Cathode rejuvenation

Abstract

A method for rejuvenation of a cathode in an electron-beam system is disclosed, wherein the cathode comprises an electron emitter embedded in a guard member, the method comprising: setting a pressure in an atmosphere surrounding the cathode to a rejuvenation pressure; and setting a rejuvenation temperature at the cathode; wherein, at the rejuvenation pressure and the rejuvenation temperature, an ablation rate of the guard member is higher than an ablation rate of the electron emitter. A method of operating an electron beam system involving such rejuvenation is also disclosed, as well as an associated electron impact X-ray source and a computer program that when executed causes a computer to perform the method.

Description

Technical field

[0001] The present disclosure relates to cathodes in electron-beam systems, particularly in electron-impact X-ray sources.Background

[0002] An electron beam is typically generated by accelerating electrons emitted from a cathode towards an anode. In some implementations the anode comprises a ring-shaped element through which the electrons can pass towards a target, and in other implementations the anode is comprised of the target itself. Examples of such cathodes, or electron emitters, may be found in US 2017 / 148605.

[0003] One problem that frequently arises in this kind of electron beam systems is that the cathode is subject to aging. A typical cathode for use in an electron gun comprises an electron emitter comprised as a low work function material crystal, e.g. a lanthanum hexaboride (LaB 6 ) crystal, embedded in high work function material guard member, e.g. a carbon guard ring. In order to emit electrons, the crystal is heated, and this heating causes a slow evaporation or ablation thereof. After continued use, an end surface of the crystal may thus recede into the carbon embedding and the diameter of the end surface thereof may decrease. Both these effects lead to a reduced electron emission angle and consequently a drift in electron beam performance.

[0004] X-ray radiation may be generated by letting the electron beam impact upon a target material. The X-ray radiation may be generated as Bremsstrahlung or characteristic line emission from the target material. The performance of the X-ray source depends inter alia on the characteristics of the X-ray spot generated by the interaction between the electron beam and the target. Generally, there is a strive for higher brilliance and smaller X-ray spots, which requires improved control of the electron beam and its interaction with the target. In particular, several attempts have been made to determine and to more accurately control the spot size and shape of the electron beam impacting the target. Improved electron beam characteristics are desired not only in X-ray sources, but more generally.

[0005] WO 2023 / 104785 discloses an X-ray source and an associated method in which cathode aging is compensated for using grid bias of a Wehnelt. A bias voltage is applied to the Wehnelt to widen the electron emission angle from the cathode as it ages.

[0006] The present disclosure provides a method for rejuvenation of the cathode, such that the working life thereof can be extended beyond what would otherwise be possible. This may be an alternative to using a grid bias as discussed above, thus enabling the provision of a less complex high-voltage generator. The method may also be performed in conjunction with an adjustable grid bias to further extend the service life of the cathode. The term rejuvenation, as used herein, means conditioning of the cathode to counteract, at least to some degree, deterioration thereof caused by aging (extended use). Rejuvenation does not necessarily mean that the cathode performance is restored to that of a newly installed cathode, but rather an improvement of the performance compared to the aged cathode.Summary

[0007] It has surprisingly been found that the working life of an aged cathode may be extended by intentionally removing guard ring material to reset the electron beam divergence to within an acceptable range. During regular operation, the cathode is present in a vacuum environment (e.g., at an operating pressure of about 5·10 -7< mbar or less, such as less than 1·10 -7< mbar). By exposing the cathode to an atmosphere, i.e., intentionally increasing the pressure in the environment in which the cathode is located, ablation of the guard ring (i.e., the carbon) will take place at a considerably higher rate than that of the LaB 6 crystal. Thereby, the recession of the LaB 6 crystal into the carbon guard ring can be counteracted, such that the working life of the cathode is extended. Effectively, this provides a rejuvenation of the cathode. The atmosphere to which the cathode is exposed for this purpose preferably contains oxygen.

