DE2308176A1 - SUPRAL CONDUCTIVE DEVICE FOR OPERATION WITH HIGH FREQUENCY ELECTROMAGNETIC FIELDS - Google Patents

Description

Superconductive device for operation with high-frequency electromagnetic fields

You invention relates to a superconducting device for Operation with high-frequency electromagnetic fields with a frequency of more than 10 MHz with at least one superconducting surface carrying high-frequency currents.

Superconducting devices for operation with high-frequency electromagnetic fields with a frequency of more than 10 MHz can be used in many different ways in technology. In particular, they can be used as resonators and separators for particle accelerators or as high-frequency resonators for other purposes, for example as frequency standards, and in particular be designed as cavity resonators or helical resonators. Superconducting cavity resonators are operated in the frequency range from 1 to 15 GHz, superconducting helical resonators in the range around 100 MHz. In these devices, the superconductivity of surface layers is used to guide the high-frequency currents associated with the high-frequency electromagnetic fields. Since the penetration depth "-etej / high-frequency currents and fields into the superconducting surface is only very small, for example only about 300 to 400 % , the nature of the thin, current-carrying surface layer is of decisive importance for the functionality of the superconducting device High-frequency surface resistance, and thus the quality Q in resonators, for example, and the critical magnetic field H_ measured under the influence of high-frequency alternating fields

depends significantly on the physical condition of the surface.

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This is particularly important when operating superconducting devices with high bit frequencies, in particular in the frequency range of more than 10 MHz, since the losses caused by the surface resistance increase approximately proportionally to the square of the frequency. A high critical magnetic field H ^ac is particularly important in order to be able to operate the superconducting devices for operation with high-frequency electromagnetic fields with the highest possible high-frequency power and, at the same time, low surface resistance. If the critical magnetic field H is exceeded, the losses increase sharply and the surface resistance increases considerably and the electromagnetic field collapses. In order to achieve a high critical magnetic field H _n and low surface resistances, efforts are made to create surfaces that are as smooth as possible and free of contamination. At every inhomogeneity of the surface, in particular at tips, electrical and / or magnetic field increases can occur, which trigger a number of effects that have not yet been fully clarified in detail, which in turn lead to

ac lead to a reduction in the critical field H _n and an increase in the surface resistance. Chemical and electrochemical polishing processes are used to smooth and clean the surfaces, and also annealing processes, especially for niobium surfaces. In addition to cleaning the niobium surface by degassing, the latter is also intended to achieve grain growth of the niobium in order to reduce the number of grain boundaries on the niobium surface. Furthermore, in the case of niobium cavity resonators, particularly high qualities and critical magnetic fields can be achieved by providing the surface delimiting the resonator cavity with a niobium oxide layer by means of anodic oxidation. As a result of the oxidation of the surface, the resonator surface that is effective for superconductivity is moved away from the actual surface into a deeper, purer layer of nlob.

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At the same time, the niobium oxide layer serves as a protective layer for this deeper-lying effective surface ("Physics Letters "34 A (1971), pp. 439-440).

It is also known that, for example in niobium cavity resonators of the TMq ₁ Q or of the HEMq ₁ ^ type, as a result of excessive electric field increases on the resonator surface caused by surface disturbances, the effect can occur that despite an increase in the power supplied to the resonator a certain limit to the power absorbed by the resonator no longer increases and a certain electric field strength at the resonator surface is initially not exceeded. As is known, an improvement is possible if the resonator is loaded for some time with this electrical field strength or power, which is not initially to be exceeded. After some time, the power absorbed by the resonator and the electric field strength on the resonator surface continue to rise until finally the critical magnetic field strength

H is exceeded and a so-called magnetic breakdown occurs. Obviously, in the treatment mentioned, which is also known under the designation “processing”, peaks or projections on the resonator surface are broken down. Under certain circumstances it can be advantageous in such a treatment to let some gaseous helium into the resonator during the treatment. Obviously, under these conditions, the helium atoms are ionized by the electrons emerging from the tips on the resonator surface as a result of field emission and accelerated towards the emitting surface, where they level the tips when they strike. After such a treatment, which can optionally be repeated several times, and after which the helium gas is pumped out of the resonator again, reproducible values for the achievable critical field strength H _c ^ac and the

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unloaded quality Q ₀ reached. The resonators pretreated in this way are actually operated

-Q Ultra-high vacuum of, for example, 10 ^v Torr within the resonators ("Applied Physics Letters" 13 (1968), pages 390 to 391 and "Proceedings of the 8 International Conference on High-Energy Accelerators - CERN 1971", Geneva 1971, pages 51 to 58 and 253 to 257).

