Abstract:
A cryo probe head for the transmission/reception of RF signals for NMR measurements with a heat exchanger ( 1 ) for cooling heat sources ( 5 ), the heat exchanger having a contact element ( 4.2 ) for thermal connection between a cryogenic fluid and the heat source, is characterized in that the heat exchanger comprises a container having an interior volume V B  into which a first cryogenic fluid F 1  that has a liquid component F 1L  and a gaseous component F 1G  flows through an inflow conduit ( 8 ) and from which a second cryogenic fluid F 2  that has liquid component F 2L  and a gaseous component F 2G  flows out through an outflow conduit ( 9 ). The inflow conduit has a flow cross-section Q Z  and a circumference U Z  from which a characteristic conduit volume V Z =4·Q 2   Z /U Z  results, wherein V B &gt;10·V Z , and the outflow conduit has a flow diameter Q A  wherein Q A ≧Q Z . The contact element is in close thermal contact with both the liquid volume component V L  of the cryogenic fluid and with the heat source. A device for setting the inflow quantity of the first cryogenic fluid F 1  into the container is provided that ensures a state F 1L /F 1G &gt;F 2L /F 2G  during operation. In this way, vibrations due to the cooling process can be largely reduced and the consumption of cryogenic fluid minimized.

Description:
[0001]    This application claims Paris convention priority to DE 10 2009 046 321.6 filled Nov. 3, 2009, the entire disclosure of which is hereby incorporated by reference 
       BACKGROUND OF THE INVENTION  
       [0002]    The invention relates to a cryo probe head for the transmission and/or reception of radio-frequency signals for nuclear magnetic resonance measurements, with at least one heat exchanger for cooling one or more heat sources, in particular, components of the cryo probe head, wherein a cryogenic fluid is supplied to the heat exchanger and the heat exchanger has at least one contact element that ensures a connection with good thermal conduction between the cryogenic fluid and the heat source. 
         [0003]    Such a configuration is known from DE 103 40 352 A1. 
         [0004]    In a configuration for measurement by means of nuclear magnetic resonance, a probe head is placed in the strong steady-state magnetic field of a typically superconducting magnet. The sample to be measured is introduced into this. The probe head contains radio-frequency coils and resonators that are used to excite nuclear spins and to receive the signals generated by the excited nuclear spins. Because the nuclear spin signals are generally very weak, there are easily prone to interference. For that reason, the signal-to-noise (S/N) ratio is a very important performance criterion in nuclear magnetic resonance. One common procedure for improving the S/N ratio is cooling the relevant components to the lowest possible temperature. This is done using cryo probe heads. 
         [0005]    To cool cryo probe heads to cryogenic temperatures, cryocoolers with closed gas circuits are generally used, preferably with helium as the fluid and based on Gifford-McMahon, Stirling, or Joule-Thomson processes. 
         [0006]    A further cooling method is evaporation of cryogenic fluids within the probe head. Liquid is run through tubes into the probe head, where the thermal load causes it to evaporate. 
         [0007]    One feature of any cooling mechanism is a fluid that provides the necessary cooling power for heat dissipation and a thermal contact system that permits the transfer of heat from the component to be cooled to the heat transfer medium=fluid. Such a device is generally termed a heat exchanger. 
         [0008]    Irrespective of the characteristics of a cooling system and its auxiliary equipment, an NMR detection system must be operated with as little disturbance as possible and with low operating and maintenance costs. 
         [0009]    This invention described below is for cooling components of an NMR detection system with the aim of providing cooling that is as simple and efficient as possible with minimum effort and minimum susceptibility to internal or external disturbances. 
         [0010]    Operation of cryocoolers requires a number of items of auxiliary equipment such as compressors, heat sinks, pumps, etc., which makes installation and operation correspondingly costly both in terms of maintenance effort and operating costs. Moreover, the use of rotating or linearly moving components in the equipment leads to transmission of mechanical vibrations into the probe head. Mechanical vibrations that are transmitted to the probe head can have a considerable adverse influence on the NMR signal. 
         [0011]    According to the usual prior art, a heat exchanger is supplied through a flow inlet with gaseous or liquid cooling fluid, which exits the heat exchanger through a flow outlet. By means of a contact element with good thermal conduction, a heat exchanger is connected to a heat source, wherein the latter is kept at a desired temperature by dissipating a certain heat load. The heat source can, for example, be an RF resonator or a signal amplifier. 
         [0012]    In available systems, the connections between the flow inlet and the flow outlet are constituted as cooling ducts that enable a fluid to flow through and heat to be transferred from a heat source to a cooling fluid in the thermal contact system through structures that may be routed in any way. The cooling conduits can extend helically around a thermal contact system or be spiral and embedded in the thermal contact system. Other embodiments of the cooling ducts are also possible. 
         [0013]    Both cooling by evaporation of a fluid cryogen and cooling with cryogenic gases in cryocoolers are used in standard commercial NMR detection systems, although considerable disadvantages are encountered in systems according to the state of the art. 
