the harmonic if they were de-coupled.

(16) The output from the accelerometer 16 provides readings of the vibrations caused by the harmonics and structural resonances on a frequency spectrum. The readings that are output are the magnitude of vibrations at respective frequencies within a frequency range. Based on the output from the accelerometer, at step 103, the first operating frequency of the PTR 12 is modulated by adjusting the first operating frequency to a second operating frequency. This is achieved by the controller 18 causing the operating frequency of the PTR 12 to alter. Additionally, in examples where external components coupled to the PTR are taken account of as part of the operating frequency of the PTR, modulation is also able to be applied to those components to adjust the frequency of the vibrations they cause and therefore modulate their contribution to the operating frequency. This also applies to examples using alternative mechanical refrigerators.

(17) By changing the operating frequency of the PTR 12, the frequency of the harmonics changes. Even a small change, such as a change of about 0.1 Hz to 0.5 Hz is sufficient to limit the extent to which any harmonic of the operating frequency couples to a structural resonance of the cryostat 14. This reduces the total amount of vibration within the cryostat thereby reducing the noise experienced by any sample at the cooling target. To avoid increasing the vibration levels when modulating the operating frequency of the PTR, the vibrations caused by the second operating frequency can be monitored in the same way as the vibrations caused by the first operating frequency. Should the vibrations be increased by the second operating frequency, the further adjustments can be made to the frequency. However, this will likely be unnecessary since it is possible to tell the effect on the vibrations of a change from the first operating frequency to a second operating frequency by reviewing the output of the accelerometer 16 while adjusting the operating frequency.

(18) Other factors also have to be taken into account when modulating the operating frequency of the PTR 12. One such factor is the thermal performance of the PTR. As mentioned above, PTRs typically have an operating frequency of about 1.40 Hz. This is because the lowest operating temperature and greatest cooling power is able to be achieved at about this operating frequency. However, we have discovered that PTR operating frequencies between about 1.20 Hz and about 1.90 Hz can be used to drive a PTR without having too detrimental an effect on the minimum temperature that is able to be achieved. This can be seen from FIG. 3, which shows a plot of the temperature of the coldest part of the PTR compared to the rotary valve frequency.

(19) From FIG. 3, it can be seen that at 1.20 Hz, the PTR head temperature is about 3.8 K (indicated by line 30 in FIG. 3); at 1.40 Hz, the PTR head temperature is about 3.6 K (indicated by line 32 in FIG. 3); and at 1.90 Hz, the PTR head temperature is about 3.8 K (again indicated by line 30 in FIG. 3). These are the maximum and minimum temperature values within this frequency range. Accordingly, it is still possible for the PTR to provide cooling to temperatures below 4.0 K while operating at a frequency other than 1.40 Hz. When choosing an operating frequency over a more limited range than 1.20 Hz to 1.90 Hz, the range in PTR head temperatures reduces. For example, in the operating frequency range of 1.30 Hz to 1.50 Hz, the range in temperature is less than 0.1 K as can be seen from FIG. 3.

(20) Outside of the frequency range of 1.20 Hz to 1.90 Hz however, the PTR head temperature increases significantly. This can be seen from FIG. 3, which shows that below a frequency of 1.20 Hz, the PTR head temperature increases to about 7.6 K at a frequency of 1.00 Hz. At a frequency of 2.00 Hz, the temperature increase of the PTR head is less significant. However, there is still an increase in the PTR head temperature, and, although not shown in FIG. 3, the temperature continues to increase as the frequency increases.

(21) The effect on the vibrations within the cryostat 14 when operating the PTR 12 coupled to the cryostat is shown by FIG. 4. This shows two plots comparing the output of the accelerometer 16 when the PTR is not operating with the output of the accelerometer when the PTR is operating at an operating frequency of 1.40 Hz.

(22) Each of the plots show that the cryostat used in generating the plots has a structural resonance at about 8.00 Hz and at about 13.00 Hz. This is indicated by the respective peak shown at each of these frequencies in each plot. While the peaks at the structural resonances are the primary features on the plot showing the accelerometer's 16 output when the PTR is not operating, the plot showing the accelerometer's output when the PTR is operating shows further peaks. These peaks are shown at regular intervals across the frequency spectrum shown in FIG. 3. These peaks at regular intervals represent vibrations caused by the PTR at the operating frequency of the PTR and at the operating frequency harmonics at each multiple of the operating frequency. Further, it can be seen from this plot that a respective harmonic coincides with each of the structural resonance at about 8.00 Hz and about 13.00 Hz causing the respective harmonic and respective structural resonance to couple.