[0008] Although the present invention has been devised in the context of, and is particularly useful for, electron-impact X-ray sources, it is applicable in broader fields and can be beneficial to any electron beam system that comprises a cathode having an electron-emitting element embedded in a guard member. Also, the present disclosure uses LaB 6 embedded in a carbon guard ring as an illustrative example, but other configurations are also conceivable such as using indium-cerium or some other low work function material instead of LaB 6 and / or using another high work function material than carbon / graphite for the guard ring.

[0009] The present invention provides a method for rejuvenating a cathode in an electron beam system wherein the cathode comprises a crystal assembled in a guard member. The method comprises applying a rejuvenation temperature to the cathode and exposing it to an atmosphere at a rejuvenation pressure, wherein the rejuvenation temperature and the rejuvenation pressure are selected so that the ablation rate of the guard member is higher than the ablation rate of the crystal. In an electron beam system, the rejuvenation procedure may be effected based on various criteria. For example, the rejuvenation may be initiated when a first criterion is fulfilled and aborted when a second criterion is fulfilled. Hence, the present invention also provides a method of operating an electron beam system in which a rejuvenation function as disclosed herein has been implemented. In another aspect, the present invention provides an electron impact X-ray source comprising an electron beam system configured to perform the inventive method.

[0010] Further, as will be understood by the skilled reader, the inventive rejuvenation process may be implemented in some electron beam systems without the addition of any specific hardware beyond what is already present. In another aspect, the present invention thus provides a computer program comprising instructions that, when executed on at least one computer in or connected to the electron beam system, causes the inventive methods to be performed.

[0011] The first criterion, i.e. the criterion for initiating the rejuvenation procedure, can be categorized as being based on run time, based on a measurement, or based on monitoring. An example of a run-time based criterion is the number of hours the cathode has been in use. An hour of usage may be weighted by cathode temperature or heater current to reflect that the crystal ablation rate may increase with temperature. The run time may also be weighted with a measured pressure to reflect that the crystal ablation rate may depend on the pressure.

[0012] In some embodiments, regular operation of the electron beam system may already involve interruptions to perform calibration and / or measurement of electron beam spot properties, wherein the procedures may comprise scanning the electron beam over an aperture and detecting the current passing through the aperture as a function of electron beam spot position. During such procedures a quantity indicative of an electron beam divergence may be obtained, an example of such a quantity being the minimum attainable spot size. Since the gradual recession of the crystal into the guard member is associated with a decrease in divergence angle, a criterion for acceptable divergence may be set up for this quantity, e.g. corresponding to a 50% reduction of the divergence as compared to a nominal value. When this criterion is fulfilled the rejuvenation process may be initiated.

[0013] In embodiments where continuous monitoring of the crystal is available, data from such monitoring may be used as a criterion for when to initiate the rejuvenation process. Such monitoring may be based on optical detection or imaging of the cathode.

[0014] The second criterion, i.e. the criterion to abort the rejuvenation procedure, may likewise be categorized in three ways: fixed, calculated, or monitored. A fixed criterion may include performing the rejuvenation for a predetermined time or until a predetermined amount of guard member material has been ablated. To measure or at least estimate the ablated amount of guard member material, a quartz micro balance may be used. Another example may include a sensor for detecting a gaseous oxide formed by the guard member material upon contact with oxygen, e.g. carbon monoxide in case the guard member is made from carbon.

[0015] In embodiments where a measure indicative of the electron beam divergence is available, this measure may be used to calculate a required rejuvenation time or an amount of guard member material that should be ablated. The calculation may be based on previous experience, or the rejuvenation procedure may be initiated for a short time, say 10 seconds, and the measurement of the divergence may be repeated to obtain a rate of change based on which it may be calculated how much more time of rejuvenation is needed to restore the electron beam divergence to a nominal value.

[0016] In embodiments where continuous monitoring of the crystal is available this may be used as an abort-criterion for the rejuvenation process.