         [0014]    Cooling of cryo probe heads using thermodynamic circulatory processes implemented in cryocoolers is mentioned in DE 103 403 52 A1 and U.S. Pat. No. 5,508,613 A. 
         [0015]    Cooling by evaporation of liquid helium or nitrogen within a cryo probe head is described in EP 0 782 005 A1. 
         [0016]    The advantage of cooling by evaporation of a fluid is that, due to the phase transition from the liquid phase to the gas phase at a constant pressure, maximum heat is extracted from the environment and thus a gas-dependent minimum temperature is achieved. Moreover, the temperature of both phases of the fluid remains constant during the phase transition. Maximum cooling effect at minimum consumption of coolant is achieved when the proportion of liquid cryogen at the outlet of the cooling system is zero. 
         [0017]    In evaporation cooling, an extreme density change of the fluid can arise within the cooling system due to the phase transition, possibly causing the fluid to undergo a velocity change in the conduits, which can result in formation of pressure waves or thermoacoustic vibrations that are propagated through the system and can result in mechanical vibrations. Moreover, 2-phase flows can occur in cryogenic liquids that are closely associated with flow instabilities causing pressure fluctuations. 
         [0018]    Problems with the two-phase flow of a cryogenic fluid in a tube, the associated unstable states, and their effect on thermal transfer properties was discussed by Qi et al. in the  International Journal of Heat and Mass Transfer  (Vol. 50, Issue 25/26, Page 4999-5016). 
         [0019]    Depending on the quantity of heat transferred per unit time (=heat flow) and the heat flow per unit area (=heat flow density), changes occur in the boiling state of the liquid, e.g. the heat transfer changes to the state of nucleate boiling due to pure convection. In consequence thereof, periodic pressure fluctuations, wall temperature fluctuations and fluctuations of the transferable heat flow are observed. 
         [0020]    Nucleate boiling is not desired in the tubing system because the gas bubbles that arise have to be entrained along the entire tube and the pressure loss increases as the proportion of gas rises. On the other hand, nucleate boiling is desirable because a greater heat flow density can be transferred due to the increased transfer of material by the gas bubbles in the fluid being released from the walls of the conduit, which results in a greater cooling power. 
         [0021]    An NMR RF coil cooled with cryogenic liquid was described by Styles at al. in the  Journal of Magnetic Resonance  (Vol. 60, pages 397-404, 1984). In the configuration described therein, the RF coil is constituted as a tube through which cryogenic fluid flows. Within the coil, evaporation of the fluid and thus cooling of the RF coil is caused by the thermal power of the RF coil. However, such a configuration has considerable disadvantages. The formal design of the coil would be heavily restricted if it had to be constituted as tubes. During evaporation and the associated rapid density change, the flow of the gas phase is greatly accelerated. Because the design does not provide for phase separation, the density change affects the liquid phase and the entire fluid transportation in the form of thermoacoustic vibrations. The cryogenic fluid compressed through the coil under pressure causes vibrations, possibly resulting in susceptibility changes in the coil and mechanical vibrations. 
         [0022]    DE 40 13 111 A1 describes a system in which the coil is not in direct contact with the cryogenic fluid but is connected to it through a heat-conducting connection. However, the problem of vibrations persists, despite physical separation of the functions “cooling” and “RF reception”, since cryogenic fluid still has to be transported to the heat-conducting connection and the problem of vibrations in the fluid conduits remains. 
         [0023]    The use of liquid helium for coil cooling is explicitly stated in JP 2008 241 493 A and WO 03/023433 A1. In both sources, liquid helium is introduced into a heat exchanger through a tube from below. Disturbances in the fluid transportation are thus propagated, undamped into the heat exchanger. Because of the flow inlet from below, gas bubbles occurring in the inflow conduit must flow through the entire liquid bath on entering the heat exchanger, resulting in very unsettled bubbling of the liquid level and causing considerable mechanical vibration. A further weakness is the insufficiently settable or non-settable level of the liquid cryogen and the fluctuating cooling power. 
         [0024]    The necessity of thermal decoupling of multiple RF coils is discussed in DE 103 40 352 A1. In the case of the device described there, a separate heat exchanger, through which gaseous fluid flows, is used for each coil, which is incompatible with a compact design. Moreover, in the proposed serial connection gas cooling for NMR detectors the inflow into one heat exchanger is always warmer than the inflow in the preceding heat exchanger. To achieve a temperature at the same level, active closed-loop control has to be used. 
       SUMMARY OF THE INVENTION  
       [0025]    In contrast to the above, the object of this invention is to cool the NMR resonator and various components in an NMR detection device to cryogenic temperatures by means of evaporation of a cryogenic fluid, while reducing the influence of vibrations that can arise due to the cooling and that considerably disturb the NMR signal. Additionally, in particular, the consumption of the cryogenic fluid is to be kept as low as possible. 
