Patent Application
20180053623
The present invention relates to an anode stack and in particular it relates to an anode stack for cooling and electrically insulating a high voltage anode of an X-ray device.
An X-ray tube is a type of vacuum tube that converts electrical power into X-rays. In such devices, the cathode is a negatively charged electron emitter and the anode is a positively charged electron collector. Between the cathode and the anode, a flow of electrical current is established using a high voltage power source, typically between 4 kV and 500 kV, which accelerates the electrons in their path. However, efficiency of the X-ray tube is very low and the yield of X-ray production is usually less than 1%, with the remaining 99% of the input energy being converted into heat.
The excess heat that is produced in the X-ray tube must be removed from the high voltage anode in order to prevent the device from overheating and to enable continuous operation. Thus, thermal management of this generated heat is an important consideration in the design and manufacture of X-ray tubes in addition to the electrical insulation considerations. For example, the high voltage anode must be electrically isolated from mechanical components of the X-ray tube including the mechanical support, and at the same time be capable of removing excess heat.
In the past, in order to avoid overheating, X-ray tubes have been immersed in cooling fluids such as oil that are capable of heat transfer through a process of heat convection and which have high dielectric strength so that heat is dissipated away from high voltage anode as the cooling fluid circulates. More recently, it has been proposed to use anode stacks that sit between the high voltage anode and the mechanical support and which are made from materials that are both electrically insulating and thermally conductive in order to achieve the same cooling effect but without the burden and constraints of a liquid coolant.
In some anode stack designs, a dielectric disc of high thermal conductivity is sandwiched between two metallic discs, also of high thermal conductivity, one of which is coupled to the high voltage anode, and the other of which is coupled to a heat sink. In these devices, the high thermal conductivity of the dielectric and metallic discs allows effective heat transfer away from the high voltage anode.
However, this design has a number of drawbacks related to high electric fields at the triple point region, i.e. the region where the dielectric, metal and vacuum meet; the electric field at the triple point regions of traditional anode stacks can be much greater than the surrounding electric field depending on the geometry of the component parts of the anode stack and the dielectric constants of the materials. These locally intensified electric fields are often the cause of electrical arcing and electrical breakdown that ultimately leads to device failure.
The present inventors have recognised that an improved and more reliable anode stack is required, which provides for reduced electrical arcing and electrical breakdown in the X-ray device.
Thus, viewed from one aspect of the present invention, there is provided an anode stack for cooling and electrically insulating a high voltage anode of an X-ray device, the anode stack comprising: a conductor member and a dielectric member, the conductor member having a main body and a peripheral portion, wherein the dielectric member overlies the main body of the conductor member, wherein the main body of the conductor member is arranged to couple with the dielectric member at one surface, and with an end of the high voltage anode at an opposing surface in use, and wherein the peripheral portion of the conductor member comprises an annular region that surrounds at least a part of the dielectric member and which is spaced therefrom.
As defined above, in its most basic configuration, the conductor-dielectric arrangement of the anode stack of the present invention is intended to be inserted between the high voltage anode and a mechanical support of the X-ray device. The anode stack may also separate the high voltage anode from the high voltage potential of the X-ray device, a heat exchanger and/or a ground plane.
The dielectric member overlies the main body of the conductor member in the sense that the surface of the main body of the conductor member that couples with the dielectric member is the geometric projection of the surface of the dielectric member that couples with the main body of the conductor member. In other words, the footprint of the dielectric member is the same as the main body of the conductor member.
The dielectric member does not overlie the peripheral portion of the conductor member, i.e. the peripheral portion is the part which is not encompassed by the main body and it includes an annular region that encircles the main body.
The shape of the anode stack at the peripheral portion of the conductor member is critical for reducing the electric field at the triple point region; the annular region that surrounds at least a part of the dielectric member and which is spaced therefrom provides screening and thus reduces electrical arcing and electrical breakdown at the triple point region. Accordingly, the reliability of the anode stack of the present invention is greatly increased compared to previous versions whilst maintaining its ability to remove heat from the high voltage anode at a very high rate and act as an electrical insulator.
