Abstract:
A compact X-ray source is disclosed, improving controllability and insulation from unwanted high voltage effects. In one aspect, an active variable conductance device ( 130, 330 ) connected in series with the cathode is used in a closed loop, feedback arrangement to control the cathode beam current; the current flowing through the device to the cathode being directly sensed and compared with a desired current level. The result of the comparison is used to control the conductance of the device, thereby directly influencing the cathode current. A second aspect provides an extension of a Faraday cage, whereby the secondary winding of a transformer used to supply power to components within the cage is shielded within a coaxial, tubular member connected to the cage and extending outwardly from it.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a National Phase Patent Application of International Application Number PCT/GB01/03274, filed on Jul. 23, 2001, which claims priority of British Patent Application Number 0017976.2, filed Jul. 22, 2000. 

   FIELD OF THE INVENTION 
   This invention relates generally to the production of X-rays, and in particular, but not exclusively it relates to a compact X-ray source. 
   BACKGROUND OF THE INVENTION 
   A typical X-ray source comprises a thermionic source (typically a heated filament), a high-voltage supply to accelerate the electrons to a high energy, and a target made of a high atomic number metal. 
     FIG. 1  depicts a simple schematic diagram of a very basic and conventional X-ray source, although it will be realised that, in practice, much more complex arrangements are generally used, including the use of additional electrodes and magnetic fields to control and focus the electron beam 
   Electrons are emitted thermionically from a hot cathode filament  30  under the action of an isolated heater supply  10  and are attracted to a metal target  70  via an intervening anode  60 . The electrons are accelerated in a beam  50  towards the target due to a high potential difference between the filament and the anode/target arrangement established by means of a high voltage supply  20 . On striking the target  70  the electrons stimulate X-ray emission by various processes, resulting in the emission of an X-ray beam  80 . 
   Since it is desirable for the anode and target to be at, or substantially near, ground potential, the cathode filament must be at a very high negative potential with respect to ground Moreover, the cathode filament requires several watts of power to reach operable temperatures. 
     FIG. 2  shows a typical X-ray source arrangement where a cathode filament  30  is heated by a voltage supplied from an isolating transformer  11 . Typically the voltage is between 2V and 6V, whilst the electrons are accelerated by a high voltage supplied from a multiplier  90 , known as a Cockcroft-Walton voltage multiplier. The high voltage maybe in the range of hundreds of kilovolts, for example 160 kV. 
   It is often required to construct an X-ray source that is compact, and this requirement introduces or exacerbates various problems, for example those associated with providing accurate and effective control over the electron beam current, particularly where the source is desired to be capable of operating reliably with a low radiation output, and those associated with achieving sufficient insulation between various components. 
   Control over the current of the electron beam  50  is usually desirable with X-ray sources in general and, in low performance X-ray sources, this is frequently achieved merely by varying the temperature of the filament; relying upon the principle that a hotter filament emits more current than does a cooler one. In higher performance systems, exemplified in very basic form in  FIG. 3 , this is achieved by controlling the beam in the space charge limited regime by means of a field control electrode  40 , often referred to as a focusing cup or Wehnelt. Such a focusing cup  40  is required to be at a negative potential with respect to the cathode filament in much the same way as the grid in a thermionic triode valve. The required potential can be supplied by either an electrically isolated bias supply, or self-biasing using a feedback resistor  120  between cathode filament  30  and focus cup  40 . Current passing through the feedback resistor generates the required negative bias. However, such a negative feedback system has the drawback that it is difficult to adjust. 
   When conventional X-ray sources are required to operate at low electron beam current levels, a problem occurs in that electron current leakage from the cathode and focus cup becomes significant compared to the total electron beam current. Often this problem arises from cold cathode discharge (field emission), ‘surface tracking’ or other such problematic phenomena. Conventional X-ray sources measure the electron beam current with a current sensing circuit located at the end of the high voltage supply that is at ground potential (shown schematically as  25  in FIG.  4 ). A problem then arises in that any current measurement at this point in the system cannot differentiate between the actual thermionic electron beam current and the leakage current. This inability to 
   separate the level of current leakage from the overall current measurement leads to variations in X-ray output since accurate control over the true electron beam current is not possible. Particularly where low radiation output levels are called for, variations in the measured electron beam current due to spurious factors such as those mentioned above can have a significant and adverse effect upon the radiation output levels and stability of operation. 
