Liquid target X-ray source with jet mixing tool

Abstract

An X-ray source and a corresponding method for generating X-ray radiation are disclosed. The X-ray source includes a target generator, an electron source and a mixing tool. The target generator is adapted to form a liquid jet propagating through an interaction region, whereas the electron source is adapted to provide an electron beam directed towards the interaction region such that the electron beam interacts with the liquid jet to generate X-ray radiation. The mixing tool is adapted to induce mixing of the liquid jet at a distance downstream of the interaction region such that a maximum surface temperature of the liquid jet is below a threshold temperature. By controlling the maximum surface temperature, vaporisation, and thus the amount of contaminations originating from the jet, may be reduced.

Claims

1. An X-ray source comprising: a target generator adapted to form a liquid jet propagating through an interaction region; an electron source adapted to provide an electron beam directed towards the interaction region such that the electron beam interacts with the liquid jet to generate X-ray radiation; and a mixing tool adapted to induce mixing of the liquid jet at a distance downstream of the interaction region such that a maximum surface temperature of the liquid jet downstream of the interaction region is below a threshold temperature.

2. The X-ray source according to claim 1, wherein the threshold temperature corresponds to the temperature when the vapour pressure of the liquid jet equals a pressure exerted on the liquid jet.

3. The X-ray source according to claim 1, further comprising a shield arranged downstream of the interaction region, wherein the shield comprises an aperture arranged to allow the liquid jet to pass through the aperture.

4. The X-ray source according to claim 3, wherein the aperture is arranged within said distance from the interaction region.

5. The X-ray source according to claim 3, wherein the shield is arranged on a collection reservoir for collecting the liquid jet.

6. The X-ray source according to claim 5, further comprising a closed-loop circulation system located between the collection reservoir and the target generator and adapted to circulate the collected liquid of the liquid jet to the target generator.

7. The X-ray source according to claim 3, further comprising a sensor for detecting contamination, originating from the liquid, on a side of the shield facing away from the interaction region.

8. The X-ray source according to claim 1, wherein the mixing tool is formed of a surface arranged to intersect with the liquid jet.

9. The X-ray source according to claim 1, wherein the mixing tool is a liquid source adapted to supply an additional liquid to the liquid jet.

10. The X-ray source according to claim 9, wherein the liquid source is formed by a pool of the additional liquid.

11. The X-ray source according to claim 9, further comprising: a sensor for measuring a level of the additional liquid of the pool; and a level controlling device for controlling said level based on output from the sensor.

12. The X-ray source according to claim 9, wherein the liquid source is adapted to supply the additional liquid in the form of an additional jet.

13. The X-ray source according to claim 12, wherein a velocity of the additional jet comprises a non-negative component in respect to a travelling direction of the liquid jet.

14. The X-ray source according to claim 9, wherein the liquid source is adapted to supply the additional liquid in the form of a liquid curtain intersecting with the liquid jet.

15. The X-ray source according to claim 9, wherein the liquid source is adapted to provide the additional liquid on a slanting surface arranged to intersect with the liquid jet.

16. The X-ray source according to claim 9, wherein the additional liquid is a liquid metal.

17. The X-ray source according to claim 1, wherein the liquid jet is a liquid metal jet.

18. A method for generating X-ray radiation, comprising the steps of: forming a liquid jet propagating through an interaction region; directing an electron beam towards the liquid jet such that the electron beam interacts with the liquid jet at the interaction region to generate X-ray radiation; and inducing, by a mixing tool, mixing of the liquid jet at a distance downstream of the interaction region such that a maximum surface temperature of the liquid jet downstream of the interaction region is below a threshold temperature.

19. The method according to claim 18, wherein the step of inducing mixing comprises the step of determining the distance based on at least one of: a penetration depth of the electron beam into the liquid jet; a velocity of the jet; an electron velocity within the liquid jet; a boiling point of the liquid jet; a vapour pressure of the liquid jet; and a heat diffusivity of the liquid jet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of embodiments of the present invention. Reference will be made to the appended drawings, on which:

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

(3) FIG. 4 illustrates the interaction region in a portion of a liquid jet according to an embodiment;

(4) FIG. 5 is a diagram illustrating the distance between the interaction region and the position of the maximum surface temperature as a function of the energy of the impacting electrons;

(5) FIGS. 6a to d illustrate the propagation of the heat induced in the interaction region according to an embodiment; and

(6) FIG. 7 is a flowchart of a method according to an embodiment of the present invention.

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

DETAILED DESCRIPTION OF EMBODIMENTS

(8) A system comprising 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 170 may be defined by an enclosure 175 and an X-ray transparent window 180 that separates the vacuum chamber 170 from the ambient atmosphere. The X-rays 124 may be generated from an interaction region I, in which electrons from an electron beam 122 may interact with a target of a liquid jet 112.

