Aligning and focusing an electron beam in an X-ray source

Abstract

A technique for indirectly measuring the degree of alignment of a beam in an electron-optical system including aligning means, focusing means and deflection means. To carry out the measurements, a simple sensor may be used, even a single-element sensor, provided it has a well-defined spatial extent. When practiced in connection with an X-ray source which is operable to produce an X-ray target, further, a technique for determining and controlling a width of an electron-beam at its intersection point with the target.

Claims

1. A method in an electron-optical system adapted to supply an outgoing electron beam in an electron-impact X-ray source operable to produce an electron target in an interaction region, the system comprising: an aligning unit for adjusting a direction of an incoming electron beam; a deflector operable to deflect the outgoing electron beam; and a focusing unit for focusing the outgoing electron beam in the interaction region; the method comprising the steps of: determining, for a plurality of focusing unit settings and aligning unit settings, a respective position of the outgoing electron beam by deflecting the outgoing electron beam into and/or out of a sensor area, which is arranged a distance downstream of the interaction region; determining, based on the plurality of positions thus determined, an adequate aligning unit setting for which the position has minimal sensitivity with respect to a change in focusing unit setting; and applying an aligning unit setting based on said adequate aligning unit setting.

2. The method of claim 1, further comprising a step of determining an orientation of the outgoing electron beam by ensuring that the electron target partially obscures the sensor area from a deflection range of the electron beam, and further by deflecting the electron beam between the electron target and an unobscured portion of the sensor area.

3. The method of claim 1, further comprising a step of determining, for at least one focusing unit setting, a width of the outgoing electron beam in the interaction region by ensuring that the electron target partially obscures the sensor area from the electron beam, and further by deflecting the electron beam between the electron target and an unobscured portion of the sensor area.

4. The method of claim 3, further comprising the steps of: receiving a desired electron beam width in the interaction region; and alternately repeating said step of determining a width of the outgoing electron beam in the interaction region and a step of adjusting, responsive thereto, the focusing unit setting with the aim of attaining the desired electron-beam width.

5. The method of claim 3, further comprising a step of minimising the width of the outgoing electron beam in the interaction region by alternately repeating said step of determining a width of the outgoing electron beam in the interaction region and a step of adjusting, responsive thereto, the focusing unit setting with the aim of reducing the width.

6. The method of claim 4, wherein the step of alternately repeating said step of determining a width of the outgoing electron beam in the interaction region and a step of adjusting the focusing unit setting includes adjusting the focusing unit setting non-monotonically for the step of adjusting the focusing unit setting and adjusting a deflection unit setting non-monotonically for the step of determining a width of the outgoing electron beam in the interaction region.

7. The method of claim 1, wherein said adequate aligning unit setting is determined subject to a condition on an offset of the electron beam with respect to an optical axis defined by the deflector and focusing unit.

8. The method of claim 1, wherein the step of determining a respective position for a plurality of focusing unit settings and aligning unit settings comprises the sub-steps, to be performed for each of said plurality of aligning unit settings, of: determining, for one focusing unit setting, a position of the outgoing electron beam by deflecting the outgoing electron beam into and/or out of the sensor area; and repeating the step of determining a beam position for at least one further focusing unit setting and the same aligning unit setting.

9. The method of claim 1, wherein the electron target is a liquid jet.

10. A non-transitory computer-readable medium storing computer-executable instructions for executing the method of claim 1.

11. An electron-optical system in an electron-impact X-ray source operable to produce an electron target in an interaction region, said system being adapted to receive an incoming electron beam and to supply an outgoing electron beam and comprising: an aligning unit for adjusting a direction of an incoming electron beam; a deflector operable to deflect the outgoing electron beam; and a focusing unit for focusing the outgoing electron beam in the interaction region; a sensor area; and a controller communicatively coupled to the aligning unit, the deflector, the focusing unit, and the sensor area; said controller being operable to: determine, for a plurality of focusing unit settings and aligning unit settings, a respective position of the outgoing electron beam by deflecting the outgoing electron beam into and/or out of the sensor area, which is arranged a distance downstream of the interaction region; determine, based on the plurality of positions thus determined, an adequate aligning unit setting for which the position has minimal sensitivity with respect to a change in focusing unit setting; and apply an aligning unit setting based on said adequate aligning unit setting.

