WO2014008943A1

WO2014008943A1 - Electrical insulator for high-voltage electrostatic generator

Info

Publication number: WO2014008943A1
Application number: PCT/EP2012/063744
Authority: WO
Inventors: Paul Beasley; Oliver Heid; Robert Gregory SELWAY; Peter Simon Aptaker
Original assignee: Siemens Aktiengesellschaft
Priority date: 2012-07-12
Filing date: 2012-07-12
Publication date: 2014-01-16

Abstract

An essentially disk-shaped solid electrical insulator, having a recessed metal flux ring (52; 54) provided in the material of the disk-shaped solid electrical insulator on an upper or lower surface thereof near an edge (28) and a high-voltage electrostatic generator comprising an assembly of concentric electrically conductive shells (42) separated by solid electrical insulators mounted between adjacent conductive shells (42), wherein at least one of the said electrical insulators is a said essentially disc-shaped solid electrical insulator.

Description

ELECTRICAL INSULATOR FOR HIGH-VOLTAGE ELECTROSTATIC GENERATOR

The present invention relates to high-voltage electrostatic particle accelerators, such as that described in XP-002665162 Proceedings of IPAC '10 Kyoto, Japan, pp. 711-713 P. Beasley, 0. Heid, T. Hughes "A new life for High Voltage Electrostatic accelerators" . In such accelerators, concentric conductive half-shells are provided, electrically isolated from one another, but interconnected with diodes and capacitors in a Cockroft-Walton (Greinacher) cascade. Application of an AC voltage to the assembly causes each shell to be charged to a certain voltage with respect to the next, resulting in a very large potential difference between the innermost and outermost shells.

By providing a path for a particle beam through the shells, a compact high-voltage electrostatic particle accelerator may be constructed. Typically, the space around and between shells is evacuated. Fig. 1 shows a drawing from the cited proceedings, showing an example of such an accelerator.

The accelerator therefore comprises a series of electrically conductive shells, spaced apart by a small gap. A significant DC potential difference accumulates across that gap. For the structural integrity of the accelerator, it is necessary to provide solid electrical insulators between half-shells. A schematic representation of an edge of such a solid electrical insulator is illustrated in Fig. 2. A higher voltage shell 20 and a lower voltage shell 22 are shown, separated and joined by a solid electrical insulator 24. Where edges of the insulator 24 meet respective surfaces of the shells 20, 22, triple points 26 occur. Here, metal, insulator and vacuum meet. At such points 26 (which, being in three-dimensional space, are in fact lines) , there is a high probability of breakdown and flashover between the adjacent shells 20, 22. This characteristic is described, for example, in Journal of Applied Physics, 102, 033301 (2007) "Electric Field and Electron Orbits near a Triple Point" Nicholas M Jordan et al .

The problem of susceptibility to breakdown and flashover at such points has conventionally been addressed by attempts to reduce voltage and voltage gradient at such points. Fig. 3 illustrates one known method for reducing electrostatic field gradient at triple points. Rather than having an edge which meets the adjacent surfaces of shells 20, 22 perpendicularly, the edges 28 of the solid electrical insulator 24 are tapered such that their surfaces meet the shells 20, 22 at an angle a to a normal to the adjacent surfaces. In the illustrated example, a=45°, but other angles may be found effective. Fig. 3 also illustrates the convention whereby angles a which cause an area of contact of the insulator with cathode 22 to be larger than the area of contact with the anode are considered to be positive angles, while angles a which cause an area of contact of the insulator with cathode 22 to be smaller than the area of contact with the anode are considered to be negative angles.

An alternative, or complementary, arrangement is illustrated in Fig. 4. Additional de-stressing components 30 are provided in the region of the triple point to reduce local electric field gradient. The electric field lines 32 illustrate how the additional de-stressing components reduce stress at the triple point by spreading the electric field over a larger volume . However, experimentation has found that for the required reduction in local field strength to be brought about by shaping the edges of solid insulators, as shown in Fig. 3, or the provision of additional de-stressing components as shown in Fig. 4, requires more space than is available between shells of a high-voltage electrostatic particle accelerator of the type addressed by the present invention, such as illustrated in Fig. 1.

