Method and device for controlling the temperature of a flow of fluid intended to be used by an NMR analysis probe, and corresponding NMR analysis system

Abstract

The method comprises an initial cooling step (E0) during which the temperature (T_ECH) of the heat exchanger (30) is lowered and, at the same time, the flow rate of the said first flow (FL.sub.S) is set to an initial flow rate. Following the initial cooling step (E0), an operation of circulating the first flow (FL.sub.S) is initiated (E1) and during this the said first flow (FL.sub.S) passes through a heat exchange (30) at a first flow rate which is higher than the initial flow rate. The initial flow rate of the first flow may be set to zero or to a value set so that the pressure inside the said circuit is greater than or equal to the pressure outside the said circuit.

Claims

1. A method for controlling the temperature of a first flow of fluid (FL.sub.S) intended to be used by a nuclear magnetic resonance analysis probe to spin a sample holder of the probe at a given rotational frequency dependent on a flow rate of the flow, comprising: circulating the first flow, during which circulation the first flow (FL.sub.S) passes through a heat exchanger at a first flow rate, and in an initial cooling, lowering a temperature (T_ECH) of the heat exchanger and, at the same time, setting the flow rate of the first flow (FL.sub.S) to an initial flow rate which is lower than the first flow rate, wherein the circulating of the first flow (FL.sub.S) at the first flow rate is initiated following the initial cooling.

2. The method according to claim 1, wherein the initial flow rate of the first flow is set to zero.

3. The method according to claim 1, wherein, while the first flow (FL.sub.S) circulates through a circuit, the initial flow rate of the first flow (FL.sub.S) is set to a value so that a pressure inside the circuit is greater than or equal to a pressure outside the circuit.

4. The method according to claim 1, wherein, during the initial cooling, the temperature of the heat exchanger is lowered to a low value which is lower than an equilibrium value Tfh_ECH reached after the intended first flow circulation at the first flow rate.

5. The method according to claim 1, wherein, during the initial cooling, a second flow (FL.sub.C) of a coolant is circulated through the heat exchanger at a flow rate (D.sub.max.sub._FL.sub.C) which is a maximum flow rate of the cooling flow (FL.sub.C).

6. The method according to claim 5, wherein, during the initial cooling, the temperature (T_ECH) of the exchanger is lowered to a low value (T.sub.min.sub._FL.sub.S) which corresponds to a minimum equilibrium temperature of the heat exchanger through which the second cooling flow (FL.sub.C) passes.

7. The method according to claim 1, wherein, following triggering of the circulating of the first flow, while the first flow rate (D_FL.sub.S) of the first flow is set to a desired value, an evolution of the temperature (T_FL.sub.S) of the first flow (FL.sub.S) with respect to time comprises in succession (i) a first phase (.sub.0) of lowering the temperature, (ii) a transient second phase (.sub.1) comprising raising the temperature to an equilibrium value, and (iii) an equilibrium third phase (.sub.2) in which the temperature is stable and equal to the equilibrium value, the transient phase being of a duration that is set so as to allow the probe to create an NMR spectrum of a sample.

8. The method according to claim 7, wherein the first flow (FL.sub.S), circulating at the first flow rate, passes through a heating module, and wherein the heating power of the module is raised to a high value then decreased so that the temperature (T_FL.sub.S) of the first flow (FL.sub.S) is temporarily stable for part of the transient phase (.sub.1).

9. The method according to claim 8, wherein the circulating first flow (FL.sub.S) passes through the heating module downstream of the heat exchanger in the direction in which the flow (FL.sub.S) circulates.

10. The method according to claim 8, wherein the heating module comprises an electrical resistance.

11. The method according to claim 10, wherein the first flow rate of the first flow is set to a value corresponding to a frequency greater than or equal to a limit rotational frequency.

12. The method of operating a nuclear magnetic resonance analysis system comprising a probe for nuclear magnetic resonance analysis of a sample, a circuit for circulating a first flow of fluid (FL.sub.S) intended to be used by the probe, and a device for controlling the temperature of the first flow (FL.sub.S) comprising a heat exchanger, said method comprising: controlling a temperature of the first flow (FL.sub.S) by the control device implementing the control method according to claim 1.