[0017] The rejuvenation process disclosed herein may be used for electron beam systems deployed in a variety of applications. A non-limiting list of examples includes X-ray generation, electron diffraction, electron beam melting (EBM), electron fluorescence, electron microscopy, electron energy loss spectroscopy (EELS), reflection high-energy electron diffraction (RHEED), particle-induced X-ray emission (PIXE), charge neutralization, and plasma excitation.Brief description of the drawings

[0018] In the following detailed description, reference is made to the accompanying drawings, on which: Fig. 1 schematically shows an electron-impact X-ray source; Fig. 2a illustrates a new cathode; Fig. 2b illustrates an aged cathode; Fig. 2c illustrates a rejuvenated cathode; Fig. 3 illustrates shank emission from a rejuvenated cathode; Figs. 4a and 4b schematically show plan views of an exemplary cathode; Fig. 5 illustrates a rejuvenation method; and Fig. 6 illustrates operation of an electron beam system including rejuvenation. Detailed description

[0019] In the following detailed description, a cathode comprising an electron emitter, shown as a generally cylindrical LaB 6 crystal, embedded in a guard member, shown as a generally cylindrical carbon guard ring, will be used as an example. It is noted, however, that other cathode configurations are also conceivable.

[0020] As noted in the summary above, one type of electron beam system in which the present invention is useful and beneficial is electron-impact X-ray sources. An example of an X-ray source is shown schematically in Fig. 1. The X-ray source comprises an electron source 110 for providing an electron beam, an electron optic arrangement 120, and an arrangement 130 for determining a cross-sectional intensity profile of the electron beam (or, more generally, an arrangement for determining a quantity indicative of a width of the electron beam).

[0021] The illustrated electron source 110 comprises an electron emitter 111 made from LaB 6 embedded in a guard member 112 comprised as a carbon guard ring, an optional Wehnelt grid 113, and an anode 114. The electron emitter 111 and the guard member 112 are herein collectively referred to as the cathode. As illustrated in Fig. 1, an acceleration voltage V acc is applied between the cathode and the anode during operation in order to accelerate the emitted electrons and form the electron beam. Typical values for the acceleration voltage may be 10 - 1000 kV. Further, a grid voltage V grid may be applied between the cathode and the Wehnelt to assist in shaping the emitted electron beam. The grid voltage may generally be within a range of + / -10 kV with respect to the cathode. It should be noted that it is not excluded that the grid voltage may be zero, i.e. that the Wehnelt has the same electrical potential as the cathode. The grid voltage may be adjusted during the operating life of the cathode.

[0022] The electron optic arrangement 120 may include various elements for shaping and focusing the electron beam onto a target 140 for generation of X-ray radiation. In the illustrated example, the electron optic arrangement includes an alignment coil 121 for aligning the electron beam along a desired beam path, stigmator coils 122 for introducing a desired astigmatism to the electron beam (which may be zero if a symmetric electron spot is desired), a focus coil 123 for focusing the electron beam, and a deflector 124 for lateral movement of the focused electron beam.

[0023] In the illustrated example, the arrangement 130 for determining a cross-sectional intensity profile of the electron beam comprises an electron blocking member or stop 131 and an electron sensor 132. To determine the cross-sectional intensity profile, the electron beam is scanned, using the deflector 124, across an edge defined by the blocking member 131. The number of electrons that reach the electron sensor 132 will thus depend on to what extent the beam cross section is blocked by the blocking member 131. If the entire electron beam passes the blocking member and reaches the sensor 132, a maximum intensity will be detected. As the electron beam is scanned over the edge of the blocking member 131, gradually fewer electrons will reach the sensor 132 and the electron beam cross-sectional intensity profile can thus be determined from the deflection angle and the corresponding number of electrons detected by the sensor 132 at different deflection angles. It is then possible to determine various characteristics of the electron beam, such as the FWHM, from the sensor signal at different deflection angles. It is noted that there is a known relationship between deflection angle and the voltage applied to the deflector 124, which makes the determination straightforward.

