Abstract:
An optical measuring device for determining temperature in a cryogenic environment includes at least one optical waveguide provided with at least one fiber Bragg grating sensor that is interrogated by a light signal. The device includes a light injector that injects light into the at least one fiber Bragg grating sensor, and an evaluation unit that determines a temperature value from the modulated light signal emanating from the at least one fiber Bragg grating sensor. The device includes at least one jacket that non-rigidly encloses the optical waveguide, at least in the region of the at least one fiber Bragg grating sensor. The jacket has a larger coefficient of thermal expansion, at least at cryogenic temperatures, than the optical waveguide. A winding arrangement for use in a cryogenic environment is provided with such a device for temperature monitoring of a conductor of the winding arrangement.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention concerns an optical measurement device for temperature determination in a cryogenic environment. The measurement device is of the type having at least one optical wave guide provided with at least one fiber Bragg grating sensor, via which the at least one fiber Bragg grating can be interrogated by means of a light signal. The measurement device furthermore has a light injector that injects the light signal into the at least one optical wave guide, and evaluation unit to determine a temperature value from a light sensor arriving from at least one fiber Bragg grating sensor. The invention also concerns a winding arrangement whose temperature can be monitored. 
         [0003]    2. Description of the Prior Art 
         [0004]    Superconductive magnets that are used, for example, in magnetic resonance tomography systems are cooled to a temperature of 120 K or lower with a cryogenic coolant, depending on the employed superconductor type. For example, liquid helium which cools the magnet to 4.2 K is suitable for a magnet executed with a low-temperature superconductor. An event known as a quench, wherein the superconductor becomes normally-conductive, can occur in such a superconductor due to the most varied disruptive influences. This quench process initially begins at a point and propagates with high speed over the entire superconductor. This is associated with a severe heating of the superconductor which results in a high vaporization loss of cryogenic coolant. The magnet must thereupon be immediately deactivated. In order to avoid damage to the magnet, it is necessary to detect the quench process as promptly as possible and with optimal spatial resolution. For example, its point of origin can be localized by acoustic emissions that are connected with the quench event. Particularly in magnetic resonance apparatuses, this proves to be quite difficult since magnetic resonance apparatuses are normally composed of numerous coils arranged in complicated geometry. An additional possibility for quench detection makes use of a differential voltage measurement at the windings. The location of the quench can therefore likewise be locally limited. However, this leads to a large number of voltage taps, particularly in magnetic resonance apparatuses, which makes the winding process very complicated. Moreover, the resistive voltages to be measured are superimposed with very high inductive portions. 
         [0005]    An optical device for temperature measurement of a normally-conductive magnetic resonance tomography coil is specified in United States Patent Application Publication 2005/0129088 A1. A tube-shaped sheath is wound around the winding body, into which sheath an optical wave guide mechanically decoupled from said sheath is inserted. The optical wave guide is provided with multiple fiber Bragg gratings with which the coil temperature (which can be at room temperature or higher) can be monitored with spatial resolution. Since the temperature-dependent wavelength change of “naked” fiber Bragg grating sensors in the range of cryogenic temperatures (i.e. temperatures that are at 120 K and lower) is not present in practice, the optical device specified in this document is not suitable for use in such a cryogenic environment. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to provide an optical measurement device that is suitable for use in a cryogenic environment. It is also an object of the present invention to provide a winding arrangement whose temperature can be monitored under cryogenic conditions. 
         [0007]    The above object is achieved in accordance with the present invention by an optical measurement device for a temperature determination in a cryogenic environment, having at least one optical waveguide provided with at least one fiber Bragg grating sensor that can be interrogated by a light signal, a light injector that injects the light signal into the optical waveguide, an evaluation unit that determines a temperature measurement value from a light signal from the fiber Bragg grating sensor, and at least one jacket element that at least partially surrounds the at least one optical waveguide, without a rigid connection thereto, at least in the region of the at least one fiber Bragg grating element, and wherein the at least one jacket element has a coefficient of thermal expansion that is larger than the coefficient of thermal expansion of the optical waveguide, at least at cryogenic temperatures. 