(23) As indicated by line 40, the peak at about 8.00 Hz when the PTR 12 is not operating shows that vibrations at this frequency cause a displacement of about 100 nanometres (nm). The peak at about 13.00 Hz when the PTR 12 is not operating shows that vibrations at this frequency cause a displacement of about 40 nm, as indicated by line 42. In comparison, the plot of the accelerometer output when the PTR is operating shows that the peak at about 8.00 Hz and the peak at about 13.00 Hz each have a displacement amplitude of at least 300 nm due to the coupling of the respective harmonic and respective structural resonance. This is indicated by line 44 in FIG. 4. These measurements were taken using an accelerometer located on an exterior of a top plate of the system, and therefore not in a cooled region and not in an environment in which a vacuum is applied.

(24) For the structural resonance at about 13.00 Hz, the increase in displacement amplitude from about 40 nm to at least 300 nm is an increase of at least 750 percent (%). While smaller, the increase in the displacement amplitude of the structural resonance at about 8.00 Hz from 100 nm to at least 300 nm is an increase of at least 300%. As set out above, the reason for these increases in displacement amplitude is that harmonics of the PTR operating frequency couple with the structural resonances of the cryostat. This leads to high amplitude vibrations within the cryostat relative to the other vibrations present in the cryostat when PTR is operating. We have discovered that these vibrations cause noise in data being output from an experiment or procedure being run in the cryostat, which significantly affects high sensitivity experiments and procedures.

(25) An example of an arrangement that would be affected by the motion within the cryostat caused by the coupling of harmonics of the PTR operating frequency to structural resonances in the cryostat is one that uses superconducting magnets. Arrangements such as these are affected because the motion causes eddy currents to be induced in the sample due to sample movement being produced relative to the magnetic field generated. These in turn cause heating of the sample, which will impact on the measurements that can be made. Another example of a vibration sensitive arrangement is free space optical measurements of a sample. In such a situation, an optical source or detector being used to carry out the optical measurements external to the sample is not fixed in position relative to the sample, so movement of the sample relative to the external optical source or detector would affect the data collected. Thus, minimising such movement induced by vibrations would improve the quality of data collected.

(26) To reduce the magnitude of the vibrations, the cryogenic cooling system needs to be “de-tuned” such that the structural resonances no longer coincide with the harmonics of the PTR operating frequency. This causes a de-coupling of the harmonic and structural resonance thereby reducing amplification of the vibrations caused by the structural resonances and the harmonics.

(27) This is able to be achieved by adjusting the operating frequency of the PTR. This allows an approximate de-tuning can be applied during manufacture and installation followed by a more accurate de-tuning by the user if they consider it necessary once they have added anything they want to into the cryostat. This is achieved by the controller 18 being programmable to modulate the operating frequency of the PTR coupled to the cryostat.

(28) By modulating the operating frequency of the PTR, the optimum operating frequency can be selected. A demonstration of this can be seen in FIG. 5. This shows plots of vibration amplitudes in a cryostat with a structural resonance at about 19.00 Hz over a number of PTR operating frequencies between about 1.43 Hz and about 1.52 Hz. These show vibrations caused by the twelfth, thirteenth and fourteenth harmonics of the PTR operating frequency and their effect on the vibration caused at the structural resonance at about 19.00 Hz.

(29) In FIG. 5, the harmonics of the PTR operating frequency are indicated by the letter “n”. This figure shows that the greatest degree of coupling between the thirteenth harmonic and the cryostat structural resonance occurs at an operating frequency of about 1.47 Hz. The vibrations caused by this coupling cause a displacement of greater than 900 nm compared to displacements of about 200 nm when the PTR operating frequency is about 1.43 Hz and about 1.51 Hz. As above, the accelerometer used to collect these readings was installed on the exterior of a top plate of the system, and therefore not in an environment that was cooled or in which a vacuum was applied.

(30) FIG. 5 also shows that shifts in operating frequency of about 0.01 Hz can also have a significant effect. This can be seen by comparing the peak of the plot for a PTR operating frequency of about 1.46 Hz to the peak of the plot for a PTR operating frequency of about 1.47 Hz. At an operating frequency of about 1.46 Hz, the greatest amplitude vibration is about 500 nm less than the greatest amplitude vibration caused when the operating frequency is about 1.47 Hz.