12. The electron-optical system of claim 11, wherein the controller is communicatively coupled to the electron target and adapted to determine an orientation of the outgoing electron beam by ensuring that the electron target partially obscures the sensor area from a deflection range of the electron beam, and further by deflecting the electron beam between the electron target and an unobscured portion of the sensor area.

13. The electron-optical system of claim 11, wherein the controller is communicatively coupled to the electron target and adapted to determine, for at least one focusing unit setting, a width of the outgoing electron beam in the interaction region by ensuring that the electron target partially obscures the sensor area from the electron beam, and further by deflecting the electron beam between the electron target and an unobscured portion of the sensor area.

14. The electron-optical system of claim 11, wherein the sensor area is delimited.

15. The electron-optical system of claim 14, further comprising an electrically conductive screen which delimits the sensor area.

16. The electron-optical system of claim 15, adapted to maintain the screen at a constant potential.

17. The electron-optical system of claim 15, wherein the screen is arranged at a distance from the sensor area.

18. The electron-optical system of claim 11, further comprising a wall having a projection on which the sensor area is provided, wherein the sensor area is electrically insulated from the wall.

19. The electron-optical system of claim 11, further comprising a recess, which is provided in a charge-sensitive surface and which forms the sensor area.

20. An X-ray source, comprising: an electron-optical system of claim 11; and a nozzle for producing a liquid jet passing through the interaction region and acting as the electron target, wherein the production of the liquid jet is controllable by the controller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described with reference to the accompanying drawings, on which:

(2) FIG. 1a is a diagrammatical perspective view of an X-ray source of the liquid-jet type, in accordance with an embodiment of the invention;

(3) FIG. 1b is another diagrammatical perspective view of an X-ray source, in a variation of that shown in FIG. 1a;

(4) FIG. 1c shows a detail of an alternative implementation of an X-ray source of the general type shown in FIG. 1a;

(5) FIGS. 2a and 2b are flowcharts showing two embodiments of the invention as a method of calibrating an electron-optical system;

(6) FIG. 3a shows, in the plane of deflection, an electron beam at three different deflector settings and the intersection of an electron target with this plane;

(7) FIG. 3b is a plot of the sensor signal (after quantization) against combinations of a deflection setting and a focusing setting;

(8) FIG. 3c is a continuous plot of the sensor signal against a range of deflection settings combined with two different focusing settings;

(9) FIGS. 4a and 4b show a two-dimensional scanning pattern relative to an aperture in a screen delimiting a sensor area, as well as sensor data acquired using this scanning pattern; and

(10) FIGS. 5a and 5b show, similarly to FIGS. 4a and 4b, a one-dimensional scanning pattern and associated sensor data.

(11) Like reference numerals are used for like elements on the drawings. Unless otherwise indicated, the drawings are schematic and not to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

(12) FIG. 1a shows an X-ray source 10, generally comprising an electron gun 14-28, means 32 for generating a liquid jet J acting as an electron target, and a sensor arrangement 52-58 for determining a relative position of an outgoing electron beam I.sub.2 provided by the electron gun. This equipment is located inside a gas-tight housing 12, with possible exceptions for a voltage supply 13 and a controller 40, which may be located outside the housing 12 as shown in the drawing. Various electron-optical components functioning by electromagnetic interaction may also be located outside the housing 12 if the latter does not screen off electromagnetic fields to any significant extent. Accordingly, such electron-optical components may be located outside the vacuum region if the housing 12 is made of a material with low magnetic permeability, e.g., austenitic stainless steel. The electron gun generally comprises a cathode 14 which is powered by the voltage supply 13 and includes an electron source 16, e.g., a thermionic, thermal-field or cold-field charged-particle source. Typically, the electron energy may range from about 5 keV to about 500 keV. An electron beam from the source 16 is accelerated towards an accelerating aperture 17, at which point it enters an electron-optical system comprising an arrangement of aligning plates 26, lenses 22 and an arrangement of deflection plates 28. Variable properties of the aligning means, deflection means and lenses are controllable by signals provided by a controller 40. In this embodiment, the deflection and aligning means are operable to accelerate the electron beam in at least two transversal directions. After initial calibration, the aligning means 26 are typically maintained at a constant setting throughout a work cycle of the X-ray source, while the deflection means 28 are used for dynamically scanning or adjusting an electron spot location during use of the source 10. Controllable properties of the lenses 22 include their respective focusing powers (focal lengths). Although the drawing symbolically depicts the aligning, focusing and deflecting means in a way to suggest that they are of the electrostatic type, the invention may equally well be embodied by using electromagnetic equipment or a mixture of electrostatic and electromagnetic electron-optical components.