[0024] The X-ray source also includes a controller 150, which is operatively connected to the electron source 110, the electron optic arrangement 120, and the arrangement 130 for determining cross-sectional intensity profiles of the electron beam. The operative connection of the controller is schematically illustrated by arrows in Fig. 1.

[0025] During normal use, the LaB 6 crystal slowly evaporates or ablates, both in a longitudinal direction and in a radial direction. Furthermore, the top face of the crystal, which may originally be flat or slightly convex, will typically attain a concave shape. As the crystal recedes into the guard ring the divergence angle of the emitted electrons decreases, and the minimum attainable electron beam spot size consequently increases. By intentionally removing material from the guard ring, a state resembling the original may be achieved. Herein, this is termed rejuvenation of the cathode.

[0026] Figs. 2a-c illustrate three states of the LaB 6 crystal electron emitter 111 in relation to the guard member 112. Fig. 2a illustrates a new, unused cathode. The electron emitter 111 has a convex shape and projects slightly from the guard member 112. After a period of use the electron emitter becomes worn down due to ablation of material, as shown in Fig. 2b, where the electron emitter 111 has receded back into the guard member 112 and has also obtained a smaller cross section at the end surface thereof. The difference in divergence for the emitted electrons is also illustrated by arrows. For the new cathode, illustrated in Fig. 2a, the divergence is larger than for the worn cathode, illustrated in Fig. 2b. As discussed herein, the reduced divergence due to recession of the emitter crystal entails that an electron beam formed by the emitted electrons cannot be focused to the same small spot as for a new cathode.

[0027] Fig. 2c illustrates the cathode after it has been subjected to rejuvenation according to the principles disclosed herein. During rejuvenation, material is ablated away from the guard member 112 to counteract the recession of the emitter crystal into the guard ring. However, since the crystal diameter has decreased and since the top surface has attained a concave shape, a slight protrusion of the electron emitter 111 outside the guard member 112 is preferred after rejuvenation in order to recover the original divergence of the emitted electrons, as illustrated in Fig. 2c. Such a protrusion will imply that the cathode will emit some electrons from the lateral surface of the crystal. This phenomenon, called shank emission, is illustrated in Fig. 3 and will result in a decreased current in the outgoing electron beam due to lateral loss of electrons as illustrated by arrows in the Fig. 3. To compensate for this, the total emission current may be increased accordingly after rejuvenation by increasing the cathode heater current compared to the heater current used before rejuvenation. Furthermore, by applying a suitable grid voltage the electric field may be arranged so that at least some of the shank emission is directed into the outgoing electron beam, although the applicability of this approach may be limited by quality requirements on the electron beam.

[0028] It may be advantageous to perform another procedure subsequent to the rejuvenation described in connection with Figs. 2a-c. By applying a cathode temperature significantly higher than a normal operating temperature, and by maintaining the atmosphere at the operating pressure, the concave end surface of the crystal may be ablated and a flat or even convex shape close to the original may be attained. This approach may be further used in combination with removing more guard ring material than necessary to obtain a suitable distance between the guard ring end surface and the crystal end surface (e.g. the crystal protrudes slightly from the guard ring). If, instead, guard ring material is ablated until the exposed crystal diameter is close to the original and a a high cathode temperature is subsequently applied at the operating pressure, leading to removal of crystal material, this may result in a rejuvenated cathode with performance closer to a new cathode.

[0029] Figs. 4a-b schematically show a plan view and a side view, respectively, of the cathode, where the electron emitter 111 is shown embedded in the guard member 112. Electrical contacts 402a and 402b are connected to the cathode for providing the heater current to the cathode. In a typical implementation, the cathode is maintained at a high negative potential and an anode (such as the anode 114 shown in Fig. 1) is maintained at ground potential for acceleration of the emitted electrons. The heater current provided by the electrical contacts 402a and 402b is floating. Figs. 4a and 4b also schematically show a partitioning plate 404 used for insulation and / or mounting of the cathode.