         [0026]    This object is solved inventively and surprisingly simply and effectively in that the heat exchanger has a container acting as a buffer reservoir for the cryogenic fluid with an interior volume V B  into which a first cryogenic fluid F 1  that has a liquid component F 1L  and a gaseous component F 1G  flows through an inflow conduit and from which a second cryogenic fluid F 2  that has liquid component F 2L  and a gaseous component F 2G  flows out through an outflow conduit, the inflow conduit having a flow cross-section Q Z  and a circumference U Z  from which a hydraulic diameter D hyd, Z =4·Q Z /V Z  and, from this, a characteristic conduit volume V Z =Q Z ·D hyd, Z  results, wherein the container is constituted in relation to the inflow conduit in such a way that V B &gt;10·V Z , and the outflow conduit has a flow cross-section Q A  that is chosen such that Q A ≧Q Z , the contact element being in close thermal contact with both the liquid volume component V L  of the cryogenic fluid in the container and with the heat source, and a device for setting the inflow quantity of the first cryogenic fluid F 1  into the container is provided that enables a state to be reached and maintained during operation of the cryo probe head at which the following applies: F 1L /F 1G &gt;F 2L /F 2G . 
         [0027]    Due to the large interior volume V B  and the gaseous volume components V G  contained therein, any pressure waves in the inflow can be absorbed by compression of the gaseous volume component V G , in particular, because V B  is constituted much larger than the characteristic conduit volume V Z . In addition, the liquid volume component V L  in the container serves as a reserve if, for a short time, the inflow of coolant is insufficient for the quantity of heat to be dissipated. By stocking liquid volume V L , the temperature of the heat source can be kept constant. The generous dimensioning of the outflow conduit prevents back-pressure of the outflow out of the container and thus the occurrence of pressure waves in the outflow and mechanical vibrations. Because the contact element is in close thermal contact with the liquid volume component V L , it is ensured that the heat flow is almost completely transferred by boiling while any heat convection by the gaseous component V G  is negligible at the flow velocities of the cryogenic fluid that are typical in an inventive device. In this way, the position and the extent of the heat dissipation is precisely defined and constant over time. Unstable and non-steady flow states in the inflow conduit and the resulting thermoacoustic oscillations that can be expected in the transportation of cryogenic fluids are damped and attenuated in the inventive device and their adverse influence on the NMR signal is minimized. With closed-loop control of the inflow quantity, it is ensured that the state and therefore also the associated advantageous effects remain largely constant in the container. 
         [0028]    In an especially preferred embodiment of the inventive device, the container and the inflow conduit are constituted such that V B &gt;20·V Z , preferably 70·V Z ≦V B ≦150·V Z  applies. These values have proven useful in practice. 
         [0029]    In a further advantageous embodiment, a closed-loop control device is provided that controls the device for setting the inflow quantity of the first cryogenic fluid F 1  into the container and controls the volume component V L  of liquid cryogenic fluid in the container in relation to the volume component V G  of gaseous cryogenic fluid at a definable value. The ratio between V L  and V G  has a considerable influence on the stability of the cooling and the damping of undesirable vibrations. It is therefore a major advantage to be able to influence this ratio by means of a closed-loop control mechanism. 
         [0030]    It is advantageous if, in the embodiment described above, the closed-loop control device controls the inflow quantity of the first cryogenic fluid F 1  into the container depending, in particular, on the heat quantity dissipated from the heat sources through the contact element to the heat exchanger in such a way that V G &gt;V L , preferably V G ≧5·V L . These ratios are especially desirable in relation to the properties of the ratio of V G  to V L  stated above. 
         [0031]    Optimally, the closed-loop control device of the two embodiments stated above controls the inflow quantity of the first cryogenic fluid F 1  into the container such that F 2L ≈0. The consumption of cryogenic liquid is thus minimized in a simple manner. 
         [0032]    In an especially preferred variant of the last 3 embodiments described above, a temperature sensor is provided to measure the temperature of the heat source whose output signal is fed to the closed-loop control device as an input signal for closed-loop control of the inflow quantity of the first cryogenic fluid F 1  into the container. In this way, a constant temperature of the heat source can be simply achieved. 
         [0033]    Ideally, the above-mentioned embodiments of the inventive device have a level sensor for measurement of the current level of the volume component V L  of liquid cryogenic fluid in the container. Using the measured value from this level sensor simplifies closed-loop control of the ratio of V G  to V L . 
         [0034]    In an optimized variant of the last embodiment described above, the fluid inflow is increased by the device for setting the inflow quantity of the first cryogenic fluid F 1  into the container as soon as it falls below a settable level and this is signaled. In this way, the inflow quantity of the first cryogenic fluid F 1  does not have to be set and monitored manually. 
         [0035]    In a further variant of the penultimate embodiment stated above, the fluid inflow is blocked by the device for setting the inflow quantity of the first cryogenic fluid F 1  into the container when it reaches or exceeds a maximum liquid level and when it reaches or falls below a minimum liquid level, the fluid inflow is opened again. This avoids the liquid level having to be monitored and set manually and considerably simplifies operation of the inventive device. 
