Abstract:
A cryostat ( 1 ) with a magnet coil system including superconductors for the production of a magnet field B 0  in a measuring volume ( 3 ) within a room temperature bore ( 2 ) of the cryostat has a plurality of radially nested solenoid-shaped coil sections ( 4, 5, 6 ) which surround the room temperature bore and which are electrically connected in series, at least one of which being an LTS section ( 5, 6 ) with a conventional low temperature superconductor (LTS) and at least one of which being an HTS section ( 4 ) including a high temperature superconductor (HTS), wherein the LTS section ( 5, 6 ) is located in a helium tank ( 9 ) of the cryostat ( 1 ) along with liquid helium at a helium temperature T L . The apparatus is characterized in that the HTS section ( 4 ) is disposed radially within the LTS section ( 5, 6 ) in a vacuum portion of the cryostat and is separated from the LTS section ( 5, 6 ) by the helium tank ( 9 ) wall ( 9   a ) facing the room temperature bore. The HTS coil section can be reliably used in the cryostat over a long period of time (FIG.  1   a ).

Description:
This application is the national stage of PCT/EP2007/001924 filed on Mar. 7, 2007 and also claims Paris Convention priority to DE 10 2006 012 509.6 filed Mar. 18, 2006. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a cryostat having a magnetic coil system including superconducting materials for generation of a magnetic field B 0  within a measurement volume, the magnet system having a plurality of radially nested solenoid-shaped coil sections connected in series at least one of which is an LTS section of a conventional low temperature superconductor (LTS) and with at least one HTS section of a high temperature superconductor (HTS), wherein the LTS portion is located in a helium tank of the cryostat having liquid helium at a helium temperature T L . 
     Cryostats of this kind are e.g. disclosed in DE 10 2004 007 340 A1. 
     By way of example, nuclear magnetic resonance systems, in particular spectrometers, require very strong, homogenous and stable magnetic fields. The stronger the magnetic field, the better the signal to noise ratio as well as the spectral resolution of the NMR measurement. 
     Superconducting magnet coil systems are used to produce strong magnetic fields. Magnetic coil systems having solenoid-shaped coil sections are widely used which are nested within each other and operated in series. Superconductors can carry electrical current without losses. The superconducting condition is established below the material-dependent transition temperature. Conventional low temperature superconductors (LTS) are normally utilized for the superconducting material. These metallic alloys, such as NbTi and Nb 3 S, are relatively easy to process and are reliable in application. An LTS coil-portion conductor usually comprises a normally conducting metallic matrix (copper) in which superconducting filaments are embedded and which, during normal operation, completely carry the current. In the case of NbTi, these are usually several tens or hundreds of filaments; in the case of Nb 3 Sn, the filament number could be more than one hundred thousand. Although the internal construction of the conductor is actually somewhat more complex, this is irrelevant within the present context. 
     The coil sections are cooled with liquid helium within a cryostat in order to cool the superconducting portions below the transition temperature. The superconducting coil sections are thereby at least partially immersed in the liquid helium. 
     In order to further increase the magnetic field strength of the magnetic coil system it is desirable to also utilize a high temperature superconductor (HTS). For a given temperature, conductors, which include HTS, can carry much more current and thereby achieve higher magnetic field strengths than those with LTS. HTS materials are thereby appropriate for use in the inner most coil sections of a magnetic coil system. 
     HTS or ceramic superconductors are currently primarily made from bismuth conductors with HTS filaments within a silver matrix. The conductors are usually stripe or band-shaped. 
     Coil sections made from HTS have turned out to be unreliable and susceptible to short lifetimes, particularly in under-cooled helium. Investigation of defective HTS portions has shown that the HTS material is split open, thereby destroying the current carrying capability of the HTS conductor. This effect, which is also known in other context, is occasionally referred to as “ballooning”. 
     It is accordingly the purpose of the present invention to present a cryostat in which HTS coil portions enjoy a long lifetime and can be utilized in a reliable manner, in particular, without ballooning. 
     SUMMARY OF THE INVENTION 
     This purpose is achieved by a cryostat of the above-mentioned kind in that the HTS section is disposed radially within the LTS section in a vacuum portion of the cryostat and is separated from the LTS section by means of a wall of the helium tank facing the room temperature bore. 