         [0008]    Due to the non-positive (non-rigid) contact of the at least one jacket element with the at least one optical wave guide, the expansion of the at least one jacket element given a temperature increase or the contraction of the at least one jacket element given a temperature drop is directly transferred to the at least one optical wave guide, and therefore to the at least one fiber Bragg grating sensor. Even if the fiber Bragg grating sensor itself has a negligible coefficient of thermal expansion in the cryogenic temperature range of 120 K and below, upon a temperature change the at least one jacket element measurably affects the focal wavelength of the at least one fiber Bragg grating due to the existing or, respectively, greater coefficient of expansion. 
         [0009]    It is thus advantageous when for the at least one jacket element to be formed from a polymer material (in particular from PMMA). Polymer material (in particular PMMA) has a high coefficient of thermal expansion in the cryogenic temperature range of 120 K and below. For example, PMMA exhibits a coefficient of thermal expansion of &gt;10 −6  per K at a temperature in the range of approximately 4 K (liquid helium) up to 20 K while the coefficient of thermal expansion of, for example, glass of an optical fiber is &lt;10 −7  per K. Such a polymer material (in particular PMMA) is additionally characterized by a low intrinsic heat capacity. 
         [0010]    Furthermore, it is advantageous for the at least one jacket element to exhibit a pronounced expansion in the length direction of the at least one optical wave guide in the region of the at least one fiber Bragg grating sensor. The thickness of the at least one jacket element is thus kept as small as possible in the region of the associated at least one fiber Bragg grating sensor in order to minimize the heat capacity of the at least one jacket element. An optimally short response time of the at least one fiber Bragg grating sensor is thereby ensured. 
         [0011]    The at least one jacket element advantageously tapers towards its ends in the length direction of the at least one optical wave guide. For example, if the at least one optical wave guide with at least one fiber Bragg grating sensor and the at least one jacket element associated with the at least one fiber Bragg grating sensor is embedded in a composite material (for example casting resin), a compression by the composite material is avoided in such an embodiment of the at least one jacket element. 
         [0012]    It is additionally advantageous for the at least one jacket element to be fashioned to be rotationally symmetrical around the at least one optical wave guide. In particular, the at least one jacket element tapers conically at both ends. Due to such a symmetrical design of the at least one jacket element, the expansion and contraction forces emanating from the at least one jacket element that act on the at least one optical wave guide are distributed uniformly over its extent. The expansion and contraction of the at least one fiber Bragg grating sensor therefore ensues uniformly so that the light signal (reflected on at least one fiber Bragg grating senor, for example) exhibits an optimally small bandwidth. 
         [0013]    Multiple fiber Bragg grating sensors are advantageously provided at different points along the at least one optical wave guide with respective associated jacket elements. A temperature distribution can thus be determined with spatial resolution, and the event location can be precisely limited given point events, for example a sudden, locally limited temperature increase. The resolution is determined solely by the spacing of the individual fiber Bragg grating sensors from one another. For example, if what is known as the wavelength multiplexer method is applied with the optical measurement device according to the invention, normally up to 10 fiber Bragg grating sensors can be arranged in succession in an optical wave guide. Each fiber Bragg grating sensor thereby has a different focal wavelength. For this the light signal injected into the optical wave guide by the injection means must exhibit a wavelength range that covers all focal wavelengths. For evaluation, the evaluation means hereby advantageously possesses a spectrometer (for example a Fabry-Perrot interferometer). 
         [0014]    Moreover, a time multiplexing method (OTDR: Optical Frequency Doman Reflectometry) can be used as an alternative to the wavelength multiplexing method, a nearly unlimited number of fiber Bragg grating sensors can be arranged in an optical wave guide. The sensors can also be spatially differentiated given an identical focal wavelength. For example, the evaluation means can exhibit an edge filter for the evaluation of the light signal scattered at the fiber Bragg grating sensors. 
