CROSS REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority under 35 U.S.C. § 119(a)-(d) to European Patent Application EP 19 209 166.8 filed on Nov. 14, 2019, and the contents of which are incorporated into the present application by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to a method for charging a superconductor magnet system, the superconductor magnet system comprising a main superconductor bulk magnet. The invention relates also to a superconductor magnet system.

BACKGROUND

Such a method, based on the so called “field cooling” process, is described in U.S. Pat. No. 7,859,374 B2, which is considered the nearest state of the art.

Superconductors carry an electric current at practically no ohmic losses. The superconducting state of a superconductor is assumed only below a critical temperature Tcrit, which is in the cryogenic range, and depends on the superconductor material used.

Superconductors can be used to generate strong magnetic fields (or magnetic flux densities). The magnetic field is generated by an electric current running within the superconductor, typically wherein the superconductor forms a coil or ring for this purpose (“superconductor magnet”), and the magnetic field is obtained in a superconductor bore. In particular, the electric current can run in a closed superconducting electric circuit at a practically constant current strength, without a need for a connection to an electric current source once charged (“persistent mode”).

Coil type superconductor magnet apparatus are typically charged with a directly connected electric current source, and when the charging is complete, a superconducting switch is closed to establish the persistent mode.

For superconductor bulk magnets, which are based on closed ring-shaped superconducting structures, no electric contacts are necessary. Superconductor bulk magnets may be charged by a procedure known as “field cooling”, e.g. described in U.S. Pat. No. 7,859,374 B2. In this procedure, the superconductor bulk magnet located within a cryostat is placed in an electrical charger magnet. Initially, the temperature of the superconductor bulk magnet is kept above Tcrit, and the electrical charger magnet is ramped up until a desired magnetic field has been reached. Then the temperature within the cryostat is lowered below Tcrit, so that the superconductor bulk magnet becomes superconducting. Then the electrical charger magnet is ramped down. During this, superconducting electric currents are induced within the superconductor bulk magnet, opposing the change of magnetic flux, and as a result trapping (or conserving) the magnetic flux within a superconductor bore of the superconductor bulk magnet. Then the superconductor bulk magnet together with its cryostat may be removed from the electrical charger magnet. As long as the superconductor bulk magnet is kept sufficiently cold, and in particular below Tcrit, the magnetic field within the superconductor bore stays constant and can be used, for example for NMR measurements. For accessing the trapped magnetic field in an experiment, such as an NMR experiment, the cryostat should provide a room temperature bore, which in turn is arranged within the superconductor bore.

T. Nakamura et al., Journal of Magnetic Resonance 259 (2015), page 68-75, also describes a superconductor magnetic system with a superconductor bulk magnet for use in NMR.

However, the superconductor bulk magnet not only generates (or conserves) a magnetic field in its superconductor bore, but also generates a magnetic field on its outside (“stray field”). This outside stray field is in general not desired, since it may disturb electrical equipment nearby, and even may be dangerous to persons carrying for example a pacemaker.

In order to reduce the stray field, it is known to surround the superconductor bulk magnet with ferromagnetic material.

U.S. Pat. No. 7,859,374 B2 proposes to place a shielding of steel sheets around the cryostat containing a superconductor bulk magnet after the electrical charger has been removed. The steel sheets will reduce the stray field in the vicinity of the superconductor bulk magnet. However, the ferromagnetic material necessary for shielding is heavy and takes a lot of space, what is particularly disadvantageous for benchtop size magnet systems.

Post-published European patent application 19 170 280.2 also describes a superconductor bulk magnet equipped with ferromagnetic shields.

The Czech company “CAN superconductors”, Kamenice, C Z, offers Bi-2223 based bulk tubes as superconductor magnetic shields, compare the company brochure (datasheet) “SUPERCONDUCTING MAGNETIC SHIELDS CST/CSV”, downloaded on Oct. 7, 2019 at https://www.can-superconductors.com/magnetic-shields.html. The bulk tubes may be used to keep external magnetic fields from reaching the inside of the bulk tubes.

For superconductor magnet systems based on solenoid type magnet coils, it is known to apply an electric current of a first polarity to the radially inner coil sections, and to apply an electric current of opposite polarity to a radially outward coil section in order to reduce the stray field, compare e.g. DE 10 2015 218 019 A1.