[0030] In one example, settings used for effecting the rejuvenation procedure comprise an air pressure of the order of 0.5 mbar and cathode temperature of the order of 1200 K (realized with heater current of 1.2 A), which is known to be just below the emission threshold for LaB 6 . This increased-pressure atmosphere can be contrasted to the normal operating pressure, which is typically below 10 -7< mbar. Normal heater currents during regular operation of the electron beam system are in the range 1.7 to 2.5 A to provide a cathode temperature of about 1700 K - 2000 K, i.e., slightly higher than the cathode temperature used during rejuvenation.

[0031] In the trials performed by the inventors, the acceleration voltage for the electron beam was turned off during the rejuvenation procedure. However, it may be possible to have the acceleration voltage turned on (at least if it is kept below some limit). The acceptable acceleration voltage during rejuvenation will be limited by arcing, which is expected to increase as pressure increases.

[0032] It is expected that, for normal working conditions, the recession of the cathode into the guard ring will be a deterministic process, meaning that the amount of recession may be predicted from run time and applied heater current. Thus, the rejuvenation procedure may be initiated when a certain run time with a certain current has been reached, e.g. as determined by heater current integrated over run time. Alternatively, a quantity indicative of the divergence angle, e.g., the minimum spot size attainable, may be measured and the rejuvenation procedure may be initiated when the quantity fulfils some pre-determined criterion.

[0033] Furthermore, it may be possible to find a sweet spot where an X-ray source can operate within an atmosphere so arranged that the crystal and the guard ring loose material at the same rate. It may, however, for other reasons be undesirable to operate at such an atmosphere; e.g. if the source comprises other parts made from carbon, they will likely deteriorate at approximately the same rate as the guard ring provided that they attain a similar temperature.

[0034] The ablation rate of the guard member 112 may be controlled by temperature and oxygen partial pressure. Several mechanisms may limit the ablation rate, such as the number of oxygen molecules colliding with the guard member, chemical reaction rate, and diffusion limited reaction rate. The collision rate gives an upper bound on how much material that may be oxidized. Taking carbon as the illustrative material for the guard member 112, every incoming oxygen molecule may combine with two carbon atoms and form two molecules of CO. For lower temperatures the chemical reaction rate may limit the amount of oxides formed. For higher temperatures a gaseous layer of CO may hinder incoming oxygen molecule from reaching the surface of the carbon, and the oxygen may instead form CO 2 when interacting with the gaseous layer. The oxidation of graphite is discussed by Gulbransen et al. in "The Oxidation of Graphite of Temperatures of 600° to 1500°C and at Pressures of 2 to 76 Torr of Oxygen", Journal of the Electrochemical Society, June 1963, p. 476-483 (Gulbransen) The diffusion limit is, according to Gulbransen, dependent on the surface area of the guard member where a smaller sample is expected to transition to the diffusion limit at a higher temperature than a larger sample. In the diffusion limit the reaction rate is a weaker function of temperature than in the chemical reaction rate limit. Thus, rejuvenation is preferably performed in the diffusion limit (if this is attainable) to be less sensitive to temperature control. The reaction rate in the diffusion limit is expected to be inversely proportional to the surface area of the guard member according to Gulbransen. Thus, a small area of the guard member may be preferable. However, the thermal properties of the cathode may also impact the available design options. An upper limit for the ablation rate, i.e. how much a guard member surface may recede per unit time, may be calculated from kinetic gas theory. The number of oxygen molecules that will impact a unit area of the guard member per unit time may be written as: J = p 2 πm o k B T where m o is the mass of an oxygen molecule, p is the partial pressure of oxygen surrounding the cathode, k B is the Boltzman constant (~1.38·10 -23< J / K), and T is the absolute temperature in K of the atmosphere surrounding the cathode. From the collision rate J the ablation rate, or recession speed, v may, under the assumption that for each impacting oxygen molecule two molecules of CO are formed, be calculated as v = 2 Jm c ρ = 2 pm c ρ 2 πm o k B T where m c is the mass of a carbon atom and ρ is the density of the guard member material (~2000 kg / m3 for graphite). Assuming that rejuvenation is initiated by admitting a small amount of air into the vacuum enclosure and pumping to a pressure of 0.5 mbar, the partial oxygen pressure may be set to 10 Pa and the temperature to 300 K to calculate an upper limit on the ablation rate v max = 5.4 µm / s. Practical tests combined with profilometry have revealed that an ablation rate of about 78 nm / s may be attained during these conditions. Since this is considerably lower than the calculated v max it may be inferred that some other process than impact of oxygen molecules limits the ablation rate. As noted above the ablation rate may also be limited by the chemical reaction rate and / or the diffusion limited reaction rate.

