DD218657A1 - PROCESS FOR PRE-COOLING HELIUM CRYSTATES - Google Patents

Abstract

The aim and object of the invention is a method for Vorkuehlung of helium cryostats, which consist of at least two nested Dewargefaessen. The process should ensure as low as possible helium filling losses and require no constructive system arrangements that limit its manageability and applicability. According to the invention, this object is achieved in that gaseous nitrogen is introduced into the dewar space of the inner dewar and then the dewar space is sealed. In the Aussendewar liquid nitrogen is filled and in the Innewewar gaseous helium. After completion of the Vorkuehlphase liquid Helium is filled into the Innendewar, thereby the nitrogen in the Dewarraum frozen and vacuum produced.

Description

Process for the precooling of helium cryostats

Field of application of the invention

The invention relates to a method for precooling of cryostats for helium temperature. It can be used in particular for glass bath cryostats, but can also be used for other types of cryostat.

Helium cryogenic systems are used to study a wide range of physical, technical and biological issues and are included in, for example, transport studies, optical investigations and magnetic resonance investigations as standard equipment.

In addition, it is also possible to use the pre-cooling process for special helium transport systems. , ·.

Characteristic of the known technical solutions.

Helium cryostats are known as glass and metal variants. They generally consist of at least two nested systems that have a large temperature gradient. guarantee. The outer system serves as radiation ^! screen and is usually cooled with a helium over boiling coolant (ia liquid nitrogen). It can . be designed as' screen with extremely high reflectivity and low heat conduction or as Dewar. The inner system is used to hold the liquid helium, as well as, the test sample and necessary for the physical examinations to the sample.

Because of the low. Heat capacity of the liquid helium'is absolutely necessary in the case of cryostats constructed according to this principle to pre-cool the system prior to filling the helium. In 'thermally unfavorable system arrangements, which can not be avoided, for example, by the special physical examination method, a filling of liquid helium is possible only with very large helium consumption without a very good precooling. For the i. a. limited to the available amounts of liquid helium, it may occur that helium can not be kept in the liquid phase by the cryostat.

As a pre-cooling process usually the passage of freshly evaporated nitrogen is applied through the inner vessel. In this case, a relatively long pre-cooling time is required during which the actual Sfeßapparatur can not be used. With this method 'can also reach only pre-cooling temperatures that are above the temperature of the liquid pre-cooling agent. , ;

Sin direct. On the other hand, filling the pre-cooling agent into the inner system does not make sense, since its controlled removal before the filling of the actual working temperature-generating gas is usually not possible. .

Lift the above. Process there is the possibility of pre-cooling by a thermal bridge or the thermosiphon principle. However, the application of these methods is associated with the known solutions with a higher technical complexity. The Varian company discloses a glass double dewar system (Varian, technical manual, Ξ-245 liquid helium accessory, publication no. 87-125-509) in which the dewar space of the inner vessel is accessible through a stopcock. About this tap, the vessel can be pumped and nitrogen introduced into the Dewarraum, which acts as a thermal bridge between the outer vessel and the .Innenraum of the inner vessel and the pre-cooling realized, laughing, completion of the pre-cooling by heat transfer, the nitrogen "is pumped through the tap and so the Dewarwirküng the inner vessel restored.

The disadvantages of this arrangement, in addition to the 'technically and economically more complex construction in the high risk of breakage, which inevitably' caused by the fused glass cock and thus makes the system unsuitable for a wide practical application, as well as in the more complicated regime of Vorkühlverfahrens. · ...

Object of the invention

The aim of the invention is the pre-cooling of helium cryostat by a method that ensures the lowest possible helium filling losses. ''

Explanation of the essence of the invention ' ,

Based on the disadvantages in 'the handling of the known Vorkühlverfahrens by heat transfer, it is an object of the invention to make the pre-cooling of helium cryostat so ^r that in the shortest possible time an optimally low pre-cooling temperature is achieved, the method should be easy to handle and reliable contamination of the actual cryogen helium is avoided. The method must not require constructive system arrangements, ie its. Limit manageability and possible use.