13. The method according to claim 12, wherein, following triggering of the circulating of the first flow, while the first flow rate (D_FL.sub.S) of the first flow is set to a desired value, an evolution of the temperature (T_FL.sub.S) of the first flow (FL.sub.S) with respect to time comprises in succession (i) a first phase (.sub.0) of lowering the temperature, (ii) a transient second phase (.sub.1) comprising raising the temperature to an equilibrium value, and (iii) an equilibrium third phase (.sub.2) in which the temperature is stable and equal to the equilibrium value, the transient phase being of a duration that is set so as to allow the probe to create an NMR spectrum of a sample, and wherein the probe creates an NMR spectrum of a sample during the transient phase (.sub.1).

14. The method according to claim 12, wherein, following triggering of the circulating of the first flow, while the first flow rate (D_FL.sub.S) of the first flow is set to a desired value, an evolution of the temperature (T_FL.sub.S) of the first flow (FL.sub.S) with respect to time comprises in succession (i) a first phase (.sub.0) of lowering the temperature, (ii) a transient second phase (.sub.1) comprising raising the temperature to an equilibrium value, and (iii) an equilibrium third phase (.sub.2) in which the temperature is stable and equal to the equilibrium value, the transient phase being of a duration that is set so as to allow the probe to create an NMR spectrum of a sample, wherein the first flow (FL.sub.S), circulating at the first flow rate, passes through a heating module, and wherein the heating power of the module is raised to a high value then decreased so that the temperature (T_FL.sub.S) of the first flow (FL.sub.S) is temporarily stable for part of the transient phase (.sub.1) and wherein the probe creates an NMR spectrum of a sample when the temperature (T_FL.sub.S) of the first flow (FL.sub.S) is temporarily stable during part of the transient phase (.sub.1).

15. A device for controlling a temperature of a first flow of a fluid intended to be used by a nuclear magnetic resonance analysis probe to spin a sample holder of the probe at a given rotational frequency dependent on a flow rate of the flow, the device comprising: at least one heat exchanger through which the first flow (FL.sub.S) is intended to circulate at a first flow rate, and a command module designed to command an initial drop in a temperature (T_ECH) of the heat exchanger and at the same time set the flow rate of the first flow to an initial flow rate lower than the first flow rate, then initiate circulation of the first flow (FL.sub.S) through the exchanger at the first flow rate.

16. The device according to claim 15, wherein the command module is designed to set the initial flow rate to zero.

17. The device according to claim 15, wherein, while the first flow (FL.sub.S) is circulating through a circuit, the command module is designed to set the initial flow rate of the said first flow (FL.sub.S) to a value that is set so that the pressure inside the circuit is greater than or equal to the pressure outside the circuit.

18. The device according to claim 15, wherein the heat exchanger is designed to have passing through it a second flow (FL.sub.C) of a coolant, and wherein the command module is designed to initially command the cooling second flow (FL.sub.C) to circulate at a flow rate which is a maximum flow rate (D.sub.nax.sub._FL.sub.C) of the cooling flow (FL.sub.C), so as to obtain the initial drop in temperature of the exchanger.

19. The device according to claim 18, wherein the command module is designed to command an initial drop in the temperature of the exchanger down to a low value (T.sub.min.sub._ECH) which corresponds to a minimum equilibrium temperature of the heat exchanger through which the second flow (FL.sub.C) passes, before initiating the circulation of the first flow (FL.sub.C) at the first flow rate.

20. The device according to claim 15, comprising a heating module supplying a thermal power to the first flow (FL.sub.S) and wherein the command module is designed to operate the heating module so that the thermal power provided is at a maximum after the initiation of the circulation of the first flow (FL.sub.S) at the first flow rate and then decreases in a suitable way so that the temperature (T_FL.sub.S) of the first flow (FL.sub.S) is temporarily stable.

21. The device according to claim 20, wherein the heating module is positioned downstream of the heat exchanger in the direction of circulation of the first flow (FL.sub.S).

22. The device according to claim 20, wherein the heating module comprises an electrical resistance.

23. A nuclear magnetic resonance analysis system, comprising: a probe for nuclear magnetic resonance analysis of a sample, a circuit for the circulation of a first flow of fluid FL.sub.S) intended to be used by the probe to spin a sample holder of the probe at a given rotational frequency dependent on a flow rate of the flow, and a device for controlling the temperature of the first flow according to claim 15.