     In accordance with the present invention, it has been discovered that the ballooning is caused by helium, which expands or evaporates within the HTS material. As is well known, helium liquefies at a pressure of 1 bar below approximately 4.2 K. HTS material is ceramic and therefore has a certain porosity. Liquid helium can pass through the pores into the inner portions of the HTS material. In particular, in a superfluid state of helium, which obtains below the λ point temperature of approximately 2.2 K, helium can pass through the smallest of gaps. In the event that subsequently heats beyond the evaporation point, it then expands rapidly in volume during evaporation. If the warming occurs too rapidly, the evaporating helium cannot escape quickly enough from the porous material and a substantial amount of pressure is built up within the pores of the HTS. Since the HTS material is ceramic and relatively brittle, the HTS can be explode in consequence of this pressure. 
     All these effects can be prevented with the cryostat configuration in accordance with the invention. The HTS section or sections of the magnetic coil system and thereby all of the HTS material is disposed in a vacuum portion of the cryostat between the inner wall of the helium tank and the room temperature bore. Clearly, no liquid helium is present in that vacuum portion. Moreover, the vacuum portion of an NMR cryostat is usually one integral piece. For example, the HTS section has an outer circularly cylindrical periphery which seats on the likewise circularly cylindrical inner wall of the helium tank and is thereby positively connected to the radially inner LTS section, which, if appropriate, seats on this wall from the helium side by means of small spacers. Since the HTS materials of the HTS section are in vacuum and do not come in contact with liquid helium, evaporation of helium, which has seeped into the inner portions of the HTS material is thereby avoided. The ballooning effect of the HTS material is therefore prevented. 
     Cooling power for the HTS section can be transferred from the helium tank wall to the HTS section by means of thermal contacts, e.g. made from copper. Towards this end, the operating temperature of the HTS section can clearly be somewhat higher than that of the LTS section in the helium tank since, at a temperature of about or below 4 K, the critical current of the HTS conductor is only weakly dependent on temperature, in contrast to the situation for the LTS sections. This is particularly true when the helium in the tank is under-cooled to approximately 2 K. 
     In an advantageous embodiment of the cryostat in accordance with the invention, the temperature of the liquid helium T L &lt;4 K, preferentially &lt;2.5 K, in particular &lt;2.2 K. At this low temperature, the danger of ballooning is particularly high without the measures in accordance with the invention so that the advantages of the invention are particularly great. Since the critical current density of the conductor of the LTS section increases with lower temperature, the lower temperatures enable higher magnetic field strengths B 0  and/or more compact LTS sections. 
     In a preferred embodiment of the cryostat in accordance with the invention, the superconducting leads to at least one HTS section also travel in the vacuum portion, at least to the extent that the leads contain HTS material. In this manner, all HTS materials, including the joints, are protected from liquid helium. Conductors generally made from a conventional superconducting material, e.g. a NbTi multi-filament wire, extend from these joints to a vacuum tight feed-through and into the helium tank, preferentially in a region of the helium tank which does not usually contain liquid helium or at least no superfluid helium. The superconductor is passed through this feed-through so that the complete magnetic current can flow to and from the HTS and LTS sections without loss. A superconducting switch for loss free persistent current mode operation is normally located in the helium tank: with vertically disposed magnet coils, above the LTS section. In the case of two coupled helium tanks with approximately 2 K (with the LTS section) and approximately 4 K, the feed-throughs are preferentially passed from the vacuum into the 4 K tank. 
     In a further preferred embodiment, the LTS section is mechanically rigidly connected to the HTS section, in particular by means of or via the wall of the helium tank. This leads to a particularly mechanically stable configuration for the magnetic coil system and thereby the magnetic field strength B 0 . 
     In a further preferred embodiment, the HTS section is thermally coupled to the wall of the helium tank. In this manner, sufficient cooling of the HTS section is achieved in a straightforward manner. 
     In a further advantageous embodiment, at least one radiative shield is located between the HTS section and the room temperature bore which is preferentially coupled to the helium tank to thereby have a temperature of approximately 4.2 K. The radiation shield reduces the radiative heat input from the room temperature bore into the HTS section. 
     If required, it is also possible to take steps to thermally shield the leads which run in the vacuum portion to the HTS section in that they pass within the radiative shield or are cooled by heat conducting leads which run parallel thereto and which have good thermal conductivity. In this manner, a quenching of these leads, including possible joints, can be avoided. 
     In an advantageous embodiment, a sorption medium for helium is disposed in the vacuum portion, in particular activated charcoal. This obviates the need for long-term use of external or transport pumps. 