It has been found unsuitable to use arrangements such as shown in Fig. 4, as there is generally insufficient space to provide additional de-stressing components 30. Furthermore, the electrostatic field lines 32 illustrated in Fig. 4 demonstrate that a taper on the edge of insulator 24 is insufficient to achieve the voltage gradient required. Accordingly, the present invention provides improved electrical isolators for use between small gap concentric shells of a high-voltage electrostatic generator, such as used in the particle accelerator discussed above. The present invention extends to high-voltage electrostatic generators employing such electrical isolators.

The present invention accordingly provides apparatus as defined in the appended claims. The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein: Fig. 1 schematically illustrates a cross-section of a conventional high-voltage electrostatic particle accelerator; Fig. 2 schematically illustrates a conventional solid electrical isolator placed between shells of a conventional high-voltage electrostatic particle accelerator; Fig. 3 schematically illustrates further conventional solid electrical isolators placed between shells of a conventional high-voltage electrostatic particle accelerator;

Fig. 4 schematically illustrates a conventional arrangement of solid electrical isolator placed between shells of a conventional high-voltage electrostatic particle accelerator with additional components for reducing electric field gradient at triple points;

Fig. 5 illustrates a solid insulator useful in a high-voltage electrostatic particle accelerator;

Fig. 6 illustrates a schematic half-axial cross-section of a solid insulator, comprising metal rings, according to an embodiment of the invention;

Fig. 7 illustrates a high-voltage electrostatic particle accelerator, employing solid insulators, according to an embodiment of the invention; and

Fig. 8 illustrates a schematic cross-section of a solid insulator, comprising metal rings, according to an embodiment of the present invention, in position between concentric conductive shells of a high-voltage electrostatic particle accelerator such as illustrated in Fig. 7.

Fig. 7 illustrates a high-voltage electrostatic particle accelerator, employing solid insulators, according to an embodiment of the invention. An outer casing 40 is provided, as a vacuum vessel. Several concentric shells 42 are shown. As explained above, these shells are connected together by diodes and capacitors (not shown) to provide a high electrostatic voltage gradient. Structural integrity is provided by solid insulators 44 separating and joining the concentric shells. As illustrated, these solid insulators 44 are preferably offset with respect to corresponding solid insulators between adjacent pairs of shells. Mechanical supports 46 position and retain the assembly of concentric shells within the vacuum vessel 40. As illustrated, similar solid insulators 48 are provided at upper and lower extremities of the vacuum vessel supports 46.

According to an embodiment of the present invention, the solid insulators 44 are essentially disc-shaped, including recessed metal field rings, with a profile as discussed below. The solid insulators 48 may have a similar profile. If required, or preferred, by the design, solid insulators 48 may be smaller, larger, thinner or thicker than solid insulators 44.

Fig. 5 shows a perspective view of a solid insulator 50 which may be employed as solid insulators 44 and/or solid insulators 48 in a high-voltage electrostatic particle accelerator such as shown in Fig. 7 according to the prior art, and which may be improved according to the present invention by incorporation of recessed metal field rings.

Preferably, the solid insulators 44, and preferably also insulators 48, have a rotationally symmetric profile which includes a tapered edge, as shown in Figs. 5 and 3, and also includes recessed metal flux rings at specific locations on the upper and lower surfaces of the solid insulator.

Fig. 6 shows an example axial half-cross section of an insulator 44 according to an embodiment of the invention. The insulator has rotational symmetry about axis A-A. According to an aspect of the present invention, the insulator is provided with a metal flux ring 52 recessed into its upper surface, and a further metal flux ring 54 recessed into its lower surface. The recessed metal flux rings are arranged such that they will be in electrical contact with the metal shells, when in use. This will cause reduction of the electrostatic potential gradient in the proximity of the triple point 26. However, the electrostatic potential gradient inside the insulator increases but these tend to have a much greater gradient breakdown strength than vacuum, and so the aim of the present invention may be achieved.