(13) Downstream of the electron-optical system, an outgoing electron beam I.sub.2 intersects with a liquid jet J, which may be produced by enabling a high-pressure nozzle 32, at an interaction region 30. This is where the X-ray production takes place. X-rays may be led out from the housing 12 in a direction not coinciding with the electron beam. The portion of the electron beam I.sub.2 that continues past the interaction region 30 reaches a sensor 52 unless it is obstructed by a conductive screen 54. In this embodiment, the screen 54

(14) is an earthed conductive plate having a circular aperture 56. This defines a clearly delimited sensor area, which corresponds approximately to the axial projection of the aperture 56 onto the sensor 52. In this embodiment, the sensor 52 is simply a conductive plate connected to earth via an ammeter 58, which provides an approximate measure of the total current carried by the electron beam I.sub.2 downstream of the screen 54. As the figure shows, the sensor arrangement is located a distance D away from the interaction region 30, and so does not interfere with the regular operation of the X-ray source 10. The screen 54 and the sensor 52 may be spaced apart in the axial direction, but may also be proximate to one another.

(15) A lower portion of the housing 12, vacuum pump or similar means for evacuating air molecules from the housing 12, receptacles and pumps for collecting and recirculating the liquid jet, quadrupoles and other means for controlling astigmatism of the beam are not shown on this drawing. It is also understood that the controller 40 has access to the actual signal from the ammeter 58.

(16) FIG. 1b shows another embodiment, largely similar to that shown in FIG. 1a, but in which the sensor 52 and the screen 54 are differently implemented. In this embodiment, there is no separate screen 54. Rather, delimitation of the sensor area 52 is effected by means of the housing 12 in a configuration where the sensor 52 projects out from the inner wall of the housing. Between the sensor 52 and the housing 12, there is electrical insulation, such that a difference in electrical potential between the sensor and the housing can be allowed. Hence, the earthed screen 54 of the embodiment as shown in FIG. 1a is not present in the embodiment shown in FIG. 1b; the delimitation of the sensor 52 is instead effected by the earthed housing 12. As for the embodiment shown in FIG. 1a, an ammeter 58 is used for determining the potential of the sensor. Although the sensor 52 is shown to project out from the inner wall of the housing 12, it should be understood that the sensor could also be mounted flush with the housing wall.

(17) FIG. 1c shows, according to a further embodiment of the invention, a detail of an X-ray source of the general type described in FIG. 1a. The sensor 52 has a different geometry compared to the previous embodiments, which causes it to produce signals that differ as a function of the location of an impinging electron beam. This also avoids the need for a screen 54 altogether. More precisely, the present embodiment includes a screen comprising a body 62 of an electrically conducting material, which is preferably heat- and vacuum-resistant, such as most metals, in particular Cu or W or an alloy containing any of these. The body 62 has a main sensor surface 64 facing the expected main direction of electron impingement (i.e., towards the cathode 14 in the X-ray source 10). In the main sensor surface, there is provided a bore 66 extending in the direction of electron impingement. The bore 66 forms a non-through hole (or recess) in the body 62. Electrons impinging in the bore 66 will experience a substantially lower backscattering rate (i.e., they will be absorbed by the sensor with a higher likelihood) than electrons impinging on the main sensor surface. Hence, the electrons impinging in the bore will not be attenuated by the effect of backscattering to a similar extent, which will manifest itself as a relatively higher response (in terms of signal level) to a given amount of irradiated charge, which achieves an amplification effect. Hence, the mouth of the bore 66 forms a delimited sensor area in the sense of the present invention. Depending on the depth/diameter ratio of the bore 66, the amplification may be made more or less dependent on the angle of incidence, as considered suitable in each intended use case. In the case of an X-ray source 10 with a non-movable cathode 14, the bore 66 is preferably deeper than its diameter, as electrons impinging from directions other than the cathode 14 can be expected to be noise and are preferably filtered out to the greatest possible extent. The geometry of the bore 66 may vary between wide limits; for instance, the shape of the bottom surface in the bore 66 is of very little consequence.

