JP4111441B2 - Large-diameter electrostatic accelerator with discharge breakdown prevention function - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is an electrostatic accelerator that accelerates a beam of charged particles such as ions and electrons to a high energy of MeV class. Conventional electrostatic accelerators widely used in high energy physics and nuclear physics insulate high voltage applied to the accelerator with an expensive ceramic tube to accelerate charged particles. The invention prevents the electric field concentration of the insulating tube by mounting a large electric field relaxation ring, and does not cause discharge breakdown even in an inexpensive FRP (glass fiber reinforced plastic) insulating tube instead of a ceramic tube. An electrostatic accelerator is provided.
[0002]
[Prior art]
In an electrostatic accelerator, an electric field is formed inside the accelerator by a high voltage applied from the outside, and charged particles are accelerated by the electric field. In a conventional electrostatic accelerator, an insulating tube made of a high-melting-point inorganic material such as ceramic is used for this high-voltage insulation. As shown in the “conventional accelerator” portion of FIG. It is joined to a flange (metal) for supplying a potential to the metal by brazing or the like.
[0003]
This structure only prevents the electric field from concentrating on the triple point, which is the contact point on the vacuum side inside the flange (metal), the insulating tube, and the accelerator due to the difference in dielectric constant between the vacuum and the insulating material. However, even if a small amount of electric field concentration and accompanying electric discharge breakdown occur, the insulating material has a high melting point, so that damage to the insulating tube is prevented.
[0004]
For example, there has been a report on an electrostatic accelerator (for example, Non-Patent Document 1) that uses alumina ceramic for the insulating tube and devised the inner surface shape to reduce the electric field at the triple point, and is an accelerator that uses FRP for the insulating tube. In addition, there is a performance report when it is difficult to stably maintain 1 MV in a report before the large electric field relaxation ring is mounted (for example, Non-Patent Document 2).
[0005]
[Non-Patent Document 1]
1979 LINEAR ACCELERATOR CONFERENCE, DESIGN CRITERIA FOR HIGH VOLTAGE, HIGH CURRENT ACCELERATING COLUMNS, AECL-6745 (p.1-4)
[Non-Patent Document 2]
REVIEW OF SCIENTIFIC INSTRUMENTS, VOLUME 69, NUMBER 2, FEBRUARY 1998
[0006]
[Problems to be solved by the invention]
However, as shown in “Conventional Accelerator” in the lower left of FIG. 1, the portion where “the electric field relaxation effect is insufficient” still remains in the vicinity of the three points, and it is difficult to manufacture a large-diameter ceramic tube. However, when the insulation tube is damaged, the insulation tube and the flange (metal) are joined by brazing or the like, so that they cannot be easily disassembled.
[0007]
[Means for Solving the Problems]
The present invention is an electrostatic accelerator that solves these problems. That is, the present invention relates to an electrostatic accelerator that accelerates a beam of charged particles such as ions and electrons to a high energy of MeV class, a metal part that supplies a potential inside the accelerator, an FRP insulating tube, and a vacuum side inside the accelerator. In the vicinity of the triple point as a contact, a metal electric field relaxation ring having an arc cross-sectional shape for reducing electric field concentration having a radius equal to or greater than the thickness of the FRP is attached to prevent electric field concentration in the vicinity. The present invention provides an electrostatic accelerator that prevents discharge breakdown of an insulating tube by reducing the electric field strength to 1.5 kV / mm or less.
[0008]
As shown in the “accelerator of the present invention” in the lower right of FIG. 1, the radius of the cross-sectional shape of the electric field relaxation ring is 120 mm (R120), and the outer shape of the ring cross-section forms a ¼ arc. On the other hand, the thickness of the FRP insulating tube is 54 mm, and the radius of the cross-sectional shape of the electric field relaxation ring is greater than the thickness of the insulating tube. Further, the radius of the roundness at the tip of the cross-sectional shape of the ring is 10 mm (R10).
[0009]
As shown in the lower right of FIG. 1, the present invention inserts a metal flange between individual large-diameter simple cylindrical FRP insulating pipes, and couples them with a clamp to form a cylindrical accelerator. Form a structure. Inside the cylindrical accelerator structure, a metal electric field relaxation ring with an arc cross-sectional shape is attached at a position in contact with the inner surface of the insulating tube at the inner extension of the metal flange inserted between the individual insulating tubes. ing. In the accelerator of the present invention, charged particles such as ions or electrons are generated from an ion source, and a high voltage is applied from the outside to a porous electrode installed at the center of the insulating tube through a metal flange, and charged particles are placed in the insulating tube. An accelerating electric field is generated to accelerate the charged particles from the ion source to a desired speed. At this time, since the electric field concentration on the insulating tube is suppressed due to the presence of the electric field relaxation ring, discharge breakdown of the insulating tube is prevented.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a cross section of a conventional accelerator and a cross section of an electrostatic accelerator according to the present invention. The features and effects of the accelerator of the present invention are as follows.
[0011]
1) In the present invention, in order to reduce electric field concentration at the triple point that is a metal part (flange) for supplying electric potential inside the accelerator, the FRP insulating tube, and the vacuum contact inside the accelerator, A metal electric field relaxation ring having an arc cross-sectional shape having a radius equal to or greater than the wall thickness is attached, and the electric field strength is reduced to 1.5 kV / mm or less. In FIG. 1, the electric field relaxation ring is attached only to the accelerator inner triple point, but even if a similar metal structure is attached to the accelerator outer side, the electric field relaxation effect of the inner triple point can be obtained.
[0012]
2) As an effect, it is possible to use FRP (glass fiber reinforced plastic) as an insulating tube material instead of a high-melting-point inorganic material such as ceramic used in a conventional accelerator. FRP is a low-melting-point organic material that softens at about 100 ° C., but performance deterioration due to micro-discharge cannot occur at the above electric field strength. Conventionally used ceramics require processes such as compression shaping and firing, so that the insulating tube is expensive and a large diameter exceeding 1.6 m cannot be manufactured at present. By using FRP, it is possible to manufacture at a low cost even with a large diameter exceeding 1.6 m.
[0013]
3) Although there are a few electrostatic accelerators that use FRP as an insulating tube in the past, the cross-sectional shape is thin or thick, and the discharge breakdown is prevented by reducing the electric field concentration and extending the creepage distance. . However, it is not easy to manufacture large-diameter FRPs having these complicated cross-sectional shapes, which causes an increase in cost. In the present invention, an inexpensive electrostatic accelerator is provided by using an FRP insulating tube having a simple cylindrical cross section.
[0014]
4) The FRP insulating tube is fixed by clamping to a metal part or metal support structure that supplies a potential to the inside of the accelerator. The airtightness inside and outside the accelerator is maintained by inserting an O-ring between the two (between the insulating tube and the flange (metal) or metal support structure). This structure has not been used heretofore because a gap is formed at the triple point due to the crushing of the O-ring and electric field concentration occurs. However, the electric field relaxation ring of 1) can reduce electric field concentration and prevent discharge breakdown. Even if the surface of the insulating tube is damaged by electric discharge breakdown, it is possible to easily disassemble the accelerator and replace only the damaged FRP insulating tube.
[0015]
5) FIG. 2 shows the effect of improving the withstand voltage performance of the accelerator according to the present invention. FIG. 2 shows the results of a withstand voltage test conducted on only one stage of a multi-stage electrostatic accelerator. When an FRP insulating tube was used in the conventional accelerator structure, the design rated voltage of 200 kV was not reached even if 6 hours were spent for conditioning (dehydration operation), and 180 kV was finally maintained.
[0016]
On the other hand, when a withstand voltage test was carried out with an electric field relaxation ring having a radius of 120 mm (FRP tube thickness of 54 mm) and a 1/4 arc cross section in the vicinity of the cathode triple point of the same accelerator structure, the design rating was only a few minutes. After reaching 200 kV, and after 6 hours of conditioning, 300 kV corresponding to 1.5 times the rating could be stably maintained.
[0017]
【The invention's effect】
In the present invention, a metal part for supplying electric potential to the accelerator, an FRP insulating tube, and a metal electric field relaxation ring having an arc cross-sectional shape for reducing electric field concentration in the vicinity of the triple point, which is a vacuum contact inside the accelerator, are mounted. Thus, even a simple cylindrical FRP insulating tube has a remarkable effect peculiar to the present invention that does not cause discharge breakdown of the insulating tube because electric field concentration near the triple point can be prevented. is there.
[Brief description of the drawings]
FIG. 1 is a diagram showing a comparison of internal structures of a conventional accelerator and an accelerator according to the present invention.
FIG. 2 is a diagram showing a withstand voltage performance of a conventional accelerator and an accelerator equipped with an electric field relaxation ring according to the present invention.
FIG. 3 is a bird's-eye view of the inside of an accelerator equipped with the large electric field relaxation ring of the present invention.