24. The system according to claim 23, comprising a command module designed to command the creation of a spectrum by the probe after the circulation of the first flow at the first flow rate has been initiated, during a transient phase (.sub.1) subsequent to an initial drop in the temperature (T_FL.sub.S) of the first flow (FL.sub.S) to a low value (T.sub.min.sub._FL.sub.S; T.sub.min.sub._FL.sub.S) and prior to a phase (.sub.2) of equilibrium of the temperature (T_FL.sub.S) of the flow intended for the probe at a value higher than the low value.

25. The system according to claim 24, wherein the device for controlling the temperature of the first flow comprises a heating module supplying a thermal power to the first flow (FL.sub.S), wherein the command module is designed to operate the heating module so that the thermal power provided is at a maximum after the initiation of the circulation of the first flow (FL.sub.S) at the first flow rate and then decreases in a suitable way so that the temperature (T_FL.sub.S) of the first flow (FL.sub.S) is temporarily stable, and wherein the command module is designed to command the creation of a spectrum by the probe when the temperature of the first flow is temporarily stable (T.sub.min.sub._FL.sub.S) during the transient phase (.sub.1).

Description

(1) The invention will be better understood from the following description of a method for controlling the temperature of a flow of fluid intended to be used by an NMR analysis probe, referred to as a first flow, and of a method of operating an NMR analysis system, according to various embodiments of the invention, and of a corresponding control device and a corresponding NMR analysis system, with reference to the attached drawings in which:

(2) FIG. 1, by way of illustrative example, depicts the evolutions with respect to time of various physical parameters, during operation of an NMR analysis system of the prior art

(3) FIG. 2 depicts the evolutions with respect to time of various physical parameters (flow rates, temperatures) when implementing the method for controlling the temperature of the first flow intended for the NMR analysis probe during operation of the NMR analysis system according to a first particular embodiment of the invention;

(4) FIG. 3 depicts the evolutions with respect to time of various physical parameters (flow rates, temperatures) when implementing the method for controlling the temperature of the first flow intended for the NMR analysis probe during operation of the NMR analysis system according to a second particular embodiment of the invention;

(5) FIG. 4 depicts an NMR analysis system according to one particular embodiment designed to operate in accordance with FIG. 3;

(6) FIG. 5A depicts a flow diagram of the steps of the method of operation of the NMR analysis system incorporating a method for controlling the temperature of the first flow corresponding to FIG. 2;

(7) FIG. 5B depicts a flow diagram of the steps of the method of operation of the NMR analysis system incorporating a method for controlling the temperature of the first flow corresponding to FIG. 3.

(8) For the sake of clarity, in the various figures, elements in the various embodiments, alternative forms and/or exemplary embodiments of the invention which are analogous or correspond to one another bear the same references, unless specifically indicated to the contrary.

(9) FIG. 4 depicts a nuclear magnetic resonance (NMR) analysis system 1. According to a first embodiment of the invention, the system comprises: a nuclear magnetic resonance analysis probe 2; a device 3 for controlling the temperature of a flow of fluid intended to be used by the probe, referred to as first flow; a circuit 4 for circulating the first flow intended to be used by the probe; a command module (not depicted).

(10) The NMR analysis probe 2, depicted schematically in FIG. 4, comprises a sample holder 20 intended to contain a sample that is to be analysed. The sample holder 20 is incorporated into a rotor (not depicted) equipped for example with a turbine for spinning it. The rotor is placed inside a stator 21. The latter is itself situated, in a known way, at the magnetic centre of the complete NMR device.

(11) The circulation circuit 4 is intended to cause a flow of a fluid, intended to be used by the probe 2, to circulate in a closed loop.

(12) The fluid is, for example, helium or nitrogen. It comes from a source such as a standard container, for example a pressurized fluid cylinder. It is intended to be circulated through the circuit 4 in the form of a flow FL.sub.S intended to be used by the probe 2. This flow may have various functions within the probe 2. In the exemplary embodiment described here, it has the function of spinning the sample holder 20 and of raising/lowering the sample to a desired temperature for conducting an NMR analysis. This temperature is, for example, comprised between 8K and 300K. The sample holder 20 spun by the flow FL.sub.S has a rotational frequency that is dependent on the flow rate of the flow FL.sub.S. The flow FL.sub.S can be divided in the probe in order to produce distinct flows respectively intended to perform the various functions.