In EP 3 492 941 A1 a magnetization method for a bulk magnetic structure is described, wherein after a basic magnetization step, the bulk magnetic structure is further magnetized by controlling at least one of a temperature controller or a magnetic field generator, for obtaining a more uniform magnetic field distribution.

US 2006/0252650 A1 describes a superconducting permanent magnet comprising a plurality of bulk superconductors arranged in parallel.

SUMMARY

It is an object of the invention to reduce stray fields of superconductor magnet systems based on a superconductor bulk magnet, wherein less weight and space are required for shielding purposes.

SHORT DESCRIPTION OF THE INVENTION

This object is achieved, in accordance with one formulation of the invention, by a method for charging a superconductor magnet system, the superconductor magnet system comprisinga main superconductor bulk magnet,a shield superconductor bulk magnet, anda cryostat system having a room temperature bore,with the cryostat system containing the main superconductor bulk magnet and the shield superconductor bulk magnet,with the shield superconductor bulk magnet radially surrounding the main superconductor bulk magnet,and with the main superconductor bulk magnet and the shield superconductor bulk magnet being arranged coaxially with the room temperature bore,the method comprising the following steps:

step a) arranging the superconductor magnet system at least partially within a charger bore of a charger magnet, in particular with the charger bore being arranged coaxially with the room temperature bore of the cryostat system;

step b) with Tmain>Tmaincritand Tshield>Tshieldcrit, applying an electrical current Ichargerto the charger magnet and increasing Ichargerto a first current I1>0,

step c) lowering Tmainto or below an operation temperature Tmainopof the main superconductor bulk magnet, with Tmainop<Tmaincrit, while keeping Tshield>Tshieldcrit;

step d) lowering Ichargerto a second current I2<0, wherein a main persistent current IPmainis induced in the main superconductor bulk magnet which stays below a critical current of the main superconductor bulk magnet at Tmain, with Tmain≤Tmainop;

step e) lowering Tshieldto or below an operation temperature Tshieldopof the shield superconductor bulk magnet, with Tshieldop<Tshieldcrit;

step f) increasing Ichargerto zero, wherein a shield persistent current IPshieldis induced in the shield superconductor bulk magnet which stays below a critical current of the shield superconductor bulk magnet at Tshield, with Tshield≤Tshieldop;

step g) removing the superconductor magnet system from the charger bore of the charger magnet, and keeping Tmainat or below Tmainopwith Tmainop<Tmaincritas well as Tshieldat or below Tshieldopwith Tshieldop<Tshieldcrit;

with Tmain: temperature of the main superconductor bulk magnet;Tmaincrit: critical temperature of the main superconductor bulk magnet;Tshield: temperature of the shield superconductor bulk magnet; andTshieldcrit: critical temperature of the shield superconductor bulk magnet.

The inventive procedure achieves the charging of both the radially inner main superconductor bulk magnet with a main persistent current, and the radially outer shield superconductor bulk magnet with a shield persistent current. Through the inventive procedure, the main persistent current and the shield persistent current have opposite current directions (“opposite polarity”), so in the surroundings of the superconductor magnet system, their corresponding magnetic fields (or magnetic flux densities) are opposed to each other and therefore cancel out each other, at least partially, and preferably practically completely. Accordingly, the stray field is reduced by an active shielding.

The main persistent current of the main superconductor bulk magnet results from a take-over of magnetic flux from the charger magnet when the charger current Ichargeris reduced from the first current I1(which is >0) to the second current I2(which is <0). Here, only the main superconductor bulk magnet is superconducting, and traps the magnetic flux in its interior.

In contrast, the shield persistent current of the shield superconductor bulk magnet results from a take-over of magnetic flux from the charger magnet when the charger current Ichargeris increased from the second current I2(which is <0) to zero current (Icharger=0). Then, also the shield superconductor bulk magnet is superconducting, in order to trap magnetic flux in its interior; here the magnetic flux inside the main superconductor bulk magnet does not change anymore.

In order to charge the main superconductor bulk magnet and the shield superconductor bulk magnet independently, the inventive method provides that the conductivity states (normally conducting or superconducting) are chosen temporarily different for the main superconductor bulk magnet and the shield superconductor bulk magnet during the charging procedure. For this purpose, their temperatures Tmain, Tshieldare set to different values during some of the steps, in particular steps c) and d). Accordingly, the main superconductor bulk magnet and the shield superconductor bulk magnet are, at least temporarily, substantially thermally decoupled during the inventive method.