     In a particular preferred embodiment of the invention, the magnetic field produced by the magnet coil system in the measurement volume B 0 &gt;20 T, in particular &gt;23 T. These strong magnetic fields can easily be achieved with the HTS section and the cryostat in accordance with the invention. In contrast thereto, conventional magnet systems that only have LTS-based sections already reach the theoretical limit at these field strengths, having a critical current density which approaches 0. 
     In a further preferred embodiment, the coil sections of the magnetic coil system are superconducting short circuited (persistent current mode) during operation. This leads to a particularly stable time dependence of the magnetic field B 0 . 
     In a further preferred embodiment, the magnetic coil system has a magnetic field B 0  homogeneity in the measurement volume and a time stability for the magnetic field B 0  that satisfy the requirements for high resolution NMR spectroscopy. 
     A further embodiment is preferred with which the cryostat has an additional helium tank with a temperature of helium held therein of approximately 4.2 K and which is connected to the (first) helium tank. The liquid helium in the (first) helium tank has a temperature of T L &lt;4 K. The use of two helium tanks permits the LTS section to be operated at a lower temperature, which leads to higher current capability. Moreover, helium evaporates from the cryostat under normal pressure and therefore can also be refilled at normal pressures. This increases the efficiency of the cooling and the operational safety. 
     In a preferred improvement in this embodiment, the series connection between the LTS section in the helium tank and the HTS section in the vacuum portion extends through the additional helium tank. The feed through of the superconducting leads into the vacuum portion is therefore effected via the additional helium tank. This is preferably the case when the (first) helium tank contains superfluid helium, since this procedure simplifies reliable sealing of the current feed-through. 
     In a preferred improvement, the additional helium tank is disposed above the helium tank about a common vertical room temperature bore. In this configuration, both tanks can be separated by a vacuum barrier and coupled through a narrow e.g. gap-shaped connection. The upper tank can be located at normal pressure or slightly above so that, in total, the refilling of helium is easier and safer. 
     In a further improvement, the wall of the additional helium tank facing the room temperature bore is separated in a radial direction from the room temperature bore by a same amount as the wall of the helium tank facing the room temperature bore. This permits a particularly simple introduction of a horizontal feed-through into the additional helium tank at a height at which liquid helium in the additional tank is normally not located. This, in turn, simplifies sealing of the feed-through. It also leads to a topologically uniform inner bore to the extent to which that bore is bordered by the helium tank. This improves accessibility and use of the bore. 
     Alternative thereto, the side of the wall of the additional helium tank facing the room temperature bore can be radially closer to the room temperature bore than the wall of the helium facing the room temperature bore. This embodiment permits a straightforward introduction of the current leads into the flat floor plate of the additional (upper) helium tank. The volume of the additional helium tank can thereby be increased which leads to a longer holding time for the helium. It is additionally preferred when the wall of the additional helium tank facing the room temperature bore is aligned with the inner bore of the HTS section. 
     Further advantages of the invention can be derived from the description of the drawings. The above-mentioned features and those to be discussed below can be utilized in accordance with the invention individually or collectively in arbitrary combination. The embodiments shown and described are not to be considered exhaustive enumeration, rather have exemplary character only for illustrating the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention is represented in the drawing and is further explained with reference to embodiments. 
         FIG. 1   a  shows a schematic representation of a first embodiment of a cryostat in accordance with the invention having an HTS section in vacuum and associated joints and feed throughs; 
         FIG. 1   b  shows a schematic representation of a second embodiment of a cryostat in accordance with the invention having an HTS section in vacuum with a common floor plate for the coil sections; 
         FIG. 2  shows a schematic representation a third embodiment of a cryostat in accordance with the invention having an HTS section vacuum and with two helium tanks; 
         FIG. 3  shows a schematic circuit diagram of the sections of  FIG. 1   a ; and 
         FIG. 4  shows a schematic circuit diagram of the sections of  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1   a  shows a first embodiment of a cryostat  1  in accordance with the invention. The cryostat  1  has a room temperature bore  2  in which a measuring volume  3  for a sample is provided. The measuring volume  3  is located in the center of a magnetic coil system, which is formed by three solenoid-shaped coil section  4 ,  5 ,  6 . The radially innermost coil section  4  has a wounding made from high temperature superconductor (HTS). The middle coil section  5  is wound with Nb 3 Sn wire and the outer most coil section  6  is wound with NbTi wire. The coil sections  5 ,  6  therefore represent low temperature superconductor (LTS) coil sections. The coil sections  4 ,  5 ,  6  are electrically connected to each other in series, as is shown in an exemplary fashion by means of superconducting joints  7   a  and  7   b . At joint  7   a , the high HTS material of a lead  4   a  is connected to a HTS coil section  4  by means of an adaptor section  8  made from NbTi. At joint  7   b , the adaptor member  8  is connected to the Nb 3 Sn wire of the LTS section  5 . The adaptor section  8  passes through a feed-through  18  in the wall  9   a  of a helium tank  9  in which mutually nested LTS sections  5  and  6  are located. The feed-through  18  is a conventional electrical low temperature vacuum feed-through having metallic tubes which are insulated which respect to each other and with respect to the wall of the tank to which the respective conductors of the adaptor piece  8  are soldered in a vacuum tight fashion. The wall  9   a  of the helium tank faces the room temperature bore  2 . 
