GB2467527A

GB2467527A - Detection of frozen deposits due to air ingress into cryogen vessels by change in resonant frequency of tensioned wire

Info

Publication number: GB2467527A
Application number: GB0901644A
Authority: GB
Inventors: Patrick William Retz
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2009-02-03
Filing date: 2009-02-03
Publication date: 2010-08-04
Also published as: GB201000354D0; CN101793494A; GB2467410A; GB0901644D0; US8146427B2; GB2467410B; US20100201379A1

Abstract

The present invention provides a sensor for detecting the build-up of frozen deposits due to air ingress in a cryogen vessel housing a superconducting magnet, comprising a tensioned wire 22, a source of variable frequency alternating current 26 and a voltage sensor. The tensioned wire is oriented perpendicular to the direction of the magnetic field produced by the superconducting magnet. By varying the frequency of the applied current, the resonant frequency of the tensioned wire may be detected as the frequency at which the wire has a peak impedance. Any variation in the frequency or magnitude of the resonant peak may be interpreted as an indication of a frozen deposit hampering the free oscillation of the tensioned wire. The cryogen vessel may used to cool superconducting magnets used in imaging systems such as nuclear magnetic resonance imaging (NMR).

Description

METHODS AND APPARATUS FOR

DETECTION OF AIR INGRESS INTO CRYOGEN VESSELS.

The present invention relates to methods and apparatus for detection of air ingress into cryogen vessels. It is particularly related to the detection of air ingress into cryogen vessels used to cool superconducting magnets used in imaging systems such as magnetic resonance imaging, nuclear magnetic resonance imaging and nuclear magnetic spectroscopy. The invention, however, may be applied to the detection of air ingress into any cryogen vessel.

Fig. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, which in this case is shown on the side of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements, by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in Fig. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first stage temperature, typically in the region of 50-lOOK. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K and may recondense the gas into liquid state.

A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by either a conductor passing through the vent tube 20 or conduction through a component of turret 19.

For fixed current lead designs, a separate vent path (auxiliary vent) (not shown in Fig.1) may be provided as a fail-safe vent in case of blockage of the vent tube 20.

The cryogen 15 is typically liquid helium at a temperature of about 4K, although other cryogens may be used such as liquid hydrogen, liquid neon or liquid nitrogen. At service intervals, it is necessary to remove the refrigerator 17, and to open the vent tube 20. There is a risk that air could enter the cryogen vessel when the refrigerator is removed, or when the vent tube 20 is opened. Furthermore, experiments have shown that air continually diffuses into the cryogen vessel through the quench valve, and the vent valve, each conventionally provided on cryogen vessels, despite the cryogen vessel being held at positive pressure.

If air enters the cryogen vessel, it will freeze onto the coldest surfaces.

With higher-boiling-point cryogens, such as nitrogen, only the water contained in air may be frozen. This may block the access between refrigerator and the cryogen vessel or, degrade the performance of the refrigerator, leading to a rise on temperature and pressure within the cryogen vessel, in turn leading to increased consumption of cryogen. The frozen deposit may also build up around the vent tube 20. The vent tube allows boiled-off cryogen gas to escape from the cryogen vessel, and is particularly important in the case of a magnet quench. During a magnet quench, a superconductive magnet suddenly becomes resistive, and loses all of its stored energy to the cryogen. This results in very rapid boil-off of cryogen. If the vent tube is constricted, or even blocked, then dangerously high pressure may build up within the cryogen vessel.

To remove a frost deposit from the inside of the cryogen vessel requires removing all of the cryogen and allowing the cryogen vessel and the magnet or other equipment within it to warm up -for example, to room temperature. This is a time consuming and costly process, as the removed cryogen will need to be replenished, and, in the case of a superconducting magnet, a shimming operation may need to be performed to correct any changes in magnetic field homogeneity which may have been brought about by the warming and re-cooling of the magnet. During this whole process, the apparatus cooled within the cryogen vessel, and the system of which it forms a part, is unusable. This may have consequential effects such as not being able to image patients, and their maladies remaining undiagnosed. It is therefore not practical to warm the cryogen vessels and their contents as a preventative service operation. However, by not performing such preventative measures, the danger of blockages and excessive cryogen pressures remains.

The present invention aims to provide apparatus and methods for detecting the presence of frost inside the cryogen vessel. The presence of a frost may then be signalled to a user or a service technician, and the warming of the cryogen vessel may be planned, for a convenient time, in order to remove the frost.

Accordingly, the present invention provides methods and apparatus as defined in the appended claims.

The above, and further, objects and characteristics of the present invention may be determined from the following description of certain embodiments thereof, in conjunction with the following description, wherein: Fig. 1 shows a conventional arrangement of a cryostat including a cryogen vessel; Fig. 2 shows a schematic drawing of a circuit for detecting frozen deposits, according to an aspect of the present invention; Fig. 3 schematically illustrates a resonance curve, illustrating the variance of the voltage across the oscillating wire of Fig. 2 as the frequency of an applied current is swept through the resonant frequency of the wire; Fig. 4 shows an example placement of a vibrating wire sensor according to an embodiment of the present invention; and Fig. 5 shows another example placement of a vibrating wire sensor according to an embodiment of the present invention.

