JP2000294398A - Surface polishing method for superconducting acceleration cavity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel surface polishing method for realizing a low-cost and high-performance superconducting acceleration cavity stably exhibiting a high-acceleration electric field. SOLUTION: In this method of polishing a surface of a superconducting acceleration cavity that is a hollow body made of metal having opening parts on its both ends and having at least an inner surface made of niobium, a process for chemically polishing/removing the inner surface and then a process for electrochemically polishing/removing it are together used. The inner surface of the superconducting acceleration cavity is made up of a niobium material or a double layer of niobium and another metal having a thickness of 0.2 to 10 mm, the polished/removed thickness of niobium by the chemical polishing is 50 to 300 μm, and the polished/removed thickness of niobium by the subsequent electrolytic polishing is 5 to 100 μm. The polishing is performed while the hollow body is rotated on its axis with the axis set parallel to the ground.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency accelerating cavity for accelerating charged particles used not only in academic applications but also in medical, engineering and agricultural fields, and in particular, superconducting acceleration used in a superconducting state. The present invention relates to a surface polishing method capable of dramatically improving the performance of a cavity (hereinafter simply referred to as “acceleration cavity”) at low cost.

[0002]

2. Description of the Related Art Conventionally, superconducting accelerating cavities using niobium as a constituent material have been known to chemically polish the surface of the cavity, particularly the inner surface thereof, for the purpose of achieving a high "Q value" and a high "acceleration electric field" ( In general, a method of performing smoothing by any of electrochemical polishing (hereinafter, referred to as “electropolishing”) using a cavity as an anode and an aluminum material as a cathode (hereinafter, referred to as “electropolishing”) is generally used.

[0003] In chemical polishing, typically, a solution using concentrated sulfuric acid instead of concentrated phosphoric acid, concentrated nitric acid, hydrofluoric acid or concentrated phosphoric acid is used, and polishing can be carried out simply by dipping in the solution. It also has advantages such as high polishing rate and difficulty in absorbing hydrogen, but the acceleration performance in a high acceleration electric field as a cavity is not so large. Although the purity of the niobium material has been improved to improve the cavity performance by the chemical polishing method, there is still a limit to the performance improvement.

On the other hand, in the case of electrolytic polishing, a polishing solution such as concentrated sulfuric acid and hydrofluoric acid, or hydrofluoric acid and butanol is generally used, and a complicated jig having a low polishing rate is required. In addition to this, there are many disadvantages, such as performance degradation due to hydrogen storage and the need for vacuum annealing for dehydrogenation. Above all, indispensability of vacuum annealing after electropolishing was considered to be one factor that increased the production cost, and the reality was that it lacked versatility. However, there is no significant decrease in the Q value of the obtained cavity at a high accelerating electric field, and there is a great advantage that this point is much superior to chemical polishing (“SUPERIORITY OF ELECTROPOLISHING OVER CHEMICAL”).
POLISHING ON HIGH GRADIENTS ”Particle Accelerat
ors Vol. 60 pp. 193-217).

[0005]

In order to meet the increasing demands for increasing accelerator construction costs and running costs in recent years, and the demand for construction of high energy accelerators, a so-called stable Q value is exhibited even at a high accelerating electric field. It is indispensable to manufacture a high-performance acceleration cavity. To this end, new development of a technique for polishing the surface of the accelerating cavity made of niobium or niobium clad material has become increasingly indispensable.

As described above, the chemical polishing method having a high polishing rate and a simple chemical polishing method has a problem that a superconducting accelerating cavity exhibiting a stable and high accelerating electric field cannot be obtained. The electropolishing method showing the accelerated electric field has a restriction that vacuum annealing must be performed for the purpose of dehydrogenation after electropolishing. However, once the vacuum annealing is performed, there is a problem of contamination of the inner surface of the cavity due to the influence of the atmospheric gas, and a short re-electropolishing treatment is required to remove the contamination inevitably. Therefore, although acceleration performance is good, not only does it take a process that requires a hand gap called vacuum annealing and reelectrolytic polishing, but vacuum annealing leads to deterioration of niobium strength, so extra materials are required by that much, etc. This was a factor that greatly increased the manufacturing cost of the accelerating cavity.

An object of the present invention is to provide a new surface polishing method for realizing a high-performance superconducting acceleration cavity which stably exhibits a high acceleration electric field at low cost. Improving the performance of the accelerating cavity is not only an academic requirement to be able to investigate the behavior of matter in a high energy state without repairing it,
Even when the same energy is applied, the number of accelerating cavities is small, and as a result, the entire accelerator can be made compact, so that there is a great economic effect that the manufacturing cost, construction cost, and operation cost of the accelerator can be significantly reduced.