   Another problem with conventional X-ray sources arises from the high voltages required to accelerate the electron beam. When employing such extreme potential differences, there is always a risk of an electrical discharge or breakdown. When such phenomena occur, rapidly changing electromagnetic fields arise. Such fields induce large currents to instantaneously flow within the electronic circuitry of the X-ray source, and these currents can damage or destroy circuit components leading to X-ray source failure. A common solution to this problem is to enclose all susceptible components and circuitry within a Faraday shield to protect them from any rapidly changing fields. 
   In known X-ray sources, the integrity of the Faraday shield is compromised by the need to leave a conduit through which power and signals can be introduced into the circuitry. The break in the shield to provide a signal path also provides a pathway for signal interference during a high voltage breakdown. The integrity of the shield is particularly compromised by the use of isolating transformers that are generally used to introduce power and signals into the Faraday shield. 
   The present invention arose in an attempt to address some or all of the above problems. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention there is provided an X-ray source comprising: a high voltage power source; a cathode filament coupled to said high voltage power source; an active variable conductance device connected between the cathode filament and the high voltage power source; means for determining the amount of current flowing into said cathode filament through said variable conductance device and for providing a signal indicative thereof; and control means for utilising said signal to control said amount of current, thereby to control the current of an electron beam emitted from said cathode. 
   This current control arrangement differs significantly, in concept and effect, from conventional circuit schemes, which typically employ a separate DC supply for the grid voltage, floating at cathode potential. The voltage levels of such supplies require accurate control and stabilisation. It has been proposed in U.S. Pat. No. 5,528,657 to use such a series-regulating element to control the operative high voltage (anode/cathode) level, but this document does not teach series regulated control of the grid voltage level. The present invention also differs substantially, in concept and effect, from circuit arrangements for pulsed grid X-ray tubes, such as those disclosed in Japanese patent application No. 59132599. This document teaches the use of a transistor as a switch in the grid circuit to effect fast beam-switching with minimal overshoot and distortion of the current pulse. 
   Preferably, the active variable conductance device is a transistor, for example either a field effect transistor (FEI) or a bipolar transistor. 
   The active variable conductance device may alternatively comprise one or more light dependent resistors. 
   The control means advantageously comprises fibre optics and electro-optical devices, or any other optical link. 
   By using an active variable conductance device instead of a passive resistor as in the prior art, control over the electron beam current is greatly facilitated. Preferably, an optical link is used to control the variable conductance device, thereby reducing the risk of electromagnetic interference. 
   In a preferred embodiment, a current detector for detecting the current flow between the high voltage supply and the cathode filament is provided, either between the output of the high voltage power supply and the active variable conductance device or between the active variable conductance device and the cathode filament. 
   By measuring the current at this point, rather than at the ground end of the high voltage power source, discrimination between the true thermionic emission from the filament and all other forms of leakage current becomes possible. Hence the true thermionic emission current can be measured and controlled. 
   In accordance with a second aspect of the present invention, there is provided an X-ray source comprising a Faraday shield, in which electrical circuitry is housed, a high voltage power supply and an isolating transformer, wherein the isolating transformer is coaxially shielded; the shielding forming a continuation of the Faraday shield. 
   The isolating transformer is preferably in electrical connection with both an electron accelerating means and a cathode filament transformer, or other cathode filament supply means. 
   The first and second aspects of the invention are valuable individually, but a preferred embodiment comprises an X-ray source including both aspects of the invention. 
   The invention further provides an X-ray source or apparatus including any one or more of the novel features described or claimed herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying schematic drawings, in which: 
       FIG. 1  shows a conventional X-ray source circuit arrangement; 
       FIG. 2  shows conventional cathode filament heating in an X-ray source incorporating a high voltage multiplier circuit and isolating heater transformer; 
       FIG. 3  shows an X-ray source utilising negative feedback biasing; 
       FIG. 4  shows an embodiment of an X-ray source in accordance with one example of the first aspect of the present invention; 
       FIG. 5  shows a further embodiment of an X-ray source in accordance with another example of the first aspect of the present invention; 
       FIG. 6  shows an embodiment of an X-ray source in accordance with one example of the second aspect of the present invention; 
       FIG. 7  shows a further embodiment of an X-ray source in accordance with another example of the second aspect of the present invention; and 
       FIG. 8  shows a preferred embodiment of an X-ray source incorporating examples of both aspects of the invention. 