     The helium tank  9  is substantially filled with liquid helium. The helium tank  9  has a temperature T L  of approximate 4 K. For insulation purposes, the helium tank  9  is completely surrounded by a radiation shield  10 , in particular, in a radially outer direction. The radiation shield  10  also extends between the HTS section  4  and the room temperature bore  2 . The radiation shield  10  is cooled with liquid nitrogen, which is filled into a container  10   a . Additional radiation shields can also be provided for which are usually cooled by the evaporating helium gas. Alternatively or in addition thereto, the radiation shields can be cooled by refrigerators, wherein the nitrogen tank  10  can then be eliminated. The refrigerator can also re-cool the evaporated helium so that the refilling interval for liquid helium can be increased or such that the refilling of helium is only necessary following a break down. 
     The LTS coil sections  5 ,  6  are immersed in liquid helium. The HTS sections  4 , including the leads  4   a  and the joints  7   a , are disposed in vacuum between the helium tank  9  and the room temperature bore  2 . In this manner, no liquid helium can flow into the HTS material of the HTS coil sections  4  or their leads  4   a . In consequence thereof, one prevents liquid helium from evaporating within the HTS material, thereby leading to an increased volume within the HTS material and possible rupturing thereof. 
     The overall evacuated inner region of cryostat  1  constitutes the vacuum portion  14  of the cryostat and within this vacuum portion  14  is an HTS section  4  in accordance with the invention. A pressure of less than 10 −5  mbar is present in vacuum portion  14 . 
     The HTS coil section  4  is preferentially positively connected to the inner wall  9   a  of the helium tank  9  and is cooled by that tank wall  9   a . Towards this end, a good conductor such as metallic copper can e.g. be utilized. The LTS section can be positively connected to the helium tank wall  9   a , optionally by means of spacer pieces  11   a . A small separation with respect to the tank wall  9   a  produces a gap  12  by means of which helium convection can be induced in a radial sense within the section  5 . Alternatively, the section  5  is directly flush with the wall  9   a . It is normally permissible for the HTS section  4  to be somewhat warmer than the LTS sections  5  and  6 . An additional radiation shield  19  is disposed between the HTS section  4  and the room temperature bore  2  for thermal shielding of section  4 . The radiation shield  19  is cooled via the liquid helium in the helium tank  9 . Additional radiative shields can be utilized for shielding the HTS section  4 . 
     A second embodiment of the cryostat  1  in accordance with the invention is shown in  FIG. 1   b . This embodiment substantially corresponds to that of  FIG. 1   a , however, entails a particular anchoring of coil sections  4 ,  5 ,  6 . 
     A radially outer portion of a floor plate  15  forms the lower wall of the helium tank  9 . The floor plate  15  extends in a radially inward direction up to beneath the HTS section  4 . Two ring flanges  16   a ,  16   b  are attached to the floor plate  15 . The LTS section  6  is directly attached to the radially outer ring flange  16   a  and the LTS section  5  is indirectly attached thereto by means of a coil support  17   a . The HTS section is attached to the radially inner ring flange  16   b  by means of a coil support  17   b . The floor plate  15  is preferentially formed from a single piece. This configuration permits simultaneous handling of all coil sections  4 ,  5  and  6  during assembly of the cryostat  1  by means of the common floor plate  15 . 
     The third embodiment of cryostat  1  in accordance with the invention shown in  FIG. 2  corresponds substantially to the cryostat of  FIG. 1 . However, cryostat  1  of  FIG. 2  has helium tank  9  as well as an additional, upwardly disposed helium tank  21 . Both helium tanks  9 ,  21  contain helium at different temperatures. The lower helium tank  9  is at approximately 2 K and the upper additional tank  21  at approximately 4 K. Cryostats  1  of this type are preferentially utilized to produce very high magnetic fields, in particular in high field NMR systems. 