[0035] Experiments were carried out to test the inventive rejuvenation method. A cathode in an electron gun designed to provide a 10 µm electron beam spot was taken out of service when the minimum attainable spot size had increased to 14 µm.

[0036] The cathode was reinstalled in an X-ray source to be subjected to a rejuvenation procedure according to the principles disclosed herein. Before rejuvenation, the focus angle was 16 mrad and the minimum attainable spot size after reinstallation was 13 µm at an acceleration voltage of 160 kV. An air pressure of 0.5 mbar was set in the atmosphere around the cathode and a heating current of 1.2 A (corresponding to a cathode temperature of about 1200 K) was applied to the cathode. After a 30 second burn (rejuvenation), the focus angle had increased to 20 mrad and the minimum attainable spot size had decreased to 12 µm. After an additional 2.5-minute burn, the focus angle had increased to 46 mrad and the minimum attainable spot size had decreased to 9 µm at 160 kV acceleration voltage. The throughput, i.e. the total current absorbed by the target, had after the procedure decreased to 80% of nominal, i.e. the expected value of the current absorbed by the target for that cathode setting.

[0037] After the rejuvenation procedure, the cathode was removed from the X-ray source and measured in a profilometer. It was found that the crystal now protruded about 10 µm from the guard ring, which may explain the reduced throughput due to shank emission (see above).

[0038] A method 500 for rejuvenation of a cathode in an electron-beam system is illustrated in Fig. 5. Rejuvenation is effected by setting a pressure in an atmosphere surrounding the cathode to a rejuvenation pressure 502, and by setting the temperature of the cathode to a rejuvenation temperature 504. According to the invention, the rejuvenation pressure and the rejuvenation temperature are selected such that the ablation rate of the guard member of the cathode is higher than the ablation rate of the emitter crystal. It is apparent to the skilled reader that the order of setting the pressure and setting the temperature is arbitrary. The rejuvenation temperature of the cathode may be set before setting the rejuvenation pressure in the surrounding atmosphere. However, since it may typically be faster to set the temperature than to set the pressure, it may be preferred to set the pressure first in order to achieve better control of the rejuvenation process. For the rejuvenation process, the atmosphere surrounding the cathode may conveniently contain oxygen, preferably at a partial pressure of at least 20% of the rejuvenation pressure. Conveniently, the rejuvenation temperature of the cathode is set by applying a rejuvenation heater current to the cathode.