   

   In all of  FIGS. 1  to  7 , identical reference numbers are used throughout to indicate similar components and features. In  FIG. 8 , however, features and components directly comparable with those in  FIGS. 1  to  7  are given reference numbers increased by 200 over those used in the earlier figures. 
   DETAILED DESCRIPTION 
   In the conventional X-ray source shown in  FIG. 1 , a cathode filament  30  is connected to an isolated power supply  10 . Encircling the cathode filament  30 , and connected to a high voltage supply  20 , is a focusing cup  40 . In operation, an electron beam  50  is accelerated through an annular anode  60  and focused onto a metal target  70  from which X-rays  80  radiate. The power supply  10  typically comprises an isolating step-down transformer (shown in  FIG. 2  as  11 ), supplying around 6V to heat the cathode filament  30 . 
     FIG. 2  shows a conventional X-ray source including a high voltage multiplier circuit  90  connected to the focusing cup  40 . Here, an isolating transformer  11  is shown connected to the cathode filament  30 . The multiplier  90  is otherwise known as a Cockcroft-Walton voltage multiplier  90 . Most modem X-ray sources use this type of multiplier, the functioning of which is well known to persons skilled in the art. 
   Included in the conventional X-ray source shown in  FIG. 3  is a variable feedback resistor  120 , which is connected between the cathode filament  30  and the focusing cup  40 . This configuration provides negative biasing to the focusing cup  40 , thus ensuring that it remains at a negative potential as compared to the potential of the cathode filament  30 . Biasing is essential if the focusing cup is to provide space-charge control of the electron beam current and is often alternatively provided by an isolated negative bias supply. 
   A problem arising from the X-ray source of  FIG. 3  stems from the difficulties associated with safely and precisely varying the value of the feedback resistor in order to maintain optimal control of the beam current. An embodiment of an X-ray source in accordance with the first aspect of the invention is shown in FIG.  4 . Here, instead of a feedback resistor, an active variable conductance device  130  is employed. This device maybe a field effect transistor (FET) for example. Alternatively, a light dependent resistor (LDR) controlled by an optical link to vary the conductance can be used. Indeed, the reader will be aware that there are many other devices that may be suitable for the particular requirements of an application. 
   In the X-ray source of  FIG. 4 , the variable conductance device  130  is a bipolar transistor, controlled (by one of a variety of known methods) by a control circuit  140  in response to control signals  150 . In the case where optical control is used, control signals  150  will be passed by one of a choice of known optical links such as a conventional fibre optic cable and transduced by suitable electro-optical devices such as light-emitting diodes (LEDs) and photodiodes. In this way it is possible to provide precise dynamic and inertialess control of the electron beam current. 
   In a further embodiment of an X-ray source according to the first aspect of the arrangement, as shown in  FIG. 5 , a current sensing circuit  160  is employed to provide a measurable indication of the electron beam current. This circuit can include an LED, the luminance of which is directly proportional to the amplified electron beam current. The circuit generates control signals  170  that are used in feedback control of the variable conductance device  130 , through control signals  150  and associated control circuit  140 . (This feedback loop is shown schematically by the broken line  155 ). In practice, other components may be included in the feedback loop, and these components may include ground circuitry  156 , so that signal  170  returns to ground and signal  150  is transmitted from ground. The current sensing circuit  160  is shown between the high voltage supply and the active conductance device. This current sensing circuit could instead be at a position indicated by  160 A, between the active conductance device  130  and the filament  30 . 
   The advantage of the above embodiment is that, in measuring the current flow at a point in the circuit shown in  FIG. 5  by circuit  160  (or alternatively  160 A), it is possible to differentiate accurately between the thermionic current flow and the leakage current which, as described earlier, can be influenced by many extraneous factors. Measured current values can then be used in a feedback control loop via optic link  150  to facilitate optimal adjustment of the biasing level. The current sensitive circuit  160  may take many different forms, and may be optical or electronic or otherwise. Many such means will be apparent to the skilled reader. 