Claims (2)

  In an electrostatic accelerator that accelerates a beam of charged particles of ions or electrons to a high energy of MeV class, a metal part that supplies a potential to the inside of the accelerator, a glass fiber reinforced plastic (FRP) insulating tube, and a vacuum side contact inside the accelerator In the vicinity of the triple point, a metal electric field relaxation ring having a radius equal to or greater than the thickness of the FRP and having an arc cross-sectional shape for reducing the electric field concentration in the vicinity is mounted, and the electric field strength is 1.5 kV / mm. An electrostatic accelerator that prevents electric field concentration in the vicinity and prevents discharge breakdown of the insulating tube by reducing to the following. A plurality of cylindrical glass fiber reinforced plastic (FRP) insulation tubes having substantially the same shape are coaxially arranged , and a metal flange for supplying a potential to the inside of the accelerator is inserted between the insulation tubes. A cylindrical accelerator structure is formed by coupling, and inside the accelerator structure, below the inner extension of the metal flange inserted between the individual insulating tubes, the metal flange, the insulating tube In addition, a metal electric field relaxation ring having an arc cross-sectional shape for reducing electric field concentration is mounted in the vicinity of the triple point that is a vacuum side contact inside the accelerator, and the radius (R120) of the cross-sectional shape of the ring is set to the thickness of the insulating tube. The electrostatic accelerator according to claim 1, wherein the electric field strength in the vicinity is reduced to 1.5 kV / mm or less by forming it to be thicker or thicker.

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