As long as a (main or shield) superconductor bulk magnet is above its critical temperature, any induced electric currents will decay quickly, and the respective superconductor bulk magnet is “transparent” during the charging procedure.

In contrast, when a (main or shield) superconductor bulk magnet is below its critical temperature, it will conserve any induced electric currents, and thus trap the magnetic flux in its interior (inside its superconductor bore). Note that the induced (persistent) current stays, in accordance with the invention, below the critical current at the respective operation temperature then, i.e. it does not reach the critical current, so the trapping of the magnetic flux may be accomplished completely. For this purpose, the operation temperature should be chosen sufficiently far below the respective critical temperature.

The inventive method does not require heavy and bulky ferromagnetic shielding plates, but instead may accomplish the shielding with a relatively small shield superconductor bulk magnet. As compared to ferromagnetic shielding material such as iron, the active shielding using the shield superconductor bulk magnet in accordance with the invention is much more efficient per weight.

Typically the shield superconductor bulk magnet is placed in the same vacuum container of the cryostat system as the main superconductor bulk magnet, but with some distance to it for thermal decoupling. However, it is also possible to have different vacuum containers for the main and the shield superconductor bulk magnets.

It should be noted that in the charged state, both the main superconductor bulk magnet and the shield superconductor bulk magnet contribute to the magnetic field at the sample volume near the magnetic center in the room temperature bore, and therefore both contribute to the homogeneity of the magnetic field there, too. Preferably, the geometry of the shield superconductor bulk magnet is adapted as to improve the homogeneity of the magnetic field that would result from the main superconductor bulk magnet alone. More specifically, the shield superconductor bulk magnet may be used to cancel out leading terms of axial gradients (even orders like Z2 or Z4) in the total magnetic field. The shield superconductor bulk magnet typically projects beyond the main superconductor bulk magnet in axial direction, typically by at least 1/10, preferably at least ⅕, of its average radius Rshield. Further, typically for the average radius Rshieldof the shield superconductor bulk magnet and the average radius Rmainof the main superconductor bulk magnet, Rshield≥1.5*Rmainapplies.

The main superconductor bulk magnet and the shield superconductor bulk magnet are in general made from a high temperature superconductor material, for example of ReBCO type (Re: rare earth element, in particular Y or Gd) or BSCCO type or of MgB2type or of pnictide type (compare WO 2016/161336 A1), with Tmaincrit>30 K and Tshieldcrit>30K. The main and shield superconductor bulk magnets are substantially of cylindrical shape and have a central (axial) bore. They are generally of closed ring shape, to allow a (main or shield) persistent circular electrical current causing a magnetic field substantially along the axis of the central bore. They may comprise a multitude of axial layers and/or radial layers, and may in particular be made of a multitude of stacked ring elements and/or may comprise a circumferential superconductor coating on a tube type carrier body or bodies.

PREFERRED VARIANTS OF THE INVENTION

In a preferred variant of the inventive method, |I2/I1|≤0.33, preferably |I2/I1|≤0.25. Such ratios of the first and second current result, for typical sizes of main and shield superconductor bores, in a high degree of stray field reduction.

Particularly preferred is a variant wherein 0.7≤|I1/I2|*|Rmain2/Rshield2|≤1.4,preferably 0.8≤|I1/I2|*|Rmain2/Rshield2|≤1.25,with Rmain: average radius of the main superconductor bulk magnet; andRshield: average radius of the shield superconductor bulk magnet. Preferably, |I1/I2|*|Rmain2/Rshield2|=1. With such ratios, an excellent degree of stray field reduction can be achieved; in general, the radius of a 5 G contour may be reduced to ¼ or even less as compared to an unshielded situation. For determining Rmainand Rshield, averaging is done across the respective superconductor bulk material over the circumferential direction and over the axial direction. For a simple circular tube type design, an average radius represents the middle between the inner radius and outer radius. Typically, Rshield>1.5*Rmainor Rshield≥2*Rmain.