(9) The electron beam 122 may be generated by an electron source, such as an electron gun 120 comprising a high-voltage cathode, directed towards the interaction region I.

(10) The interaction region I may be intersected by the liquid jet 112, which may be generated by a target generator 110. The target generator 110 may comprise a nozzle through which liquid, such as e.g. liquid metal may be expelled to form a jet 112 propagating towards and through the interaction region I.

(11) A shield 140, having an aperture 142, may be arranged downstream of the interaction region I such that the liquid metal jet 122 is allowed to pass through the aperture 142. In some embodiments, the shield 140 may be arranged at the end of the liquid metal jet 122, preferably in connection with a collection reservoir 150. Debris, splashes and other particles generated from the liquid metal downstream of the shield 140 may be deposited on the shield and thus prevented from contaminating the vacuum chamber 170.

(12) The system may further comprise a closed-loop circulation system 160 located between the collection reservoir 150 and the target generator 110. The closed-loop system 160 may be adapted to circulate the collected liquid metal to the target generator 110 by means of a high-pressure pump 162 adapted to raise the pressure to at least 10 bar, preferably at least 50 bar or more, for generating the target jet 112.

(13) Further, a mixing tool may be provided for inducing mixing of the liquid metal of the jet 112 at a certain distance downstream of the interaction region I. The mixing tool may e.g. be a liquid metal source 130 for supplying additional liquid 132 to the liquid jet 112 at the said distance. The additional liquid 132 may be provided to induce mixing of the liquid of the jet 112 and/or to absorb or redistribute at least some of the heat induced in the liquid jet 112 by the electrons impinging the interaction region I. The distance is preferably selected such that a maximum surface temperature of the liquid jet 112 downstream of the interaction region I is kept below a threshold temperature so as to reduce the amount of vapour originating from the liquid jet.

(14) In FIG. 1, the additional liquid 132 is supplied in the form of an additional liquid metal jet 132. The additional jet 132 may be formed by an additional nozzle 130 configured to direct the additional jet 132 to intersect the liquid metal jet 112 at a desired position downstream of the interaction region I. Referring to the exemplary embodiment in FIG. 1, the additional jet 132 may be oriented to intersect a plane coinciding with the electron beam 122 and the liquid metal jet 112 so as not to interfere with the electron beam 122 (or shadow the generated X-ray beam 124). It will however be appreciated that other configurations also are conceivable, wherein the additional liquid 132 e.g. is supplied in the form of a liquid curtain intersecting with the liquid metal jet 112. The liquid curtain (or liquid veil or film) may e.g. be formed by a slit-shaped additional nozzle 130 or an array of nozzles 130 generating an array of additional jets 132 merging into a substantially continuous curtain or sheet of liquid metal.

(15) FIG. 2 discloses a similar system as the one described with reference to FIG. 1. However, in the present embodiment the liquid source 130 is realised by a pool 130 of additional liquid, such as liquid metal 132, arranged such that a surface of the pool 130 intersects the liquid metal jet 112 at the desired position downstream of the interaction region I to keep the maximum surface temperature below the threshold. As indicated in FIG. 2, the pool 130 may be combined with a collection reservoir 150 for collecting the liquid metal at the end of the liquid metal jet 112, and a shield 140. The shield 140 may be arranged such that the aperture 142 is located between the interaction region I and surface of the pool 130. The pool 130 may further comprise a sensor for measuring the level of the additional liquid metal 132 of the pool, and a level controlling device for controlling said level based on output from the sensor (sensor and level controlling device not shown in FIG. 2).

(16) FIG. 3 shows a further embodiment of a system that may be similarly configured as the embodiments described with reference to FIGS. 1 and 2. According to this embodiment, the system may comprise a mixing tool 130 arranged to interact or interfere with the liquid jet 112 such that mixing of the liquid jet is induced at a certain distance downstream of the interaction region I. The certain distance, or point of mixing, may correspond to the position in which the additional liquid 132 is supplied to the liquid jet 112 according to the embodiments of FIGS. 1 and 2. The mixing tool 130 may e.g. comprise an edge being inserted into at least a portion of the propagating liquid jet 112, or be formed by a surface onto which the entire jet 112 or at least a part of the jet 112 is impacting so as to induce mixing within the liquid of the jet 112. The mixing may also be realised, or induced, by supply of an additional liquid metal 132 as described above in connection with FIGS. 1 and 2.