(18) FIG. 2a illustrates in flow-chart form an algorithm of operating the X-ray source 10 for evaluating a plurality of aligning-means settings and finding an adequate setting. Starting from point A 201, the aligning means is set to a first setting a.sub.1 in step 202. In step 203, the position of the electron beam relative to the screen 54 is determined for a first focusing-means setting f1, and the result is stored in a positions memory 251. The step 203 of determining a relative position is repeated for at least a second focusing-means setting f.sub.2. If there are no further focusing-means settings to be used, which is established in step 204, the algorithm proceeds, in step 205, to computing a sensitivity for this aligning-means setting using the general formula S=p/f and storing the result in a sensitivities memory 252. In step 206, it is checked whether the steps up to this point are to be repeated for further alignment-means settings. If not, the algorithm goes on to step 207, where it processes the sensitivity data as a function of the alignment-means setting. In this embodiment, the data points stored in the sensitivities memory 252 are fitted to a function expected to model the behavior of the electron-optical system for the interesting range of values. For example, the data may be fitted to a second-order polynomial 253, the minimum of which is easy to establish. The minimum is determined in step 208 and forms the output of the algorithm. It is noted that the minimum may or may not coincide with any of the alignment settings tried empirically in step 203.

(19) FIGS. 4 and 5 illustrate two possible measuring schemes for determining the relative electron beam position using deflection of the electron beam I.sub.2 over a limited sensor area. FIG. 4a shows a pixel pattern 401 together with a deflection curve (dotted arrows) to be followed by the electron beam spot on the sensor area. The sensor area is defined as that portion of the sensor 52 which coincides with (the projection of) the aperture 56 in the screen 54. While the pixel pattern 401 is purely imaginary, the deflection curve is shown with a realistic orientation in the plane of the screen 54. FIG. 4b shows the pixel pattern 401 with an indication of the measurement results 403 from the scanning shown in FIG. 4a. The orientation of the pixel pattern has been adjusted for visibility (by a clockwise rotation of about 45 degrees) and now corresponds to a plot of the presence of a non-zero sensor signal in each signal, which is visualized as a binary-valued function of two variables, namely the X and Y deflector settings. In this example, the relative position of the electron beam is measured by the center of mass CM 402 of the non-zero pixels. The position of the center of mass may be expressed as fractions of a pixel. As a further development, the center-of-mass computation may become more accurate if the sensor signal is regarded as a continuous quantity rather than a binary quantity. In this further development, pixels that overlap with the aperture 56 only partially will contribute to a smaller extent to the location of the center of mass.