         [0015]    It is advantageous when the light signal from the injection means is injected in pulses into the at least one optical wave guide with a pulse frequency in a range from 500 Hz to 10 kHz. It is thus ensured that the change of the temperature distribution can be temporally resolved given a high propagation speed of a temperature change, as it occurs given a quench process in a superconductor, for example. 
         [0016]    The above object also is achieved in accordance with the present invention by a winding arrangement with at least one winding body composed of a number of windings of at least one electrical conductor, and an optical measurement device as described above that determines a temperature of the electrical conductor in a cryogenic environment, the jacket element of the optical waveguide being in thermal contact with the winding body. 
         [0017]    The advantages explained above for the optical measurement device according to the invention are applicable to the winding arrangement as well. 
         [0018]    It is advantageous to arrange the at least one optical wave guide internally and/or externally on the winding body. 
         [0019]    The winding body is advantageously provided with a composite material, in particular with casting resin (for example epoxy resin). The composite material primarily serves for mechanical stabilization of the at least one conductor in the winding body. The composite material additionally serves for electrical insulation of two adjacent windings. Moreover, the composite material advantageously possesses a good heat conductivity. It is therefore ensured that an initially locally limited temperature increase propagates quickly and thus can be detected early by the nearest fiber Bragg grating sensor. 
         [0020]    It is advantageous when at least one optical wave guide is embedded in the composite material. The at least one optical wave guide can thus be positioned optimally close to the at least one conductor, and the at least one optical wave guide is protected from external influences and additionally is mechanically stabilized by the composite material. Due to the embedding it is additionally ensured that the at least one optical wave guide and in particular the at least one fiber Bragg grating sensor are arranged at a fixed, invariable distance from the at least one electrical conductor to be monitored. 
         [0021]    The composite material of the winding body advantageously simultaneously serves as a jacket element of the at least one fiber Bragg grating sensor. This can be ensured a suitable composite material, in particular a casting resin. 
         [0022]    The at least one electrical conductor is advantageously at least one superconductor. The at least one superconductor can thereby be a low-temperature or even a high-temperature superconductor. It is thus possible to promptly detect a quench event occurring in at least one superconductor and to localize it in an optimally precise manner given the use of sufficiently many distributed fiber Bragg grating sensors. A thermal stress of the superconductor by the at least one optical wave guide is nonexistent in principle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0023]      FIG. 1  illustrates an optical measurement device with a winding arrangement in a cryogenic medium, constructed and operating in accordance with the present invention. 
           [0024]      FIG. 2  is a cross-section through the winding arrangement shown in  FIG. 1 . 
           [0025]      FIG. 3  is a longitudinal section through an optical waveguide embedded in composite material, having a fiber Bragg grating sensor and a jacket element associated with the fiber Bragg grating sensor, in accordance with the present invention. 
           [0026]      FIG. 4  is a cross-section through the optical waveguide of  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    According to the invention, an optical measurement device with a winding arrangement  30  in a cryogenic medium  4  (for example liquid helium or liquid nitrogen) is shown in  FIG. 1 . The winding arrangement  30  thereby exhibits a winding body  31  arranged on a winding support  32 . However, the winding body can also be executed in a self-supporting manner, i.e. without winding support  32  (not shown in  FIG. 1 ). The winding body  31  is thereby fashioned from a plurality of windings of a superconductive conductor  34  (see  FIG. 3 ). The superconductive conductor  34  can thereby be a low-temperature superconductor or a high-temperature superconductor. Depending on the superconductor type, the conductor  34  can be band-shaped, be executed with rectangular cross-section or even exhibit a round cross-section. Both winding supports  32  and winding bodies  31  are of a hollow cylinder shape in the presented exemplary embodiment. The winding body  31  respectively possesses an optical wave guide  20   i,    20   a  both on its inner side facing towards the winding carrier  32  and on its outer side  35  facing away from the winding carrier  32 . According to the exemplary embodiment in  FIG. 1 , the outer optical wave guide  20   a  is shown wound around the winding body  32  [sic]. The inner optical wave guide  20   i  can likewise be arranged wound parallel to this (not shown in  FIG. 1 ). However, other embodiments to arrange the optical wave guides  20   i  and  20   a  parallel to the inner or, respectively, outer winding body surface are also conceivable. For example, the optical wave guides  20   i,    20   a  could also be arranged in a meandering shape. The optical wave guides  20   i,    20   a  are provided with numerous temperature-sensitive fiber Bragg grating sensors  21 . The respective optical wave guide  20   i,    20   a  and the associated fiber Bragg grating sensors  21  are advantageously arranged such that the fiber Bragg grating sensors  21  form a “blanketing” sensor network. The fiber Bragg grating sensors  21  are advantageously arranged equidistant from one another. If a quench event in which the superconductor  34  suddenly becomes normally-conductive at a point occurs in the superconductor  34 , such that what is known as a “hot spot” forms at the event location, this can be detected by a fiber Bragg grating sensor  21  or multiple fiber Bragg grating sensors  21 . 
