EP2225769A1

EP2225769A1 - Precession anode x-ray tube

Info

Publication number: EP2225769A1
Application number: EP08854029A
Authority: EP
Inventors: Damian Kucharczyk; Paul William Loeffen
Original assignee: Oxford Diffraction Ltd
Current assignee: Oxford Diffraction Ltd
Priority date: 2007-11-30
Filing date: 2008-11-28
Publication date: 2010-09-08
Also published as: JP2011505657A; GB0723513D0; GB2455140A; WO2009068891A4; WO2009068891A1

Abstract

An X-ray tube is described having a filament for generating electrons (5), a cathode, and an anode (8) which comprises an elongate member (17) having a fixed point relative to the sealed housing of the tube and an end that is moveable relative to the sealed housing. The longitudinal axis of the elongate member in a rest position is laterally displaced relative to the path of the accelerating electrons, whereby a spherical target surface on the moveable end of the member may be moved relative to the impinging electrons such that the spatial location of X-ray generation in the tube is invariant relative to the sealed housing, thereby spreading localised heating caused by the X-ray generation over a greater portion of the target surface. The member is typically driven such that the anode precesses about the X-ray beam.

Description

Precession Anode X-Ray Tube

Field of the Invention

The present invention relates to the field of X-ray tubes, and particularly X-ray tubes for use in the field of X-ray diffraction. It may also relate to X-ray tubes for other analytical applications including X-ray fluorescence, radiography, tomography, topography, or other applications involving the generation of a small point or line source of high brilliance X-rays.

Background to the Invention

X-rays are produced when a beam of electrons, accelerated by a high voltage, impinges on a metal target and is therefore rapidly decelerated by collision with the metal atoms. The process of generating X-rays in this way is highly inefficient and the majority of the power in the electron beam is dissipated as heat. In order to avoid melting, the metal target therefore requires cooling. X-ray tubes typically comprise a cathode with a filament that emits electrons. The electrons are accelerated by a high voltage applied between the cathode and the metal target, or anode. The anode and the cathode are enclosed in a vacuum.

A very common embodiment of an X-ray tube widely used in X-ray diffraction and other analytical applications comprises a stationary anode (also referred to as a fixed anode) in a sealed vacuum vessel typically made of glass or ceramic with water cooling of the anode via feed-throughs into the vacuum enclosure. A typical example of such a sealed X-ray tube is the XRD C-Tech Tube manufactured by PanAlytical B.V. which operates with a maximum high voltage of 60 kV for a Molybdenum target and an anode current of 55 mA, which corresponds to a maximum power of 3,300 Watts. Such a tube has a line focus of 0.4 x 12 mm which can produce a line or point projection of the X-ray source when viewed through beryllium windows surrounding the anode.

A more recent version of the sealed X-ray tube operates at much lower power (typically around 50 Watts) producing a micro-focus of the electron beam on the metal anode. A typical example of such a microfocus sealed X-ray tube is the Apogee Tube manufactured by Oxford Instruments, Inc. X-ray Technologies Group which operates at 50 kW with an anode current of 1 mA producing a round focus of the electron beam of diameter 35 - 50 microns. Both the standard sealed X-ray tube and the microfocus sealed X-ray tube have a typical lifetime of about 10,000 hours before the tube needs replacing, at which time the entire X-ray tube is replaced as a consumable. The advantages of stationary anode sealed X-ray tubes are their simplicity, reliability, compactness and low cost.

Another very common type of X-ray tube, which is also widely used in X-ray diffraction and other analytical applications, comprises a rotating anode in a vacuum enclosure. The anode is rotated in order to spread the heat load caused by the electron beam across a wider area thereby facilitating cooling and allowing the application of much higher heat loads per unit focal area. Consequently, rotating anode X-ray tubes have the advantage of producing X-rays at intensities that are several times greater than those from sealed X-rays tubes. Modern rotating anode X-ray tubes may also exploit a micro-focus electron beam in the same way as the sealed tubes, thereby permitting a lower power. An example of a microfocus rotating anode X-ray tube is the MicroMax007HF generator from Rigaku Corporation, which typically operates at 1 ,200 Watts with a focal spot of diameter 70 microns.

The operation of a rotating anode within a high vacuum, whilst applying cooling to the anode, necessarily requires a mechanical solution that is considerably more complicated than a sealed X-ray tube with a stationary anode, and typically requires frequent and demanding maintenance. For example a rotating anode X- ray tube will typically require a dynamic feed-through with bearings and seals that require regular maintenance. Therefore, the advantages of higher X-ray intensity available from a rotating anode have to be weighed up against the drawbacks of increased cost, complexity and time commitment. There is a considerable body of prior art dealing with the field of rotating anode X-ray tubes. The application of sealed X-ray tubes and rotating anode X-ray tubes to the field of X-ray diffraction is described in C. Giacovazzo, H. L. Monaco, D. Viterbo, F. Scordari, G. GiIIi, G. Zanotti, M.Catti "Fundamentals of Crystallography", Edited by C. Giacovazzo, Oxford University Press, 1992.

A number of variants of the above X-ray tubes and anode types have been developed over the years in attempts to achieve improved performance. For example GB2381432A describes an X-ray emission device, which employs a rotating anode with a roughly cylindrical surface. The peripheral cylindrical edge portion of the rotating anode is bombarded with a bema of electrons to generate X- rays and the position of the focal point on the anode relative to a reference position is dynamically controlled.

US3753020 describes a multi-anode X-ray tube having a plurality of anodes disposed at the circumferential portion of adjacent sectors having a common centre which is concentric with a gear non-moveable fitted to the sectors. The gear is in a mating arrangement with a worm screw for permitting selection of one of the anodes to be in line with a stream of electrons from the cathode and thereby selecting spectrum of the generated X-rays according to the material of the selected anode.