(20) Analogous to FIG. 4, FIG. 5 shows a pixel pattern 501 in an electron-optical system capable of deflecting the outgoing electron beam in one dimension only. The aperture 56 in the screen 54 is circular and centered on an optical axis of the electron-optical system. The circle is advantageous as an aperture shape since there no need to compensate the relative rotation of the images which may ensue when different focusing settings are used. As shown in FIG. 5a, which (apart from the imaginary pixel pattern 501) is a true illustration of the geometry in the plane of the screen 54 or the sensor 52. Apparently, the respective focusing settings F.sub.1 and F.sub.2 cause the electron beam to rotate by different amounts. Nevertheless, each of the distances d.sub.1, d.sub.2 from the aperture center to each of the pixel patterns can be estimated on the basis of the radius R of the aperture and the length L of the pattern that overlaps with the aperture, namely by {square root over (R.sup.2L.sup.2/4)}. The overlapping length can be estimated by counting the number of pixels for which a non-zero sensor signal is obtained. Thus, for focusing setting F.sub.1, L.sub.1=11 pixel widths and for focusing setting F.sub.2, L.sub.2=9 pixel widths. Although the distances d.sub.1 and d.sub.2 do not provide complete information of the relative beam position, they may be used as a relative measure for the purpose of determining which one of two aligning means settings is least sensitive to a change in focusing setting, and thus, which one provides the best beam parallelity.

(21) FIG. 2b shows an algorithm for associating a focusing-means setting with a beam width at the level of the interaction region. The algorithm may be a continuation of the algorithm explained above with reference to FIG. 2a, as the letter B suggests, or may be carried out independently. In a first step 210, the arrangement of aligning plates 26 is adjusted to an adequate setting, so that the electron beam I.sub.1 travels substantially parallel to the optical axis of the electron-optical system and that the position of the outgoing beam I.sub.2 depends on the setting of the deflection means 28 but substantially not on the setting of the focusing lenses 22. Then, in step 211, the liquid jet is enabled and, in step 212, the orientation of the deflecting capacity of the deflection means 28 is determined. In normal circumstances, the lenses 22 rotate the electron beam about the lens center during its passage through the focusing field, so that orientation in the outgoing electron beam I.sub.2 will differ from that in the incoming beam I.sub.1 by an angle that is related to the intensity and axial extent of the focusing field. The liquid jet beam may appear in the measurements as an elongated region of non-filled pixels (that is, pixels having a reduced or near-zero sensor signal E). The direction in which the elongated region extends can be readily determined by processing the values, such as by fitting them to a straight line, whereby the direction of the liquid jet may be related to the coordinate system of the deflection means. This implies in particular that the preferred scanning direction in later step 214, normal to the jet, is known. After this, in step 213, the focusing means 22 is set to a first value F.sub.1. In step 214, the electron beam I.sub.1 is scanned (deflected) into and/or out of the jet. FIG. 3a is drawn in the plane of deflection which is perpendicular to the liquid jet J. The figure shows the beam in three different deflection positions, I.sub.1, I.sub.1 and I.sub.1, each of which corresponds to a setting of the deflection means 28. It is emphasized that the angle of the beam has not been drawn to scale, but the beam positions above (I.sub.1), inside (I.sub.1) and below the beam (I.sub.1) represent a small angular range, so the beam can be captured by the sensor 52 (not shown in FIG. 3a) located further downstream. The quantity to be measured in step 214 is the width W.sub.1 of the electron beam at the interaction region. Expressed in deflector setting units, the width W.sub.1 is related to each edge of the curve of sensor signal values E when plotted against deflector settings d (e.g., the deflection voltage U.sub.28 indicated in FIG. 3a). The relationship between deflector settings angles or actual lengths at the level of the interaction region can be established by scanning objects located in the interaction region that have known dimensions. In step 215, the beam width is determined and stored in a beam-widths memory 255, either in deflector-settings units or in angular or length units. In step 216 it is determined whether the beam-width scan is to be repeated for other focusing settings F.sub.2, F.sub.3, . . . . The collection of focusing settings to be examined may be a predefined data set or may determine dynamically, such as by fulfilling the condition of examining both focal lengths that are less than the distance to the liquid jet and focal lengths that are greater than this distance. Such a condition ensures that data sufficient for determining the location of the beam waist are collected. If a desired beam width has been input, the algorithm, in a final step 217, determines at least one focusing-means setting that will produce the desired beam width. Point C 218 is the end of the algorithm.