The present invention provides equipment for detecting the presence of frozen deposits in a cryogen vessel, using the physical effect that a length of tensioned wire will resonate when an alternating current is passed through it, in the presence of a static magnetic field if frequency of the current matches the natural frequency of the wire. For a given wire material this frequency will depend critically on the inverse of the free length of the wire.

This principle of a current-carrying wire vibrating in a background magnetic field has been used extensively in viscometer designs. The present invention applies this effect to the detection of solid deposits in cryogenic vessels of superconducting magnets. In operation, the superconducting magnet itself provides the constant background

magnetic field.

Fig. 2 schematically illustrates an electrical circuit of an embodiment of the present invention. A wire 22 is held under tension between two fixing points 24. An alternating current 1(f), of frequency 1, is provided by current source 26 and applied through the wire 22. The background magnetic field, provided by the superconductive magnet, is perpendicular to the plane of the drawing. The voltage V across the wire may be monitored in order to detect resonance.

Fig. 3 shows an example of a typical voltage V response to the frequency I of the alternating current 1(f). The resonant frequency is shown at Ir. As shown in the drawing, the impedance of the wire will rise dramatically as the frequency of the applied current 1(f) approaches the natural (resonant) frequency Ir of the tensioned wire. If the frequency I of the applied current 1(f) is increased through the natural frequency Ir, the peak measured voltage V corresponds to the resonant frequency. For wires of the type and length envisaged for measurement of deposits inside cryogen vessels of superconducting magnets, the frequency Ir will typically be in the kHz range.

Once the resonant frequency has been determined, any variation in this resonant frequency may be detected. Such variation may be used to infer the presence of solid deposits in the cryogen vessel. For example, a solid deposit on the wire will reduce its resonant frequency, as the wire will be made heavier by the deposit. On the other hand, a solid deposit may reduce the free length of the tensioned wire, which will increase the resonant frequency of the wire.

Fig. 4 illustrates an example sensor arrangement of the tensioned wire according to the present invention. Wire 22 is held under tension between fixing points 24 which are held a fixed distance apart. Electrical connections 30 are made to a voltage detector and a current source in a circuit similar to that of Fig. 2. The sensor is shown installed within a cross-section of a tube 32, representing a critical aperture within which it is desired to monitor the build-up of frozen deposits. The magnetic field from the superconducting magnet is perpendicular to the plane of the drawing, that is, perpendicular to the length of the wire.

When frozen deposits are absent, or are present to a level below the wire, for example to level a, the wire 22 is free to oscillate freely. The voltage V across the wire would peak at the natural frequency Fr shown in Fig. 3. As frozen deposits build up and reach the level b, the deposits touch the wire, and cause damping of the oscillations. This will cause a reduction in the sharpness of the peak in the voltage V against frequency F response shown in Fig. 3. By applying the current 1(t) through the illustrated spectrum of frequencies, a lower peak voltage may be detected. This may be interpreted as an indication that a solid deposit has built up, and this may be signalled to an operator or a service technician. The oscillations will ultimately stop when the deposits become more extensive still, and no voltage peak will be observed at all.

Fig. 5 illustrates another example arrangement of the tensioned wire sensor according to the present invention. Again, the magnetic field from the superconducting magnet is perpendicular to the plane of the drawing, that is, perpendicular to the length of the wire. Wire 22 is held under tension between fixing points 24 which are held a fixed distance apart.

Electrical connections 30 are made to a voltage detector and a current source in a circuit similar to that of Fig. 2. This arrangement differs from the arrangement of Fig. 5 in that the tensioned wire is oriented vertically.

When frozen deposits are absent, or are present to a level below the wire, the wire 22 is free to oscillate freely. The voltage V across the wire would peak at the natural frequency fr shown in Fig. 3. As frozen deposits build up, for example to level a, the deposits touch the wire 22, and cause damping of the oscillations. This in effect causes a shortening free length of the tensioned wire. By applying the current 1(t) through the spectrum of frequencies illustrated in Fig. 3, a peak voltage V may be detected at a higher frequency t'. This may be interpreted as an indication that a solid deposit has built up along part of the length of the tensioned wire, and this may be signalled to an operator or a service technician.

The resonant frequency of the tensioned wire will continue to rise as the level of deposits rises. The oscillations will ultimately stop when the deposits become more extensive still, and no voltage peak will be observed at all. When the deposits become very extensive, for example reaching level b, the oscillations may cease completely.

The advantage of the embodiment of Fig. 5 is that it provides an indication of the thickness of the frozen deposit, which may easily be derived from the change in resonant frequency of the tensioned wire. The embodiment of Fig. 4 provides a more basic indication of whether the frozen deposit has reached the height of the sensor.

