Adiabatic Demagnetization Apparatus

This application is a continuation of U.S. patent application Ser. No. 17/626,166, filed Jan. 11, 2022, which is a national phase application of International Patent Application No. PCT/EP2020/069792 filed on Jul. 13, 2020 the entire content and disclosure of which is incorporated herein by reference.

The present disclosure relates to a method of controlling an adiabatic demagnetization apparatus and an adiabatic demagnetization apparatus. The present disclosure particularly relates to a method of controlling a multi-stage adiabatic demagnetization apparatus within a predetermined temperature range.

A cryostat is generally used to maintain low temperatures of samples mounted within the cryostat. Low temperatures may be achieved by using, for example, a cryogenic fluid bath such as liquid helium. However, the cooling medium, such as liquid helium, continuously evaporates due to external and/or internal heat input in the cryostat and therefore needs to be refilled regularly. This requires considerable time and resources, whereby the operating costs of such cryostats are high.

In order to overcome the above drawbacks, cryogen-free cryostats have been developed. Cryogen-free cryostats may employ a cryogen-free closed cycle system, such a pulse tube cryocooler. Modern pulse tube cryocoolers can achieve temperatures down to 1.2 K. In order to achieve sub-Kelvin temperatures, a magnetic cooling stage can be used in addition to the cryogen-free closed cycle system. The magnetic cooling stage may be an adiabatic demagnetization refrigerator (ADR), which can achieve temperatures down to a few milli-Kelvin. ADR is based on the magneto-caloric effect. When a medium is magnetized its magnetic moments get aligned and the heat of magnetization is released. Vice versa, if the medium is demagnetized its temperature drops.

Conventional ADR systems are operated in single-shot mode. This means that low temperatures are achieved only for a short time and are not maintained stably for a longer time. However, in many applications it is considered beneficial to maintain low temperatures e.g. in the sub-Kelvin range for a long time and in a stable manner.

In view of the above, new methods of controlling an adiabatic demagnetization apparatus and adiabatic demagnetization apparatuses that overcome at least some of the problems in the art are beneficial.

In light of the above, a method of controlling an adiabatic demagnetization apparatus, a non-transitory machine readable medium, a controller, an adiabatic demagnetization apparatus, and a cryostat are provided.

It is an object of the present disclosure to provide a method of controlling an adiabatic demagnetization apparatus, a non-transitory machine readable medium, a controller, an adiabatic demagnetization apparatus, and a cryostat, which can continuously and variably achieve low temperatures, in particular in the sub-Kelvin range. Further aspects, benefits, and features of the present disclosure are apparent from the claims, the description, and the accompanying drawings.

According to an independent aspect of the present disclosure, a method of controlling an adiabatic demagnetization apparatus is provided. The method includes varying at least on operational parameter of the adiabatic demagnetization apparatus.

According to some embodiments, which can be combined with other embodiments described herein, the at least one operational parameter is selected from the group including, or consisting of, a cycling frequency of at least one adiabatic demagnetization unit, a switching mode of a plurality of thermal switches, and at least one of a maximum cycling temperature and a minimum cycling temperature of at least one adiabatic demagnetization unit.

According to an independent aspect of the present disclosure, a method of controlling an adiabatic demagnetization apparatus is provided. The method includes: (i) cycling at least one adiabatic demagnetization unit of the adiabatic demagnetization apparatus between a first temperature and a second temperature with varying frequency; and/or (ii) operating a plurality of thermal switches of the adiabatic demagnetization apparatus in a first switching mode if a first target temperature is set and/or a first heat load is applied, and operating the plurality of thermal switches in a second switching mode if a second target temperature is set and/or a second heat load is applied; and/or cycling at least one adiabatic demagnetization unit of the adiabatic demagnetization apparatus between a first temperature and a second temperature, wherein the first temperature and/or the second temperature is varied or variable.

According to an independent aspect of the present disclosure, a method of controlling an adiabatic demagnetization apparatus is provided. The method includes cycling at least one adiabatic demagnetization unit of the adiabatic demagnetization apparatus between a first temperature and a second temperature with varying frequency.

According to some embodiments, which can be combined with other embodiments described herein, the frequency is varied over time.