(31) A method of achieving further de-coupling of a harmonic from a structural resonance can be applied in addition to adjusting the operating frequency of the PTR. This additional method is to alter the mass of the cryogenic cooling system since this will affect the frequency of the structural resonances.

(32) FIG. 6 shows the effect on the vibrations in a cryostat where this method is applied. The plot in the upper half of FIG. 6 shows the output of an accelerometer attached to a cryostat to which a PTR is coupled and operating at a frequency of about 1.40 Hz. In the cryostat used for this example, there is a resonance at about 8.60 Hz. In the upper plot of FIG. 6, it can be seen that the resonance at about 8.60 Hz is coupled to one of the harmonics of the PTR operating frequency (each of which are again represented by peaks at regular intervals across the frequency spectrum shown in the figure), which has been amplified.

(33) The lower plot shown in FIG. 6 shows the accelerometer output for same cryostat with the same PTR operating at the same frequency. However, in this plot, the structural resonance is shifted to about 7.60 Hz, which means that it is no longer coupled with a harmonic of the PTR operating frequency. To achieve this, a mass of about 100 kilogrammes (kg) was attached to the cryostat, and has resulted in a reduction in the amplitude of the vibrations in the cryostat.

(34) While this method achieves a reduction in the amplitude of the vibrations, we have discovered that adjusting the operating frequency of the PTR provides a greater flexibility than is possible to achieve using this additional method. This is because each individual cryostat has its own unique structural resonances that are determined by how the cryostat is constructed and the arrangement and mass of its components, which varies (even if only slightly) from system to system. Additionally, anything that is added to a cryostat for an experiment or procedure, such as a sample, changes the frequency of the structural resonance due to the corresponding mass that is added to the cryostat. Since it is not known during the manufacture or installation exactly what a user will add to a cryostat when they use it, it is therefore not possible to accurately de-tune the cryostat by altering the mass of the cryostat, so any additional de-tuning applied by altering the mass of the cryostat has the potential to have a less significant effect than intended once the cryostat is set up as the user wishes.

(35) Returning to the example method of reducing noise and example frequency adjuster, there are two procedures that can be used to achieve the modulation of the operating frequency. The first of these procedures is for a user to review the output of the accelerometer. The controller of the frequency adjuster is then used to adjust the operating frequency of the PTR coupled to the cryostat to which the accelerometer is attached to a suitable frequency based on the accelerometer's output. This is achieved by use of a dial or user interface (not shown) on the controller that is linked to the stepper motor (not shown) that rotates the rotary valve of the PTR causing the rotation rate to be adjusted in response to a corresponding signal from the controller.

(36) The second procedure is an automated procedure where software is used to modulate the PTR operating frequency instead of the user. In this procedure, the output of the accelerometer is analysed using software held by the controller of the frequency adjuster. This identifies peaks caused by vibrations across the frequency spectrum and adjusts the operating frequency of the PTR to a frequency with the lowest or a lower level of vibration using frequency scanning and spectral analysis techniques, such as Fast Fourier Transforms. Of course, in some examples the user is able to override the software to choose an alternative operating frequency for the PTR if desired.

(37) Should there be a part of the cryostat that is considered particularly sensitive to vibrations, or is of greater importance, the accelerometer is able to be placed at that location so that the user is able to focus their efforts of reducing vibrations on that part of the cryostat.

(38) In some examples, a Gifford-McMahon (GM) refrigerator, Stirling cooler or dilution refrigerator, for example operable with a pressure pump and/or a compressing system, is used in place of (or in addition to) a PTR. In a GM refrigerator, the operating frequency of the rotary valve is modulated to reduce the vibrations it generates; in a Stirling cooler, the operating frequency of the pistons is modulated for the same reason; and in a dilution refrigerator, the operating frequency of a pressure pump and/or compressing system coupled to and being used with the dilution refrigerator to assist its operation is modulated for the same reason.

(39) In addition to the operating frequencies set out above, the operating frequency of 3 K mechanical refrigerators used in examples described herein, most “higher power” refrigerators (in other words those considered to be able to cool to temperatures as low as 3 K or lower and/or with a cooling power considered to be high) all have operating frequencies of about 1 Hz to 2 Hz. Some specialised 3 K coolers (for example those used for space applications) operate at higher frequencies of typically tens or even hundreds of Hertz.