[0008]

Means for Solving the Problems The present inventors have investigated and examined in detail the problems of the surface state of niobium and acceleration performance in the accelerating cavity. It became clear that it affected the performance of the cavity. That is, as shown in FIG. 1, the accelerating cavity has a structure in which a number of cylindrical shapes with a bulged central portion are combined.
First of all, it was said that the generation of strain (deformed layer) during the rolling of the sheet material, the subsequent bending of the pipe, and the formation of surface wrinkles and scratches and the occurrence of surface cracks caused by the press forming for forming a dish-like object. In particular, processing distortion and various surface defect layers occur.
According to the results of surface observation and X-ray observation by EPMA, ESCA, optical microscope and the like, and further, actual polishing and dissolution test, various affected layers are 30 to 30
It exists up to about 200 μm, and it is also clear from the processing history of niobium that the thickness of the affected layer varies between 30 and 200 μm. The degree of removal of the defective layer greatly affects the Q value of the accelerating cavity and the stability of the accelerating electric field.

[0009] Then, regardless of whether chemical polishing or electrolytic polishing is used, the reason why the presence of a surface defect layer deteriorates the performance of the accelerating cavity is as follows. It has been found that it greatly contributes to the storage of hydrogen. Fig. 2 shows the relationship between the heating temperature and the amount of released hydrogen, based on the difference in the amount of hydrogen absorbed due to the difference between the methods of chemical polishing and electropolishing. Is clearly shown.

[0010] Here, the amount of stored hydrogen is significantly different between the electrolytic polishing and the chemical polishing. It is considered that this is simply due to the difference in the time of immersion in the polishing liquid, that is, the difference in the polishing rate. In general, it has been confirmed that the removal rate of niobium by chemical polishing is 10 to 20 times faster than that of electrolytic polishing, and this is one reason that vacuum annealing is essential after electrolytic polishing. Was found.

For example, FIGS. 3 and 4 show that a niobium accelerating cavity is polished by electropolishing to 100 μm in the conventional manner, then vacuum annealed at 700 ° C., the cavity performance is confirmed in advance, and the cavity itself is placed in a container. Using SiC as a polishing medium, only the inner surface is mechanically polished while rotating to form a surface defect layer artificially. FIG. 3 is electrolytically polished, FIG. 4 is chemically polished, and 20 μm so that the defect layer remains. The figure shows the results of measuring the performance of the target, which is obtained by polishing and removing niobium and smoothing the niobium. As is clear from these results, if the amount of removal is insufficient even by chemical polishing or electrolytic polishing, only unsatisfactory cavity performance can be obtained.However, if these cavities are vacuum-annealed and appropriately post-treated, , Found that the cavity performance was restored. This means that the hydrogen stored by niobium was released and the performance was restored.

Therefore, if the surface defect layer of the niobium material can be completely removed, there is a possibility that it is not necessary to worry about the problem of hydrogen occlusion by electrolytic polishing alone. Polishing has a disadvantage that the polishing removal rate is extremely slow, and the length of polishing time leads to an increase in cost again.

Based on the above findings, as a result of various studies on low-cost methods for obtaining an acceleration cavity having high acceleration performance,
If polishing is performed in two stages by combining chemical polishing and electrolytic polishing, the advantages of both methods are combined, and in a short time, not only the vacuum annealing after polishing can be completely eliminated, but also the high Q value and acceleration electric field It has been found that an excellent acceleration cavity can be obtained.

That is, the surface defect layer is removed by chemical polishing, and subsequently, the accelerating cavity made of niobium or niobium cladding is smoothed and finished by using electrolytic polishing together, thereby performing vacuum annealing or electropolishing for a long time. Even if it is not necessary, a high-performance acceleration cavity can be obtained in a short time, so from the viewpoint of the manufacturing method and the effect of shortening the processing time, furthermore, there is no reduction in mechanical strength due to vacuum annealing, The use and processing of an extra material for maintaining strength can be made unnecessary, and it can be said that this is an extremely useful polishing method.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. First, after bending the niobium plate, press forming, electron beam welding, etc. to shape the accelerating cavity and mechanically maintain the inner surface,
First, 50 to 300 μm is removed by chemical polishing. 50-300 to the niobium thickness removed by chemical polishing
The reason for having a width of μm is that the depth of the surface defect layer is slightly different depending on the method of manufacturing the cavity.

In the chemical polishing, the entire cavity may be immersed in a polishing solution, or the polishing may be performed by injecting the polishing solution only into the cavity while using the cavity as a container. What can be removed is that the polishing liquid whose temperature has been controlled from one opening toward the other while rotating in the circumferential direction with the axis of the cavity placed parallel to the ground. It turned out that the method of polishing while flowing was the best. Then, when the polishing is completed, the polishing liquid is quickly discharged from the cavity, washed with water, an aluminum counter electrode is inserted, and the electrolytic polishing liquid is caused to flow from the one-way opening as in the case of chemical polishing. Then, electrolytic polishing is performed using the cavity as an anode. The thickness of niobium dissolved and removed by electrolytic polishing is preferably in the range of 5 to 100 μm.

The reason why the electrolytic polishing is performed after the chemical polishing is to reduce the surface residual resistance by smoothing the niobium surface and obtain a high Q value and a high accelerating electric field. Therefore,
Unnecessarily long polishing times not only unnecessarily thins niobium, but also increases polishing costs. On the other hand, if the polishing time is short, there is a problem that the cavity performance cannot be sufficiently improved.

