Ion generation system with efficient ion collector

Abstract

The system includes an ion source configured to generate ions having a first polarity, one or more extraction electrodes configured to extract ions from the ion source as an ion beam having an extraction energy, a mass resolving slit or aperture configured to select a desired isotope from the ion beam such that the desired isotope ion beam passes through the mass resolving slit or aperture, a target positioned relative to the mass resolving slit or aperture such that the desired isotope ion beam is incident on the target, and a voltage source coupled to the target and configured to maintain the target at a first voltage having a first polarity. The first voltage causes a decrease in extraction energy as the desired isotope ion beam approaches the target, minimizing sputtering and maximizing collection of ions on the target to reorganize the ionized material.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 188,729, filed May 14, 2021, the entire disclosure of which is incorporated herein by reference.

[0002] technology The present disclosure relates generally to the field of heavy metal ion generation, such as for use in medical applications. More specifically, the present disclosure relates to improving the efficiency of collecting and arranging ions accelerated from an ion source at high extraction energies. [Background technology]

[0003] background Conventional techniques related to ion beams typically aim to produce high energy collisions between the ion beam and the substrate material to create changes in the substrate material. The ions themselves in such systems are not efficiently retained within the substrate material, and they may be sputtered, sublimated, or scattered. Such systems and methods do not provide for highly efficient collection of the ions themselves. In contrast, one objective of the present application is the collection of the ions of the ion beam as a material of construction that can be collected, stored, transported, used, etc., for example, in medical applications. Summary of the Invention

[0004] overview One embodiment of the present disclosure is a system, for example an ion generation system. The system includes an ion source configured to generate ions having a first polarity, an extraction electrode configured to extract ions from the ion source as an ion beam having an extraction energy, a target positioned relative to the extraction system such that the ion beam is incident on the target, and a voltage source coupled to the target and configured to maintain the target at a first voltage having a first polarity. The first voltage causes a reduction in the kinetic energy of the ions at the extraction energy, such that the effective potential energy of the target ions at the target is the difference between the extraction voltage and the first voltage of the target. Such an effective low energy minimizes sputtering, reduces the heat load on the target, and maximizes the collection of ions on the target. When the effective energy is equal to or less than the thermal energy of the evaporated ions, the ions can be deposited as a thin film on a substrate, thus maximizing the capture of ions.

[0005] In some embodiments, the first voltage is less than the extraction energy. The first voltage may also be greater than 95% of the extraction potential, or greater than 99% of the extraction potential. The first voltage may be less than the extraction potential by an amount corresponding to the thermal energy of the ions. The first voltage may be about 100V less than the extraction potential, such that the ion beam has an energy of about 100V when it reaches the target (it should be noted that ion kinetic energy is often expressed in volts in this context, with one volt being equal to one joule per coulomb, although in other conventions these values ​​may be expressed in eV).

[0006] In some embodiments, the extraction energy is between 20 kV and 80 kV, for example, between 40 kV and 60 kV. In some embodiments, the first polarity is positive.

[0007] In some embodiments, the target is configured to collect the ionized material as a film on the target. In some embodiments, the system also includes a mass resolving aperture between the extraction electrode and the target, the aperture configured to block an undesired subset of ions from reaching the target.

[0008] In some embodiments, the ions include ytterbium ("Yb") ions. The target may include a crystalline structure. The target may include a carbon fiber material.

[0009] Another embodiment of the present disclosure is a method of collecting ions. The method includes providing an extraction energy to an ion beam. The ion beam includes ions having a first polarity. The method also includes positioning a target such that the ion beam is incident on the target, holding the target at a first voltage having a first polarity such that as the ion beam approaches the target, a first voltage causes a decrease in extraction energy, and collecting the ions on the target.

[0010] In some embodiments, the method also includes removing the ions from the target as constituted neutral material. The ions may include ytterbium atoms.

[0011] The extraction potential may be greater than the target voltage. In some embodiments, the first voltage and the extraction potential differ by an amount corresponding to the thermal energy of the ions. Providing an extraction energy to the ion beam may include accelerating the ions out of the ion source using an electrode held at the extraction voltage. The extraction voltage may have a first polarity and be greater than the target voltage.