The present invention accordingly provides a sensor for detecting the build up of frozen deposits in a cryogen vessel housing a superconducting magnet, comprising a tensioned wire, a source of variable frequency alternating current and a voltage sensor. The tensioned wire is oriented perpendicular to the direction of the magnetic field produced by the superconducting magnet. By varying the frequency of the applied current, the resonant frequency of the tensioned wire may be detected as the frequency at which the wire has a peak impedance. Any variation in the frequency or magnitude of the resonant peak may be interpreted as an indication of a frozen deposit hampering the free oscillation of the tensioned wire.

The frozen deposit sensor of the present invention provides an active measurement of the depth of ice. The sensor itself is very small and very simple, being only a tensioned wire and using the background field of the superconducting magnet. Electrical power dissipation of the sensor can be made extremely small by suitable choice of wire type and dimensions, and magnitude of the applied alternating current. By using wires whose resonant frequencies are a few kHz, stray-capacitance circuit issues may be avoided.

While operation of the sensor of the present invention has been described by applying an alternating electrical current through the wire, and varying the frequency of the current through a certain spectrum, an alternative method of operation may be as follows. An alternating current of a frequency corresponding to the approximate expected resonant frequency of the wire may be applied. The actual resonant frequency may be determined by monitoring the voltage V across the wire, and varying the frequency of the applied alternating current until a maximum value of voltage V is detected. Intermittently, or constantly, the applied alternating current may be varied slightly in frequency, to ensure that its frequency tracks the resonant frequency of the tensioned wire. A data output from the alternating current generator may be employed to indicate the resonant frequency of the tensioned wire. -10-

Claims

CLAIMS: 1. Apparatus for detecting air ingress into a cryogen vessel, comprising: a frozen deposit sensor, itself comprising a tensioned wire held perpendicular to a magnetic field, through which an alternating current is passed, to induce mechanical and electrical resonance in the wire.
2. Apparatus according to claim 1, installed in a cryogen vessel housing a superconducting magnet, wherein the tensioned wire is arranged to be perpendicular to a magnetic field produced by the superconducting magnet when in use.
3. Apparatus according to claim 1 or claim 2, wherein the wire is held perpendicular to an expected direction of accretion of the frozen deposit.
4. Apparatus according to claim 1 or claim 2, wherein the wire is held in an expected direction of accretion of the frozen deposit.
5. A method for detecting air ingress into a cryogen vessel, comprising the steps of: a)-providing apparatus according to any of claims 1-4, the frozen deposit sensor being located at a position of interest within the cryogen vessel; b)-applying an alternating current of a first frequency to the tensioned wire, and measuring a resulting voltage across the wire; c)-varying the frequency of the alternating current through an expected resonant frequency of the wire, while measuring the resulting voltage across the wire; -11 -d)-determining a resonant frequency of the wire as a frequency of the applied current at which the impedance of the wire is a maximum; e)-repeating steps (b) to (d) and, in response to a determined resonant frequency differing from an initially determined resonant frequency, signalling the presence of a frozen deposit, thereby indicating air ingress into the cryogen vessel.
6. A method for detecting air ingress into a cryogen vessel, comprising the steps of: a)-providing apparatus according to any of claims 1-4, the frozen deposit sensor being located at a position of interest within the cryogen vessel; b)-applying an alternating current of a first frequency to the tensioned wire, and measuring a resulting voltage across the wire; c)-varying the frequency of the alternating current through an expected resonant frequency of the wire, while measuring the resulting voltage across the wire; d)-determining a resonant frequency of the wire as a frequency of the applied current at which the impedance of the wire is a maximum; e)-repeating steps (b) to (d) and, in response to a maximum impedance differing from an initially determined maximum impedance, signalling the presence of a frozen deposit, thereby indicating air ingress into the cryogen vessel.
7. A method for detecting air ingress into a cryogen vessel, comprising the steps of: a)-providing apparatus according to any of claims 1-4, the frozen deposit sensor being located at a position of interest within the cryogen vessel; b)-applying an alternating current of a first frequency to the tensioned wire, and measuring a resulting voltage across the wire; c)-varying the frequency of the alternating current through an expected resonant frequency of the wire, while measuring the resulting voltage across the wire; d)-determining a resonant frequency of the wire as a frequency of the applied current at which the impedance of the wire is a maximum; e)-varying the frequency of the alternating current through the wire, while measuring the resulting voltage across the wire, thereby detecting any variation in determined resonant frequency from an initially determined resonant frequency, and in response thereto, signalling the presence of a frozen deposit, thereby indicating air ingress into the cryogen \ressel
8. A method for detecting air ingress into a cryogen vessel, comprising the steps of: a)-providing apparatus according to any of claims 1-4, the frozen deposit sensor being located at a position of interest within the cryogen vessel; b)-applying an alternating current of a first frequency to the tensioned wire, and measuring a resulting voltage across the wire; c)-varying the frequency of the alternating current through an expected resonant frequency of the wire, while measuring the resulting voltage across the wire; d)-determining a resonant frequency of the wire as a frequency of the applied current at which the impedance of the wire is a maximum; e)-varying the frequency of the alternating current through the wire, while measuring the resulting voltage across the wire, thereby detecting any variation in maximum impedance from an initially-determined maximum impedance, and in response thereto, signalling the presence of a frozen deposit, thereby indicating air ingress into the cryogen vessel.