Other approaches to the particular problem of ameliorating localised heating have been proposed. For example, US 6154521 describes a form of gyrating anode X- ray tube. The anode takes the form of a large cup-shaped body portion with spherical target surface and is attached to the main body of the tube via a bellows assembly, which can be driven so as to rock the anode in two axes simultaneously. Together with the main body of the tube, the cup-shaped body portion of the anode forms the sealed surface that defines an interior vacuum envelope when evacuated. However, the large size of the cup-shaped body portion and the use of connecting bellows may make accurate control of the target surface difficult. Moreover, evacuation of the body to high vacuum will cause compression of the bellows, which could significantly limit flexibility.

US3794872 describes a sealed X-ray tube having a flat target surface facing an electron beam source and carried by a rigid support mounted for sliding reciprocally lateral motion against a sliding surface. A bellows is sealed to the target such that, along with the envelope of the tube, forms a closed surface. The axis of the target reciprocates laterally in a direction perpendicular to the axis of the tube but the target does not rotate on its own axis. FR2803432A1 describes an X-ray tube that employs an anode in the form of a mechanically operated arm on which is located a spherical target surface. The anode is mounted on a hinge having at least two degrees of freedom, whereby the target surface can follow a cycloidal or circular path. In its central rest position, the arm is aligned with the path of a focussing electron beam along a central axis of the tube. Motion of the arm about this axis causes movement of the target surface relative to the X-ray beam according to a particular trajectory and aids the dissipation of heat. DE3638378 describes a similar arrangement.

However, although the devices described above facilitate improved performance, accurately maintaining a well-defined focal region for the efficient generation of high intensity X-rays can be difficult and the moving anode can shadow the X-ray path to the exit window. As such, it will be apparent to the skilled person that there is currently a need for a compact, cost-effective X-ray tube similar to a fixed electrode tube, but which is adapted for efficient heat dissipation so as to permit the generation of X-rays at intensities approaching those achievable in larger, high- maintenance rotating anode tubes.

Summary of the Invention According to a first aspect of the present invention, an X-ray tube comprises a sealed housing defining an interior vacuum envelope when evacuated, the X-ray tube further comprising within the housing: a filament for generating electrons; a cathode; and, an anode spatially separated from the cathode, the anode and the cathode adapted to define a potential difference therebetween for accelerating electrons from the filament so as to impinge on a target surface of the anode and thereby generate X-rays, wherein the anode comprises an elongate member having a first end and a second end, the anode being configured such that: a longitudinal axis of the elongate member in a rest position is laterally displaced relative to the path of the accelerating electrons a point on the length of the elongate member from the first end to the second end is substantially fixed relative to the sealed housing; the target surface of the anode is spherical and is disposed on the second end of the elongate member; and, the second end of the elongate member is moveable relative to the sealed housing, whereby the spherical target surface may be moved relative to the impinging electrons such that the spatial location of X-ray generation in the tube is invariant relative to the sealed housing, thereby spreading localised heating caused, by the X-ray generation over a greater portion of the target surface.

In this way, the present invention provides a different solution to the problem of localised heating of the anode in the region where X-rays are generated. The moving target surface of the anode spreads the heat loading, but the absolute spatial location of X-ray generation in the tube remains invariant relative to the housing, including the X-ray exit window, and therefore also remains invariant relative to the components that generate and shape the electron beam. As such, the precise alignment of the desired focal region of the electron beam for generating the X-rays with respect to the exit window is not compromised by movement of the target surface.

The longitudinal axis of the elongate member in its rest position is laterally displaced relative to the path of the accelerating electrons and therefore is generally coaxial with this path. The offset rest position facilitates the driving of the member to achieve a desired locus for the target surface. The use of a spherical target surface on the offset member facilitates maintenance of the absolute spatial invariance in the location of the X-ray generation. Moreover, the requirement that the target surface be inclined such that X-rays generated can exit the tube via an X- ray exit member is automatically achieved with the curved, spherical surface. In addition, the use of an offset spherical target surface ensures that it does not shadow the X-ray path to the exit window.

Preferably, the longitudinal axis of the elongate member lies along a radius of the spherical target surface and the radius is equal to the distance from the anode surface to the substantially fixed point on the length of the elongate member. The anode may be adapted to be driven in a number of ways. Preferably, the anode is adapted to be< driven such that the target surface of the second end moves along an oscillatory path relative to the impinging electrons. More preferably, the anode is adapted to be driven such that the target surface of the second end moves along an elliptical oscillatory path relative to the impinging electrons. The anode may be adapted to be driven such that the target surface of the second end moves along a gyratory path relative to the impinging electrons. In order to spread the heat loading is preferred that the anode is adapted to be driven such that the target surface is exposed to the impinging electrons for an exposure time which is varied in a predetermined manner.

When the electron beam forms a line focus at the target surface, it is preferred that the target surface moves along an elliptical oscillatory path having an ellipticity that is opposite to the aspect ratio of the focussed X-ray beam. It is preferred that the ellipticity ratio is generally the reciprocal of the electron beam aspect ratio, thereby spreading the heat load in a manner that takes account the non-point like nature of the focussed X-ray beam. For example, with a standard X-ray beam having an aspect ratio of 10:1 , an ellipticity ratio of approximately 1 :8 would be suitable. It is further preferred that this primary elliptical oscillatory motion is superposed with another slower oscillatory motion along an axis substantially parallel to the minor axis of the faster elliptical movement. The slower oscillatory motion may simply follow a linear path or else it may follow an elliptical path having an opposite ellipticity to the faster elliptical motion.

The anode may be adapted in a number of ways to provide a target surface for X- ray generation. Preferably, the anode comprises a cap disposed over the second end of the elongate member and coated with copper (Cu), Molybdenum (Mb) or other target materials for providing the target surface.

In order to control the driven movement of the anode, it is preferred that the X-ray tube further comprises means for anti-resonant damping of the motion of the elongate member.

The elongate anode member may take many forms. In one embodiment, the elongate anode member comprises a flexible monolithic rod which is fixed at the first end relative to the housing and is moveable relative to the housing at the second end by a flexing action. Preferably, the rod passes through an airtight aperture in the housing and is formed of a thermally conductive material for conducting heat away from the target surface and to the exterior of the housing.