According to the invention, this object is achieved in that a gas filling is introduced between the walls of the internal end-effector vessel known per se and then the space between the walls of the interior finisher is sealed off, for example by glass melting vessels The coolant is transferred from the pre-cooling agent into the outer Dewar vessel by heat transfer from the outer system connected to the pre-cooling means to the inner walls of the inner vessel . the outer and the inner system, a temperature equilibrium 'By following it, filling the liquid helium working gas in the inner vessel, the gas filling is excluded in Dewarraum ^then: frozen, so that the inner vessel again acts as a Dewar.

. '.' s ' · ·

  - 4 -

Thus, the pre-cooling process after your one-time introduction of a gas filling in the Dewarraum of the inner vessel only in the filling of the pre-cooling agent into the outer system. The design of the dewar and measuring system is retained. As a result, a considerable reduction in the workload is achieved. The temperature equilibrium between the outer and inner system ¹ is established by itself after a relatively short time, with the temperature in the inner vessel being close to the temperature of the pre-cooling means due to the optimally short cooling path Sine contamination risk of the inner vessel by the pre-cooling agent is excluded.

embodiment

The invention will be explained in more detail below using the example of an EPR glass bath cryostat for liquid helium. The entire measuring system consists of two Dewar flasks and one, measuring insert, which is arranged in the inner vessel. It is shown schematically in Fig. 1.

The outer dewar 1 serves as a radiation shield. It is filled with liquid nitrogen and thus provides sufficient reservoir for pre-cooling of the inner vessel 2. The inner vessel 2 made of glass material with low helium permeability serves to receive the actual measuring system 3 (EPR resonator with measuring sample, waveguide, sample rotating device, etc.) and becomes filled with liquid helium during the actual measurement. This vessel according to the invention in Dewarraum 4 with a nitrogen filling, at room temperature about 200 Torr provided. After the liquid nitrogen has been introduced into the outer dewar 1, it will take over the transport of the inner vessel and the measuring system. * The heat transfer between the walls of the inner vessel and the measuring insert is ensured by the admission of helium gas into the inner system. After about -15 minutes, a sufficient pre-cooling of the system is achieved and thus a low-loss helium filling possible. Due to the liquid helium, the nitrogen in the dewar space is frozen out, so that this again acts as a dewar vessel with vacuum in the Dewar space.

.The nitrogen filling of the inner vessel brings next to the 'pre-cooling function described yet another advantage, since. it restricts helium diffusion through the glass walls into the dewar space. Thus, a regeneration of the vessel is necessary only after much more hours of operation than without nitrogen filling. .

Außenendewar 1, Innendewar 2 and measuring system 3 are connected by a suitably constructed lid 5 and sealed vacuum-tight, so that a pumping of the plonium in Inn.engefäß and thus a temperature reduction below 1.5 K is possible *

Claims (3)

Invention claim. 1. A method for precooling of helium cryostats, which consist of at least two nested Dewar flasks, of which the inner contains the measuring system, which is to be cooled with liquid helium, characterized in that in the space between the walls of the Innendewars (Dewarraum) gaseous nitrogen is introduced, that the Dewarraum the Innendewars after filling the gaseous nitrogen. is tightly closed, that after the arrangement of the indoor Endewar in Äußendewar and the measuring system in the inside of the outer end. liquid nitrogen is filled and in the Innendewar gaseous helium that is filled after completion of the pre-cooling phase in the Inüendewar liquid helium, whereby in the Dewarraum of the Innendewars the gas filling is frozen and Va- '. Kuwait is being made. 2. The method according to item Λ , characterized in that when using Dewar gef made of glass Dewarraum the Innende-. was sealed after filling the gaseous nitrogen. ν 3. The method according to item 1 and 2, characterized in that the process steps gas filling and closing the Dewarraumes of the Innendewars be repeated only after a large number of measurements / need. For this a sheet drawing