         [0036]    A preferred embodiment has closed-loop temperature control of the heat source by means of a closed-loop control device and a heater that is thermally well connected to the heat source. In this case, the controlled temperature is necessarily always higher than the temperature that is achieved without a heater. The temperature can thus be controlled at a constant value in a simple but effective way in response to an increase in the heat load at the heat source. 
         [0037]    In an especially preferred embodiment, the heat source is directly connected to the lower cover of the container belonging to the heat exchanger, wherein this lower cover performs the function of the contact element and is therefore preferably made of a material with the lowest possible thermal resistance and having a wall thickness that is as small as possible to ensure the best possible thermal connection between the heat source and the liquid fluid V L . This is especially advantageous because the material for a separate contact element is avoided. 
         [0038]    A further preferred embodiment has a heat source that is directly connected to the upper cover of the container belonging to the heat exchanger, wherein the upper and the side covers of this container perform the function of the contact element and therefore preferably consist of a material with the lowest possible thermal resistance and having a wall thickness that is as large as possible so that the best possible thermal connection between the heat source and the liquid fluid V L  is thereby ensured. As in the embodiment above, a separate contact element can also be avoided. 
         [0039]    In a further preferred embodiment of an inventive device, the heat source is directly connected to the upper cover of the container belonging to the heat exchanger and this container also accommodates, in its interior, a contact element with good thermal conduction that is permanently connected to the upper cover of the container and is immersed in the liquid fluid V L , wherein the contact element is preferably arranged directly below the heat source and the upper cover preferably has the smallest possible wall thickness to provide for the best possible thermal connection between the heat source and the liquid fluid V L . In this embodiment, the contact surface between the liquid component of the fluid V L  and the contact element is especially large, which advantageously reduces bubbling in the liquid phase. 
         [0040]    A further advantageous embodiment has a heat exchanger with a contact element that protrudes from the top of the heat exchanger, is attached to the upper cover of the heat exchanger and has a good thermal connection to the heat source, wherein the lower part of the contact element is immersed in the liquid fluid V L , so that the best possible thermal connection between the heat source and the liquid fluid V L  is thereby ensured. This variant can prove advantageous because the heat source has no direct contact with the container of the heat exchanger and heating of the latter can be minimized. 
         [0041]    In an alternative embodiment, the heat source is attached above the fluid level to the side wall of the container belonging to the heat exchanger, wherein this wall performs the function of the contact element and is therefore preferably made from a material with the lowest possible thermal resistance and with a wall thickness that is as large as possible. 
         [0042]    In a similar alternative embodiment, the heat source is attached below the fluid level to the side wall of the container belonging to the heat exchanger, wherein the wall performs the function of the contact element and is therefore preferably made of a material with the lowest possible thermal resistance and a wall thickness that is as small as possible. 
         [0043]    Optimally, the contact element is connected to a thermally insulating fastening element wherein this fastening element is constituted such that the upper part of the contact element is separated from the liquid fluid V L  and only the lower part is immersed in the fluid V L  so that, in this way, the temperature of the heat source is less influenced by changes of the level of V L . This embodiment minimizes the problem of temperature fluctuations of the heat source in a surprisingly simple manner. 
         [0044]    A variant of the last embodiment stated above has a heat exchanger with two contact elements. In this way, two heat sources can advantageously be cooled using one heat exchanger. 
         [0045]    An especially preferred embodiment of the inventive device has multiple, serially connected reservoirs, wherein the outflow of the i-th reservoir constitutes the inflow of the (i+1)-th reservoir. Moreover, at least the last reservoir in the series is equipped with temperature control/level control. A serial connection is especially advantageous because it enables multiple heat sources to be cooled without the need for a separate cooling device for each of them. 
         [0046]    Further advantages of the invention result from the description and the drawing. Moreover, according to the invention, the characteristics stated above and further below can be used singly or in any combination. The embodiments shown and described are not intended to be an exhaustive list but are examples used to explain the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING  
         [0047]    The figures show: 
           [0048]      FIG. 1  Method of functioning of the heat exchanger 
           [0049]      FIG. 2  Prior art: heat exchanger with helically wound cooling conduit 
           [0050]      FIG. 3  Prior art: heat exchanger with two parallel and spirally wound cooling conduits 
           [0051]      FIG. 4  Inventive heat exchanger in which the heat source is connected to the lower wall of the container with good thermal conduction 
           [0052]      FIG. 5  Inventive heat exchanger in which the heat source is connected to the upper wall of the container with good thermal conduction 
           [0053]      FIG. 6  Inventive heat exchanger as in  FIG. 5 , in which a subcomponent of the contact element is also present that is immersed in the liquid volume component 
           [0054]      FIG. 7  Inventive heat exchanger as in  FIG. 6 , in a 3D view and with partially removed cover 
           [0055]      FIG. 8  Inventive heat exchanger similar to  FIG. 6  but in which the heat source is directly connected to the contact element. The contact element is immersed in the liquid volume component 
           [0056]      FIG. 9  Inventive heat exchanger according to  FIG. 6  with a sensor that responds on reaching the maximum level of V L    
           [0057]      FIG. 10  Inventive heat exchanger according to  FIG. 6  with two sensors wherein one sensor responds on reaching the maximum level and one sensor responds on reaching the minimum level of V L . 
