X-ray source with ionisation tool

Abstract

An X-ray source and a corresponding method for generating X-ray radiation are disclosed. The X-ray source includes a chamber comprising an interaction region, and a first electron source operable to emit a first electron beam, including electrons of a first energy, towards the interaction region such that the first electron beam interacts with a target to generate X-ray radiation. The X-ray source further includes a second electron source adapted to be independently operated to emit a second electron beam including electrons of a second energy for ionising particles in the chamber, and an ion collection tool that is adapted to remove the ionised particles from the chamber by means of an electromagnetic field. By ionising particles and preventing them from moving freely in the chamber, problems related to contamination of the chamber may be mitigated.

Claims

1. An X-ray source comprising: a chamber comprising an interaction region; a first electron source operable to emit a first electron beam, comprising electrons of a first energy, towards the interaction region such that the first electron beam interacts with a target to generate X-ray radiation; a second electron source adapted to be independently operated to emit a second electron beam comprising electrons of a second energy for ionising particles in the chamber; and an ion collection tool adapted to remove the ionised particles from the chamber by means of an electromagnetic field; wherein said first electron source comprises a first electron emitter and a first anode electrode for generating a first acceleration potential; wherein said second electron source comprises a second electron emitter, a second anode electrode for generating a second acceleration potential, and a deflector; wherein the first energy is 1 keV or higher and the second energy is lower than 1 keV.

2. The X-ray source according to claim 1, wherein said second electron emitter comprises a filament for emitting electrons when heated by a heater current.

3. The X-ray source according to claim 2, further comprising a controller arranged to adjust at least one of said heater current, said second acceleration potential, and said deflector.

4. The X-ray source according to claim 3 wherein the ion collection tool is arranged to provide a measure of the number of ionized particles and wherein said controller is arranged to control the second electron source to increase said measure.

5. The X-ray source according to claim 1, wherein the ion collection tool comprises a getter material.

6. The X-ray source according to claim 1, wherein the ion collection tool comprises a conductive element for generating the electromagnetic field directing the ionised particles towards an ion dump.

7. The X-ray source according to claim 1, wherein the ion collection tool is adapted to generate an electric field that is oriented transversally to the first electron beam.

8. The X-ray source according to claim 1, wherein said electromagnetic field is arranged rotationally symmetric with respect to an optical axis of the first electron source.

9. The X-ray source according to claim 1, further comprising a target generator adapted to form a stream of a target material propagating through the interaction region so as to form the target.

10. The X-ray source according to claim 9, wherein the target is formed of a liquid metal jet.

11. The X-ray source according to claim 10, wherein the ion collection tool is connected to a liquid jet material system for resupplying the material to the target generator.

12. The X-ray source according to claim 1, further comprising an X-ray window, wherein the second electron source is adapted to direct the second electron beam towards the X-ray window.

13. A method for generating X-ray radiation, comprising the steps of: directing a first electron beam, comprising electrons of a first energy, towards an interaction region in a chamber such that the electron beam interacts with a target to generate X-ray radiation; directing by means of a deflector, independently from the first electron beam, a second electron beam comprising electrons of a second energy for ionising particles in the chamber, such that the second electron beam interacts with debris generated from the interaction between the first electron beam and the target, thereby ionising at least some of the particles in the chamber; and wherein said first electron beam is generated using a first electron source comprising a first electron emitter and a first anode electrode for generating a first acceleration potential; wherein said second electron beam is generated using a second electron source comprising a second electron emitter and a second anode electrode for generating a second acceleration potential; wherein the first energy is 1 keV or higher and the second energy is lower than 1 keV.

14. The method according to claim 13, further comprising collecting the ionized particles, measuring a rate at which ionized particles are collected, and controlling said second electron beam so that the rate is increased.

15. The method according to claim 13, further comprising forming a stream of a target material propagating through the interaction region in the chamber so as to form the target.

16. The method according to claim 15, further comprising resupplying the collected particles to the stream of target material.

17. The method according to claim 13, further comprising resupplying the particles removed from the chamber to the target.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described for the purpose of exemplification with reference to the accompanying drawings, on which:

(2) FIGS. 1-3 are schematic, cross sectional side views of X-ray sources according to some embodiments of the present invention;

(3) FIG. 4 is a cross sectional side view of a second electron source according to an embodiment; and

(4) FIG. 5 schematically illustrates a method for generating X-ray radiation according to an embodiment of the present invention.

(5) All the figures are schematic, not necessarily to scale, and generally only show parts that are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION OF EMBODIMENTS

(6) An X-ray source 100 according to an embodiment of the invention will now be described with reference to FIG. 1. As indicated in FIG. 1, a vacuum chamber 110 may be defined by an enclosure 112 and an X-ray transparent window 180 that separates the vacuum chamber 110 from the ambient atmosphere. The X-rays 150 may be generated from an interaction region I, in which electrons from a first electron beam may interact with a target 120.

(7) The electron beam may be generated by a first electron source 130, such as an electron gun 130 comprising a high-voltage cathode, directed towards the interaction region I.