Particularly preferred is a variant wherein Tmainop≤0.75*Tmaincritand Tshieldop<0.75*Tshieldcrit,preferably Tmainop≤0.60*Tmaincritand Tshieldop≤0.60*Tshieldcrit. With these operation temperatures Tmainopand Tshieldop, the main and shield persistent current typically stay well below the critical current at the respective operation temperature. Note that the temperatures are noted in Kelvin here.
Variant with a Single Normal Cryocooler

An advantageous variant providesthat the cryostat system comprises a normal cryocooler and thermal connections of the normal cryocooler to a main bulk thermal stage connected to the main superconductor bulk magnet and to a shield bulk thermal stage connected to the shield superconductor bulk stage,that during step c) and step d), a shield thermal switch in the thermal connection to the shield bulk thermal stage is open, andthat during step e) and after step e), the shield thermal switch is kept closed. A “normal” cryocooler is designated for use (at least) in the steady state from step g) on. In this variant, independent temperature levels of the main and shield superconductor bulk magnet can be achieved with a single normal cryocooler. In particular, the shield thermal switch avoids a thermal short-circuit of the thermal stages during the cooling procedure. This design is relatively inexpensive, but requires a relatively large cooling power of the normal cryocooler if the normal cryocooler is intended as the only source of cooling power during the entire method.
Variants Using an Auxiliary Cryocooler and a Normal Cryocooler

A preferred variant providesthat at least during steps c) through f), the cryostat system is equipped with an auxiliary cryocooler,that during steps c) through f), cooling power of the auxiliary cryocooler is directed to a main bulk thermal stage connected to the main superconductor bulk magnet,that during steps e) and f), cooling power of the auxiliary cryocooler is directed to a shield bulk thermal stage connected to the shield superconductor bulk magnet,that after step f), the auxiliary cryocooler is removed from the cryostat system,and that the cryostat system comprises a normal cryocooler and thermal connections of the normal cryocooler to the main bulk thermal stage and to the shield bulk thermal stage,and that after step f), the normal cryocooler provides cooling power to the main bulk thermal stage and the shield bulk thermal stage. In this variant, the cooling power of the normal cryocooler can be chosen relatively small, enough so during the steady state from step g) on, it is capable of keeping the main and shield superconductor bulk magnets at their respective operation temperatures; note that the normal cryocooler is in general permanently installed at the cryostat system. During the charging procedure, when the thermal stages have to be cooled down from temperatures above the respective critical temperatures, the cooling power of the auxiliary cryocooler can be used; since the auxiliary cryocooler is removed after charging (i.e. typically still on the premises of the manufacturer), its size does not influence the size of the superconductor magnet system in practical use at the site of application. A design for an auxiliary cryocooler that can be applied in the present invention is described in EP 1 655 616 A1, the entire content of which is hereby incorporated by reference.

In an advantageous further development of the above variant, the normal cryocooler provides cooling power to the main bulk thermal stage and the shield bulk thermal stage only after step f). Then malfunctions of the normal cryocooler, which may be caused by the stray field of the superconductor magnet system during charging (i.e. before the active shielding is in force), can be avoided. The normal cryocooler does not need to be operational in strong magnetic fields.

Another further development of the above variant provides that the thermal connection between the normal cryocooler and the main bulk thermal stage and/or the thermal connection between the normal cryocooler and the shield bulk thermal stage include a weak thermal link or a thermal switch,

and that the weak thermal link or the thermal switch slows down or blocks heat conduction between the normal cryocooler and the main bulk thermal stage and/or between the normal cryocooler and the shield bulk thermal stage during at least some of step c), step d), step e) and/or step f). The thermal switch(es) and weak link(s) avoid a thermal short-circuit of the main bulk thermal stage and the shield thermal stage via the normal cryocooler, and thus facilitate keeping the thermal stages (and thus the main and shield superconductor bulk magnet) temporarily at different temperatures in the course of the inventive method. The weak thermal link slows down heat conduction through the weak link to a degree such that this heat conduction is small (e.g. 20% or less) as compared to a heat conduction (cooling power input) between the auxiliary cryocooler and the thermal stage connected to said weak link.
Variant with Two Normal Cryocoolers