It is preferable that the annular region of the peripheral portion of the conductor member surrounds a joining region between the dielectric member and the main body of the conductor member, the surface perimeter of said joining region preferably coinciding with the triple point region of the anode stack. Thus, the shape of the anode stack at the peripheral portion of the conductor member reduces the electric field at the joining region. Preferably, the joining region has a perimeter surface comprising surfaces of the dielectric member and of the main body of the conductor member.
Thus, when travelling along the perimeter surface of the dielectric member within the joining region to reach the perimeter surface of the main body of the conductor member within the joining region, or vice versa, the path taken is smooth and does not involve any sharp angles. The path may advantageously be substantially linear, or in other words, the surface of the dielectric member within the joining region and the surface of the main body of the conductor member within the joining region are at 180 degrees to one another.
In a particularly preferred embodiment, the joining region is cylindrically shaped. In this arrangement as well as other prismatically shaped arrangements of the joining region, all normal axes to the perimeter surface of the joining region may be coplanar or lie in parallel planes. As a result, the electric field in the joining region is reduced further by the removal of sharp edges in said region.
An anode stack direction may be defined from the dielectric member to the conductor member along a major axis shared by the dielectric member and the conductor member. Therefore, the perimeter surface of the joining region may coincide with the stack direction.
The annular region of the peripheral portion of the conductor member may define a trench in the conductor member that extends in the anode stack direction. Advantageously, the trench removes any sharp angles from the joining region and so the electric field in this region is reduced even further by virtue of the shape of the trench. More preferably, the trench height may be at least equal to the trench width and yet more preferably, the trench height may be substantially equal to the trench width. These trench dimensions have been selected in such a way that electric field at the triple point region is reduced by at least two times compared to the case when no trench is used.
The dielectric member and the main body of the conductor member may each be prismatically shaped. However, in a particularly preferred embodiment, the dielectric member and the main body of the conductor member are each cylindrically shaped, and not just at the joining region. Thus, the entire external surface perimeter of the dielectric member and the main body of the conductor member have no sharp edges and so the electric field in the proximity of these anode stack parts is greatly reduced compared with an anode stack having sharp interface regions between the dielectric member and the main body of the conductor member.
The ratio of thermal expansion coefficient between the conductor member to the dielectric member is preferably less than or equal to 3:1, in particular, when the ratio of conductor to dielectric thickness is 1:2. This reduces the mechanical stress imparted on the anode stack.
The anode stack may further comprise an attachment member arranged on an opposing surface of the conductor member to that which couples with the dielectric member, for attachment with the high voltage anode in use. Mechanically, an attachment member advantageously provides much more support between the conductor member and the high voltage anode than mere placement of the high voltage anode onto the anode stack, for example. Preferably, this attachment member may be a metal screw, which allows heat transfer from the high voltage anode to the conductor member and/or the dielectric member via the attachment member.
The conductor member, that is, either one of or both of the main body and the peripheral portion, preferably comprises a material having thermal conductivity above 20 Wm−1K−1. Suitable conductor member materials include but are not limited to tungsten, molybdenum and copper, or alloys thereof.
The dielectric member preferably also comprises a material having thermal conductivity above 20 Wm−1K−1. Suitable dielectric member materials include but are not limited to aluminium nitride (AlN), aluminium oxide (Al2O3), beryllium oxide (BeO) and diamond.
The anode stack may further comprise a brazing, gluing or soldering material between the main body of the conductor member and the dielectric member for joining the main body of the conductor member and the dielectric member. These techniques for attaching the main body of the conductor member to the dielectric member are advantageous over other attachment techniques, such as bolting, which are not as practical for joining dissimilar materials. Some examples of suitable braze materials include copper, Cusil and Nichoro. However, the braze materials are not limited to these, and depending on whether the dielectric member insulator is coated or uncoated, active or non-active braze materials may be used.
In some examples, the anode stack may further comprise a base conductor member arranged to couple with an opposing surface of the dielectric member to that which couples with the main body of the conductor member. This results in a conductor-dielectric-conductor arrangement of the anode stack.
Preferably, the base conductor member has a main body and a peripheral portion, wherein the dielectric member overlies the main body of the base conductor member, wherein the main body of the base conductor member is arranged to couple with the dielectric member, and wherein the peripheral portion of the base conductor member comprises an annular region that surrounds at least part of the dielectric member and which is spaced therefrom. In effect, the shape of the base conductor member may comprise many similarities with the shape of the conductor member.