(8) According to the present embodiment, the target 120 may e.g. be formed of a liquid jet 120 intersecting the interaction region I. The liquid jet 120 may be generated by a target generator 140 comprising a nozzle through which e.g. a gas or a liquid, such as e.g. liquid metal may be expelled to form the jet 120 propagating towards and through the interaction region I.

(9) The X-ray source 100 may further comprise a closed-loop circulation system 142 located between a collection reservoir for collecting the material of the liquid jet 120 and the target generator 140. The closed-loop system 142 may be adapted to circulate the collected liquid metal to the target generator 140 by means of a high-pressure pump adapted to raise the pressure to at least 10 bar, preferably at least 50 bar or more, for generating the target jet 120.

(10) Further, the X-ray source may comprise an ionisation tool 160 adapted to ionise particles in the chamber 110. The ionisation tool 160 may e.g. be formed of a second electron source 160 that may be operable to emit one or several second electron beam(s), preferably independently from the operation of the first electron source 130, comprising electrons of a second energy suitable for ionising e.g. debris that may be generated upon the interaction between the first electron beam and the target material. In the example illustrated by the present figure, the second electron source 160 may comprise an electron emitter, one or several anode electrodes, and a deflector. The second electron source 160, or electron gun, may be arranged to emit at least one electron beam in a direction intersecting the direction of the first electron beam, i.e., oriented transversally to the first electron beam. Further, the transversal second electron beam may be oriented to interact with particles at a position between the X-ray window 180 and the interaction region I, so that particles may be ionised on their way from the interaction region I towards the X-ray window 180. Furthermore, a guiding electromagnetic field (not shown) may be provided ensuring that the electrons emitted from the second electron source travel along a path increasing the probability that they will encounter a neutral particle, e.g. a circular or helical path in the vicinity of the interaction region or the X-ray window.

(11) The X-ray source 100 may further comprise a controller 190 or controlling circuitry 190 that may be operably connected to the ionising tool 160. The controller 190 may be configured to control the operation of the ionising tool 160 and allow e.g. the second electron beam to be directed at a desired position. It will also be appreciated that the controller may be further connected to e.g. the ion collection tool or a particle sensor (not shown) to retrieve a measure of the number of ionized particles generated in, or present in, the chamber 110. This measure may e.g. be used as input when controlling the operation of the ionising tool.

(12) FIG. 1 also indicates that the X-ray source 100 may comprise an ion collection tool 170 for removing, or at least immobilising, ionised particles. The collection tool 170 may utilise an electromagnetic field E for controlling, or at least affecting, the movement of the particles. The electromagnetic field E may e.g. be provided with a transversal component relative to the optical axis of the X-ray source, so that charged particles may be deflected away from trajectories that lead up to e.g. the X-ray window 180 or the first electron source 130. The electromagnetic field E may be generated between two electrodes 172, which e.g. may be formed of a first and a second electrically conductive plate arranged at opposing sides of the optical axis. A bias voltage may be applied to the electrodes 172 by means of a voltage source 174 that is electrically connected to the electrodes 172.

(13) In the present embodiment, one of the electrodes 172 may be combined with an ion collector, or ion dump 176, adapted to collect the ionised particles. Thus, the charged particles may be captured by the electric field E and directed towards the ion collector 176 at which they may be trapped or collected by means of e.g. condensation, electrostatic attraction and/or a getter material. Further, the ion collector 176 may be connected to the closed-loop recycling system 142 such that the collected particles may be reused in the generation of the target 120. Alternatively, or additionally, the ion collector 176 may be combined with a measuring device (not shown) for measuring an amount of collected particles. The measuring device may e.g. comprise a current measuring device, such as an ammeter, for measuring the electric current produced by the charged particles, thus providing a measure of the ionization rate within the chamber. The measuring device may further be connected to the controller 190.

(14) FIG. 2 discloses an X-ray source according to an embodiment that may be similarly configured as the embodiment described with reference to FIG. 1. In the present embodiment, the ion collection tool 170 may be arranged to generate an electric field E along the direction of the first electron beam. The electric field may preferably be generated by means of a rotationally symmetric electrode 172. With this setup, the electric field will disturb the first electron beam to a limited extent or in a way that can be easily compensated for by defocusing or refocusing. In particular, the primary effect of a rotationally symmetric electrode is to change the divergence of the electron beam. In the present example, the ion collection tool 170 comprises an electrode 172 having an aperture through which the first electron beam may propagate on its way to the target 120 (which may be an arbitrary target, such as e.g. a stationary solid target or a liquid jet target). Depending on the size of the aperture, the first electrode 172 may thus form a mechanical shield preventing at least some particles from propagating towards the first electron source 130. Further, the geometric configuration of the ion collection tool 170 and the magnitude of the bias voltage may be selected in order for the resulting electric field E to prevent charged particles from entering the region of the first electron source 130 via the aperture. The bias voltage to be applied to generate the electric field E is to be selected in such manner that the act of moving a singly charged positive ion with a kinetic energy below a maximum energy from the interaction region I through the electric field E to the aperture of the electrode 172 requires a work greater than said maximum energy. In other words, a parallel electric field may be designed such that it realises an energy threshold high enough to stop all ions with kinetic energies below the maximum energy.