The object of the invention is to further increase ^{the critical magnetic field strength H ac} in superconducting devices for operation with high-frequency electromagnetic fields having a frequency of more than 10 MHz with at least one superconducting surface carrying high-frequency currents

raise.

This is achieved according to the invention in that the high-frequency Currents carrying superconducting surface during operation of the device at least partially with gaseous Helium is in contact, the gas pressure of which is less than the lower limit pressure for the gas discharge and greater than 10 Torr is.

"Operation" of the device is not to be understood as a pretreatment - such a pretreatment is part of the manufacture of the device - but normal operation in which the superconductive device is used for its intended purpose. Surprisingly, it has now been found that the operation of such facilities can be reached by helium gas much higher critical magnetic fields H_ ^ac pressure range specified as the operation under ultra high vacuum. This applies even if the superconducting devices, in particular resonators, had been subjected to the aforementioned pretreatment by loading under vacuum or helium gas. According to the invention

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The helium gas pressure to be used must be less than the lower limit pressure for the gas discharge, i.e. less than the pressure if it were exceeded, a gas discharge would set in at lower pressures. A gas discharge would namely lead to the collapse of the electromagnetic field in the superconducting device. The lower limit pressure for the gas discharge, which depends in particular on the electrical fields prevailing in the device determined in individual cases in a simple manner by measurement

-5 become. Furthermore, the helium gas pressure should be greater than 10 Torr. This is because no increases could be achieved at lower pressures of the critical magnetic field H "can be observed. In addition, the respective gas pressure should be at the respective Operating temperature of the superconducting device must be present, for example in the case of superconducting niobium resonators is usually around 1.4 to 1.5 K.

If one does not bring the entire current-carrying superconducting surface of the device into contact with helium gas you should preferably keep those areas of the surface under helium gas where experience has shown the collapse of the magnetic field strength begins. As a rule, the critical field strength is the first exceeded in a smaller area of the surface, for example in places with a slightly higher surface resistance. These areas then become normally conductive and can, as there are increased losses in them, with the development of heat occur as germs for the propagation of the normally conducting state over the entire superconducting device.

In order to avoid a gas discharge with certainty, it is particularly advantageous if the helium gas pressure is at most is half to a third of the lower limit pressure for the gas discharge. Usually a particularly advantageous one The range for the helium gas pressure is between approximately

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- ■ 5—4 · 5 x 10 and 10 torr. It is particularly favorable if the High-frequency currents carrying superconducting surface is made of niobium. The already very high critical magnetic field H of Ni oboberflächen when operating under ultra-high vacuum is namely further increased by the contact with the helium gas. Of course, the entire superconducting device, for example the entire resonator body, can also consist of niobium.

Furthermore, it has surprisingly been shown that operation under helium gas is then also in the sense of a. . Increasing the critical magnetic field H affects when the High-frequency currents carrying superconducting niobium surface is covered with a niobium oxide layer, preferably a niobium pentoxide layer.

With certain superconducting devices, in particular with resonators and separators for particle accelerators, However, the case can now arise that helium gas atoms would interfere if they were to fill the entire interior space of a cavity resonator, which is bounded by the current-carrying surfaces. The accelerated particles flying through this interior could namely the helium atoms ionize and would be slowed down themselves. In such cases, however, one can proceed as follows: that a vacuum space is provided along the particle path, as has been customary up to now, and at least part of the superconducting surface of which carries high-frequency currents separates the neighboring vacuum space by a partition made of electrically non-conductive material, one of them has the lowest possible dielectric loss factor. The gaseous helium is then located between these Partition wall, which in a cavity resonator, for example in Shape of a tube can be inserted, and the superconducting surface.