(13) The circulation circuit is designed so that the flow FL.sub.S passes through the control device 3 and the probe 2. It comprises a compression module 5 comprising at least one compressor of the compressor-pump type. This compression module 5 is arranged in the pressurized closed loop so as to circulate the flow FL.sub.S through this loop. In the exemplary embodiment described here, it is arranged between a flow outlet of the NMR analysis device and a flow inlet of the control device 3. However, it could be arranged in some different position along the circuit 4. It could notably be placed inside the cryostat 3.

(14) The control device 3 is intended to control (namely set or regulate) the temperature of the first flow FL.sub.S so as to set the temperature of this first flow FL.sub.S. In this instance it is a cryostat. In a first particular embodiment of the invention, the device 3 incorporates a heat exchanger 30. In this exemplary embodiment, the heat exchanger 30 is arranged in such a way as to perform an exchange of heat between a cooling flow, referred to as second flow and the first flow FL.sub.S intended for the probe 2. The cooling second flow, also referred to as cryogenic flow and denoted FL.sub.C, comes from an external cryogenic fluid cold source (not depicted). In operation, it passes through the heat exchanger 30 at a given flow rate D_FL.sub.C equal to or below a maximum flow rate D.sub.max.sub._FL.sub.C. The flows FL.sub.C and FL.sub.S are intended in this instance to pass through the heat exchanger 30 in opposite directions.

(15) In another exemplary embodiment, the heat exchanger 30 is cooled by a cold-producing machine interposed between the downstream side of the exchanger and the upstream side of the probe (downstream and upstream referring to the direction in which the first flow FL.sub.S circulates). In such a case, there is no second flow FL.sub.C.

(16) The command module comprises a microprocessor and software elements designed to command the operation of the various elements of the NMR analysis system 1, notably the thermal control device 3. As an alternative, two command modules may be provided, one intended to command the operation of the probe and the other intended to command the circulation of the fluids FL.sub.C and FL.sub.S and the operation of the thermal control device. The command module or modules are designed to command the execution of the steps of the method of controlling the temperature of the first flow and the steps of the method of operating the NMR analysis system 1, which steps will be described hereinafter.

(17) The method of operating the NMR analysis system 1 according to a first particular embodiment of the invention will now be described with reference to FIG. 5. This method of operation involves implementing a method of controlling the temperature of the first flow that is intended to be used by the probe FL.sub.S, so as to spin a sample holder at a rotational frequency dependent on the flow rate of the flow FL.sub.S and to cool the sample, according to a first embodiment, as will become apparent from the description that follows.

(18) With reference to FIGS. 2 and 5A, the method of operating the NMR analysis system 1 comprises an initial step E0 of lowering the temperature T_ECH of the heat exchanger.

(19) The temperature of the exchanger is measured by one or more probes placed on or in the body of the exchanger 30. There is no need to measure the absolute value of the temperature T_ECH. All that is required here is to measure a variation in this temperature T_ECH so as to monitor how it evolves with respect to time. It is for example possible to make a spot measurement of the temperature and/or to determine a mean temperature from a number of measurement points.

(20) The step E0 consists in circulating the cryogenic flow FL.sub.C through the exchanger 30 but in this instance without circulating the first flow FL.sub.S intended for the probe. In other words, the initial flow rate of the first flow is equal to zero: D_FL.sub.S=0. Initially, at an instant t.sub.0, the flow rate of the cryogenic flow, or second flow, denoted D_FL.sub.C, is zero. During the step E0, from this instant t.sub.0 on, the circulation of the cryogenic fluid FL.sub.C is initiated and its flow rate D_FL.sub.C passes rapidly from zero to a desired value, for example equal to the maximum flow rate of the fluid FL.sub.C (namely the highest flow rate at which this fluid can circulate through the exchanger 30), denoted D.sub.max.sub._FL.sub.C. During the step E0, the exchanger 30 therefore operates with the cryogenic second fluid FL.sub.C circulating, but without the circulation of the first flow FL.sub.S. This results in a drop in the temperature of the exchanger T_ECH until it reaches a desired low value. This value may correspond to a stable (or equilibrium) minimum temperature T.sub.min.sub._ECH of the heat exchanger 30 through which only the cryogenic flow FL.sub.C passes. However, it is possible for the temperature of the exchanger 30 not to be decreased down to the minimum equilibrium temperature T.sub.min.sub._ECH. The temperature of the exchanger 30 could, for example, be lowered to a low temperature chosen according to an end-application. In any event, during the initial cooling step the temperature of the heat exchanger 30 is advantageously lowered to a low value which is below an equilibrium value T.sub.fh.sub._ECH that will be reached later after the first flow FL.sub.S has circulated with an operational flow rate or first flow rate.