           [0058]      FIG. 11  Inventive heat exchanger according to  FIG. 6  with one sensor for detection of the liquid component F 2L  in the outflow conduit 
           [0059]      FIG. 12  Inventive heat exchanger with a temperature sensor and controllable heater, both thermally connected to the heat source. 
           [0060]      FIG. 13  Multiple inventive heat exchangers disposed side by side that are serially interconnected through conduits transporting fluid, wherein level sensors are only contained in the last heat exchanger 
           [0061]      FIG. 14  As  FIG. 13 , but with heat exchangers disposed one below the other 
           [0062]      FIG. 15  Heat exchanger with two contact elements that are attached to a thermally insulating element 
           [0063]      FIG. 16  Heat exchanger with a contact element that is attached to a thermally insulating element and this element is constituted such that the cooling of the contact element is performed as independently as possible from the level of the liquid volume component 
           [0064]      FIG. 17  As  FIG. 16 , but with two contact elements that are attached to two insulating elements 
           [0065]      FIG. 18  Cooling of an NMR resonator with a mounting support and a signal amplifier similar to  FIG. 14 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0066]    The inventive device is intended to cool in an NMR detection device the NMR resonator and various components to cryogenic temperatures by means of evaporation of a cryogenic fluid largely reduce the influence of vibration that can arise due to the cooling operation and interferes with the NMR signal, and, in particular, keep consumption of the cryogenic fluid as low as possible. 
         [0067]      FIG. 1  shows a heat exchanger according to the prior art with a heat exchanger  1  into which an inflow  2  of coolant flows and from which an outflow  3  of coolant exits. A heat source  5  is connected thermally conductively and permanently to a contact element  4  made of material with good thermal conduction that ensures heat transfer from the heat source to the heat exchanger. 
         [0068]    A further embodiment of the prior art is shown in  FIG. 2 . Here, the contact element  4  with a T shape is located in the interior of the heat exchanger  1  while the coolant is routed helically around the contact element  4 . 
         [0069]      FIG. 3  shows a different variant according to the prior art, wherein the coolant is routed spirally and horizontally embedded between the heat source  5  and the contact element  4 . 
         [0070]    The basic structure of the inventive cooling device is shown in  FIG. 4 . The heat exchanger  1  is implemented as a container having an interior space with a volume component of liquid fluid V L    6  and a volume component of gaseous fluid V G    7 , wherein the spaces of V L    6  and V G    7  are connected to an inflow conduit  8  and an outflow conduit  9 . 
         [0071]    The inflow F 1    2  of cryogenic fluid enters the heat exchanger  1  through an inflow conduit  8 . The inflow F 1    2  of the cryogenic fluid has a liquid component F 1L  and a gaseous component F 1G . 
         [0072]    If the heat source  5  is heated, the heat flow emanating from it is directly carried through the thin wall of the heat exchanger  1  to the volume component of liquid cryogen V L    6  in the interior of the heat exchanger  1 . Here, a certain quantity of the liquid component V L    6  evaporates to form the gaseous phase, thus reducing that liquid component and increasing the gaseous component V G    7 . 
         [0073]    The extent of gas bubble formation in the liquid volume component V L    6  can be determined according to the boiling curve of the cryogenic fluid by adapting the surface available for heat transfer to the heat flow to be transferred. By appropriately dimensioning the contact surface between thermal contact element  4  and the liquid component V L    6 , the boiling state in the steady-state condition and therefore the intensity of the gas bubble formation are defined. In this way, excessive nucleate boiling of the fluid in V L    6  and therefore the occurrence of vibrations can already be avoided in the design of an inventive cooling device. 
         [0074]    In the gas space V G    7  in the interior of the heat exchanger  1 , gases arising from evaporation from the liquid component V L    6  collect, together with the gaseous component F 1G  brought in by the inflow F 1    2 . In the interior of the heat exchanger  1 , there is a defined separation between the gaseous phase and the liquid phase at any moment in time. 
         [0075]    The outflow F 2    3  of the cryogenic fluid exits the heat exchanger  1  through the outflow conduit  9 . The outflow F 2    3  of the cryogenic fluid has a liquid component F 2L  and a gaseous component F 2G . The gaseous component F 2G  of the fluid exiting in F 2    3  is composed of the gaseous component F 1G  of the entering fluid plus the quantity evaporated from the liquid component V L    6  in the interior of the heat exchanger  1 . The outflow conduit  9  is dimensioned in relation to the inflow conduit  8  such that the gas quantity F 2G  can flow out without a pressure increase occurring in the interior of the heat exchanger. 