(22) Alternatively, above steps 213, 214 and 215 are performed jointly by recording the sensor signal value E for each of a plurality of points (U.sub.28, U.sub.22), where U.sub.28 is a deflection-means setting and U.sub.22 is a focusing-means setting. Such a data set is plotted in FIG. 3b. If the liquid jet J overlaps with the sensor area, its presence will manifest itself as an area in which the sensor signal E is reduced or near-zero, such as the shaded central region of FIG. 3b. At the level of line B, the region has a relatively distinct waist, which corresponds to the electron beam's I.sub.1 passage through the liquid jet J when the beam is focused at the liquid jet itself. FIG. 3b shows quantized sensor-signal values, which for the sake of clarity have been rounded to either zero or a single non-zero value. A detail of FIG. 3b is shown more realistically in FIG. 3c, which is a plot of the original (non-quantized) sensor-signal values E against the deflection-means setting U.sub.28 for two representative focusing-means settings. A first curve A corresponds to the data located on line A-A in FIG. 3b, and a second curve B corresponds to the data located on line B-B. It is clear from FIG. 3c that the relatively smaller width of the electron beam when optimally focused leads to a sharper transition between the unobscured and the obscured portion of the curve. In other words, a larger portion of the range of deflection-means settings will correspond to either a completely unobscured or a completely obscured position of the electron beam I.sub.1 in relation to the liquid jet J.

(23) It is emphasized that the recording of the sensor-signal values E need not proceed along any line similar to lines A-A or B-B or in any particular order. It is in fact preferable to record the values in a non-sequential fashion, so that the impact of any hysteresis in the deflection or focusing means is obviated. In electron-optical equipment, elements containing ferromagnetic material may give rise to such hysteresis due to residual magnetization (or remanence). For instance, it may be advantageous to adjust the focusing-means setting or the deflection-means setting non-monotonically during the measurement session. More precisely, a measurement scheme may be devised in which the share of measuring points for which the concerned focusing-means setting is reached by way of an increment is approximately equal to the share of measuring points for which the setting is reached by way of a decrement. A similar condition may be integrated into the measurement scheme for the deflection-means settings, at least if the deflection means is known to have non-negligible hysteresis. Advantageously, the measuring points reached by way of increments in the concerned quantity are located in substantially the same area and are distributed in a similar manner as the measuring points reached by way of decrements. Put differently, there is a low or zero statistic correlation between the sign of the increment in the concerned quantity (deflection-means setting or focusing-means setting) and the value of the quantity. Alternatively, there is a low or zero statistical correlation between the sign of the increment in the concerned quantity (either of the deflection-means setting and the focusing-means setting) and the combined values of the deflection-means and focusing-means settings.

(24) In a further development of the method described with reference to FIG. 2b, the actual liquid jet width is also determined. This may be effected in an analogous fashion, namely by estimating the width of the portion of reduced signal in the curve 254 of sensor-signal values E against deflector settings d.

(25) The following items define further advantageous embodiments.

(26) 1. A method of evaluating a setting of aligning means (26) for adjusting a direction of an incoming electron beam (I.sub.1) in an electron-optical system adapted to supply an outgoing electron beam (I.sub.2) to an electron-impact X-ray source (10), which system further comprises:

(27) a deflector (28) operable to deflect the outgoing electron beam, and

(28) focusing means (22) for focusing the outgoing electron beam in an interaction region (30) of the X-ray source,

(29) wherein the method comprises the steps of:

(30) determining, for one focusing-means setting, a relative position of the outgoing electron beam by deflecting the outgoing electron beam into and/or out of a sensor area (52) arranged a distance (D) downstream of the interaction region;

(31) repeating the step of determining a relative beam position for at least one further focusing-means setting and the same aligning-means setting; and

(32) evaluating the aligning-means setting by determining the sensitivity of the relative beam position to a change in focusing-means setting.

(33) 2. The method of item 1,

(34) wherein the step of determining a relative beam position includes using a sensor area (52) delimited by a conductive screen (54) and maintaining the conductive screen at a constant potential.

(35) 3. The method of item 1 or 2,

(36) wherein the step of determining a relative beam position includes using a sensor area delimited by a proximate screen.

(37) 4. The method of any one of the preceding items,

(38) wherein the step of determining a relative beam position includes using a sensor area delimited by a screen which surrounds the sensor area completely.