(21) After the temperature of the exchanger 30 has reached the desired low value (here T.sub.min.sub._ECH), the initial step E0 may be continued for a short length of time in order temporarily to maintain the low temperature (in this instance T.sub.min.sub._ECH) of the exchanger 30 while waiting to initiate the next step E1.

(22) In one particular alternative form of embodiment, the initial step E0 of cooling the exchanger 30 is performed with the first flow FL.sub.S circulating but at a low flow rate D_FL.sub.S. For example, the flow rate D_FL.sub.S is set to a low value allowing the circuit 4 to be placed at a slight overpressure with respect to the outside thus avoiding the ingress of external contaminants.

(23) Because the heat exchanger 30 has a certain mass, of the order of several tens of kilograms, this initial, or prior, cooling step E0 allows the exchanger 30 to be charged with frigories. At the end of the step E0, the exchanger 30 constitutes a reservoir of thermal energy, of the cold reservoir type. Because the exchanger 30 has a certain thermal inertia, this reservoir can be used subsequent to step E0 to accentuate the cooling of the first flow intended for the probe FL.sub.S, as will become apparent in the description that follows.

(24) At the end of the step E0, at an instant t.sub.1, the command module initiates a circulation of the first flow of fluid FL.sub.S intended to be used by the probe 2, during a step E1.

(25) During a step E2, the flow rate of the first flow intended for the probe FL.sub.S, zero prior to the instant t.sub.1, is rapidly increased and set to a fixed first flow rate strictly higher than zero and constituting an operational flow rate. This set first flow rate denoted D.sub.fh.sub._FL.sub.S, here is set to cause the sample holder 20 to spin at a high rotational frequency f.sub.h, for example in excess of 10 Hz, for example again comprised between 10 and 20 kHz. The first flow rate of the first flow FL.sub.S is thus set to a value corresponding to a frequency higher than or equal to a limit rotational frequency (this limit frequency being for example equal to 10 kHz). By definition, the limit rotational frequency is the rotational frequency reached when the exchanger 30 is temperature stabilized for this rotational frequency without implementation of the invention (and therefore without the initial step E0). The high flow rate D.sub.fh.sub._FL.sub.S therefore corresponds to a high rotational frequency f.sub.h of the probe 2.

(26) Circulation of the first flow FL.sub.S in the circuit 4 is then maintained, at a stable flow rate equal to D.sub.fh.sub._FL.sub.S, during a step E3.

(27) Let us note that, following the step E0 and simultaneously with steps E1 et seq, the circulation of the second flow FL.sub.S through the exchanger 30 at a flow rate here equal to D.sub.max.sub._FL.sub.C is maintained, as represented by step E4 in FIG. 5A.

(28) As circulation of the first flow FL.sub.S at the set flow rate, here equal to D.sub.fh.sub._FL.sub.S, is established by executing steps E0 to E4, the temperature of the flow FL.sub.S is controlled, or regulated, by the control device 3, notably by passing through the heat exchanger 30. This results in an evolution with respect to time of the temperature of the first flow FL.sub.S which comprises three successive phases .sub.0, .sub.1 and .sub.2, described hereinafter.

(29) The first phase, or initial phase .sub.0, is a cooling phase during which the temperature of the first flow FL.sub.S drops progressively to a low value as depicted in FIG. 2. During this initial phase .sub.0, the cooling of the first flow FL.sub.S passing through the exchanger 30 is brought about by two types of heat transfer comprising: in the conventional way, an exchange of heat between the first flow FL.sub.S and the second flow FL.sub.C, the two flows circulating through the exchanger 30 (in this instance in opposite directions), and according to the invention, a transfer of heat between the heat exchanger 30, previously cooled and acting as a cold reservoir, and the flow FL.sub.S.

