FR2657195A1 - Anode comprising cavities for X-ray tubes - Google Patents

Abstract

The invention relates to X-ray tubes and, more particularly, to improvements to the anodes of such tubes for increasing the thermal emissivity thereof. The invention resides in the fact that the surface of the anode is pierced with cavities whose depth H is large compared to the radius R of the base. Experiments and calculations show that the gain in emissivity increases as the emissivity of the material of the anode decreases, the ratio H/R increases and the number N of faces of the cone decreases.

Description

CELL ANODE FOR
X-RAY TUBES
The invention relates to X-ray tubes and more particularly to improvements to the anodes of such tubes to increase their thermal emissivity.

An X-ray tube comprises, in a vacuum enclosure, a cathode made up of a heated filament which emits electrons and of a concentration device backed by the filament which focuses the emitted electrons towards an anode brought to a positive potential compared to at the cathode. The point of impact of the electron beam on the anode constitutes the source of X-radiation.

The anode generally consists of a metal block on which is deposited a layer of a target material of the type with a high atomic number, refractory and good conductor of heat such as, for example, tungsten, molybdenum or an alloy containing at least one of these elements.

The target layer is bombarded by the electron beam over a small area called the focal point. As a result of the low energy efficiency, only one percent of the energy is transformed into X-rays, most of which is transformed into heat. The metal block of the anode must therefore be provided to dissipate the stored heat.

The heat which is stored in the anode is transferred in two ways to the external medium on the one hand, by conduction by the anode material towards the axis of the rotor and the medium external to the tube and, on the other hand, by the radiation from the anode in all directions. In the latter case, the radiation is proportional to the fourth power (T4) of the temperature T of the surface of the anode, to the surface of the anode and to the emissivity of the anode material.

We therefore understand that we tried to increase these three elements to increase the radiation but certain limitations exist. Thus, the operating temperature of the anode cannot exceed certain values which depend, in particular, on the material used and on its mechanical behavior at high temperatures.

The increase in the area of the anode goes hand in hand with its dimensions, but this limits its speed of rotation.

Also, the object of the present invention is to provide an anode for an X-ray tube which has increased emissivity.

The invention relates to an anode for an X-ray tube, characterized in that all or part of the surface of the anode is pierced with cells.

The cells are preferably in the form of cones, the base of which is on the surface of said anode.

The base of the cone can be circular, elliptical, polygonal or other.

For better emissivity, the base is triangular and the depth must be greater than the radius of the circle containing the base of the cone.

Other objects, characteristics and advantages of the present invention will appear on reading the following description of a particular embodiment, said description being made in relation to the accompanying drawings in which - Figure 1 is a perspective view of a
cell anode for X-ray tube according to
present invention, - Figure 2 shows a form of cells based
polygonal, - figures 3, 4 and 5, represent diagrams
equivalent emissivity as a function of emissivity
material for different parameters.

FIG. 1 represents an anode 9 of an X-ray tube which has the general shape of a disc 10 comprising two faces 11 and 12 parallel to each other and an inclined face 14 over all or part of which is deposited a layer 15 of a X-ray emissive material. The connection between the inclined face 14 and the face 11 is effected by a face 16 formed by the periphery of the disc 10. The disc 10 is designed to rotate around an axis of rotation 13 which is perpendicular with parallel faces 11 and 12 in their center.

The disc 10 is made of metallic material such as molybdenum, tungsten or an alloy containing at least one of these elements, or graphite or a composite material of the carbon-carbon type. The material of layer 15 is of the type with a high atomic number such as tungsten.

When an electron beam bombards layer 15, almost all (around 99%) of the beam energy is transformed into heat absorbed by the disc 10.

This absorbed heat is removed towards the outside of the tube, as indicated in the preamble, by conduction via the axis of the rotor and by radiation towards the envelope of the tube.

The invention proposes to increase the dissipation of heat by radiation by increasing the emissivity of the disc, the temperature and the area of the disc remaining equal elsewhere.

To increase this emissivity, the invention proposes to modify the surface of the disc by creating cells 17, for example of conical shape with a polygonal base.

The increase in the emissivity of the disc depends on the disc material, the characteristics of the cells and their density on the surface of the disc.

The characteristics of the cells and their influence on the emissivity were determined using the diagram in FIG. 2 which represents a cone 18 with vertex 0 having a hexagonal base 19. This six-sided cone 20 has a height H between the vertex 0 and base 19 and a radius R which defines the circumference passing through the vertices of the regular hexagon of base 19.

As a result of the existence of the faces 20 of the cone, the emissivity of the base 19 of the cone is greater than that which it would have in the absence of the cone. The calculations make it possible to determine that the emissivity Ep of the cell "brought back" to the surface at the base 19 is given by the formula
#i
#p = (1)
1 - (1 - #i) (1 - α) formula in which - Ei is the emissivity of the anode material, - a = Sp with Sp the area of the polygon in area NSi - If the area of a face of the cone and N the number of
cone faces.

et l'on peut alors tracer les courbes représentatives de la fonction
Ep = f (Ei) pour N = 12 et H/R variable (figure 3) pour N = 3 et H/R variable (figure 5) pour H/R= 2 et N variable (figure 4)
Ces différentes familles de courbes montrent que le gain en émissivité est d'autant plus grand que - l'émissivité du matériau Éi est faible, - le nombre N de faces du cône est petit, - le rapport H/R est grand avec une saturation pour un
rapport H/R > 25.If we replace Sp and Si by their value expressed as a function of N, the radius R of the base polygon and the height H of the C cone, we obtain

and we can then draw the curves representative of the function
Ep = f (Ei) for N = 12 and variable H / R (figure 3) for N = 3 and variable H / R (figure 5) for H / R = 2 and variable N (figure 4)
These different families of curves show that the gain in emissivity is all the greater as - the emissivity of the material Ei is low, - the number N of faces of the cone is small, - the H / R ratio is large with saturation for a
H / R ratio> 25.

Preferably, the depth H of the cells is between two and twenty-five times the radius R.

The emissivity Ep is the equivalent emissivity of a cell; for the entire surface of the disc, the total emissivity eT is given by
ET = (ld) Ei + d.Ep (3) with d the ratio of the total area of the base polygons and the area of the disc.

As an indication, for a molybdenum disc with
Ei = 0.2, a density d = 0.5 and H / R = 2, we get
ET = 0.35, i.e. a gain close to 2.

To make cells in the surface of the disc, several methods can be implemented such as chemical attack by means of a mask or mechanical percussion using needles whose points have the shape of cones that we want to get on the surface of the disc.

Furthermore, if it is desired to obtain a variable emissivity according to the position on the disc, it is possible to modify the characteristics of the cells so that this is so.

The invention has been described in the case of cells in the form of cones with a regular polygonal base, but it is clear that it also applies to other forms of cells, in particular of conical shape with circular base or other.

Claims (6)

1. Anode for X-ray tube, characterized in that all or part of the surface of the anode (9) is pierced with cells (17). 2. Anode according to claim 1, characterized in that the cells (17) are in the form of cones (18) whose base (19) is on the surface of said anode. 3. Anode according to claim 2, characterized in that the cones (19) have a polygonal base. 4. Anode according to claim 3, characterized in that the base of the cones (18) is triangular. 5. Anode according to any one of the preceding claims, characterized in that the depth (H) of the cells is greater than the radius (R) of the circle containing the opening of the cells (17) at the surface of said anode. 6. Anode according to claim 5, characterized in that the depth (H) of the cells is between two and twenty-five times the radius (R).