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Except for the critical magnetic field H_, this can also affect helium gas in contact with the superconducting surface also affects the high-frequency surface resistance have a beneficial effect on the superconducting surface in terms of reducing resistance. This has for example occasionally with resonators that are below their critical

QA

see magnetic field H operated, again one

Increase in the unloaded quality Q ₀ .

The invention is intended to be based on a few figures and examples will be explained in more detail.

Fig. 1 shows in section a niobium cavity resonator of the TM ₀₁₀ type.

Fig. 2 shows schematically a cavity resonator with pump and waveguide connections. Figures 3 and 4 show different embodiments of the Device according to the invention, in which the current-carrying superconducting surfaces are completely or partially covered by partition walls are separated from an adjacent vacuum space. Fig. 5 shows schematically a further embodiment of a Device according to the invention, in which only small parts of the current-carrying superconducting surfaces with helium gas stay in contact.

The niobium cavity resonator of the TM ₀₁₀ type shown in FIG. 1 is machined in one piece from solid niobium by turning. It works in the X-band at a frequency of 9 to 5 GHz. The interior 1 of the cavity resonator is 15 mm long and has a circular cross-section with a diameter of 25 mm. The channels 2 and 3 leading into the interior 1 are 19 and 16 mm long and have a circular cross-section with a diameter of 12 mm. By means of the flanges 4 and 5, the resonator is on during operation via coupling parts also made of niobium without the interposition of cold, vacuum-tight microwave windows

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X-band waveguide, for example made of copper, connected. During operation, the resonator is advantageously permanently connected to an ultra-high vacuum pump, for example a turbo molecular pump, via the waveguide. If the superconducting surfaces of the interior 1 and the channels 2 and 3 of the cavity resonator, which carry high-frequency currents during operation, are to be brought into contact with gaseous helium, this can be done in a simple manner by supplying gaseous helium on the forepump side of the turbo-molecular pump . This is shown schematically in FIG. Waveguides 12 and 13 are connected to both sides of the cavity resonator designated by 11 in this figure. With a high frequency generator 14 is designated. The pump line 15 leads to a turbo molecular pump 16, which in turn is connected to a backing pump 18 via a line 17. A helium supply line 19 opens into the line 17, that is to say on the forepump side of the turbo-molecular pump 16. At the end of the waveguide 13 there is a measuring device 20. 2 cryostat, not shown, which contains liquid helium, the temperature of which is lowered to about 1.5 K by pumping out. If, for example, so much gaseous helium is allowed to flow in through line 19 per unit of time that a helium gas pressure of about 0.5 Torr is established in line 17 as a result of the suction effect of backing pump 18, and the pressure ratio between the backing pump and the high vacuum side of the turbo molecular pump 16 is about ^{100 to 200, a helium gas pressure of about 2.5 to 5-1Ο "3} Torr is obtained within the cavity resonator 11, which is at a temperature of about 1.5 K.

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In the following, the improvements are to be explained in more detail, which on various examples of the in FIG cavity resonator shown were achieved when operating with helium gas within the resonator cavity.

A copy of the rotated solid niobium cavity was degassed first under ultra-high vacuum at 1900 ⁰ C and then concentrated in a solution of 50 vol .-% nitric acid and 50 parts by volume of 96 40 # hydrofluoric acid starting at a temperature of 5 ° C in chemically polished in several steps. Overall, a surface layer with a thickness of about 100 / u was removed. The resonator was then immersed in an approximately 6-5 # (wt .- #) aqueous hydrogen peroxide solution and then rinsed with distilled water. When wet, the resonator was then connected to the flanges and waveguides using indium gaskets. The turbo molecular pump was switched on for evacuation. After a pumping time of about 30 hours, during which had been heated, the resonator multiple times to a temperature of 100 ⁰ C, the resonator was cooled to the temperature of liquid helium of 4.2 K. The temperature was then lowered to about 1.4 K by pumping the helium out of the cryostat. Then the microwave generator was switched on and the power supplied to the cavity resonator was steadily increased. The energy absorption of the cavity resonator was initially limited by electron processes such as field emission and so-called "multipactoring" to a value that corresponds to a magnetic flux density of about 75 mT. An increase in the power at the resonator input did not initially result in any further increase in the field strength in the resonator after this value had been reached. This limitation could be canceled or suppressed by a pretreatment, which consisted in evacuating the person