According to some embodiments, which can be combined with other embodiments described herein, the frequency is varied based on a heat load.

According to some embodiments, which can be combined with other embodiments described herein, the frequency is increased when the heat load increases and/or wherein the frequency is decreased when the heat load decreases.

According to some embodiments, which can be combined with other embodiments described herein, the first temperature is higher than the second temperature.

According to some embodiments, which can be combined with other embodiments described herein, the adiabatic demagnetization apparatus includes a total number n of adiabatic demagnetization units, wherein n≥1, 2 or 3.

According to some embodiments, which can be combined with other embodiments described herein, the n adiabatic demagnetization units are connectable in series.

According to some embodiments, which can be combined with other embodiments described herein, the n adiabatic demagnetization units are connectable in series by thermal switches.

According to some embodiments, which can be combined with other embodiments described herein, a number m of adiabatic demagnetization units of the n adiabatic demagnetization units is cycled between respective first temperatures and second temperatures, wherein m≤n, in particular wherein m=n−1.

According to some embodiments, which can be combined with other embodiments described herein, the frequency is varied based on a heat load applied to a last stage n of the n adiabatic demagnetization units.

According to an independent aspect of the present disclosure, a method of controlling an adiabatic demagnetization apparatus is provided. The method includes operating a plurality of thermal switches of the adiabatic demagnetization apparatus in a first switching mode if a first target temperature is set and/or a first heat load is applied; and operating the plurality of thermal switches in a second switching mode if a second target temperature is set and/or a second heat load is applied.

According to some embodiments, which can be combined with other embodiments described herein, the first switching mode is different from the second switching mode.

According to some embodiments, which can be combined with other embodiments described herein, the first target temperature is different from the second target temperature and/or the first heat load is different from the second heat load.

According to some embodiments, which can be combined with other embodiments described herein, the plurality of thermal switches includes a total number a of thermal switches, wherein a≥2.

According to some embodiments, which can be combined with other embodiments described herein, a number b of the a thermal switches is operated in the first switching mode, and a number c of the a thermal switches is operated in the second switching mode, wherein b T c.

According to some embodiments, which can be combined with other embodiments described herein, thermal switches, which are not operated in the first switching mode and/or the second switching mode, are closed.

According to some embodiments, which can be combined with other embodiments described herein, more thermal switches are operated when the target temperature is changed to a lower target temperature and/or when a heat load increases. Additionally, or alternatively, less thermal switches are operated when the target temperature is changed to a higher target temperature and/or when a heat load decreases.

According to some embodiments, which can be combined with other embodiments described herein, the method further includes cycling at least one adiabatic demagnetization unit of the adiabatic demagnetization apparatus between a first temperature and a second temperature with varying frequency.

According to an independent aspect of the present disclosure, a method of controlling an adiabatic demagnetization apparatus is provided. The method includes cycling at least one adiabatic demagnetization unit of the adiabatic demagnetization apparatus between a first temperature and a second temperature, wherein the first temperature and/or the second temperature is varied or variable.

According to some embodiments, which can be combined with other embodiments described herein, the first temperature is higher than the second temperature.

According to some embodiments, which can be combined with other embodiments described herein, the first temperature and/or the second temperature is varied based on a heat load and/or a target temperature.

According to some embodiments, which can be combined with other embodiments described herein, the first temperature and/or the second temperature is decreased if the heat load is increased, and/or the first temperature and/or the second temperature is increased if the target temperature is increased.

According to some embodiments, which can be combined with other embodiments described herein, the first temperature and/or the second temperature is increased if the heat load is decreased, and/or the first temperature and/or the second temperature is decreased of the target temperature is decreased.

According to some embodiments, which can be combined with other embodiments described herein, based on a change of the heat load and/or the target temperature, both of the first temperature and the second temperature can be increased or decreased, or the first temperature can be increased and the second temperature can be decreased, or the first temperature can be decreased and the second temperature can be increased.

According to some embodiments, which can be combined with other embodiments described herein, the cycling of the at least one adiabatic demagnetization unit of the adiabatic demagnetization apparatus between the first temperature and the second temperature includes: cycling the at least one adiabatic demagnetization unit of the adiabatic demagnetization apparatus between the first temperature and the second temperature with varying frequency.