Similarly to the shape of the anode stack at the peripheral portion of the conductor member, the shape of the anode stack at the peripheral portion of the base conductor member also reduces the electric field at the triple point region formed at its interface with the dielectric member and the vacuum of the X-ray device; the annular region that surrounds at least a part of the dielectric member and which is spaced therefrom provides screening and thus reduces electrical arcing and electrical breakdown at the triple point region.
In some examples, the largest diameter of the base conductor member may at least five times larger than the largest diameter of the conductor member. In addition or otherwise, the thickness of the base conductor member may be at least five times larger than the thickness of the conductor member. Each of these dimensions are selected in order to increase the heat dissipation capacity of the base conductor member.
In other examples, the shape of the base conductor member may be substantially identical to the shape of the conductor member. This provides for simpler manufacturing of the anode stack since both the conductor member and the base conductor member can be formed using the same mould.
As is the case with the conductor member, the ratio of thermal expansion coefficient between the base conductor member to the dielectric member may be less than or equal to 3:1, in particular, when the ratio of conductor to dielectric thickness is 1:2, for the same reasons.
As is the case with the conductor member, the base conductor member preferably comprises a material having thermal conductivity above 20 Wm−1K−1, for the same reasons. Suitable base conductor member materials include but are not limited to tungsten, molybdenum and copper, or alloys thereof.
As is the case between the main body of the conductor member and the dielectric member, the anode stack may further comprise a brazing, gluing or soldering material between the main body of the base conductor member and the dielectric member for joining the main body of the conductor member and the dielectric member, for the same reasons. Some examples of suitable braze materials include copper, Cusil and Nichoro. However, the braze materials are not limited to these, and depending on whether the dielectric member insulator is coated or uncoated, active or non-active braze materials may be used.
In each of the examples of the anode stack described above, the anode stack may further comprise a layer of material comprising a thermal transfer material arranged between the main body of the conductor member and the high voltage anode in use. In particular, the thermal transfer material may be a graphite based material, which has high thermal conductivity and acts as a good heat exchanger. These graphite based materials are also compliant and fill any small air gaps that arise due to surface roughness between the main body of the conductor member and the high voltage anode in use.
Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:
A representation of a prior art anode stack 1000 is shown in
Although this thermal management system overcomes some of the disadvantages of oil based cooling systems, it also introduces some disadvantages itself. In particular, the perpendicular angles at the edges of the interface between the dielectric 1150 and conductor plates 1140, 1145 causes high electric field strengths to exist at the respective triple point regions, i.e. the regions where the dielectric, metal and vacuum meet. Since the electric field at the triple point region is much greater than the surrounding electric field, electrical arcing and electrical breakdown ensue, which leads to device failure, and this is a prominent problem with the prior art anode stack 1000 of
As explained above with reference to
In
The conductor member 23 has a main body 23a and a peripheral portion 23b. The dielectric member 21 overlies the main body 23a of the conductor member 23 in the sense that the main body 23a has a shape which is the geometrical projection of the dielectric member 21, that is, the footprint of the dielectric member 21 is the same shape as the main body 23a. The main body 23a of the conductor member 23 is coupled with the dielectric member 21 at one planar surface X, and with an end of the high voltage anode at an opposing planar surface Y in use. The peripheral portion 23b of the conductor member 23 comprises an annular region that surrounds at least a part (in a vertical direction with respect to
The anode stack 20 of this first example further comprises a base conductor member 22 arranged to couple with an opposing surface of the dielectric member 21 to that which couples with the main body 23a of the conductor member 23. The base conductor member 22 is identical in shape to the conductor member, i.e. it also has a main body and a peripheral portion.
In the conductor-dielectric-conductor anode stack arrangement of
Two trenches 24, 25 exist in the first example of the present invention, the first grooved in the conductor member 23 and the second grooved in the base conductor member 22. These trenches 24, 25 have a rectangular cross section in the first example of the present invention, but in other examples of the present invention, they may have a square or dome-shaped cross section.