In another embodiment, the elongate anode member comprises a rigid member having a joint at the first end for pivoting about an inflexion point substantially fixed relative to the housing and which is moveable relative to the housing at the second end by a pivoting action. Preferably, the joint comprises a ball and socket type joint which allows a universal rocking of the rigid member about the inflexion point. Preferably, the X-ray tube further comprises a resilient means for retaining the first end in the substantially fixed relative position. In a preferred implementation, the resilient retaining means comprises a helical spring.

Although the moving target surface ensures that excessive localised heating is avoided and the heating loading during X-ray generation is spread over the target surface, it is still also desirable to provide means to cool the surface and conduct away heat generated there. It is therefore preferred that the rigid elongate anode member comprises an internal pathway for the flow of a liquid coolant to and from the proximity of the target surface for conducting heat away from the target surface. In a preferred implementation, the internal pathway comprises a coaxial structure for the flow of the liquid coolant to and from the proximity of the target surface. Preferably, the internal pathway comprises a diffusing structure proximate the target surface for diffusing the coolant to ensure uniform cooling.

It is further preferred that the X-ray tube comprises a flexible tubing coupled to the internal pathway of the rigid elongate member and to the exterior of the housing for carrying the coolant to and from the rigid member. Preferably, the tubing is configured as a double helix around the rigid member at the first end. The liquid coolant flowing in or around the rigid member may act as anti-resonant damping means by damping motion of the elongate member. Alternatively, or in addition, the anode may be adapted to be driven by the passage of the liquid coolant so as to move the target surface relative to the impinging electrons. The elongate anode member of the present X-ray tube may be driven by any suitable means, including mechanical, electromechanical and electromagnetic driving mechanisms.

For electromagnetic driving mechanisms, it is preferred that the X-ray tube further comprises means within the housing for conveying an electromagnetic driving force to the second end of the elongate member for moving the second end. In one implementation, the force conveying means comprises a solid ring of magnetic material disposed circumferentially about a portion of the elongate member proximate the second end. In another implementation, the force conveying means comprises a disc of magnetic material having a central aperture through which the elongate member passes proximate the second end and having a peripheral region retained in an annular slot in the housing.

According to a second aspect of the present invention, an X-ray tube assembly comprises an X-ray tube according to the first aspect and means disposed external to the housing for driving the anode so as to move the target surface relative to the impinging electrons. By separating the driving means from the X-ray tube, the latter may easily be removed and replaced in the assembly in the event of failure or wearing out of the tube. In this way, the X-ray tube of the present invention retains the ease of maintenance and replacement associated with current sealed X-ray tubes employing fixed anodes.

For such electromagnetic driving mechanisms, the X-ray tube assembly preferably comprises an X-ray an electromagnet arrangement disposed on the exterior of the

X-ray tube housing for generating an electromagnetic driving force to be conveyed to the second end by the force conveying means. In one implementation, the electromagnet arrangement comprises one or more electromagnetic coils with a multi-pole core disposed adjacent the force conveying means. In another implementation, the electromagnet arrangement comprises two or more discrete electromagnets disposed circumferentially about the exterior of the housing adjacent the force conveying means. Alternatively, the X-ray tube assembly comprises a mechanical actuator disposed external to the X-ray tube housing for driving the anode so as to move the target surface relative to the impinging electrons.

As will be appreciated by the skilled person, the present invention provides a new compact, cost-effective X-ray tube, which combines the best features of many existing X-ray tubes. This new X-ray tube maintains the advantages of conventional sealed X-ray tubes with stationary anodes, whilst generating considerably higher X-ray intensity comparable to that generated from a rotating anode X-ray tube. The present invention utilises a sealed vacuum enclosure of similar dimensions to a conventional sealed X-ray tube, which may be easily exchanged as a complete unit when the tube is worn, thereby eliminating complicated maintenance associated with rotating anodes. The present invention is also considerably less expensive to manufacture than a rotating anode solution thereby maintaining the economic advantage.

It is anticipated that the present invention will substitute current sealed X-ray tubes and provide a low cost, low maintenance alternative to rotating anode X-ray tubes. The elongate anode member may be implemented in various forms providing it is adapted to be driven so as to move the offset spherical shaped target surface of the anode in a manner that maintains the spatial location of X-ray generation in the tube invariant. Similarly, a variety of suitable driving, damping and cooling mechanisms may be combined with the core invention to provide a high- performance practical device, having a range of applications.

Brief Description of the Drawings

Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows an X-ray tube assembly according to the present invention; Figure 2 shows a close-up of the electron gun and anode arrangement of figure 1 ; Figures 3A and 3B illustrate precession of the anode electromagnetically driven via a magnetic vibration disc;

Figure 4 shows an embodiment of the present invention with monolithic flexible rod anode member; and, Figure 5 shows an example of an electromagnetic driving mechanism with magnetic vibration disc and anti-resonant damping ring.

Figures 6A, 6B and 6C, respectively, show different target surface loci for an electron beam with aspect ratio 10:1 moving on an ellipse with ellipticity ratio of 1 :8; Figures 7A, 7B and 7C, respectively, show different target surface loci for an electron beam with aspect ratio 10:1 moving on an ellipse with ellipticity ratio of 1 :3; and,

Figures 8A, 8B and 8C, respectively, show different target surface loci for an electron beam with aspect ratio 3:1 moving on an ellipse with ellipticity ratio of 1 :3.

Detailed Description

Figure 1 illustrates a complete X-ray tube according to the present invention, showing the key components. The X-ray tube is comprised of two main modules: the electron beam generator 1 and the anode driving mechanism 2. The two modules are joined together to create a sealed vacuum enclosure.