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Resonator first under vacuum and for about an hour. then a few minutes under helium gas with a gas pressure of 10 Torr with slowly increasing power derait was operated so that the electric field strength in the resonator is constantly in the vicinity of its respective limit value lay. After this pretreatment, the helium gas was pumped out of the resonator cavity and this was evacuated.

In the normal treatment that follows this pretreatment

In operation with a vacuum of about 10 to 10 Torr in the resonator cavity, the energy supply could be increased rapidly, and without being limited by processes associated with the electric field strength, until a critical positive magnetic density B _ß of about 128 was reached mT the field collapsed. Helium gas was then admitted into the resonator cavity at a pressure of about 2.5 * 10 7 Torr. With this helium gas filling, a critical magnetic flux density of about 150 mT has now surprisingly been achieved. By letting in and pumping out helium gas, it was possible to switch back and forth reproducibly between the two critical magnetic flux densities of 128 mT and 150 mT. When the helium was admitted more quickly, only a fraction of a second was required to change the critical magnetic flux density, while the "switching process" took about 10 to 15 seconds during pumping due to the somewhat longer pumping time. This result clearly shows that the critical magnetic flux density is brought into contact with the superconducting surfaces of the resonator carrying high-frequency currents with gaseous helium

on on

B ₀ and thus also the critical magnetic field H _c can be increased considerably compared to operation under vacuum.

Other examples of the resonator shown in FIG. 1 were made of fine-grain niobium with a medium grain size

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of about 0.2 mm and then, without any heat treatment, merely according to that in "Physics Letters" 37 A (1971), pages 139 to 140 or in the German Offenlegungsschrift 2 027 156 described method electropolished, a surface layer of about ^ 200 / µm thick was removed. Then the inner surface of the resonator initially by the method described in "Physics Letters" 34 A (1971), pages 439 to 440, in an NH, solution oxidized anodically and then redissolved the oxide layer in 50 # plus acid. This treatment, i.e. Oxidation and detachment of the oxide layer were repeated several times. A resonator was then made with a bare inner surface, a others with anodically oxidized surface according to the same Rinses connected to the waveguides and then treated further in the same way, as has already been explained. After the pretreatment explained, these resonators also showed critical magnetic properties when operated under vacuum Flux densities of about 130 mT, which after the admission of gaseous helium into the resonator cavity even to about 160 mT rose. This is astonishing for resonators that haven't even been heat treated high value.

The unloaded quality Q in the various measurements, which were made just below the critical magnetic flux density B _n , was in each case between about 3 ³

Q C

and 5 * 10. Whether the resonator cavity was filled with helium gas or evacuated had no significant influence. In some cases, however, the unloaded quality Q _{0 could be increased,} for example, from a value of 8.2 · 10 ⁹ under vacuum to a value of 4.7 · 10 9 under a helium gas filling.

Figures 3 and 4 schematically represent two embodiments of the device according to the invention, which are in particular

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particularly suitable for resonators that are to be used in particle accelerators. In both figures, a cavity resonator 21 or 31 is flanged to tubes 22 and or 32 and 33, in which a particle beam of an accelerator runs in the direction of arrows 24 and 34-. These tubes and the internal spaces 25 and 35 of the resonators are evacuated and separated from part of the superconducting surface of the resonator (Fig. 3) or from the entire superconducting surface of the resonator (Fig. 4) by separating pipes or 36. severed. The space 27 and 37 between the respective resonator surfaces and the separating tubes 26 and 36 is filled with gaseous helium, which can be supplied via lines 28 and 38, respectively. With these arrangements, the particle beams can pass through the resonators without encountering helium atoms. Which of the embodiments shown in FIGS. 3 and 4 is advantageously to be used in the special case depends in particular on the course of the electric field strength within the resonator cavity and on the properties of the superconducting surfaces. The dividing tubes or dividing walls are preferably arranged in those areas in which the electric field strength is still as small as possible. The material for the walls, such as glasses, quartz or synthetic substances can be considered must be electrically non-conductive and the lowest possible dissipation factor tgti have to make it little affects the quality of the resonator as possible. With greater electrical field strengths, the dielectric strength of the material must also be sufficiently high. Act larger electric field strength at the location of the partition walls or separating pipes in the longitudinal direction thereof, it may further be advantageous to design the partitions or separating pipes bulbous, so that they are inclined to the electric field and the potential drop over the partition wall is as small as possible. The partitions can be proportionate