(39) 5. The method of item 4,

(40) wherein the step of determining a relative beam position includes using a sensor area delimited by a screen which defines a circular aperture (56).

(41) 6. The method of any one of the preceding items,

(42) wherein the deflector and focusing means define an optical axis of the electron-optical system, and wherein the step of determining a relative beam position includes using a sensor area delimited by a screen that has an aperture (56) which is centered on the optical axis.

(43) 7. A method of calibrating an electron-optical system for supplying an electron-impact X-ray source, comprising the steps of:

(44) defining a plurality of aligning-means settings;

(45) evaluating each of the aligning-means settings by the method of any one of the preceding items; and

(46) determining, on the basis of the sensitivities of said plurality of aligning-means settings, an adequate aligning-means setting which yields a minimal sensitivity.

(47) 8. A method of calibrating an electron-optical system for supplying an electron-impact X-ray source, wherein the source is operable to produce an electron target in the interaction region, comprising:

(48) performing the method of item 7 and applying said adequate aligning-means setting; and

(49) determining, for at least one focusing-means setting, a width of the outgoing electron beam in the interaction region by enabling the electron target, so that it partially obscures the sensor area from the electron beam, and deflecting the electron beam between the electron target and an unobscured portion of the sensor area,

(50) wherein preferably the electron target is a liquid jet.

(51) 9. The method of item 8,

(52) further comprising the step of determining an orientation of the outgoing electron beam by enabling the electron target, so that it partially obscures the sensor area from the electron beam, and deflecting the electron beam between the electron target and an unobscured portion of the sensor area,

(53) wherein the step of determining a width of the electron beam includes deflecting the electron beam in a normal direction of the electron target.

(54) 10. A data carrier storing computer-executable instructions for executing the method of any one of the preceding items.

(55) 11. An electron-optical system in an electron-impact X-ray source (10), said system being adapted to receive an incoming electron beam (I.sub.1) and to supply an outgoing electron beam (I.sub.2) and comprising:

(56) aligning means (26) for adjusting a direction of the incoming electron beam;

(57) a deflector (28) operable to deflect the outgoing electron beam; and

(58) focusing means (22) for focusing the outgoing electron beam in an interaction region (30) of the X-ray source,

(59) a sensor area (52) arranged a distance (D) downstream of the interaction region; and

(60) a controller (40) communicatively coupled to the aligning means, the focusing means and the sensor area, said controller being operable to:

(61) determine, for one focusing-means setting, a relative position of the outgoing electron beam by causing the deflector to deflect the outgoing electron beam into and/or out of the sensor area;

(62) repeat said determining a relative beam position for at least one further focusing-means setting and the same aligning-means setting; and

(63) evaluate the aligning-means setting by determining the sensitivity of the relative beam position to a change in focusing-means setting.

(64) 12. The electron-optical system of item 11,

(65) further comprising an electrically conductive screen (54) which delimits the sensor area.

(66) 13. The electron-optical system of item 12,

(67) wherein the screen is maintained at a constant potential.

(68) 14. The electron-optical system of item 12 or 13,

(69) wherein the screen is proximate to the sensor area.

(70) 15. The electron-optical system of any one of items 12 or 14,

(71) wherein the screen surrounds the sensor area completely.

(72) 16. The electron-optical system of item 15,

(73) wherein the screen defines a circular aperture (56).

(74) 17. The electron-optical system of any one of items 12 to 16, wherein:

(75) the deflector and focusing means define an optical axis of the electron-optical system; and

(76) the screen has an aperture (56) which is centered on the optical axis.

(77) 18. An X-ray source, comprising:

(78) an electron-optical system of any one of items 11 to 16; and

(79) a nozzle (32) for producing a liquid jet passing through the interaction region,

(80) wherein the controller is further operable to cause the nozzle to produce said liquid jet, so that the jet partially obscures the sensor area from the electron beam, and to cause the deflector to deflect the electron beam between the liquid jet and an unobscured portion of the sensor area.

(81) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. Any reference signs in the claims should not be construed as limiting the scope.