The electron beam generator 1 is constructed using well-known high-voltage materials such as high voltage resistant ceramic 3 and/or glass and also employs well-known high-voltage technologies such as vacuum feed-throughs and ceramic- metal and/or glass-metal junctions 4. The main function of the electron beam generator module 1 is twofold. On the one hand, it is used to produce an electron beam 5, and on the other hand it is used to isolate the system from high voltage discharges. The shape of the enclosure is important in minimising such discharges. The electron beam 5 is generated by an electron gun which comprises a filament coil 6 generally made of tungsten and mounted on a filament electrode 7, whereby the filament 6 emits electrons when operated at low voltage and high current. The electrons are focused by means of a HV and VF negative electrode arrangement, and are attracted to the anode cap 8 when a voltage is applied.

The electrons emitted from the filament coil 6 are shaped by an electrostatic field generated by an electrode known as a Wehnelt Cylinder 9, which produces a beam of electrons with a rectangular cross-section of a few tens of microns by a few hundred of microns 10, as shown in more detail in Figure 2. The aspect ratio of the rectangular cross section of the electron beam is typically about 10:1 , which is an industry standard. The electron beam emerging from the Wehnelt Cylinder 9 is divergent and consequently a refocusing electrostatic field is applied by the focussing slit 11. The focussing slit 11 has a separate electrical feed-through in order to apply an independent high voltage. Stray electrons from the beam are blocked by the anti-sputtering ring 12 from impinging on the anode 8 and thereby creating unwanted X-rays or ions, which could pollute the anode, or sputtering ions, which could pollute the cathode 7.

Some of the X-rays 13 generated by the electron beam 5, and impinging on the surface 8a of the anode cap 8, have an unobstructed trajectory to exit the X-ray tube through the beryllium window 14. The surface 8a of the anode cap 8 has a spherical shape, wherein the radius of the spherical target surface is equivalent to the distance from the anode surface to the pivot point 16 of the elongate member 17 supporting the anode cap. For example, the radius of curvature of the target surface may be greater than or equal to approximately 100mm. The anode cap is typically made of copper, due its superior thermal conductivity, with a thin coating of a given metal deposited on the surface of the anode facing the electron beam. The type of metal coating is selected so as to generate an emission of X-rays with a characteristic wavelength spectrum, as required. In the preferred embodiment, the rectangular source of the X-ray beam at the electron beam focal point 15 on the anode will have a square aspect ratio when projected through the beryllium window 14.

It is a key aspect of this invention that the electron beam 5 impinges on the surface of the anode 8 off-centre from the axis of the elongate member 16 in its rest position and towards the beryllium window 14, as shown in figure 3A, in comparison with the on-axis arrangement shown in figure 3B. Consequently, the electron beam is not normal to the surface 8a of the anode and, at the focal point 15, the target surface of the anode is effectively inclined towards the beryllium window. In this way, the curvature of the anode surface 8a does not block any of the X-rays from exiting through the beryllium window 14. It should be noted that, in a stationary anode sealed X-ray tube, the anode is flat and inclined towards the beryllium window. Due to the requirement for a spherical curvature of the anode surface 8a, an important practical requirement in manufacturing an X-ray tube of the present invention is to minimise the shape errors in the anode surface, since these will otherwise distort the projection of the X-ray beam. To minimise the shape errors it is expected that the anode surface will need to be extensively polished.

The anode driving mechanism 2 module contains the anode, which comprises an elongate member 17 with an anode cap 8 mounted on one end, including the target surface for X-ray generation. The anode is configured such that the target surface

8a of the anode is able to precess about a fixed pivot point, which may be at any point along the longitudinal axis of the elongate member. In a preferred embodiment, the elongate member is a hollow cylinder, which is able to precess about a ball and socket support 16. However, other variants may include an elongate member which is rigidly and monolithically attached to the housing of the

X-ray tube, as shown in figure 4, whereby the precession of the anode is possible due to the elastic flexion of the elongate member material. In the preferred embodiment, the anode cap 8 is electrically grounded via a conducting spring 21 connecting the elongate member to the X-ray tube housing. As well as ensuring a constant electrical contact, the spring 21 also secures the hemispherical base of the elongate member in position in the ball socket.

It is desirable in all embodiments of the invention that the anode be cooled during operation to transfer away the heat deposited in the anode material by the impinging electron beam. In the preferred embodiment, this cooling is achieved by passing coolant fluids (typically water) on the underside of the anode. Preferably, the coolant is provided in turbulent flow by the use of a turbulence slit 18. In the preferred embodiment, the coolant fluid is transferred to the underside of the anode inside a pipe which is fixed concentrically with the elongate member 17 such that the return flow is outside of the pipe but still within the elongate member. In the preferred embodiment, the coolant fluid flows in and out of the X-ray tube through fixed feed-throughs 19a and 19b in the wall of the vessel, and the coolant pipes connecting the feed-throughs 19a and 19b with the elongate member 17 are formed to make coils 20 around the base of the elongate member, and thereby provide some flexibility of the elongate member to precess without excessive damping.

In an alternative embodiment, shown in figure 4, the cooling pipes 21 inside the elongate member 17 are fixed with respect to the X-ray tube housing, and therefore move with respect to the elongate member when it is allowed to precess. This embodiment has the advantage that the cooling water exits the cooling pipes 17 in a position that is stationary with respect to the electron beam position and is consequently more efficient from a cooling perspective. However, in the embodiment shown in figure 4, it is possible that the coolant fluid itself will damp or amplify the precession of the elongate member. Indeed, in the embodiment shown in figure 4, it is even possible that the flow of the coolant fluid may be used to drive the precession of the elongate member, although such a drive mechanism can be unpredictable.

The present invention includes a mechanism to drive the elongate member so that the anode is made to precess in a controlled manner with respect to the electron beam. Various driving mechanisms can be envisaged in different embodiments, including mechanical actuation of the elongate member from outside of the X-ray tube. In general, at least part of the driving mechanism will be located external to the housing of the X-ray tube and be detachable therefrom. In this way, the overall X-ray tube assembly comprises an external drive mechanism and a replaceable X- ray tube, which includes the movable anode.