(17) The above-discussed embodiments may be combined with the shield 140 described with reference to FIG. 1. The shield 140 may be arranged downstream of the position in which the additional liquid metal 132 is supplied to the liquid metal jet 112 and/or in which the mixing is induced. However, it will be appreciated that the shield 140, according to alternative embodiments, may be arranged such that the aperture 142 is located between the interaction region I and the position for supply of the additional liquid metal 132 and/or the position at which mixing may be induced.

(18) FIG. 4 illustrates a cross sectional side view of a portion of the liquid jet 112 according to any one of the previously described embodiments. In this example, the liquid jet 112 propagates through the interaction region I at speed v.sub.j. Further, an electron beam 122 is illustrated, in which electrons propagate towards the liquid jet at speed v.sub.e and interacts with the liquid of the jet 112 in the interaction region I. The penetration depth of the electrons into the jet 112 is in the present FIG. 4 indicated by . In the following, an example of how to estimate the position of the maximum surface temperature of the jet is given. It should however be noted that this is merely an example based on a physical model for illustrating the underlying heat diffusion process resulting in the maximum surface heat of the jet being located at a certain distance downstream of the interaction region. It should also be noted that this model may not be applicable for cases wherein the temperature within the liquid jet exceeds the boiling point of the liquid jet. Other methods of determining the distance between the interaction region I and the position having the maximum surface temperature are conceivable.

(19) The electrons impacting the liquid jet 112 may have a characteristic penetration depth that depends, inter alia, on the energy of the impacting electrons. The time it will take for the electrons to penetrate the liquid depends e.g. on the scattering events they experience. A conservative estimate of this time may be obtained by using the incoming electron velocity v.sub.e. The estimate can be improved by considering the amount of scattering essentially perpendicular to the incoming direction of the electrons. This gives the following relation:

(20) $=\frac{0.1E_0^{1.5}}{}$
where E.sub.0 is the energy of the incoming electrons in keV, p is the target density in g/cm.sup.3, and is the penetration depth in m. The width of the interaction volume can in a similar approximation be written as

(21) $y=\frac{0.077E_0^{1.5}}{}$
where y is in m. Thus, the electrons may be distributed within a cone having an angle of tan.sup.1(0.077/(20.1)) from the incoming direction. If the incoming linear momentum is partitioned accordingly the resulting velocity in the forward direction is the cosine of this angle times the incoming velocity. Thus the velocity in the impact direction can be estimated as 93% of the velocity of the incoming electrons. To calculate the velocity of the electrons from the acceleration voltage, relativistic effects might have to be considered. According to special relativity the velocity of an electron with energy E.sub.0 keV can be written as

(22) $v=c\sqrt{1-\frac{1}{{\left(1+\frac{E_0}{511}\right)}^2}}$
where c is speed of light in m/s, the rest mass of the electron has been set to 511 keV, and v is in m/s. Putting all of this together gives the following estimate for the time required for electrons to penetrate into the jet:

(23) ${}_e=\frac{}{0.93v_e}=\frac{\frac{0.1E_0^{1.5}}{}}{0.93c\sqrt{1-\frac{1}{{\left(1+\frac{E_0}{511}\right)}^2}}},$
where .sub.e is in s.

(24) The time required for the heat to reach the surface of the jet and thus cause vaporization of the liquid can be estimated by solving the heat equation

(25) $\frac{T}{t}={}^{2}T,$
where the temperature T is a function of time and three spatial dimensions (x, y, and z), is the heat diffusivity in units of m.sup.2/s. If an initial temperature distribution corresponding to a temperature elevation T in a point at a distance into the liquid jet is assumed one may write the excess temperature as

(26) $T=T\frac{1}{{\left(4t\right)}^{3/2}}{e}^{-\frac{x^2+y^2+z^2}{4t}}.$

(27) By seeking the time where this function reaches its maximum for a spatial coordinate corresponding to the jet surface an estimate on the time when maximum evaporation rate occurs can be obtained. By selecting the coordinate system so that (x,y,z)=(,0,0) on a point on the jet surface closest to the point where the initial elevated temperature is applied, deriving T with respect to t, and setting the derivative to zero one obtains

(28) ${}_T=\frac{{}^2}{6}$
where .sub.T is the time when temperature on the jet surface reaches its maximum.