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be made thin because at a helium gas pressure of about

-3 -Q

10 Torr and a vacuum of about 10 * Torr on the

ο Only a pressure of about "1 mg / cm acts on the partition wall.

Finally, FIG. 5 schematically shows an embodiment a device according to the invention in which only relatively small parts of the high-frequency currents carry superconducting surface are in cortact when operated with helium gas, which is also outside the main part of the resonator. The resonator 41 shown in Fig. 5 is via superconducting tubes 42 and 43 to normal conducting or superconducting waveguides 44 and 45 connected via which microwave energy is supplied to him will. Within the superconducting tubes 42 and 43 vacuum-tight microwave windows 46 and 47 are attached, the the inner space 49 of the resonator 41 evacuated through the line 48 against the end parts of the superconducting tubes 42 and 43 and the waveguides 44 and 45 complete over the waveguides 44 and 45 are in operation gaseous helium at a pressure between about 10 and 10 torr which is then in contact with the end portions of the superconducting tubes 42 and 43. Such an arrangement is advantageous, for example, when the end parts of the superconducting tubes or the waveguides, if these are superconducting, have a particularly low critical magnetic field and trigger the magnetic Field collapse, for example in connection with field increases, takes place at these end parts. Through the gaseous This is because helium can also increase the critical magnetic field at these end parts.

It has not yet been possible to clarify exactly what the beneficial effect of helium gas on the superconducting surfaces is based on. Just like the strong increase in the critical field strength, the rapid ones are also rapid

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Changes in the critical magnetic field strength within. half of a few. Seconds and even shorter times depending on the intake and pumping out of the helium completely surprised. In no case is it an improvement in the electrical insulation effect of the Vacuum inside the resonators due to the helium gas, as is the case with superconducting alternating and three-phase

_p cables for helium gas with a pressure between 10 and

—S
10 Torr is known from German Offenlegungsschrift 2 217. The field collapse in the examined resonators does not occur through electrical flashover due to insufficient dielectric strength of the vacuum, but clearly through exceeding the respective critical magnetic flux density. An increase in this critical flux density could not be explained by an improvement in the dielectric strength of the vacuum.

6 claims
5 figures

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Claims (6)

VPA 73/7525 - 15 - Claims (i J Superconducting device for operation with high frequency electromagnetic fields with a frequency of more than 10 MHz with at least one high-frequency currents leading superconducting surface, characterized in that the high-frequency currents leading superconducting Surface is at least partially in contact with gaseous helium during operation of the device, the gas pressure of which less than the lower limit pressure for gas discharge -5
charge and is greater than 10 Torr. 2. Superconductive device according to claim 1, characterized characterized in that the helium gas pressure is at most half to a third of the lower limit pressure for the gas discharge amounts to. 3. Superconductive device according to claim 1 or 2, characterized in that the helium gas pressure is between 5 · 10 ^J and 10 * Torr. 4. Superconducting device according to one of claims 1 to 3, characterized in that the superconducting surface carrying high-frequency currents consists of niobium. 5. Superconductive device according to claim 4, characterized in that the high-frequency currents leading superconducting surface is covered with a niobium oxide layer. 6. Superconductive device according to one of claims 1 to 5, characterized in that at least part of the high frequency currents carrying superconducting surface - 16 ^ 409636/0483 VPA 73/7525 - 16 - from an adjacent vacuum space through a partition . the gaseous helium istjund electrically non-conductive material separated with a minimum of dielectric loss factor is located between said partition and the superconductor surface. 409836/0483