In the preferred embodiment, the driving mechanism is electromagnetism applied by electromagnetic coils 22, each with an electromagnetic core 23 positioned on the outside of the X-ray tube which may be detached from the body of X-ray tube. In the preferred embodiment, the electromagnetic coils 22 activate a magnetic disc 24 attached rigidly to the anode by attraction or repulsion causing the disc to incline with respect to the X-ray tube, and thereby driving an angular displacement of the elongate member about its pivot point. Such actuation might typically cause the magnetic disc 24 to be displaced by the order of 1 mm. In a preferred embodiment, eight such electromagnetic poles are positioned octagonally around the magnetic disc 24 and may be sequentially activated to drive a precessional motion of the magnetic disc, and hence the anode 8.

In an alternative embodiment, two magnetic poles may be activated simultaneously to create a bi-polar actuation with attraction and repulsion used on opposite sides of the disc 24. This provides the possibility of damping vibrations of the magnetic disc by reversing direction of the electromagnetic field. In another embodiment, permanent magnets could be arranged around the disc, in segments, with alternate polarity, in order to create magnetic levitation of the disc. In one embodiment, an anti-resonant damping ring 25 encircles the elongate member and is so positioned as to damp the resonant vibrations, as shown in figure 5. Such damping can be achieved via the driving mechanism, but the antiresonant damping ring 25 minimises the power required by the driving mechanism to achieve this.

We now consider in some detail the actual precessional motion of the anode and example loci that provide high performance in terms of distributed localised heating of the surface. Given the typical aspect ratio (10:1) of an electron beam focus at the target surface, the preferred locus of movement is predominantly elliptical. It should be noted that, as the focus is typically not point-like, it is even more important to consider how long the electron beam focus spends impinging on a particular region of the target surface, as this will determine the actual localised heating load. This is particularly true at turning points in the anode oscillatory motion, where the target surface decelerates and then accelerates.

We consider three examples, each having a particular combination of beam aspect ratio and underlying ellipticity of movement, which is broadly designed to complement the beam shape. In each example we analyse three different modes of vibration, including the basic fast elliptical motion and also the fast elliptical motion superimposed on two forms of other slowly-varying motion. In each case we consider the implications for localised heating.

In the first example, we consider a beam having an aspect ratio of 10:1 (beam height=0.3 mm, beam width=0.03 mm) and subject to a generally elliptical vibration having an ellipticity ratio of 1:8 {r{.r₂ where o is the x-axis radius of the ellipse and r₂ is the y-axis radius of the ellipse. For each mode of operation, the underlying elliptical motion has a relatively short oscillation period of T= 0.02s, corresponding to a frequency of 50Hz, and has elliptical radii of r_r=0.1 mm and r₂=0.8 mm, respectively.

In the first mode of operation for this example, a simple pure elliptical motion is analysed by calculating the equations of motion in Cartesian coordinates. The time-varying coordinates, x and y (in mm), of the locus relative to the centre of the ellipse are calculated as a function of time t (in seconds) to be:

2π x = r. cos — t T

^{<1 1 a)} y = r, sin — t T

The linear speed components, V_x and v_y, are given by:

2π . 2π v = — r. — sin — t

^{T T} (1.1 b) v, = r, — cos — t

and the resultant speed is given by:

_ 2π ■, . -, 2π -, -> 2π v = Jv ⁺ V ⁼ — _Λ T sin^" — f + r,^" cos^" — t (1.1c)

Figure 6A shows the locus of relative movement between the electron beam and the target surface. From (1.1b) the maximum linear speeds are V_x = 31.416 mm/s and Vy = 251.327 mm/s. Energy concentration may be expressed in terms of the time spent by the beam over a given region of the target surface as a fraction of a single period of the elliptical oscillation. This gives a measure of how long that region of the target surface is exposed to the electron beam. A higher value means longer exposure, with a value of 100% indicating that the area is continuously exposed to the beam. For the above mode of oscillation, the minimum and maximum exposure fractions are calculated to be 4.2%T and 7.2%T, respectively. Thus, the parts of the target surface that are most exposed to the electron beam get an exposure dose that is approximately 71% greater than those parts of the parts of the target surface that are least exposed to the electron beam. Overall, the high frequency elliptical oscillation at 50 Hz reduces the heat load by a factor of between 14 and 24, as compared to a stationary anode.

In the second mode of operation for this example, the pure elliptical motion of the first mode is superposed on a very slow, side-to-side linear vibration along the x- axis having a period of T₂ = 500x7 = 10s and a magnitude of R = r₂= 0.8mm. In this mode the Cartesian coordinates of the locus of motion are given by:

2π _ 2π x = r cos — t + Rcos — t T T

(1.2a)

. 2π y = r_^ sin — t T

and the linear speed components are given by:

2;r . 2;r _n 2π . 2π v_v = -r, — sin — t - R — sin — i 1 T T T₂ T₂

(1.2b)

2π 2π v >_v = r -, — _τ cos — _τ t

Figure 6B shows the locus of relative movement between the electron beam and the target surface for this mode. From (1.2b) the maximum linear speeds are again calculated to be V_x = 31.416 mm/s and v_y = 251.327 mm/s for the fast elliptical motion, and V_x = 0.503 mm/s for the slow linear motion. The minimum and maximum exposure fractions for the fast motion cycle are again calculated to be 4.2%T and 7.2%T, respectively. However, for the full motion cycle, the worst case scenario (i.e. at one of two extreme positions) gives a minimum and maximum exposure fraction of 0.6%T₂ and 0.96%T_2l respectively, whereas the best case scenario (i.e. at the centre between two extreme positions) gives a minimum and maximum exposure fraction of 0.1 %T₂ and 0.3%T₂, respectively.