         [0028]    The fiber Bragg grating sensors  21  can respectively exhibit different specific focal wavelengths (what are known as Bragg wavelengths). The fiber Bragg grating sensors  21  are interrogated by a light signal LS that is generated by a broadband light source  51 . The light signal LS is injected into the fiber Bragg grating sensors  21  via a coupler  52  and one or more optical wave guides  20   i,    20   a.  A portion of the injected light signal LS with the respective focal wavelength is reflected back as a partial reflex signal in each fiber Bragg grating sensor  21 . In contrast to this, the remaining part of the light signal LS passes the appertaining fiber Bragg grating sensor  21  and, if applicable, strikes the next fiber Bragg grating sensor  21 . A light signal LS′ reflected back by the fiber Bragg grating sensors  21  is then present at the coupler  52 , which light signal LS′ is composed of the partial reflex light signals of the individual fiber Bragg grating sensors  21 . However, the focal wavelengths of multiple fiber Bragg grating sensors of an optical wave guide do not necessarily need to be different when, for example, what is known as an “optical time domain reflectometer” is used to differentiate the response signals of different fiber Bragg grating sensors. 
         [0029]    If a fiber Bragg grating sensor  21  experiences a temperature change, its focal wavelength changes corresponding to the magnitude of the temperature change, and therefore to the wavelength yield (=the wavelength spectrum) of the partial reflex light signal reflected by the appertaining sensor  21 . This variation in the wavelength yield serves as a measure for the temperature change to be detected. However, a transmission mode (not shown in Figures) is also conceivable. In contrast to the reflection mode, here the entire wavelength spectrum emitted by the light source  51  must be examined for missing wavelength ranges. These missing wavelength ranges correspond to the respective focal wavelengths of the individual sensors  21 . 
         [0030]    The light signal LS′ arriving from the fiber Bragg grating sensors  21  and injected again into the coupler  52  is directed by the coupler  52  to an evaluation unit  53 . This in particular comprises an optical transducer, an analog/digital converter and a digital signal processor. The optoelectronic transducer advantageously has a spectrally sensitive element for selection of the individual partial reflex light signals, for example in the form of a polychromator, and a light receiver (possibly also in multiple parts). Grid or diffraction spectrometers for analysis of the light spectrum are also conceivable. Given the use of an “optical time domain reflectometer”, for example, a cost-effective edge filter is also sufficient. An analog/digital conversion occurs in the analog/digital converter, following the optoelectronic transduction. The digitized output signal of the analog/digital converter is supplied to the digital signal processor, by means of which measurement values M 1 , M 2 , . . . for the temperatures detected in the fiber Bragg grating sensors  21  can be determined. In contrast to this, the coupler  52  can be omitted in the transmission mode. Here the light signal LS is injected at one end of the optical wave guide(s)  20   a,    20   i  by means of the light source  51  and is detected by an optoelectronic transducer at the other end of the optical wave guide(s)  20   a,    20   i.    