         [0076]    With respect to the quantity of liquid component F 1L  transported in the steady state, the following operating states are conceivable: 
         [0077]    i) exactly the same amount of liquid is transferred into the heat exchanger  1  as has to evaporate to cool the heat source  5 . 
         [0078]    ii) more liquid is transferred into the heat exchanger  1  than has to evaporate to cool the heat source  5   
         [0079]    iii) less liquid is transferred into the heat exchanger  1  than has to evaporate to cool the heat source  5   
         [0080]    Comments about the above three states: 
         [0081]    i) In an ideal ratio between the inflowing liquid component F 1L  and the evaporated liquid quantity, the gaseous component F 2G  in the outflow conduit F 2    9  is 100%, that is, no excess liquid flows out of the interior of the heat exchanger  1 . The volume components of liquid V L    6  and gas V G    7  in the interior of the heat exchanger  1  remain constant over time. 
         [0082]    ii) If more liquid F 1L  enters through the inflow F 1    2  than has to be evaporated by the heat flow from heat source  5  into the liquid volume component V L    6  in the interior of the heat exchanger  1 , the volume components of liquid V L    6  and gas V G    7  remain constant over time in the interior of the heat exchanger  1 , as in operating state i), because not only gas but also liquid flows out through the outflow opening  9 , serving as an overflow. In this case, the outflow F 2  flowing out through outflow conduit  9  contains not only the gaseous component F 2G  but also a liquid component F 2L . The liquid component F 2L  of the fluid exiting in F 2    3  is then composed of the liquid component F 1L  of the entering fluid minus the quantity evaporated in V L    6 . This operating state is conceivable if the heat flow is reduced or the quantity transported in the inflow F 1    2  is increased. 
         [0083]    iii) If less liquid F 1L  enters through the inflow F 1    2  than has to be evaporated by the heat flow from heat source  5  into the liquid volume component V L    6  in the interior of the heat exchanger  1 , outflow F 2    3  contains 100% gaseous component F 2G . The liquid component V L    6  inside the heat exchanger  1  is then reduced until no liquid component is present. As long as a liquid component V L    6  is present, the heat flow can still be dissipated. This operating state is conceivable if the heat flow is increased or the quantity transported in the inflow F 1    2  is reduced or interrupted. The liquid volume component V L    6  acts as a sort of buffer so that disturbances in the form of non-constant transported quantity are not passed on to the heat source in the fluid transfer. Moreover, thanks to this thermal buffering effect, temporary increases in the heat input are compensated for without a need for active closed-loop control. 
         [0084]    In addition to the liquid component, the gaseous volume component V G    7  in the heat exchanger  1  is also used as a buffer or damping element for vibrations. 
         [0085]    An approximately ideal thermal insulation is technically extremely costly and unsuitable for a system whose main purpose is economic viability and low operating costs. For that reason, heat input from the environment or from the conduit walls must be expected, which is also often described as thermal loss. For this reason, the gas phase occurs in the conduit for cryogenic liquids and therefore also formation of a 2-phase flow comprising gaseous and liquid components. The gaseous components may be present in the flow as dispersed bubbles but they may also occupy the entire cross-section of the outflow conduit  9  in some sections. The gas phase and liquid phase can move at different velocities and absorb heat from the walls to different degrees. As a consequence, the transferable heat flow varies along a conduit, which results in poorly defined local heat transfer, especially in cryogenic tube heat exchangers according to prior art. Unstable and non-steady flow states can occur in the inflow conduit  8  that are propagated through the conduit system as thermoacoustic oscillations until they are evaporated due to wall friction or due to suitable design measures. 
         [0086]    Quantitative analysis of such effects would be extremely costly in any case. It is therefore expedient to reduce any interfering consequences of these effects by providing design elements for this purpose. Such a design element is the gaseous volume component V G    7  in the interior of the heat exchanger  1  or the separation of the gaseous component V G    7  from the liquid component V L    6 . By positioning of the heat source and adaptation of the wall thickness, it is ensured that the heat dissipation from the heat source  5  is always performed through the liquid component V L    6 . In this way, the heat flow is always constantly transferred by boiling. Time-variable heat transfer by convection may also occur in the gaseous component V G    7  in the upper part of the heat exchanger  1  but is negligible in comparison to the heat transfer due to boiling in the liquid component V L    6  at the flow velocities that would prevail in a cooling device for coil resonators in NMR detection devices. Therefore, in the inventive cooling device, the position and the extent of the heat dissipation are precisely defined and constant over time because the evaporation location, the heat transfer contact surfaces, and the liquid volume component V L    6  are invariable over time. 
         [0087]    Pressure variations that enter the heat exchanger  1  from the conduits cause a minimum compression of the volume in the space, which is large compared to the incoming volume of gas. Further, by rapid venting to the atmosphere through the outflow conduit  9 , a still larger buffer is available if a pressure variation in the outflow conduit  9  persists. 