Finally, in the third mode of operation for this example, the pure elliptical motion of the first mode is superposed on very slow motion about another elliptical shape. The period of the slow elliptical motion is T₂ = 500x7 = 10s and the x-axis and y- axis elliptical radii are R₁ = r₂ = 0.8mm and R₂ = 4Xr₁= 0.4mm, respectively, giving an ellipticity ratio R₁: R₂ of 2:1 for the slow motion ellipse. In this mode the Cartesian coordinates of the locus of motion are given by: 2π 2π x = r, cos — t + R, cos — t

(1.3a)

. 2π „ . 2π y = r₂ sin — t + R₂ sm — t

and the linear speed components are given by:

2Λ: . 2Λ- _ 2π . 2π V₁. = —r, — sin — t — R, — sin — t

^{T T T}' ^T' (1 3b)

2π 2π „ 2π 2π v_v = r_> — cos — t + R₇ — cos — t r T T, T,

Figure 6C shows the locus of relative movement between the electron beam and the target surface for this mode. From (1.3b) the maximum linear speeds are again calculated to be V_x = 31.416 mm/s and v_v = 251.327 mm/s for the fast elliptical motion, and v_κ = 0.503 mm/s and v_y = 0.251 mm/s for the slow linear motion.

The minimum and maximum exposure fractions for the fast motion cycle are again calculated to be 4.2%T and 7.2%T, respectively. However, for the full motion cycle, the worst case scenario (i.e. at one of two extreme positions) gives a minimum and maximum exposure fractions of 0.47%T₂ and 0.75%T₂, respectively, whereas the best case scenario (i.e. at the centre between two extreme positions) gives a minimum and maximum exposure fractions of 0.06%T₂ and 0.23%T₂, respectively.

In the second and third mode of oscillation described above, the basic elliptical locus is either slowly oscillated either side to side or else on a separate ellipse, the frequency of the second oscillation being much longer than the first. This has the function of slowly distributing the heat load further over the target area. As described above, in this example, the high frequency elliptical first mode of oscillation at 50 Hz reduces the heat load by a factor of between 14 and 24 compared to a stationary anode, effectively allowing the power density to be increased. Nevertheless, the heating is still spread over a relatively small area of the target surface. The superposed slow oscillations at 0.1 Hz, in the second and third mode of oscillation, have the effect of spreading the cumulated dose over a larger area, which does not allow the power density to be increased, but does extend the lifetime of the anode by spreading the damage to the anode surface.

We now move on to the second example, where we consider a beam having the same aspect ratio of 10:1 (beam height=0.3 mm, beam width=0.03 mm), but subject to a generally elliptical vibration having an ellipticity ratio of 1 :3 (o:r₂ where rj is the x-axis radius of the ellipse and r₂ is the y-axis radius of the ellipse. For each mode of operation, the underlying elliptical motion still has an oscillation period of T= 0.02s, corresponding to a frequency of 50Hz, but has elliptical radii of r,=0.22 mm and r^=0.66 mm.

In the first mode of operation, a simple pure elliptical motion is analysed by calculating the equations of motion in Cartesian coordinates. The coordinates, x and y, of the locus of motion (in mm), where t is time (in seconds), are given by:

2π x — r_x cos

(2.1a)

. 2π y = r₂ sm — t

The linear speed components, v_x and v_y, are given by:

2π . 2π v_v = -r, — sin — t T T _{(2 1 b)}

V i₁₁ = r, - — _τ cos — _τ t and the resultant speed is given by:

2π 2π 2π

V = Jy + v = — Ji^" sin- — f + r-r cos- — t (2.1c)

Figure 7A shows the locus of relative movement between the electron beam and the target surface. From (2.1 b) the maximum linear speeds are V_x = 69.115 mm/s and V_y = 207.345mm/s. The energy concentration for this mode of oscillation, in terms of the minimum and maximum exposure fractions, is calculated to be 1.93%T and 7.5%T, respectively. In the second mode of operation for this example, the pure elliptical motion of the first mode is superposed on a very slow, side-to-side linear vibration along the x- axis having a period of T₂ = 500x7= 10s and a magnitude of R = r₂ = 0.66mm. In this mode the Cartesian coordinates of the locus of motion are given by:

2π _ 2π x = r, cos — t + Rcos — t 1 T T₂

(2.2a)

. 2π y = r, sin — t T

and the linear speed components are given by:

2ΛΓ . 2^- Jlπ . 2π v, = —r, — sin — t — R — sin — t

^{1 l 2} *² (2.2b)

2π 2π v = r, — cos — t , - T T

Figure 7B shows the locus of relative movement between the electron beam and the target surface for this mode. From (2.2b) the maximum linear speeds are again calculated to be V_x = 69.115 mm/s and v_y = 207.345mm/s for the fast elliptical motion, and V_x = 0.415mm/s for the slow linear motion. The minimum and maximum exposure fractions for the fast motion cycle are again calculated to be 1.93%T and 7.5%T, respectively. However, for the full motion cycle, the worst case scenario (i.e. at one of two extreme positions) gives a minimum and maximum exposure fraction of 0.22%T₂ and 0.83%T₂, respectively, whereas the best case scenario (i.e. at the centre between two extreme positions) gives a minimum and maximum exposure fraction of 0.12%T₂ and 0.42%T₂₎ respectively.

Finally, in the third mode of operation for this example, the pure elliptical motion of the first mode is superposed on very slow motion about another elliptical shape. The period of the slow elliptical motion is T₂ = 500x7 = 10s and the x-axis and y- axis elliptical radii are R₁ = r₂ = 0.66mm and R₂ = n= 0.22mm, respectively, giving an ellipticity ratio RUR₂ of 3:1 for the slow motion ellipse. In this mode the Cartesian coordinates of the locus of motion are given by: 2π _n 2π x = κ cos — 1 + /?, cos — t

T T₂

(2.3a)

. 2π _ . 2π y = r, sin — t + R_^ sni — t

and the linear speed components are given by:

2π . 2π „ 2π . 2π v, = -r. -sin- t- R. — sin — t

T T 1 -, T 1 -,

(2.3b)

2π 2π _ 2π 2π v_v = r, — cos — t + R, — cos — t

- - T T T_n r,

Figure 7C shows the locus of relative movement between the electron beam and the target surface for this mode. From (2.3b) the maximum linear speeds are again calculated to be V_x = 69.115 mm/s and v_y = 207.345mm/s for the fast elliptical motion, and V_x = 0.415 mm/s and v_y = 0.138mm/s for the slow linear motion.