[0012] Another embodiment of the present disclosure is a system including an ion source configured to generate ions having a first polarity, one or more extraction electrodes configured to extract ions from the ion source as an ion beam having an extraction energy, a mass resolving slit or aperture configured to select a desired isotope from the ion beam such that the desired isotope ion beam passes through the mass resolving slit or aperture, a target positioned relative to the mass resolving slit or aperture such that the desired isotope ion beam is incident on the target, and a voltage source coupled to the target and configured to hold the target at a first voltage having a first polarity. The first voltage causes a decrease in extraction energy as the desired isotope ion beam approaches the target, minimizing sputtering and maximizing collection of ions on the target to reconfigure the ionized material. In some embodiments, a magnetic analyzer is positioned between the one or more extraction electrodes and the mass resolving slit or aperture, the magnetic analyzer configured to generate a magnetic field configured to cause separation of the desired isotope from other isotopes of the ion beam. [Brief description of the drawings]

[0013] The present disclosure will become more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which like reference numerals refer to like elements and in which:

[0014] [Figure 1] 1 is a schematic diagram of an ion generation system according to an example embodiment. [Diagram 2] FIG. 2 is a schematic diagram of a target and voltage source of an ion generating system in an embodiment relating to positive ions, according to an exemplary embodiment. [Diagram 3] FIG. 2 is a schematic diagram of a target and voltage source of an ion generating system in an embodiment relating to negative ions, according to an exemplary embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Detailed Description Before turning to the figures which illustrate in detail certain embodiments, it is to be understood that the disclosure is not limited to the details or methodology set forth in the specification or illustrated in the drawings. It is also to be understood that the terminology used herein is for the purpose of description only and should not be regarded as limiting.

[0016] Referring generally to the figures, systems and methods relating to ion generating systems, e.g., heavy metal ion generating systems, according to various exemplary embodiments are shown. In particular, the figures generally illustrate systems and methods relating to highly efficient collection of ions (e.g., heavy metal ions) at a target of the ion generating system such that the ions are reconstituted as materials that can be collected, stored, transported, used, etc. for various applications.

[0017] As described in more detail below, ions are generated in an ion source and extracted from the ion source as an ion beam with high extraction energy, e.g., 20 kV to 80 kV (e.g., 40 kV to 60 kV) (note that ion kinetic energy is often expressed in volts in this context, with one volt equal to one joule per coulomb, and in other conventions these values ​​may be expressed in keV). The ion beam may pass through a magnetic analyzer, which uses a magnetic field to sort ions by momentum (or atomic mass, if all ions generated have the same charge), and a mass resolving aperture, which is positioned relative to the beam and the magnetic analyzer to primarily pass desired ions (e.g., ions of a desired isotope) while preventing other ions from passing through the aperture. Thus, the ion beam with only the desired ions, or a high percentage of the desired ions, passes through the aperture. A target (target substrate, substrate, etc.) is positioned such that the beam of desired ions is incident on the target.

[0018] The approach described herein results in a higher percentage of ions that attach to the target and a lower sputtering rate at the target, which increases the percentage of ions that are configured as neutral material (e.g., of the desired isotope) and reduces the percentage of ions that are lost. Thus, the overall efficiency (e.g., power and resource consumption per unit of material generated) of the entire ion generation system is improved. The accumulation of ions (which may otherwise result from ion scattering) on ​​other undesirable surfaces of the ion generation system is also minimized or prevented, thereby reducing downtime and maintenance of the ion generation system. In addition, the approach described herein reduces heat transfer to the target substrate (heat load on the target, thermal energy load on the target), thus reducing or eliminating temperature management issues that may otherwise be caused by high energy collisions at the target.