The annular region of the peripheral portions of the two conductor members 22, 23 surround respective joining regions 26, 27 between the dielectric member 21 and the main body of one of the two conductor members 22, 23, said joining regions 26, 27 coinciding with the triple point region of the anode stack 20. The two joining regions 26, 27 each have a perimeter surface which comprises surfaces of the dielectric member 21 and the respective main body of one of the two conductor members 22, 23. As shown in the example of
In this cylindrical arrangement, all normal axes to the perimeter surface of the dielectric member 21 and the two conductor members 22, 23 are coplanar. As a result, the electric field in the proximity of these anode stack components and in the joining regions 26, 27 in particular, is reduced further by the removal of sharp edges in said regions. In this example, the joins between the dielectric member 21 and the two conductor members 22, 23 are substantially at 180 degrees. In contrast, the sharp 90 degree angles shown in
An anode stack direction is defined from the dielectric 21 member to each of the two conductor members 22, 23, as shown by the direction of the two arrows in
The anode stack 20 of
Machined plane surfaces between the dielectric beryllium oxide disc and the conductive tungsten discs further reduce the electric fields at the triple point regions. Tungsten and beryllium oxide each have thermal conductivities above 20 Wm−1K−1 and so they are good thermal conductors.
The ratio of conductor to dielectric thickness is about 1:2 in the example of
Now turning to
The trench dimensions that are selected determine how greatly the electric field at the triple point regions is reduced compared to an anode stack example without a trench. Its dimensions are preferably selected in such a way that electric field at the triple point regions is reduced by at least two times compared to the case when no trench is used. Electrostatic simulations showed that this two times reduction of electric field is achieved when the extension of conductor above conductor-dielectric interface, height (h), is at least one width (d) of the trench (that is, h≧d).
Trenches of anode stacks according to examples of the present invention generally have a rectangular or square cross section and the top of the peripheral portion is formed generally as a domed or semi-circular shape in cross section.
In the second example of the present invention of
In the third example of the present invention shown in
In this example, it can be seen that a thin layer of active braze material such as copper or Cusil is provided between the main body of the conductor member 42 and the dielectric member 41. In the other examples, the braze material may be non-active. Selection of braze material depends on thermal expansion coefficients of the materials of the dielectric and conductor members and such braze materials are selected in order to minimise stresses between the dielectric and conductor members. Instead of brazing, other techniques such as gluing or soldering may be used for joining the main body of the conductor member 42 and the dielectric member 41 in other examples of the present invention. The ceramic disc 41 of
Electrostatic simulations of this two-part anode stack 40 of the third example of the present invention show maximum electric field to be about 4 V/μm, which is a similar result to the case of three-part anode stack 50 shown in
The anode stack 50 according to the fourth example of the present invention shown in
Finally, in
In this example, the large metallic disc 63 may connect to a heat sink, and it is larger in both diameter and thickness compared to the other metallic disc 62. This configuration enables increased heat dissipation capacity of the anode stack 60. The largest diameter of large metallic disc 63 can be up to 6 inches, whereas the metallic disc 62 is about 1.5 inches in diameter.
As shown in
Graphite based products have extremely high thermal conductivity of between 5,000 and 240,000 Wm−1K−1 (depending on the crystal direction), so use of graphite based materials is particularly beneficial for heat exchange applications. The graphite based layer may be brazed onto the metallic disc 62 and the brazing is performed in a vacuum braze oven using a tightly calibrated braze process. This brazing technique is advantageous over techniques, such as bolting, which are less effective for joining electrically dissimilar components together.
The attachment member, which in this case is a metal screw 67, is arranged on the opposing surface of the metallic disc 62 to that which couples with the dielectric disc 61, and it is for attachment with the high voltage anode 67 in use. Mechanically, the metal screw 67 provides much more support between the metallic disc 62 and the high voltage anode 68 than for example mere placement of the high voltage anode onto the anode stack. The metal screw 67 also allows effective heat transfer from the high voltage anode 68 to the conductor member 62.
Aside from use in X-ray devices, other engineering applications that will benefit from the anode stack as described above include heat spreaders for cooling integrated chips, RF transmitting tubes, microwave klystrons, microwave travelling wave tubes, or in other applications where high voltage and high power are used. Additionally, the invention may have uses in high voltage power supplies or in high voltage solid state relays.