The minimum and maximum exposure fractions for the fast motion cycle are again calculated to be 1.93%T and 7.5%T, respectively. However, for the full motion cycle, the worst case scenario (i.e. at one of two extreme positions) gives a minimum and maximum exposure fractions of 0.2%T₂ and 0.73%T₂, respectively, whereas the best case scenario (i.e. at the centre between two extreme positions) gives a minimum and maximum exposure fractions of 0.11%T₂ and 0.31%T₂, respectively.

If the results of the above analysis are compared with those of the first example, it becomes apparent that the smaller ellipticity ratio of 1 :3 tends to lead to a more uneven spread of the power (for a given beam shape), with greater variation between the maximum and minimum target surface exposure to the focused electron beam.

We now move on to the third and final example, where we consider a beam having a smaller aspect ratio of 3:1 (beam height=0.165 mm, beam width=0.055 mm), and subject to a generally elliptical vibration having an ellipticity ratio of 1 :3 (^r₂ where n is the x-axis radius of the ellipse and r₂ is the y-axis radius of the ellipse). For each mode of operation, the underlying elliptical motion still has an oscillation period of T= 0.02s, corresponding to a frequency of 50Hz, and has elliptical radii of 0=0.22 mm and r^=0.66 mm.

> In the first mode of operation for this example, a simple pure elliptical motion is analysed, calculating the equations of motion in Cartesian coordinates. The coordinates, x and y, of the locus of motion (in mm), where t is time (in seconds), are given by:

2π

X = K COS 1

(3.1a)

. 2π y = r, sin — t T

The linear speed components, V_x and v_y, are given by:

2π . 2π v_v = -r, — sin — t

^{T T} (3.1 b) v_v = r, — cos — t

^{■ "} T T 5 and the resultant speed is given by:

2π , . , 2π , , 2π v = _Λ v +v = — _Λ Ki^' sin^* — t + r,^" cos^~ — t (3.1c) 0 Figure 8A shows the locus of relative movement between the electron beam and the target surface. From (3.1b) the maximum linear speeds are V_x = 69.115 mm/s and V_y = 207.345mm/s. The energy concentration for this mode of oscillation, in terms of the minimum and maximum exposure fractions, is calculated to be 4.0%T and 5.5%T, respectively. 5

In the second mode of operation for this example, the pure elliptical motion of the first mode is superposed on a very slow, side-to-side linear vibration along the x- axis having a period of T₂= 500x7 = 10s and a magnitude of R = r₂= 0.66mm. In this mode the Cartesian coordinates of the locus of motion are given by: 2π _ 2π x= r. cos — t + Rcos — t T T

(3.2a)

. 2π y = r, sin — r t

and the linear speed components are given by:

2π . 2π _n 2π . 2π v. = -r, — sin — t -R — sin — t

T T % (3.2b)

2π 2π v_v = r₂ -cos — t T T

Figure 8B shows the locus of relative movement between the electron beam and the target surface for this mode. From (3.2b) the maximum linear speeds are again calculated to be V_x = 69.115 mm/s and v_y = 207.345mm/s for the fast elliptical motion, and v_x = 0.415mm/s for the slow linear motion. The minimum and maximum exposure fractions for the fast motion cycle are again calculated to be 4.0%T and 5.5%T, respectively. However, for the full motion cycle, the worst case scenario (i.e. at one of two extreme positions) gives a minimum and maximum exposure fraction of 0.26%T₂ and 1.28%T₂, respectively, whereas the best case scenario (i.e. at the centre between two extreme positions) gives a minimum and maximum exposure fraction of 0.21 %T₂ and 0.61 %T₂, respectively.

Finally, in the third mode of operation for this last example, the pure elliptical motion of the first mode is superposed on very slow motion about another elliptical shape. The period of the slow elliptical motion is T₂ = 500x7 = 10s and the x-axis and y- axis elliptical radii are R₁ = r₂ = 0.66mm and R₂ = r_r= 0.22mm, respectively, giving an ellipticity ratio Ri.R₂ of 3:1 for the slow motion ellipse. In this mode the Cartesian coordinates of the locus of motion are given by:

2π _ 2π x = r. i cos — j 1 + R, i cos — j t

(3.3a)

. 2π „ . 2π v = r, sin — 1 + R-, sin — t T ^" T_n and the linear speed components are given by:

2π . 2π _ 2π . 2π v = —r, — sin — t - R, — sni — t 1 T T T₂ T₁

(3.3b)

2π 2π _ 2π 2π V₁, = r, — cos — 1 + R₁ — cos — t r T T, T.

Figure 8C shows the locus of relative movement between the electron beam and the target surface for this mode. From (3.3b) the maximum linear speeds are again calculated to be V_x = 69.115 mm/s and v_y = 207.345mm/s for the fast elliptical motion, and V_x = 0.415 mm/s and v_y = 0.138mm/s for the slow linear motion.

The minimum and maximum exposure fractions for the fast motion cycle are again calculated to be 4.0%T and 5.5%T, respectively. However, for the full motion cycle, the worst case scenario (i.e. at one of two extreme positions) gives a minimum and maximum exposure fractions of 0.24%T₂ and 0.94%T₂, respectively, whereas the best case scenario (i.e. at the centre between two extreme positions) gives a minimum and maximum exposure fractions of 0.11%T₂ and 0.42%T₂, respectively.

If the results of the above analysis for the third example are compared with those of the first example, it becomes apparent that with the smaller beam aspect ratio of 3:1 , and a elliptical motion ellipticity ratio of 1 :3, there is a more even spread of power over the basic fast ellipse, with a difference between maximum and minimum of just 37%. This is not surprising as the closer the "line" focus of the electron beam becomes to a square focus, the easier it becomes to avoid overlap in the locus of the beam. As such, a lower aspect ratio is desirable.