According to some embodiments, which can be combined with other embodiments described herein, the method further includes: operating a plurality of thermal switches of the adiabatic demagnetization apparatus in a first switching mode if a first target temperature is set; and operating the plurality of thermal switches in a second switching mode if a second target temperature is set.

According to an independent aspect of the present disclosure, a machine readable medium (e.g. a memory) is provided. The machine readable medium includes instructions executable by one or more processors to implement the embodiments of the method of the present disclosure.

According to an independent aspect of the present disclosure, a controller is provided. The controller includes one or more processors and a memory coupled to the one or more processors and comprising instructions executable by the one or more processors to implement the embodiments of the method of the present disclosure.

According to an independent aspect of the present disclosure, an adiabatic demagnetization apparatus is provided. The adiabatic demagnetization apparatus includes the controller.

According to an independent aspect of the present disclosure, a cryostat is provided, including the adiabatic demagnetization apparatus.

Embodiments are also directed at apparatuses for carrying out the disclosed methods and include apparatus parts for performing each described method aspect. These method aspects may be performed by way of hardware components, a computer programmed by appropriate software, by any combination of the two or in any other manner. Furthermore, embodiments according to the disclosure are also directed at methods for operating the described apparatus. It includes method aspects for carrying out every function of the apparatus.

Reference will now be made in detail to the various embodiments of the disclosure, one or more examples of which are illustrated in the figures. Within the following description of the drawings, the same reference numbers refer to same components. Generally, only the differences with respect to individual embodiments are described. Each example is provided by way of explanation of the disclosure and is not meant as a limitation of the disclosure. Further, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the description includes such modifications and variations.

shows an operation principle of an adiabatic demagnetization refrigerator.

Adiabatic demagnetization refrigeration (ADR) is a cooling method which uses the entropy-dependence of a paramagnetic spin system (e.g. the magnetic moments due to the electronic orbital motion and electron spin, or nuclear spins) to provide low-temperature or ultra-low temperature cooling. The method allows to generate low or ultra-low temperatures of some milli-Kelvin or even some micro-Kelvin. An exemplary implementation of ADR makes use of a single ADR unit which includes a heat switch, a cooling medium, and a magnet. Low or ultra-low temperatures are generated by demagnetizing the cooling medium. An exemplary cooling procedure is shown in more detail in.

In boxof, the heat switch is closed, and the cooling medium is coupled to a pre-cooling unit. In box, a local magnetic field is increased to maximum and heat of magnetization is released, whereby the sample is heated. In box, thermalization takes place and the heat of magnetization is removed by means of the pre-cooling unit. In box, the heat switch is open, and the magnetic field is still applied. In box, the magnetic field is reduced, and the sample is cooled. In box, the sample temperature is constant, and the magnetic field is decreased. In box, the magnetic field is zero and the cooling process terminates. In box, the cooling medium regenerates and the sample warms up to base temperature.

ADR can be operated in a single-shot mode, e.g. using a single ADR unit. The single-shot mode can achieve low temperatures only short-term and not continuously. Such a short-term cooling may limit the use and commercial application of ADR. Instead, He-3 based techniques, such as dilution refrigerators, are often used for providing sub-Kelvin temperatures in a continuous manner.

shows a schematic view of multi-stage adiabatic demagnetization refrigerator.

The drawbacks of short-time cooling with ADR may be solved by using multi-stage ADR, i.e., refrigerators which have two or more mutually connected ADR units.illustrates multi-stage ADR where n ADR units are connected as a chain. By dissipating the heat of magnetization of ADR unit n in ADR unit n−1 it is possible to provide a residual magnetic field and hence cooling power at the last ADR unit n. Besides the simple chain schematically illustrated in, more complex configurations of multi-stage ADR including e.g. multiple ADR chains, each having multiple ADR units, can be provided, wherein the ADR chains may be operated in parallel or in series.

A first ADR unit (“” in) of the multiple ADR units can be connected to a heat sink. The heat sinkcan be provided by a cryogen-free closed cycle system, such a pulse tube cryocooler. The heat sinkcan be maintained at an essentially constant temperature e.g. in a range between 1 K and 4 K. For example, the heat sinkcan be maintained at an essentially constant temperature of about 4 K.