[0019] As described in more detail below, the advantages herein are achieved in part by keeping the target at a voltage with the same polarity as the ions and a magnitude slightly below the potential of the ion beam. The target voltage (and the electric field created by that voltage) causes a reduction in the energy of the ion beam as it approaches the target. For example, the target voltage may be selected to (at least partially) offset the energy added to the ion beam provided by the extraction electrode, such that the energy of the ions is reduced to thermal energy as they reach the target. In such a case, both the electronic blocking of the ions at the target (i.e., the interaction between the ions themselves and the target interaction between the electrons of the ions and the electrons of the target substrate) and the nuclear blocking (i.e., the interaction between the nuclei of the ions and the nuclei of the target substrate) are reduced to zero or near-zero levels. Reducing these interactions via the target potential causes the ions to attach to the target and form as a film, rather than colliding with the target with high energy and sputtering or scattering.

[0020] 1, a block diagram of an ion generation system 100 is shown according to an exemplary embodiment. The ion generation system 100 includes an ion source 102, an extraction electrode 104, a magnetic analyzer 106, a mass resolving aperture 108, a target 110, and a voltage source 112 connected to ground 114 and the target 110. The ion source 102, the extraction electrode 104, the magnetic analyzer 106, the mass resolving aperture 108, and the target 110 are sequentially arranged such that ions are generated in the ion source 102 and pass sequentially through the extraction electrode 104, the magnetic analyzer 106, and the mass resolving aperture 108 before reaching the target 110. The ion generation system 100 is configured to provide efficient collection of desired ions at the target 110 as configured neutral material that can be removed, stored, transported, etc., and ultimately used in a number of applications, e.g., medical applications, as described in more detail below.

[0021] The ion source 102 is configured to generate ions. For example, the ion source 102 may be configured as a Bernas or Freeman ion source that includes a filament operable to emit electrons that ionize a gas supplied to the ion source 102, e.g., a heavy metal gas, such as ytterbium vapor. Other metals (Lu, Tc, etc., may also be used). The interaction between the electrons and the gas ionizes the gas to generate ions. In some embodiments, the ion source 102 generates positive ions (i.e., "cations," ions with a positive polarity). In other embodiments, the ion source generates negative ions (i.e., "anions," ions with a negative polarity). The ion source 102 includes an exit slit or opening so that ions may be extracted from the ion source 102. In some embodiments, the ion source 102 includes an auxiliary heater to protect elements of the ion source 102 and improve uniformity of ions for extraction from the ion source 102, as described in detail, for example, in U.S. Provisional Patent Application No. 63 / 122,699, filed December 8, 2020, the entire disclosure of which is incorporated herein by reference.

[0022] The extraction electrode 104 includes one or more electrodes configured and operated to provide an electric field that extracts ions from the ion source 102. Because ions have a charge of a first polarity (positive or negative in different embodiments), a voltage of opposite polarity at the extraction electrode 104 extracts ions from the ion source as an ion beam. The extraction electrode 104 may include one or more electrodes for accelerating the ion beam, decelerating the ion beam, shaping the beam, aiming the beam, etc. By providing an electric field that accelerates the ion beam out of the ion source 102, the extraction electrode 104 provides an extraction energy to the ion beam of the same or similar magnitude as the voltage of the extraction electrode 104. For example, an electrode with a voltage of 55 kV can provide an extraction energy of 55 kV to the ion beam as it passes through the extraction electrode 104 (note that ion kinetic energy is often expressed in volts in this context, and one volt is equal to one joule per coulomb).

[0023] Thus, an ion beam having a high extraction energy is provided as an output of the extraction electrode 104. In various embodiments, the high extraction energy can be in the range of 20 kV to 80 kV, for example, 40 kV to 60 kV (e.g., 55 kV). In such embodiments, the voltage applied at the extraction electrode 104 can be selected to provide the ion beam with a desired extraction energy for a particular scenario.

[0024] In the example of FIG. 1, the ion beam passes from the extraction electrode 104 to the magnetic analyzer 106. In other embodiments, the magnetic analyzer 106 is omitted. The magnetic analyzer 106 is configured to provide a magnetic field that creates a magnetic force on the ion beam. The magnetic force on each ion may be approximately equal, but the ion beam may contain ions of different isotopes such that the mass of the ions varies. The magnetic force provided by the magnetic analyzer 106 may result in separation of the ions by mass. Thus, after passing through the magnetic analyzer 106, different regions of the cross section of the ion beam may contain ions of different isotopes, i.e., different masses.