However, as an electron beam line focus with aspect ratio of 1:10 is fairly standard using current beam shaping technology, it is necessary to optimize for operation with such a beam. In this regard, the first example with either the second or third mode of operation is preferred, namely a basic fast ellipse with 1 :8 ellipticity ratio superposed on another slower moving shape to spread the heat load further, As will be appreciated, many other trajectories are possible, which may be optimized according to electron beam focal shape and the spherical shape of the anode target surface onto which the beam is projected.

Claims

1. An X-ray tube comprising a sealed housing defining an interior vacuum envelope when evacuated, the X-ray tube further comprising within the housing: a filament for generating electrons; a cathode; and, an anode spatially separated from the cathode, the anode and the cathode adapted to define a potential difference therebetween for accelerating electrons from the filament so as to impinge on a target surface of the anode and thereby generate X-rays, wherein the anode comprises an elongate member having a first end and a second end, the anode being configured such that: a longitudinal axis of the elongate member in a rest position is laterally displaced relative to the path of the accelerating electrons a point on the length of the elongate member from the first end to the second end is substantially fixed relative to the sealed housing; the target surface of the anode is spherical and is disposed on the second end of the elongate member; and, the second end of the elongate member is moveable relative to the sealed housing, whereby the spherical target surface may be moved relative to the impinging electrons such that the spatial location of X-ray generation in the tube is invariant relative to the sealed housing, thereby spreading localised heating caused by the X-ray generation over a greater portion of the target surface.

2. An X-ray tube according to claim 1 , in which the longitudinal axis of the elongate member lies along a radius of the spherical target surface and the radius is equal to the distance from the anode surface to the substantially fixed point on the length of the elongate member.

3. An X-ray tube according to claim 1 or claim 2, in which the anode is adapted to be driven such that the target surface of the second end moves along an oscillatory path relative to the impinging electrons.

4. An X-ray tube according to claim 3, in which the anode is adapted to be driven such that the target surface of the second end moves along an elliptical oscillatory path relative to the impinging electrons.

5. An X-ray tube according to claim 1 or claim 2, in which the anode is adapted to be driven such that the target surface of the second end moves along a gyratory path relative to the impinging electrons.

6. An X-ray tube according to claim 1 or claim 2, in which the anode is adapted to be driven such that the target surface is exposed to the impinging electrons for an exposure time which is varied in a predetermined manner.

7. An X-ray tube according to any preceding claim, wherein the anode comprises a cap disposed over the second end of the elongate member and coated with copper (Cu) or Molybdenum (Mb) for providing the target surface.

8. An X-ray tube according to any one of claims 1 to 7, further comprising means for anti-resonant damping of the motion of the elongate member.

9. An X-ray tube according to any one of claims 1 to 8, in which the elongate member of the anode comprises a flexible monolithic rod which is fixed at the first end relative to the housing and is moveable relative to the housing at the second end by a flexing action.

10. An X-ray tube according to claim 9, in which the rod passes through an airtight aperture in the housing and is formed of a thermally conductive material for conducting heat away from the target surface and to the exterior of the housing.

11. An X-ray tube according to any one of claims 1 to 8, in which the elongate member of the anode comprises a rigid member having a joint at the first end for pivoting about an inflexion point substantially fixed relative to the housing and which is moveable relative to the housing at the second end by a pivoting action.

12. An X-ray tube according to claim 11 , in which the joint comprises a ball and socket type joint which allows a universal rocking of the rigid member about the inflexion point.

13. An X-ray tube according to claim 11 or claim 12, further comprising a resilient means for retaining the first end in the substantially fixed relative position.

14. An X-ray tube according to claim 13, wherein the resilient retaining means comprises a helical spring.

15. An X-ray tube according to any one of claims 11 to 14, in which the rigid member comprises an internal pathway for the flow of a liquid coolant to and from the proximity of the target surface for conducting heat away from the target surface.

16. An X-ray tube according to claim 15, in which the internal pathway comprises a coaxial structure for the flow of the liquid coolant to and from the proximity of the target surface.

17. An X-ray tube according to claim 15 or claim 16, in which the internal pathway comprises a diffusing structure proximate the target surface for diffusing the coolant to ensure uniform cooling.

18. An X-ray tube according to any one of claims 11 to 17, further comprising flexible tubing coupled to the internal pathway of the rigid member and to the exterior of the housing for carrying the coolant to and from the rigid member.

19. An X-ray tube according to claim 18, wherein the tubing is configured as a double helix around the rigid member at the first end.

20. An X-ray tube according to any one of claims 15 to 19 when dependent on claim 8, wherein the anti-resonant damping means comprises liquid coolant flowing in or around the rigid member.

21. An X-ray tube according to any one of claims 15 to 19, wherein the anode is adapted to be driven by the passage of the liquid coolant so as to move the target surface relative to the impinging electrons.

22. An X-ray tube according to any one of claims 1 to 20, further comprising means within the housing for conveying an electromagnetic driving force to the second end of the elongate member for moving the second end.

23. An X-ray tube according to claim 22, in which the force conveying means comprises a solid ring of magnetic material disposed circumferentially about a portion of the elongate member proximate the second end.

24. An X-ray tube according to claim 22, in which the force conveying means comprises a disc of magnetic material having a central aperture through which the elongate member passes proximate the second end and having a peripheral region retained in an annular slot in the housing.

25. An X-ray tube assembly comprising an X-ray tube according to any one of claims 20 to 24 and an electromagnet arrangement disposed on the exterior of the housing for generating an electromagnetic driving force to be conveyed to the second end by the force conveying means.

26. An X-ray tube assembly according to claim 25, in which the electromagnet arrangement comprises one or more electromagnetic coils with a multi-pole core disposed adjacent the force conveying means.

27. An X-ray tube assembly according to claim 25, in which the electromagnet arrangement comprises two or more discrete electromagnets disposed circumferentially about the exterior of the housing adjacent the force conveying means.

28. An X-ray tube assembly comprising an X-ray tube according to any one of claims 1 to 20 and a mechanical actuator disposed external to the housing for driving the anode so as to move the target surface relative to the impinging electrons.

9. An X-ray tube or assembly as described herein with reference to the figures.