(Embodiment) Total cavity length 570 mm, maximum cavity diameter 210 mm, beam pipe diameter 80 mm, wall thickness 2.5 m
As shown in FIG. 5, a 5-unit cavity of 1300 MHz of m is placed on a gantry having a rotation imparting function and an inverted function, and is rotated at 1 rpm while keeping 1 volume of 89% phosphoric acid kept at 30 ° C., 67
A chemical polishing solution consisting of 1 volume of 1% nitric acid and 1 volume of 40% hydrofluoric acid was passed for 30 minutes (polishing target of 300 μm) while passing at 120 ml / min. After that, while rotating the cavity, the polishing liquid is rapidly discharged, and the water is repeatedly washed over with pure water by repeatedly turning over and inverting, then the aluminum electrode pipe is attached, and the cavity is returned to a horizontal state at 0.4 rpm. 85 volumes of 98% sulfuric acid kept at 25 ° C. while rotating, 4
An electropolishing solution consisting of 10 volumes of 0% hydrofluoric acid was added at 4 l /
The average current density is 50 mA / cm
^{In step 2} , electrolytic polishing was performed for 120 minutes (15 μm polishing target).
Thereafter, the cavity was turned over or turned over, and then washed with pure water and ultrapure water. When the total polishing amount of the cavity was measured with an ultrasonic film thickness meter, an average of 310 μm was obtained, and a high-performance acceleration cavity having a Q value of 10 ¹⁰ at 2 ° K and an acceleration electric field of 35 MV / m was obtained.

For comparison, only 32 chemical polishing liquids were used.
For one minute (320 μm polishing target) and washed,
A sample was continuously polished for 32 hours (320 μm polishing target) only by electrolytic polishing, and the cavity performance was measured at 2 ° K.
The Q value was 10 ¹⁰ , and the acceleration electric fields were 25 MV / m and 34 M / m, respectively, which caused an extreme difference in acceleration performance.

The polishing apparatus shown in FIG. 5 is disclosed in Japanese Patent Application No. Hei 10-160.
No. 446, in which 1 is a metal hollow body, 2 is a gantry, 3 is a motor, 4 is a hollow body holding bracket, 5
a and 5b are sleeves, 6 is a liquid supply pipe, 7a and 7b are cathode terminals, 8a and 8b are carbon brushes, 9a and 9
b is a liquid return pipe, 10 is an internal pressure control port, 11 is an exhaust port, 1
2 is a liquid supply port, 13 is a polishing liquid, 14 and 15 are spur gears, 16
Is a drain port, and 17 is a hydraulic cylinder.

[0021]

According to the first or second aspect of the present invention, the affected layer is effectively removed without impairing the mechanical strength of the niobium material itself and minimizing the influence of the stored hydrogen. This has the effect of achieving smoothing and remarkably improving high-frequency acceleration performance. Further, according to the invention of claim 3, there is an effect that vacuum annealing after polishing is not required.
Further, according to the invention of claim 4, there is an effect that the shape of the accelerating cavity can be effectively utilized, and the polishing can be performed efficiently and accurately.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a sectional configuration of an acceleration cavity to which a polishing method of the present invention is applied.

FIG. 2 is an explanatory diagram showing a difference in hydrogen storage amount between chemical polishing and electrolytic polishing.

FIG. 3 is a characteristic diagram showing the performance of an acceleration cavity subjected to electrolytic polishing after mechanical polishing.

FIG. 4 is a characteristic diagram showing the performance of an acceleration cavity chemically polished after mechanical polishing.

FIG. 5 is a front view of a polishing apparatus for performing the polishing method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal hollow body 2 Stand 3 Motor 13 Polishing liquid

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Shuhei Nomura 5-12-20 Himejima, Nishiyodogawa-ku, Osaka-shi, Osaka F-term Co., Ltd. F term (reference) 2G085 BA04 EA01 EA04

Claims (4)

[Claims] 1. A method for polishing a surface of a superconducting accelerating cavity which is a metal hollow body having openings at both ends and at least an inner surface of which is made of niobium, wherein the inner surface is first chemically polished and removed. And a subsequent step of electrochemically polishing and removing the surface. 2. The superconducting accelerating cavity has an inner surface of 0.2 to 10
2. The method for polishing a surface of a superconducting accelerating cavity according to claim 1, wherein the method comprises a niobium material having a thickness of 1 mm or a multilayer of niobium and another metal. 3. The superconducting accelerating cavity according to claim 1, wherein the removal thickness of niobium by chemical polishing is 50 to 300 μm, and the removal thickness of niobium by subsequent electrolytic polishing is 5 to 100 μm. Surface polishing method. 4. A metal hollow body having openings at both ends, wherein a superconducting acceleration cavity having at least an inner surface made of niobium is set so that an axis thereof is horizontal with the ground. After the inner surface of the niobium material is polished and removed while flowing a chemical polishing liquid from one opening to the other while applying rotation around the center, then an aluminum electrode is attached and electrolytic polishing is performed. A method for polishing a surface of a superconducting acceleration cavity, wherein electrolytic polishing is performed again with a liquid.