[0025] In FIG. 1, the ion beam is illustrated passing from a magnetic analyzer 106 to a mass resolving aperture 108 that is configured to block an undesired subset of ions from passing through the mass resolving aperture 108 while allowing desired ions to pass through the mass resolving aperture 108. Specifically, ions that are allowed to pass through the mass resolving aperture 108 are primarily ions of the desired isotope (or two desired isotopes), while ions of one or more other isotopes are blocked by the mass resolving aperture 108. This is accomplished by positioning the mass resolving aperture 108 relative to the magnetic analyzer 106 to take advantage of the separation of isotopes by mass achieved by the magnetic analyzer 106. In various embodiments, various geometries are possible. Thus, in an example that includes a magnetic analyzer 106 and a mass resolving aperture 108, the ion beam that reaches the target 110 contains a high percentage of the desired isotope (single species) or desired isotopes (several species) and a low percentage of contamination with ions of different isotopes.

[0026] The ion beam from the mass resolving aperture 108 is incident on the target 110. The target 110 is configured to receive and collect ions of the ion beam. The target 110 can include a substrate material suitable for receiving and holding ions, including as a membrane on the surface of the target 110 and / or embedded in the lattice structure of the target 110. For example, the substrate material of the target 110 can have a crystalline structure. As another example, the substrate material of the target 110 can include a carbon fiber material (e.g., carbon fiber cloth). The material of the target 110 can also be selected to allow the target 110 to be held at a substantially constant voltage even when ions are collected on the target 110, embedded in the target 110, or otherwise received by the target 110. The material of the target 110 can be selected to help cause attachment of ions to or within the target 110. The target 110 may be removable and replaceable in the ion generating system 100 to facilitate obtaining ionic material that accumulates on the target 110 during operation of the ion source 102 .

[0027] The target 110 is shown coupled to a voltage source 112 that is connected between the target 110 and a ground 114. Other elements of the ion generating system 100 also include suitable electronics, power sources, etc., to enable its operation. The voltage source 112 is configured to keep (place, establish, maintain, etc.) the target 110 at a voltage (referred to herein as the target voltage) having the same polarity as the ion beam. For example, as illustrated in FIGS. 2-3 and discussed below, a positive voltage is supplied to the target 110 by the voltage source 112 in a scenario in which positive ions are generated by the ion source, while a negative voltage is supplied to the target 110 by the voltage source 112 in a scenario in which negative ions are generated by the ion source.

[0028] The target voltage is preferably less than the extraction energy of the ion beam so that the ion beam can reach the target without being biased in the opposite direction by the target voltage, while being high enough to reduce the energy of the ion beam enough to minimize both electronic and nuclear stopping of the ion beam at the target 110 (thereby minimizing scattering or sputtering that would otherwise be caused by high energy collisions between the ions and the target 110). For example, the target voltage may be less than the extraction energy by an amount corresponding to the thermal energy of the ions, such that the energy of the ions is reduced to thermal energy just as they reach the target 110. In various embodiments, the target voltage is less than the extraction energy and greater than 95% of the extraction energy, e.g., greater than 99% of the extraction energy (while being less than the extraction energy). In some examples, the target voltage is less than about 100V less than the extraction energy, such that the ion beam has an energy of about 100V when it reaches the target (e.g., extraction energy minus target voltage equals about 100V). In one embodiment, the extraction energy is 55 kV and the target voltage is 54.9 kV.

[0029] In some embodiments, the voltage source 112 and the target 110 are configured such that the voltage of the target 110 is substantially constant during operation of the ion generating system 100 and as ions collect on the target 110 (e.g., as a film on the target 110 and embedded in the target 110) and are configured into neutral material (e.g., of a desired isotope). In some examples, the target 110 may be removable from the ion generating system 100 to facilitate removal of the configured ionized material from the target 110. In some such examples, the voltage source 112 is controlled to gradually decrease the target voltage towards zero to allow the target 110 to be disconnected from the voltage source 112 without disturbing the ionized material collected thereon. In some embodiments, the target 110 (or a portion thereof) is removed for use in transport and further processing of the ionized material and replaced with a new target 110 (or a new portion thereof) for use in subsequent operation of the ion generating system 100. In other embodiments, the ionized material may be removed from the target 110 and collected in a receptacle (or other collection and holding device) so that the target 110 may be reused in a subsequent operation of the ion generating system 100 to collect more ions.