(15) It will however be realised that the conductive element or electrode may be arranged inside an aperture of a shield that does not form part of the electric field generating means. As indicated in the present figure, the electric E field may be generated between an electrode 172 and a portion of the housing, which may be kept at ground potential or at any other potential suitable for generating a desired electric field E.

(16) Further, the ionisation tool 160 may comprise a plurality of second electron sources arranged to irradiate particles passing between the interaction region I and the first electron source 130. The ionisation tool 160 may e.g. be arranged in a passage between the interaction region I and the ion collection tool 170.

(17) FIG. 3 illustrates an X-ray source 100 that may be similar to the embodiments described in connection to FIGS. 1 and 2, wherein the ionising tool (comprising e.g. a second electron source 160) is arranged upstream of the interaction region I, as seen from the first electron source 130. As indicated in the present figure, one or several electric coils 170 may be arranged to at least partly enclose the first electron beam. In FIG. 3, a cross section of a coil is indicated, wherein the coil 170 may be configured to generate a magnetic field B that may be parallel with the optical axis of the X-ray source 100. The coil 170 may form part of an electron-optical system for controlling and improving a quality of the electron beam. Alternatively, or as a consequence, the coil 170 may be arranged to deflect at least some charged particles entering the coil. Referring to the example illustrated by the present figure, charged particles having a trajectory that is non-parallel to the magnetic field B may interact with the magnetic field within the coil 170 such that they may be prevented from reaching the first electron source 130. Particles travelling along the optical axis may however be less affected by the coil 170, since they travel along the magnetic field B. They may on the other hand be bombarded by electrons of the first electron source and possibly be given a non-zero velocity component perpendicular to the optical axis.

(18) Further, an ion dump 178 or aperture, which e.g. may be a negatively charged plate, may be arranged upstream of the coil 170 to collect at least some of the particles that are deflected by the magnetic field B. Thus, particles generated in the vicinity of the target 120 need to pass the magnetic field B and the aperture of the ion dump 178 before they reach the first electron source 130.

(19) According to an embodiment, the magnetic field B as e.g. shown in FIG. 3 may be combined with an electric field with an orientation similar to what is shown in FIG. 2. In that case the ion dump 178 may be replaced with e.g. a rotationally symmetric electrode having an aperture through which the first electron beam may propagate on its way to the target.

(20) FIG. 4 is a cross section of a second electron source 160 according to an embodiment. The electron source 160 may be similarly configured as the ionising tool discussed in connection with the previous figures. The second electron source 160 may comprise an electron emitter 162, such as e.g. a filament 162, which may be configured to emit electrons when heated by a heater current. Further, an anode arrangement 164 may be provided in order to generate an electric field EA for accelerating the emitted electrons. The emitted electrons may then pass a deflector arrangement 166, which e.g. may be formed of a plurality of plates for directing the electron beam in different directions. Even though not illustrated in the present figure, the controller may be arranged to control the heater current, the acceleration potential and/or the deflector 166 so as to provide a second electron beam having the desired properties for ionising particles in the chamber.

(21) FIG. 5 is a flowchart illustrating a method for generating X-ray radiation according to an embodiment of the present invention. The method may comprise the steps of forming 10 a stream of a target material propagating through the interaction region I in the chamber 110 so as to form the target 120, and directing 20 a first electron beam, comprising electrons of a first energy, towards the interaction region I such that the electron beam interacts with the target 120 to generate X-ray radiation. The method may further comprise the steps of ionising 30 particles in the chamber, and removing 40 the ionised particles from the chamber 110 by means of an electric field E. The steps of ionising 30 particles may, in some embodiments, comprise the step of directing 30 a second electron beam comprising electrons of a second energy suitable for ionising the particles, such that the second electron beam interacts with debris generated from the interaction between the first electron beam and the target, thereby ionising at least some of the particles in the chamber. The method may further comprise the steps of collecting 50 the ionized particles, measuring 60 a rate at which ionized particles are collected, and adjusting 70 said second electron beam so that the rate is increased. In yet a further embodiment the method may comprise the step of resupplying 80 the particles removed from the chamber to the target. For embodiments comprising the step of forming 10 a stream of a target material propagating through the interaction region I in the chamber 110 so as to form the target 120 comprising the method may further comprise a step of resupplying 80 the collected particles to the stream of target material.

(22) The person skilled in the art realises that the present invention by no means is limited to the example embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the ionisation tool and/or the electrodes of the ion collection tool may be arranged in other geometric positions. The applied electromagnetic field need not be purely axial or purely transversal, but may be oriented in different ways provided it is effective in limiting the mobility of debris particles, notably by accelerating them away from sensitive parts the X-ray source or immobilising them by adsorption onto a surface or in an ion dump. In particular, the ionisation tool and/or the electromagnetic field may be deployed in a time varying fashion, which provides for more sophisticated ways of diverting debris particles from sensitive parts (e.g. the X-ray window or the cathode) into regions where they are harmless. Time-varying deployment may also be used to clear the irradiation region from freely moving debris more thoroughly at periodic intervals.

(23) Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.