(30) Thanks to the thermal inertia of the previously-cooled exchanger 30, the first flow FL.sub.S intended for the probe drops to a temperature far lower than it would have reached in conventional operation of the exchanger 3 consisting in initiating circulation of each of the first and second fluids FL.sub.S and FL.sub.C substantially at the same time, without the preliminary step E0 of cooling the heat exchanger 30. In the example described here, during the initial phase of cooling the flow FL.sub.S, the heat exchanger 30 supplies a thermal power of the order of 2 kW.

(31) The cooling phase .sub.0 finishes when the thermal energy accumulated in the mass of the heat exchanger 30 is spent. It is followed by a second phase referred to as a transient phase .sub.1 which involves a raising of the temperature of the first flow FL.sub.S to an equilibrium value denoted T.sub.fh.sub._FL.sub.S. In parallel, the temperature of the exchanger T_ECH also rises during the transient phase .sub.1, to an equilibrium value denoted T.sub.fh.sub._ECH.

(32) The transient phase .sub.1 is followed by a third phase referred to as an equilibrium phase .sub.2, during which the temperature of the first flow FL.sub.S remains stable and equal to the value T.sub.fh.sub._FL.sub.S. In parallel, the temperature of the exchanger T_ECH also stabilizes at the equilibrium value T.sub.fh.sub._ECH. The equilibrium phase .sub.2 is thus characterized by a first temperature plateau for the flow FL.sub.S at the value T.sub.fh.sub._FL.sub.S and by a second temperature plateau of the exchanger at the value T.sub.fh.sub._ECH. These two temperature plateaus correspond to thermal equilibrium between the first and second flows FL.sub.S and FL.sub.C circulating at respective flow rates D.sub.max.sub._FL.sub.C and D.sub.fh.sub._FL.sub.S through the heat exchanger 30. Remember that the flow rate D.sub.fh.sub._FL.sub.S corresponds to a high rotational frequency of the sample holder 20 of the probe 2.

(33) During the transient phase .sub.1, the temperature of the first flow FL.sub.S passes through a range of temperatures, here comprised between T.sub.min.sub._FL.sub.S and T.sub.fh.sub._FL.sub.S. This range of temperatures is situated below the equilibrium temperature T.sub.fh.sub._FL.sub.S which corresponds to a high rotational frequency f.sub.h of the probe even though the flow rate of the first flow FL.sub.S is already high (here equal to D.sub.fh.sub._FL.sub.S) and therefore causes the sample holder 20 of the probe 2 to spin at the high rotational frequency f.sub.h.

(34) The transient phase .sub.1 is of a duration that is set in such a way as to allow the probe 2 to create an NMR spectrum of a sample.

(35) According to the first embodiment of the invention, the probe 2 creates an NMR spectrum of a sample during this transient phase .sub.1, during an NMR analysis step E5. Indeed the NMR measurement may be able to cope with a variation for example of a few degrees K per minute (for example 5K/min).

(36) The invention has another advantage: it allows probe temperature fluctuations to be decreased (both in amplitude and in frequency) thereby stabilizing the speed at which the sample holder spins because the thermal inertia (or specific heat) of the material of which the heat exchanger is made is far higher than the thermal inertial (or specific heat) of the gas. For example: at 20K, the specific heat of the exchanger material is of the order of twice that of He (helium); at 40K, the specific heat of the exchanger material is of the order of 10 times that of He; at 60K, the specific heat of the exchanger material is of the order of 25 times that of He.

(37) The steps E0 to E5 are executed at the command of the command module(s).

(38) Note that the temperature of the first flow FL.sub.S intended to be used by the probe 2 is controlled or regulated by a control method which comprises steps E0 to E4 and phases .sub.0 to .sub.2. This method for controlling or regulating the temperature of the first flow FL.sub.S forms part of the method for operating the NMR analysis system. The method for operating the probe 2 according to the first embodiment comprises steps E0 to E5 and phases .sub.0, .sub.1, .sub.2.