   As discussed above, it is conventional to enclose all sensitive circuitry and components in a Faraday shield. However, it is not normally possible to completely electrically screen the components from potentially damaging electromagnetic fields, since a break in the Faraday shield is necessary to allow access to the circuit for power lines, control inputs etc. 
   Referring to  FIGS. 6 and 7 , a transformer primary winding  180  is coupled to a transformer secondary winding  190  via a transformer core  200 . The transformer secondary winding  190  feeds power into circuitry within a Faraday shield  210 . 
   In an embodiment of the second aspect of the invention, a toroidal metal sheath  193  surrounds the transformer secondary winding  190 , and extends as a tube  194  from the secondary circuit  190  towards the main Faraday shield  210 . For practical shielding purposes, the toroidal sheath  193  and tube  194  form an integral part of the Faraday shield  210 . Tube  194  serves as a conduit, screening wires  195  connecting (or continuing) winding  190  to circuitry within the Faraday shield. The toroidal sheath has a discontinuity, or electrical break,  196 , preventing it from acting as a shorted turn. The discontinuity is, however, such that total shielding is still obtained. 
     FIG. 7  shows a variant of  FIG. 6 , in which the outer coaxial conductor forms part of the secondary winding; it connects to the secondary winding at point  197 . Thus, the outer conductor forms part of the winding and its extension towards the Faraday shield. 
   It is to be noted that, in  FIGS. 6 and 7 , only one turn is shown for the primary and secondary windings, for clarity. In practice, more than one turn may be present for either or both of these. 
   Referring now to  FIG. 8 , there is shown a preferred embodiment of the invention in which developed forms of both aspects of the invention are incorporated into an integrated high voltage generator and x-ray source. 
   The electron beam is produced by thermionic emission from a cathode  230 , which is made from tungsten wire or other material typically formed into the shape of a hairpin. In order for it to emit electrons, the cathode must be heated to incandescence. The required cathode temperature is produced by resistive self-heating. Electrons are extracted from the cathode  230  by means of an electric field applied, in known manner, between the cathode  230  and an anode (not shown in FIG.  8 ). As explained previously, the arrangement is such that the anode is at ground potential and the cathode is raised to a high negative potential. The magnitude of the beam current is controlled by a “bias” voltage imposed onto an annular grid electrode or Wehnelt  240  that surrounds the cathode. The bias voltage is always negative with respect to the cathode. The bias voltage also serves to produce a focussing electric field for the emitted electron beam, thereby controlling its diameter and ultimately the size of the x-ray source. The cathode  230  and the annular grid electrode  240  are, as is conventional, maintained in vacuum; the vacuum wall being shown in part as  235  in FIG.  8 . 
   The grid bias voltage is obtained by a technique, known as self-bias, which is commonly used on triode devices including, in particular, electron microscopes. The electron beam current passes through a resistor connected between the grid and the cathode and develops, across the resistor, a voltage which constitutes the grid bias voltage. The system is thus self-stabilising and a separate power supply for the grid voltage is not required. The magnitude of the electron beam current depends on the size of the resistor and on physical characteristics of the gun which are geometry dependent. 
   In accordance with this embodiment, the resistor is replaced by a device whose resistance can be altered electronically. A preferred device is a Field Effect Transistor (FET)  330 , but the principle of operation could also be implemented using other devices, such as light dependent resistors. 
   The beam current flows in series through a resistor  325 , the FET  330  and a resistor  335 . A Zener diode  336  protects the FET  330  from excessive voltage. 
   As discussed above, this arrangement differs significantly, in both concept and effect, from conventional circuit schemes, which typically employ a separate DC supply for the grid voltage floating at cathode potential, and which may utilise a series-regulating element for voltage control and stabilisation. 