Another preferred variant provides that the cryostat system includes two normal cryocoolers, with a first of the normal cryocoolers having a thermal connection to a main bulk thermal stage connected to the main superconductor bulk magnet, and a second of the normal cryocoolers having a thermal connection to a shield bulk thermal stage connected to the shield superconductor bulk magnet,

and that the first normal cryocooler provides cooling power to the main bulk thermal stage from step c) on and later, and wherein the second normal cryocooler provides cooling power to the shield bulk thermal stage form step e) on and later. This design is relatively simple. In order to reduce the required cooling power of the normal cryocooler connected to the main bulk thermal stage, a shield thermal switch may be integrated in the thermal connection of the second normal cryocooler connected to the shield bulk thermal stage.
Inventive Superconductor Magnet Systems

Within the scope of the present invention is further a superconductor magnet system comprisinga main superconductor bulk magnet,a cryostat system having a room temperature bore,and a cooling system adapted for cooling the main superconductor bulk magnet, with the cryostat system containing the main superconductor bulk magnet,and with the main superconductor bulk magnet arranged coaxially with the room temperature bore,characterized inthat the superconductor magnet system further comprises a shield superconductor bulk magnet, with the cryostat system also containing the shield superconductor bulk magnet,with the shield superconductor bulk magnet radially surrounding the main superconductor bulk magnet,and with the shield superconductor bulk magnet also being arranged coaxially with the room temperature bore,and that the cooling system is further adapted for cooling the shield superconductor bulk magnet and for selectively maintaining the following states:state a) Tmain>Tmaincritand at the same time Tshield>Tshieldcrit, furtherstate b) Tmainis at or below an operation temperature Tmainopof the main superconductor bulk magnet, with Tmainop<Tmaincritand at the same time Tshield>Tshieldcrit, and furtherstate c) Tmainis at or below the operation temperature Tmainop, with Tmainop<Tmaincritand at the same time Tshieldis at or below an operation temperature Tshieldopof the shield superconductor bulk magnet, with Tshieldop<Tshieldcrit,with Tmain: temperature of the main superconductor bulk magnet;Tmaincrit: critical temperature of the main superconductor bulk magnet;Tshield: temperature of the shield superconductor bulk magnet; andTshieldcrit: critical temperature of the shield superconductor bulk magnet. With the inventive superconductor magnet system described above, the inventive charging method may be performed. State a) can be used during step b), further state b) can be used during step d), and state c) can be used during steps f) and g). The charger current Ichargeris typically controlled via an electronic control device, which preferably also monitors and controls the temperatures Tmainand Tshieldand automatically conducts the inventive charging procedure.

In a preferred embodiment of the inventive superconductor magnet system, the cooling system comprises a main bulk thermal stage connected to the main superconductor bulk magnet and a shield bulk thermal stage connected to the shield superconductor bulk magnet. The main bulk thermal stage and the shield bulk thermal stage are arranged at a distance from each other, in particular the main bulk thermal stage and the shield bulk thermal stage are arranged on opposite axial sides of the main superconductor bulk magnet and the shield superconductor bulk magnet. By using the thermal stages and arranging the thermal stages at a distance, setting (and in particular differently setting) the temperatures of the main and shield superconductor magnets is simplified. Arranging the main bulk thermal stage and the shield bulk thermal stage at opposing axial sides improves the thermal decoupling of the thermal stages.

In a preferred further development, the main bulk thermal stage and the shield bulk thermal stage comprise separate electrical heaters. The heaters allow a finer control of the temperatures of the thermal stages or the main and shield superconductor bulk magnets, and in particular if the respective cooling powers provided to the two thermal stages cannot be controlled independently, or can only coarsely be controlled, then the heaters may guarantee a temperature of the respective thermal stages above the respective critical temperature as required by the inventive method. Note that in another further development in accordance with the invention, only the shield bulk thermal stage comprises an electrical heater, but not the main bulk thermal stage.

In a preferred further development, the cooling system comprises a normal cryocooler and thermal connections of the normal cryocooler to the main bulk thermal stage and to the shield bulk thermal stage. The thermal stages facilitate setting the Tmainand Tshieldindependently during charging. The normal cryocooler may provide cooling power at least in the steady state use from step g) on.