(39) A second embodiment of the method for controlling or regulating the temperature of the first flow FL.sub.S intended to be used by the NMR probe 2 and the method for operating the NMR analysis system 1 will now be described with reference to FIGS. 3 and 5B. For the sake of clarity, only elements of this second embodiment which differ from the first embodiment will be described in detail.

(40) To implement this second embodiment, the control device 3 comprises, in addition to the heat exchanger 30, a heating module 31 depicted in dotted line in FIG. 4. The device 3 here is a cryostat. The heating module 31 comprises, for example, an electrical resistance. The circuit for circulating the first flow FL.sub.S passes not only through the heat exchanger 30 but also through the heating module 31. In the example described here, the heating module 31 is placed downstream of the heat exchanger 30 in the direction in which the first flow FL.sub.S circulates. It is operated by the command module. The thermal heating power supplied by the heating module 31 can be set and adjusted so as to contribute to setting the temperature of the first flow FL.sub.S, as will become apparent in the following description of the method for controlling the temperature of the flow FL.sub.S which follows.

(41) With reference to FIG. 5B, the control method comprises an initial step E0 of cooling the heat exchanger 30, here without the circulation of the first flow FL.sub.S. In other words, the initial flow rate of the first flow FL.sub.S is equal to zero: D_FL.sub.S=0. During the step E0, the second flow FL.sub.S circulates at a flow rate here at the maximum flow rate D.sub.max.sub._FL.sub.C. The initial step E0 is followed by a step E1 of initiating circulation of the first flow FL.sub.S then by a step E2 of setting the flow rate of the flow D_FL.sub.S to a first flow rate strictly higher than zero. This first flow rate, denoted D.sub.fh.sub._FL.sub.S, here corresponds to a high rotational frequency f.sub.h of the probe 2. Next, the method comprises a step E3 of circulating the flow FL.sub.S at the fixed first flow rate D.sub.fh.sub._FL.sub.S.

(42) According to the second embodiment of the invention, the first flow FL.sub.S circulating at the first flow rate passes through the heating module 31, in this instance after having passed through the heat exchanger 30. To start with (namely from the initial instant t.sub.0 onwards), the heating module 31 here is switched off, the thermal heating power supplied being zero. During a step E6 (FIG. 5B), the thermal power of the thermal heating module 31, denoted P_res, is increased sharply until it reaches a high power level. In other words, the heating power of the module 31 is raised to a high value. This high level of power is in this instance kept stable for a short period of time T1, during a step E7, and thus forms a power plateau as depicted in FIG. 3. Here it corresponds to the maximum thermal power P.sub.max.sub._res that the thermal resistance can supply. This thermal power P.sub.max.sub._res here is of the order of a few hundred watts. Step E6 here is initiated slightly after the circulation of the first flow FL.sub.S is initiated during step E2.

(43) As an alternative, step E6 could be initiated simultaneously with step E2 or substantially before the latter. It is also possible to conceive of rising the thermal heating power P_res to the desired plateau value, in this instance P.sub.max.sub._res, at any time prior to the step E1 of triggering the circulation of the flow FL.sub.S.

(44) Steps E6 and E7 are advantageously executed in such a way that the thermal power supplied by the heating module 31 is at a maximum substantially at the start of or shortly after the start of the step E3 of circulating the flow FL.sub.S at the set fixed flow rate (here D.sub.fh.sub._FL.sub.S).

(45) Step E7 is followed by a step E8 of lowering the thermal heating power supplied by the heating module 31. During this step E8, the heating module 31 (in this instance the electrical resistance) is operated in such a way as to reduce the thermal heating power P_res in order to lower it from P.sub.max.sub._res to zero.

(46) Thus, the first flow FL.sub.S circulating at the first flow rate is first of all cooled by passing through the heat exchanger 30 down to a very low temperature, in this instance T.sub.min.sub._FL.sub.S, then heated up slightly as it passes through the heating module 31, the power of which has been raised to P.sub.max.sub._res. In the example described here, the exchanger 30 provides a power of the order of 2 kW whereas the heating module 31 supplies a few hundred watts. The reduction in thermal heating power P_res from the high power level (in this instance P.sub.max.sub._res) is adjusted or set so that the temperature of the flow FL.sub.S intended for the probe is temporarily stable and equal to T.sub.min.sub._FL.sub.S for part of the transient phase .sub.1, in this instance for a first part of the transient phase. The command module thus operates the heating module 31 in such a way that the thermal heating power supplied is at a maximum just after circulation of the said flow FL.sub.S intended for the probe is initiated and then decreases in a way that is set so that the temperature of the flow FL.sub.S intended for the probe 2 is temporarily stable.