   In conventional x-ray generators, the beam current sensing is typically achieved by measuring the current flowing at the bottom of the diode capacitor bank forming the high voltage multiplier (often called a Cockroft-Walton multiplier). In the present system, such a high voltage multiplier  290  is employed. A conventional sense resistor  300  is also shown. However, as described above, there is a serious disadvantage to using the voltage on sense resistor  300  as the means of measuring and controlling the electron beam current; namely that the current flowing at this point may include extraneous components in addition to the true electron beam current. These extraneous currents often include currents emitted from the vacuum facing surface of the housing surrounding the filament. The locations producing such emission are known as cold cathode or field emission sites, and are well known to those skilled in the art of the design of high voltage vacuum devices. Field emission sites are unstable and can be neither predicted nor eliminated If the control signal for beam current stabilisation is derived from a sense resistor  300  then the control of the true electron beam, that is emitted thermionically from the cathode  230 , will be corrupted by the unquantifiable inclusion of extraneous currents from field emission sites. This makes stable control at low operating beam currents and high cathode voltages very difficult and degrades x-ray image quality under such conditions. The present invention permits the true current flowing from the cathode to be measured. This allows very precise control of the beam current even under usually difficult conditions, such as when operating at extreme high voltage with low beam currents, and even with field emission sites present. 
   The true electron beam current is sensed as a voltage across resistor  325  and is fed into an integrated circuit  361  configured as a voltage to frequency converter. The frequency output of integrated circuit  361  drives an LED  362 , which sends a frequency modulated light signal  371  down an optical fibre  355   a . At the other end of the fibre  355   a , the optical signal is incident upon a photodiode  363 . This converts the optical signal back into an electrical signal which accurately represents the measured electron beam current and is applied, via a buffer amplifier  364 , to circuitry (not shown) which interfaces in a known manner with a computer. Computer commands input by a user of the system are used to effect adjustment of the electron beam current. However, if a computer is not used, appropriate circuitry is presented at a location convenient for direct or remote manual adjustment by an operator, thus allowing the beam current to be controlled, which may be either in real time, or to predetermined values. 
   It is necessary to provide a feedback signal for precise closed-loop control of the beam current against the predetermined demand level selected by the operator. Advantageously, since the resistance of the FET  330  may be varied by adjusting its gate voltage, this is accomplished by means of another photodiode  365  using optical signals  351  generated by a second LED  366 ; these optical signals  351  being amplitude modulated in a sense effective to indicate any desired change of the beam current. The signals are delivered into a second optical fibre  355   b , the output of which illuminates the photodiode  365 . 
   Optical fibres are used to provide electrical isolation between electronic circuits at the high and low voltage ends of the high voltage multiplier  290 . 
   The current sensed on resistor  300  is not used for control or measurement, but may be used by circuits designed to protect the high voltage generator in the event of a fault causing excessively high current in the multiplier  290 . 
   Occasional electrical discharges can be expected to occur within the x-ray source. Such discharges lead to rapidly changing transient currents, and it is necessary to protect active electronic components from the potentially damaging effects of radiated and conducted electromagnetic interference generated by these transients. The electronic circuits associated with the cathode and grid are contained in a metal walled chamber  410 . The whole of this container is connected to the grid and is therefore at a very high voltage with respect to ground. This container provides very substantial screening for the sensitive circuits within it, and acts as a “Faraday shield”. 
   Although it does not need to be hermetically sealed, the container is constructed in such a way that its openings are of minimal size. The integrity of such a Faraday shield may be compromised by the need to bring electrical signals in and out. 
   In this embodiment, the power for all of the circuits within the shield is provided by a high voltage isolation transformer. The secondary winding  390  of the transformer is insulated so as to provide the required high voltage isolation, and is constructed as a co-axial system. The outer conducting member  393  of this co-axial arrangement forms a continuous extension of the main Faraday shield  410 . Furthermore, only the outer conductor of the co-axial arrangement winds around the transformer core  400 . 
   The inner conductor  390  emerges from a hole in the side of the outer conductor and is then joined to the end of outer conductor  393 . The length of inner conductor  390  and the size of the hole in the outer conductor  393  are kept very small. The co-axial self screening construction of the secondary winding ensures that conducted and radiated signals into the Faraday shield are so small that the reliability of the sensitive components housed within can be guaranteed. 
   The core  400  of the isolating transformer lies outside the boundary of the Faraday shield  410 ; only the outer co-axial member  393  of the secondary winding  390  is integrated into the continuum of the Faraday shield wall. 
   The Faraday shield may advantageously contain certain additional electronic circuits which might, for example, be used to monitor, control or stabilise the cathode filament voltage, current or power. Such circuitry, floating at high voltage, may also utilise fibre optics as the means of conveying signals to other electronic circuits operating near to ground potential.