[0030] 2, there is shown a schematic diagram of the target 110 and voltage source 112 of the ion generating system 100 in one embodiment associated with a positive ion beam 200, according to one exemplary embodiment. Figure 2 shows the positive ion beam 200 (i.e., a beam of positively charged ions) being directed towards and incident on the target 110.

[0031] Since the positive ion beam 200 has a positive polarity, the voltage on the target 110 is also provided with a positive polarity. Figure 2 illustrates that the target 110 is connected to the positive terminal of a voltage source 112, the negative terminal of which is connected to ground 114. The voltage source 112 keeps the target 110 at a positive potential, i.e., a potential of the same polarity as the positive ion beam 200.

[0032] The positive potential of the target 110 provides an electric field that resists the movement of the ion beam 200 towards the target 110. The ion beam 200 must cross this electric field to reach the target 110. The kinetic energy of the ion beam 200 is then converted into the electric potential of the ions in the electric field created by the positive potential of the target 110. This can be considered similar to ions rolling "uphill" to reach the target 110. As discussed above, the target voltage is selected and maintained such that the positive ions reach a low energy, e.g., thermal energy, just as they reach the target 110. Once reduced to thermal energy, the positive ion beam 200 does not have the extra kinetic energy that would cause it to leave the target 110 or cause sputtering or scattering, and therefore the ions of the positive ion beam 200 are deposited at the target 110 to form, for example, as a positive ion film 202, as shown in FIG. 2.

[0033] 3, there is shown a schematic diagram of the target 110 and voltage source 112 of the ion generation system 100 in one embodiment associated with a negative ion beam 300, according to one exemplary embodiment. FIG 2 shows a negative ion beam 300 (i.e., a beam of negatively charged ions) being directed towards and incident on the target 110.

[0034] Since the negative ion beam 300 has a negative polarity, the voltage on the target 110 is also provided with a negative polarity. Figure 3 illustrates that the target 110 is connected to the negative terminal of a voltage source 112, the positive terminal of which is connected to ground 114. The voltage source 112 keeps the target 110 at a negative potential, i.e., a potential of the same polarity as the negative ion beam 300.

[0035] The negative potential of the target 110 provides an electric field that resists the movement of the ion beam 300 towards the target 110. The ion beam 300 must cross this electric field to reach the target 110. The kinetic energy of the ion beam 300 is then converted into the electric potential of the ions in the electric field created by the negative potential of the target 110. This can be considered similar to ions rolling "uphill" to reach the target 110. As discussed above, the target voltage is selected and maintained such that the negative ion beam 300 reaches a low energy, e.g., thermal energy, just as the negative ions reach the target 110. Reduced to thermal energy, the negative ion beam 300 does not have the extra kinetic energy that would cause it to leave the target 110 or cause sputtering or scattering, and therefore the ions of the negative ion beam 300 are deposited at the target 110, for example forming as a negative ion film 302 as shown in FIG. 3.

[0036] Thus, the ion generating system 100 is configured for high efficiency generation and collection of ions as configured ionizable material. By setting the target 110 at a target voltage as described above, a high percentage of ions incident on the target 110 are deposited at the target 110, e.g., forming a film on the target 110. Thus, the efficiency of the ion generating system 100 is improved by providing collection of a high percentage of the desired ions produced by the ion source 102. In addition, because material is sputtered or scattered at a low or zero rate, accumulation on other undesirable surfaces in the ion generating system 100 is also substantially prevented, thereby reducing downtime, cleaning, maintenance, etc. of the ion generating system 100. In addition, electronic or nuclear stopping (i.e., collisions between atoms) of energetic ions at the target would significantly increase the thermal energy of the target, but embodiments herein use the potential provided by the voltage source 112 to reduce the energy of the ions, thereby avoiding the accumulation of thermal energy at the target.