         [0031]    The light source  51 , the coupler  52  and the evaluation unit  53  are combined into a transmission/reception unit  50 , wherein the light source  51  and the coupler  52  can be considered as injection means to inject the light signal LS into the fiber Bragg grating sensors  21 , and the evaluation unit  53  with optoelectronic transducer, analog/digital converter and digital signal processor can be considered as an evaluation means to determine a measurement value M 1 , M 2 , . . . for the respective temperature detected by the fiber Bragg grating sensors  21 . In another exemplary embodiment (not shown), these sub-units or parts of these can be fashioned separate from one another, thus not as a joint transmission/reception unit  50 . Moreover, a purely analog evaluation is also possible, for example by means of a hard-wired electronic circuit. No analog/digital converter would then be present, and the evaluation unit  53  would be realized by means of analog technology. 
         [0032]    The measurement values M 1 , M 2 , . . . generated in the transmission/reception unit  50  are transmitted (for example by means of a radio transmission) to a data acquisition unit (not shown in  FIG. 1 ). However, in principle the data transmission can also ensue via wires, electrically or optically. Moreover, the transmission/reception unit  50  and the data acquisition unit can also be fashioned as a common unit. 
         [0033]    A cross-section through the winding arrangement  30  shown in  FIG. 1  is depicted in  FIG. 2 . The optical wave guide segments of the individual windings of a respective optical wave guide  20   a,    20   i  are arranged equidistantly. 
         [0034]    An optical wave guide  20   a,    20   i  is presented in longitudinal section in  FIG. 3 . The optical wave guide  20   a,    20   i  is thereby embedded in a composite material  33  (in particular casting resin, for example epoxy resin) with which the superconductor  34  is mechanically stabilized in a winding body  31 . The optical wave guide  20   a,    20   i  thereby runs essentially parallel to the adjacent superconductor  34 . Also shown is a fiber Bragg grating sensor  21  that is surrounded by a jacket element  22 . The jacket element  22  is thereby non-positively connected with the optical wave guide  20   a,    20   i,  and therefore is also non-positively connected with the fiber Bragg grating sensor  21 . While the optical wave guide (which is normally produced from glass) at ≦120 K experiences nearly no expansion given a temperature change—the coefficient of thermal expansion is negligible—the jacket element  22  is fashioned from a material that directly exhibits a relatively large coefficient of thermal expansion at such low temperatures. In particular a polymer (for example PMMA; polymethylmethacrylate) is thereby considered as a jacket element material. While a temperature rise from 2 K to 20 K cannot be measured in practice with a “naked” fiber Bragg grating sensor  21  [sic], for example, this is possible without further measures with a fiber Bragg grating sensor  21  provided with a jacket element  22 . Due to the non-positive connection of the jacket element  22  with the fiber Bragg grating sensor  21 , the fiber Bragg grating sensor  21  likewise also expands with the jacket element  22  given a temperature increase. The expansion in particular ensues in the length direction  23  of the optical wave guide  20   a,    20   i  since the jacket element  22  exhibits a pronounced expansion in the length direction. The grating constant of the fiber Bragg grating sensor  21  (and therefore the focal wavelength) changes (i.e. increases) due to the expansion. This variation can be directly interrogated by the injected light signal LS. The jacket element  22  shown in  FIG. 3  is additionally arranged rotationally symmetrical around the optical wave guide  20   a,    20   i.  The jacket element  22  narrows towards both sides in the length direction  23  of the optical wave guide  20   a,    20   i,  such that it tapers conically in the depicted example. The jacket element  22  is thickest in the region of the fiber Bragg grating sensor  21 , meaning that the distance between the optical wave guide  20   a,    20   i  and the outer surface of the measurement element is maximum in the region of the fiber Bragg grating sensor  21 , at least in the direction of the nearest superconductor  34 . 
         [0035]    Such a fiber Bragg grating sensor  21  can typically exhibit a diameter of approximately 200 μm and a length of approximately 10 mm. The thickness of the jacket element  22  is thereby at maximum 1 mm. 
         [0036]    A cross-section through the optical wave guide  20   a,    20   i  depicted in  FIG. 3  is shown in  FIG. 4 . As already specified, the jacket element  22  is fashioned to be rotationally symmetrical relative to the optical wave guide  20   a ,  20   i.    
         [0037]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.