         [0088]    Influence of the environment on the state of the liquid component is limited solely to the ambient pressure whose variation does influence the evaporation temperature but negligibly compared with the temperature change in the RF pulses in the resonator  16   a.  To provide compensation for this temperature change, an embodiment of the inventive device is conceivable in which the pressure inside the heat exchanger  1  is kept constant by a closed-loop pressure control device. 
         [0089]    The heat transfer from the thermal contact element  4  to the liquid volume component V L    6  is considerably better than to the gaseous volume component V G    7 . It is important in this respect that the heat flow is mainly dissipated to the liquid volume component V L    6  and therefore special attention should be paid to the best possible transportation of the heat quantity from the heat source  5  to the liquid volume component V L    6 . 
         [0090]      FIG. 5  shows a preferred embodiment of the inventive cooling device. Here, the heat source  5  contacts the upper wall of the heat exchanger  1  with good thermal conduction. In this embodiment, the upper wall and the side walls of the heat exchanger  1  perform the function of a thermal contact element  4  so that a good thermal connection between the heat source  5  and the liquid volume component V L    6  in the interior of the heat exchanger  1  is ensured. 
         [0091]      FIG. 6  shows a further advantageous embodiment of the inventive cooling device in which the heat source  5  is connected with good thermal conduction to the upper wall of the container  1 . A subcomponent of the contact element  4 . 1  is connected to the outer side of the container  1  with good thermal conduction with the heat source  5 . A further subcomponent of the contact element  4 . 2  is in close thermal contact with the liquid volume component V L    6  in the interior of the container  1 . In this embodiment, the heat flow can be routed directly to the liquid volume component V L    6 , which results in low thermal resistance. The subcomponent  4 . 2  in the interior of the container  1  is disposed such that the gaseous volume component V G    7  is always connected to the inflow conduit  8  and the outflow conduit  9 , as is shown in the three-dimensional, partly cut-away view in  FIG. 7 . 
         [0092]      FIG. 8  shows an advantageous variant of the embodiment shown in  FIG. 6 . Here, the contact element  4  is no longer connected via the wall of the container  1  but directly with good thermal conduction to the heat source  5  and the liquid volume component V L    6 . This embodiment causes a further reduction of the thermal resistance between heat source  5  and the liquid volume component V L    6 . The wall of the container  1  is connected to the contact element  4  on the side and is now used only for mechanical fixture. The wall of the container no longer has any function with regard to the heat transfer between the heat source  5  and liquid volume component V L    6 . 
         [0093]    In an especially advantageous embodiment, the inventive cooling device has a closed-loop control device for adjusting the fluid inflow F 1    2 . The aim of such a measure is to minimize the fluid consumption by adjusting the inflow F 1    2  depending on the liquid volume component V L    6  in such a way that the residual quantity of liquid F 2F  present in the outflow F 2    3  is extremely small or equal to zero. 
         [0094]    One way of reducing the residual quantity of liquid F 2F  present in the outflow F 2    3  is to define a maximum tolerable level of the liquid volume component V L    6  in the container  1 , which is defined by the installation position of a sensor  10  (see  FIG. 9 ) and is below the level of the outflow opening  9 . 
         [0095]    To be able to additionally measure the minimum level, in a specially preferred embodiment ( FIG. 10 ), besides the sensor  10  for detecting a defined maximum tolerated level, a sensor  11  for detecting a defined minimum tolerated level in the heat exchanger  1  is also present. 
         [0096]    One alternative embodiment is shown in  FIG. 11 . This variant is based on the embodiment described in  FIG. 6  that is augmented by a sensor  12  in the outflow opening  9 . This sensor  12  detects whether a liquid component F 2L  is present in the outflow F 2    3 . If the liquid component F 2F  is greater than zero, the closed-loop control device reduces the inflow F 1    2 . 
         [0097]    A further possible embodiment is shown in  FIG. 12 . Here, a sensor  13  and a heater  14  are located at the heat source  5 . The sensor  13  measures the temperature at the heat source  5 . If the measured temperature deviates from a defined target value, the closed-loop control device compensates for the temperature change by increasing or reducing the heating power on the heater  14 . 
         [0098]    The closed-loop control device, the sensor  13 , and the heater  14  can be configured such that the same heat flow is always transferred in the heat exchanger  1  and the same quantity of liquid component V L    6  evaporates. This variant can be especially advantageously combined with the embodiments according to  FIG. 9 ,  FIG. 10 , and  FIG. 11 . 