(47) The way in which the temperature of the first flow FL.sub.S intended for the probe 2 evolves with respect to time comprises, as in the first embodiment, the following three successive phases: a phase .sub.0 of cooling the temperature; a transient phase .sub.1, involving a rise in temperature; an equilibrium phase .sub.2, during which the temperature remains stable and equal to an equilibrium value.

(48) These phases .sub.0, .sub.1, .sub.2 are analogous to those of the first embodiment described but differ therefrom in the following respect: the low value of the temperature reached at the end of the cooling phase .sub.0, T.sub.min.sub._FL.sub.S, according to the second embodiment, is slightly higher than the low value T.sub.min.sub._FL.sub.S reached at the end of the phase .sub.0 according to the first embodiment; the transient phase .sub.1 comprises a stable temperature plateau (denoted .sub.1.sub._.sub.1) here equal to the minimum temperature T.sub.min.sub._FL.sub.S, prior to a rise (denoted .sub.1.sub._.sub.2) in the temperature T_FL.sub.S of the first flow FL.sub.S so as far as the equilibrium temperature T.sub.fh.sub._FL.sub.S.

(49) In the embodiment described here, the thermal plateau relating to the first flow FL.sub.S during the transient phase .sub.1 is situated in the first part of this transient phase .sub.1 which causes the temperature of the first flow FL.sub.S to move from the lowest value achieved T.sub.min.sub._FL.sub.S to the equilibrium value T.sub.fh.sub._FL.sub.S. As an alternative, it might be possible to consider the thermal plateau being situated subsequently in the transient phase .sub.1 after a first rise in temperature T_FL.sub.S and before a second rise in temperature T_FL.sub.S to the equilibrium temperature T.sub.fh.sub._FL.sub.S.

(50) Let us emphasise that, during the transient phase .sub.1, and notably during the thermal plateau relating to the temperature T_FL.sub.S, the temperature of the first flow FL.sub.S, here equal to the value T.sub.min.sub._FL.sub.S, is lower than the equilibrium temperature T.sub.fh.sub._FL.sub.S (corresponding to a high rotational frequency f.sub.h of the probe 2) even though the flow rate of the first flow FL.sub.S has already been set to the desired value (or first flow rate) here equal to D.sub.fh.sub._FL.sub.S, and therefore causes the sample holder 20 of the probe 2 to spin at the desired rotational frequency, in this instance a high frequency f.sub.h.

(51) According to the second embodiment of the invention, the probe 2 creates an NMR spectrum of a sample during the thermal plateau relating to the first flow FL.sub.S created during the transient phase .sub.1, in an NMR analysis step E9.

(52) The method of controlling the temperature of the first flow FL.sub.S according to the second embodiment comprises the steps E0-E4 and E6-E8 and the phases .sub.0, .sub.1, .sub.2 which have just been described with reference to FIGS. 3 and 5B. The method of operating the probe 2 according to the second embodiment comprises the steps E0-E4 and E6-E8, phases .sub.0, .sub.1, .sub.2 and the analysis step E9.

(53) In the two embodiments that have just been described with reference to FIGS. 2, 5A and 3, 5B, during the initial step E0 of cooling the exchanger there is no circulation of the first flow FL.sub.S. In other words, the initial flow rate of the flow FL.sub.S is zero. As an alternative, during the initial step E0, the first flow FL.sub.S could be circulated at an initial flow rate D.sub.0.sub._FL.sub.S strictly lower than the first flow rate (which is for example equal to the flow rate D.sub.fh.sub._FL.sub.S) set subsequently to the initial step E0. The initial flow rate D.sub.0.sub._FL.sub.S of the first flow FL.sub.S may notably be set to a value that is set in such a way that the pressure inside the circuit 4 in which the flow FL.sub.S circulates is higher than or equal to the ambient pressure of this circuit 4 (i.e. greater than or equal to the pressure outside the circuit).