The presence of the flux rings spreads the potential gradient over a larger volume and so reduces the electric field strength at the triple point when such insulators are installed. The presence of each flux ring 52, 54 displaces the position of the electrostatic potential of the connected shell to inside the insulator (increasing its local gradient) in order to draw the flux away from the triple point. So the flux rings act as an extension to the shells. A further preferred feature of this insulator is the rounded upper and lower extremities 56, 58 of the edge 60 of the insulator. This has also been found to reduce the electric field strength at the triple point.

When used as shown in Fig. 7, placed between concentric shells at differing voltages, the insulators of the present invention, such as illustrated in Fig. 6, serve to reduce the electric field strength in the region of the triple points 26.

The optional rounding at upper and lower extremities 56, 58 and tapered edge surfaces 60 (Fig. 6) add to the reduction in electrostatic potential gradient at the triple points 26. The tapered edge surfaces may have an angle a of 30° - 60° with respect to the normal 62 to the upper or lower surface of the insulator 44.

While the invention has been discussed with particular reference to high voltage electrostatic particle accelerators, it may be applied to high-voltage electrostatic generators employing concentric electrically conductive shells as used for any purpose. Furthermore, the solid electrical insulators of the present invention, incorporating flux rings near their edge, may find application in any assembly where large potential differences are experienced between closely spaced parallel conductive surfaces, similar to the arrangement exemplified in Fig. 4.

While the solid electrical insulators 44 as employed in the present invention are essentially disc-shaped, and may have rotational symmetry about an axis (A-A) , they are not necessarily circular in shape, such as oval, and may have non- planer upper and lower surfaces. It is believed preferable, however, for the insulators to have a shape which does not include any sharp angles, which may attract high electric field strength.

Although it may be preferable for all electrical insulators 44, 48 to include the recessed flux rings 52, 54, the present invention extends to embodiments in which at least one of the electrical insulators is provided with at least one such flux ring. In certain embodiments, more than one flux ring 52, 54, may be provided in any one upper or lower surface of an electrical insulator, being concentrically arranged on that surface. Alternatively, some embodiments of the solid electrical insulator of the present invention may be provided with one or more metal flux ring on an upper or lower surface and no metal flux rings on the other of the upper and lower surfaces .

Claims

CLAIMS :

1. An essentially disc-shaped solid electrical insulator (44), having a recessed metal flux ring (52; 54) provided in the material of the electrical insulator on an upper or lower surface thereof near an edge (28) .

2. An essentially disc-shaped solid electrical insulator according to claim 1, having a recessed flux ring (52; 54) provided in the material of the electrical insulator on each of an upper surface and a lower surface thereof near a respective edge (28) .

3. An essentially disc-shaped solid electrical insulator according to claim 1 or claim 2, wherein more than one flux ring (52; 54) is provided in a selected one of an upper surface and a lower surface of the electrical insulator, said flux rings being concentrically arranged on the selected surface .

4. An essentially disc-shaped solid electrical insulator according to any preceding claim, having a tapered edge surface (60), the tapered edge surface being at an angle (a) of 30° - 60° with respect to a normal (62) to the upper or lower surface of the electrical insulator.

5. An essentially disc-shaped solid electrical insulator according to any preceding claim, wherein the at least one electrical insulator having rounded upper and lower extremities (56, 58) of an edge surface (60) .

6. A high-voltage electrostatic generator comprising an assembly of compact concentric electrically conductive shells (42) separated by solid electrical insulators (44) mounted between adjacent conductive shells or plates, characterised in that at least one of the electrical insulators is essentially disc-shaped, having a recessed flux ring (52; 54) provided in the material of the electrical insulator on an upper or lower surface thereof near an edge (28) .

7. A high-voltage electrostatic generator according to any preceding claim, comprising a vacuum vessel (40) and mechanical supports (46) which position and retain the assembly of concentric shells within the vacuum vessel.

8. A high-voltage electrostatic particle accelerator comprising a high-voltage electrostatic generator according to claim 6 or claim 7.

9. An assembly comprising two conductive plates (20, 22) separated and joined by an essentially disc-shaped solid electrical insulator according to any of claims 1-5.