     In the embodiment shown in  FIG. 2 , the additional helium tank  21  extends further in the inward direction (towards the room temperature bore  2 ) than does the lower helium tank  9 , wherein a feed through  18  for the adaptor piece  8  is disposed on the floor of the additional helium tank  21 . The inner wall  21   a  of the additional helium tank  21  which faces the room temperature bore  22  is located at an equal radial separation from the room temperature inner bore  2  as the radiation shield  19 . The inner wall  9   a  of the helium tank  9  is set back with respect thereto. This configuration permits efficient use of the region within the cryostat  1 . 
     In an alternative embodiment (not shown) the inner wall of the additional helium tank can also have the same radial separation from the room temperature bore  2  as the inner wall  9   a  of the helium tank  9  in  FIG. 1   b . This provides space for feeding through the superconducting adaptor pieces from the vacuum portion into the upper additional helium tank in a horizontal direction through the inner wall of the additional helium tank. The feed-through is thereby preferentially located above the surface of the liquid helium in the additional helium tank. This facilitates secure sealing of the feed-through. 
       FIG. 3  schematically shows the electrical connection of the coil sections  4 ,  5 ,  6  of  FIG. 1   a . The inner region of the helium tank  9  (helium region) is shown as a block of dotted lines as is the region of the vacuum portion  14  of the cryostat (vacuum region). 
     The LTS section  6  (NbTi) and  5  (Nb 3 Sn) are disposed within the helium region  9  in liquid helium at approximately 4 K. The LTS sections  5 ,  6  are connected in series by means of a superconducting connector  7   b ′ (joint). The LTS section  5  is connected, via an additional superconducting connection  7   b ″, with an adaptor piece  8 ′ which is fed through the vacuum feed-through  18  out of the helium region  9  into the vacuum region  14 . At this location, the adaptor member  8 ′ is connected, via a superconducting connection  7   a ′, with a lead at one side of the HTS section  4 , which is located in the vacuum region  14 . An HTS conductor on the other side is superconductively connected to an adaptor member  8 ″ at joint  7   a ″. The adaptor piece  8 ″ leads back through the vacuum feed through  18  into the helium region  9  at joint  7   b ′″, and is connected to the end of a superconducting short circuiting switch  20  for the overall coil configuration  4 ,  5 ,  6 . The other end of the short circuiting switch  20  is once more connected by means of joint  7   b ″″ to the section  6 . 
       FIG. 4  shows corresponding circuiting of the sections  4 ,  5  and  6  for the embodiment in accordance with  FIG. 2 . In this case, however, there are two helium regions  9  and  21  located at different temperatures (approximately 2 K and approximately 4 K) in correspondence with helium tank  9  (at 2 K) and the additional helium tank  21  (at 4 K). Moreover, there is a vacuum region  14  corresponding to the previous vacuum region  14 . 
     The feed-through  18  extends from the vacuum portion  14  into the 4 K region  21  in which the superconducting switch  20  is also located. The LTS sections  5 ,  6  are located in the 2 K region  9 . Some or all of these components can be located in the 4 K region  21  or in the 2 K region  9  in dependence on the topological conditions and on the configuration of the joints  7   a ,  7   b  and/or of the switch  20  with regard to their temperature and field carrying capacities. The feed-through  18  preferentially feeds into the 4 K region  21  where no superfluid helium is located. This increases the safety. In principle, the feed through  18  can also be directly fed into the 2 K region in the event that sealing of superfluid helium is guaranteed. 
     The cryostats  1  of  FIGS. 1   a ,  1   b , and  2  are preferentially parts of an NMR apparatus such as an NMR spectrometer or an NMR tomography apparatus, in particular, a high field NMR spectrometer having a magnetic field in the measuring volume B 0 &gt;20 T, preferentially &gt;23 T, wherein the magnetic coil system satisfies the requirements of high resolution NMR spectroscopy with regard to the magnetic field B 0  homogeneity in the measuring volume and the temporal stability of B 0 , which, in general requires that the coil sections of the magnetic coil system be operated in persistent current mode. As is shown in the examples, the coil axes and the room temperature bore are normally vertical. The invention, however, also concerns cryostats having horizontal bores which are preferentially utilized in imaging applications (MRI) or for ion cyclotron resonance from spectrometers.