(29) The distance from the interaction point until maximum jet surface temperature occurs can thus be written as

(30) $d=v_J\left({}_T+{}_e\right)=v_J\left(\frac{{}^2}{6}+\frac{}{v_e^{}}\right)=v_j\left(\frac{}{6}+\frac{1}{v_e^{}}\right)$
where v.sub.e.sup. is the electron velocity inside the jet in the direction perpendicular to the jet surface. By applying the expressions for penetration depth and electron velocity from above this can further be written as

(31) $d=v_J\frac{0.1E_0^{1.5}}{}\left(\frac{10^{-7}{E}_0^{1.5}}{6}+\frac{1}{0.93c\sqrt{1-\frac{1}{{\left(1+\frac{E_0}{511}\right)}^2}}}\right)$
where again should be in g/cm.sup.3, E.sub.0 in keV, and d is in m. By inserting realistic values for a liquid gallium jet X-ray source (=6 g/cm.sup.3, 1.210.sup.5 m.sup.2/s, E.sub.0=50 keV, v.sub.j=100 m/s) a distance of about 50 m is obtained. If the electron energy can be raised to 100 keV the distance would, according to this example, increase to almost 400 m; if the jet velocity can be increased to 1000 m/s in the same setting the distance may increase to close to 4 mm.

(32) It turns out that for most practical purposes the second term in the parenthesis above, corresponding to the time it takes for the electrons to reach their penetration depth, gives a negligible contribution. For simplicity we can thus estimate the distance d as

(33) 0$d{v}_J\frac{E_0^3}{6{}^2}10^{-8}.$

(34) The relation between electron energy and distance d according to this model is illustrated in the FIG. 5, which shows, for two different velocities v.sub.j of the liquid jet, the distance d (in mm) between the interaction region and the location of the maximum surface temperature T.sub.max (i.e., when no additional liquid or mixing is employed) as a function of the electron energy E.sub.0 (in keV). Curve A represents the distance d for the exemplary system described above, i.e., for =6 g/cm.sup.3, 1.210.sup.5 m.sup.2/s and a liquid jet velocity v.sub.j of 100 m/s. As indicated, this may result in a distance d of about 50 m for electron energies of 50 keV and a distance d about 0.4 mm for electron energies of 100 keV. Increasing the velocity v.sub.j of the liquid jet to 1000 m/s would, according to the present model as represented by curve B, result in a distance d of about 0.5 mm for electron energies of 50 keV and a distance d of about 3.8 mm for electron energies of 100 keV. This relation, or other estimations of the distance d, may be employed to determine where on the propagating jet to supply the additional liquid so as to prevent the maximum surface temperature from exceeding the threshold value. The additional liquid may in other words be supplied at a position between the interaction region and the estimated distanced so as to reduce the maximum surface temperature. Examples of suitable distances may be included in the range of 50 m to 4 mm.

(35) FIGS. 6a to d are a sequence of figures illustrating the diffusion over time of the heat induced in the interaction region I by the impacting electrons. Similar to FIG. 4, FIGS. 6a to d show a cross sectional side view of a portion of the liquid jet 112 according to an embodiment of the present invention. The expansion and propagation of the heated portion or region H of the liquid is indicated in relation to the position of the interaction region I. FIG. 6a illustrates the heated region H shortly after impact, showing a relatively small region H located at the interaction region I. Over time, the heated region expands due to heat diffusion, and propagates downwards with the velocity v.sub.j of the jet 112. This is illustrated in FIGS. 6b and c, showing a slightly increasing region H being located further and further downstream of the interaction region I. Finally, in FIG. 6d, the heated region H has expanded all the way to the surface of the jet 112. This occurs at the distance d downstream of the jet, wherein the surface reaches its maximum temperature T.sub.max and, accordingly, its vaporisation maximum. Thus, by inducing mixing, e.g. by supplying the additional liquid, at a position upstream of the position where the maximum temperature T.sub.max otherwise would occur, the vaporisation from the exposed surface may be reduced.

(36) According to an example, the threshold temperature may be based on the vapour pressure for the particular type of liquid used in the vacuum chamber. For a liquid metal jet exposed to a typical vacuum chamber pressure of 510.sup.7 mbar, this would result in a temperature of about 930 K for Ga, 1015 K for Sn, 850 K for In, 660 K for Bi and about 680 K for Pb. Thus, for a chamber pressure of 510.sup.7 mbar, mixing of the liquid metal jet may preferably be provided such that the maximum surface temperature of the liquid metal jet is kept below the above mentioned temperatures so as to reduce vaporisation of the liquid metal.

(37) FIG. 7 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 710 a liquid jet propagating through an interaction region, directing 720 an electron beam towards the liquid jet such that the electron beam interacts with the liquid jet at the interaction region to generate X-ray radiation, and supplying 730 additional liquid to the liquid jet at a distance downstream of the interaction region such that a maximum surface temperature of the jet downstream of the interaction region is below a threshold temperature.

(38) The person skilled in the art 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. In particular, X-ray sources and systems comprising more than one electron beam and/or liquid jets are conceivable within the scope of the present inventive concept. Additionally, variation to the disclosed embodiments can be understood and effected by the skilled person in practising 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.