[0039] Fig. 6 illustrates a method 600 of operating an electron beam system that implements cathode rejuvenation as disclosed herein. First, the electron beam system is operated to generate an electron beam, wherein a first operating temperature is set to the cathode and wherein the cathode is surrounded by an atmosphere at a first operating pressure 602. After some run time, the divergence of electrons emitted from the cathode may have decreased to a level where the electron beam can no longer be focused to a sufficiently small spot. The deterioration of the cathode may manifest in different ways depending on the application. Even if the smallest attainable spot is not used the recession of the crystal into the guard ring may result in a decreased current extracted from the cathode since the electric field at the emitting surface may be reduced. Furthermore, the electron beam profile may change, e.g. in such a way as not affecting the measured spot size while having more power in the tails of the distribution which may also reduce the electron beam throughput. Based on one or more criteria, rejuvenation as described above is thus initiated 604 to bring the cathode back to an acceptable divergence for the emitted electrons. After rejuvenation, an electron beam may again be generated by setting the temperature of the cathode to a second (post-rejuvenation) operating temperature and by setting the pressure in the atmosphere surrounding the cathode to a second (post-rejuvenation) operating pressure 606. In many situations the first and second operating pressures (i.e., before and after rejuvenation) will be equal, while the second (post-rejuvenation) operating temperature may be set higher than the first operating temperature to compensate for shank emission so that a nominal absorbed target current is maintained. Various criteria for initiating and aborting the rejuvenation process are discussed above.Conclusion

[0040] A method for rejuvenation of a cathode in an electron-beam system has been disclosed, wherein the cathode comprises a electron emitter embedded in a guard member, the method comprising: setting a pressure in an atmosphere surrounding the cathode to a rejuvenation pressure; and setting a rejuvenation temperature at the cathode; wherein, at the rejuvenation pressure and the rejuvenation temperature, an ablation rate of the guard member is higher than an ablation rate of the electron emitter. A method of operating an electron beam system involving such rejuvenation is also disclosed, as well as an associated electron impact X-ray source and a computer program that when executed causes a computer to perform the method.

Claims

1. A method for rejuvenation of a cathode in an electron-beam system, wherein the cathode comprises an electron emitter embedded in a guard member, the method comprising: setting a pressure in an atmosphere surrounding the cathode to a rejuvenation pressure; and setting a rejuvenation temperature at the cathode; wherein, at the rejuvenation pressure and the rejuvenation temperature, an ablation rate of the guard member is higher than an ablation rate of the electron emitter.

2. The method of claim 1, wherein, at the rejuvenation pressure, the atmosphere surrounding the cathode comprises oxygen.

3. The method of claim 2, wherein a partial oxygen pressure at the rejuvenation pressure is at least 20% of the rejuvenation pressure.

4. The method according to any one of the preceding claims, wherein the electron emitter is comprised as a LaB6 crystal.

5. The method according to any one of the preceding claims, wherein the guard member is provided as a carbon ring.

6. The method according to any one of the preceding claims, wherein the rejuvenation temperature is set by applying a rejuvenation heater current to the cathode.

7. A method of operating an electron beam system, comprising: generating an electron beam, wherein generating the electron beam comprises setting a first operating temperature to a cathode surrounded by an atmosphere at a first operating pressure; initiating rejuvenation of the cathode according to any one of claims 1-6; and after rejuvenation, setting a second operating temperature to the cathode and setting the pressure in the atmosphere surrounding the cathode to a second operating pressure.

8. The method of claim 7, wherein the first operating pressure and the second operating pressure are equal.

9. The method of claim 7 or 8, wherein the first and second operating pressures are less than 5·10-7 mbar.

10. The method of any one of claims 7-9, wherein the second operating temperature is adjusted so as to achieve an absorbed target current substantially equal to a nominal absorbed target current.

11. The method of any one of claims 7-10, wherein the rejuvenation is initiated after a predetermined run time; when an electron beam divergence below a first limit has been detected; or based on a direct observation of the electron emitter.

12. The method of claim 11, wherein the rejuvenation is aborted after a predetermined rejuvenation time; when a predetermined amount of material has ablated from the guard member; when the electron beam divergence is larger than a second limit; or based on a direct observation of the electron emitter.

13. The method of claim 12, wherein the predetermined amount of material or the predetermined time is calculated based on the electron beam divergence.

14. An electron impact X-ray source comprising an electron beam system configured to perform the method of any one of the preceding claims.

15. A computer program comprising instructions that when executed on at least one computer performs the method according to any one of claims 1-13.

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