         [0099]    In  FIG. 13 , multiple inventive heat exchangers are serially disposed side by side and each connected through fluid bearing conduits. The level of the liquid component V L    6  in each heat exchanger  1  of the serial configuration is at a maximum so that enough liquid component F 2L  is present in the outflow F 2    3  of each heat exchanger to ensure sufficient liquid component V L    6  in the following heat exchangers. The last heat exchanger in the flow direction of a serial configuration can have a closed-loop control device, for example, as in  FIG. 9 ,  FIG. 10 ,  FIG. 11  or a closed-loop control device derived from any of these, to minimize the fluid consumption. In the embodiment according to  FIG. 13 , for example, the last heat exchanger in the flow direction contains a sensor  10  for detection of a maximum tolerated level of V L    6  and a sensor  11  for detection of a minimum tolerated level of V L    6  based on the variant in  FIG. 10 . 
         [0100]    A further serial configuration is shown in  FIG. 14  in which multiple heat exchangers disposed one above the other are interconnected by conduits carrying fluid and are equipped with sensors  10  and  11  for detection of a maximum or minimum level of V L    6  in the last heat exchanger of the configuration. 
         [0101]    Any combinations of serial and parallel configurations are conceivable depending on the space available in an application. 
         [0102]    An especially advantageous embodiment is shown in  FIG. 15 . Here, two separate heat sources  5   a  and  5   b  are connected by separate contact elements  4   a  and  4   b  with good thermal conduction. Both are immersed in a heat exchanger  1  in the same liquid component V L    6 . The two contact elements  4   a  and  4   b  with good thermal conduction are connected by thermally insulating fastening elements  15   a  and  15   b  with the wall of the heat exchanger  1 . 
         [0103]      FIG. 16  shows an embodiment of the inventive device with only one thermally insulating fastening element  15 . The fastening element  15  is fastened to the wall of the heat exchanger  1  and is then brought into the interior space of the heat exchanger  1  as meander-shaped continuation of the heat exchanger wall and connected there with the contact elements  4   a  and  4   b.  The location of the contacting of the insulating fastening element  15  and contact element  4  is below the inflow conduit  8  and below the outflow conduit  9  so that the level of the liquid component V L    6  is always above the contacting location. In this configuration, the contact surface between the liquid component V L    6  and contact element  4  and the distance between heat source  5  and contact surface remain constant when the level of V L    6  changes. 
         [0104]      FIG. 17  shows a combination of the embodiments according to  FIG. 15  and  FIG. 16 . In this variant, two thermally decoupled heat sources  5   a  and  5   b  are connected with the same liquid component V L    6  through two separate contact elements  4   a  and  4   b  and fastened to the wall of the heat exchanger  1  using two separate thermally insulating fastening elements  15   a  and  15   b.  This variant permits simultaneous cooling of two heat sources  5   a  and  5   b  with simultaneous thermal decoupling and decoupling from the level of the liquid component V L    6 . 
         [0105]    The RF resonators  16   a  including the mounting support  16   b  or the RF resonators  16   a  together with the signal amplifier  17  are cooled in an NMR probe head ( FIG. 18 ). In this case, first the resonators  16   a  and then the signal amplifiers  17  can be supplied with fluid, or vice versa. 
         [0106]    RF resonators and signal amplifiers each act as a heat source. The thermal load due to RF pulse sequences is absorbed by a cryogenic fluid in which the liquid component evaporates. 
       LIST OF REFERENCE SYMBOLS  
       [0107]      1  Heat exchanger or container of the heat exchanger 
         [0108]      1   a  Heat exchanger for cooling the NMR resonator 
         [0109]      1   b  Heat exchanger for cooling the NMR signal amplifier 
         [0110]      2  Inflow F 1  of the cryogenic fluid with the components F 1L  and F 1G , wherein F 1L  means the liquid and F 1G  means the gaseous component 
         [0111]      3  Outflow F 2  of the cryogenic fluid with the components F 2L  and F 2G , wherein F 2L  means the liquid and F 2G  means the gaseous component 
         [0112]      4 ,  4   a,    4   b  Contact element with good thermal conduction in various embodiments 
         [0113]      4 . 1  Subcomponent of the contact element 
         [0114]      4 . 2  A further subcomponent of the contact element 
         [0115]      5 ,  5   a,    5   b  The heat source to be cooled 
         [0116]      6  Liquid volume component V L  of the cryogenic fluid in the container 
         [0117]      7  Gaseous volume component V G  of the cryogenic fluid in the container 
         [0118]      8  Inflow conduit for F 1    
         [0119]      9  Outflow conduit for F 2    
         [0120]      10  Sensor for detecting the maximum desired level of the liquid volume component V L    
         [0121]      11  Sensor for detecting the minimum desired level of the liquid volume component V L    
         [0122]      12  Sensor for detecting the liquid component F 2L  in the outflow conduit  9   
         [0123]      13  Temperature sensor on the heat source  5   
         [0124]      14  Heater for closed-loop control of the temperature of the heat source  5   
         [0125]      15 ,  15   a,    15   b  Thermally insulating fastening element for the contact element 
         [0126]      16   a  RF resonator 
         [0127]      16   b  Mounting support of the RF resonator 
         [0128]      17  Signal amplifier