Advantageously in this further development a shield thermal switch is arranged in the thermal connection of the normal cryocooler to the shield bulk thermal stage. Then achieving different temperatures at the two thermal stages, in particular for step d), is facilitated. In particular, in a relatively simple design, a single normal cryocooler may provide cooling power both during charging and during steady state use. Note that alternatively to the thermal switch, also a weak thermal link can be arranged in the thermal connection of the normal cryocooler to the shield bulk thermal stage, e.g. reducing the cooling power input from the normal cryocooler to the shield bulk thermal stage to 20% or less as compared to the cooling power input through the thermal connection from the normal cryocooler to the main bulk thermal stage.

Another further development of the above described embodiment provides that the cooling system further comprises an auxiliary cryocooler,wherein the auxiliary cryocooler is removably attached to the cryostat system,and that the cooling system is adapted to provide cooling power of the auxiliary cryocooler independently to the main bulk thermal stage and the shield bulk thermal stage. Then the normal cryocooler, which is in general permanently installed in the cryostat system may have a relatively small cooling power, and the superconductor magnet system, in use, may be relatively compact. Further, the normal cryocooler does not need to be operated during the charging process when the active shielding is not yet in force.

Advantageously in this further development, the thermal connection between the normal cryocooler and the main bulk thermal stage and/or the thermal connection between the normal cryocooler and the shield bulk thermal stage include a weak thermal link or a thermal switch. This avoids a thermal short-circuit of the thermal stages via the normal cryocooler.

In an advantageous embodiment, the cooling system includes two normal cryocoolers, with a first of the normal cryocoolers having a thermal connection to the main bulk thermal stage, and a second of the normal cryocoolers having a thermal connection to the shield bulk thermal stage. This design is relatively simple. If desired, the thermal connection of the second of the normal cryocoolers to the shield bulk thermal stage may include a thermal switch.

Inventive Use of a Superconductor Magnet System

Further within the scope of the present invention is the use of a superconductor magnet system,the superconductor magnet system comprisinga main superconductor bulk magnet,a cryostat system having a room temperature bore,and a cooling system adapted for cooling the main superconductor bulk magnet,with the cryostat system containing the main superconductor bulk magnet, and with the main superconductor bulk magnet arranged coaxially with the room temperature bore,characterized inthat the superconductor magnet system further comprises a shield superconductor bulk magnet, with the cryostat system also containing the shield superconductor bulk magnet,with the shield superconductor bulk magnet radially surrounding the main superconductor bulk magnet,and with the shield superconductor bulk magnet also being arranged coaxially with the room temperature bore,that the cooling system is further adapted for cooling the shield superconductor bulk magnet, and that the shield superconductor bulk magnet carries a shield persistent currentIPshieldwhich has an opposite current direction as compared to a main persistent current IPmaincarried by the main superconductor bulk magnet. With the main persistent current and the shield persistent current having opposite current directions (“signs”), an active shielding of the stray field is achieved. This state of use can be achieved by applying an inventive charging method described above to the superconductor magnet system. Note that for typical superconductor magnet sizes, often0.08≤|IPshield/IPmain|≤0.33 or 0.1≤|IPshield/IPmain|≤0.25 applies.

In a preferred variant of the above described use,0.6≤|IPmain/IPshield|*|Rmain2/Rshield2|≤1.6,preferably 0.7≤|IPmain/IPshield|*|Rmain2/Rshield2|≤1.4,with Rmain: average radius of the main superconductor bulk magnet; andRshield: average radius of the shield superconductor bulk magnet. With these dimensions and persistent current values, a particularly good active shielding can be achieved. Preferably, |IPmain/IPshield|*|Rmain2/Rshield2|=1.
Inventive Charger System and Use

Within the scope of the present invention is further a charging system, comprisingan inventive superconductor magnet system as described above, anda charger magnet with a charger bore, for arranging the superconductor magnet system at least partially within the charger bore,in particular wherein the charger magnet comprises a low temperature superconductor magnet coil. With the charger system, the superconductor magnet system may be charged according to the inventive method described above. The low temperature superconductor magnet coil may be, in particular, of solenoid type and may, in particular, be wound from a NbTi or Nb3Sn superconductor wire. The low temperature superconductor magnet coil typically has a critical temperature below 30 K.

Within the scope of the present invention is further the use of an inventive charging system described above in an inventive method as described above.

Further advantages can be extracted from the description and the enclosed drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any combination. The embodiments mentioned are not to be understood as exhaustive enumeration but rather have exemplary character for the description of the invention.