[0037] While the above discussion provides a general overview of the laws of physics associated with the operation of the ion generation system 100 and the effect of the target voltage on the ion beam, it should be understood that ion beam behavior is complex and that additional or alternative theories or experimental results may be used to provide further or alternative explanations for the various advantages of the systems and methods described herein. Experimental results, as discussed above, show that the advantages described herein can be obtained by providing a target voltage to the target 110 of the same polarity as the ion beam.

[0038] As used herein, the terms "approximately," "about," "substantially," and similar terms are intended to have a broad meaning consistent with common and accepted usage by those of ordinary skill in the art to which the subject matter of the present disclosure pertains. Those of ordinary skill in the art who review this disclosure will understand that these terms are intended to enable the description of certain features described and claimed without limiting the scope of these features to the precise numerical values ​​or ideal geometric forms provided. These terms should therefore be interpreted to indicate that insubstantial or insignificant modifications or variations of the subject matter described and claimed are considered to be within the scope of the present disclosure as recited in the appended claims.

[0039] The term "coupled" and variations thereof, as used herein, refers to the joining of two members directly or indirectly to each other. Such joining may be fixed (e.g., permanent or permanent) or movable (e.g., removable or releasable). Such joining may be achieved by two members directly joined to each other, two members joined to each other using a separate intervening member and any additional intermediate member joined to each other, or two members joined to each other using an intervening member integrally formed as a single unit with one of the two members. When "coupled" or variations thereof are modified by an additional term (e.g., directly coupled), the general definition of "coupled" given above is modified by the plain language meaning of the additional term (e.g., "directly coupled" means the joining of two members without a separate intervening member) and is narrower than the general definition of "coupled" given above. Such joining may be mechanical, electrical, or fluid.

[0040] References herein to the location of elements (e.g., "top", "bottom", "above", "below") are merely used to describe the orientation of the various elements in the drawings. It should be noted that the orientation of the various elements may differ according to other exemplary embodiments, and such variations are intended to be encompassed by the present disclosure.

[0041] Although the figures and description may illustrate a particular order of steps of the method, the order of such steps may differ from that depicted and described unless otherwise specified above. Also, two or more steps may be performed simultaneously or with partial concurrence unless otherwise specified above. Such variations may depend, for example, on the software and hardware systems selected, as well as designer choice. All such variations are within the scope of this disclosure. Similarly, software implementations of the described methods may be accomplished by standard programming techniques using rule-based logic and other logic to accomplish the various connecting, processing, comparing, and determining steps.

[0042] Other embodiments are within the scope of the following claims.

Claims

1. 1. A system comprising: an ion source configured to generate ions having a first polarity; one or more extraction electrodes configured to extract the ions from the ion source as an ion beam having an extraction energy; a mass resolving slit or aperture configured to select a desired isotope from the ion beam such that the desired isotope ion beam passes through the mass resolving slit or aperture; a target positioned relative to the mass resolving slit or aperture such that the desired isotope ion beam is incident on the target; and a voltage source coupled to the target and configured to maintain the target at a first voltage having the first polarity; Equipped with the first voltage causes a decrease in extraction energy as the desired isotope ion beam approaches the target and at least one of: minimizing sputtering, reducing the heat load on the target, or maximizing collection of the ions on the target, reorganizing ionized material; system.

2. The system of claim 1 , wherein the first voltage is less than the extracted energy.

3. The system of claim 2 , wherein the first voltage is greater than 95% of the extracted energy.

4. The system of claim 2 , wherein the first voltage is greater than 99% of the extracted energy.

5. 2. The system of claim 1, wherein the first voltage is less than the extraction energy by an amount corresponding to a thermal energy of the ions.

6. The system of claim 1 , wherein the extraction energy is between 20 kV and 80 kV.

7. The system of claim 1 , wherein the extraction energy is between 40 kV and 60 kV.

8. 2. The system of claim 1, wherein the first voltage is about 100V less than the extraction energy such that the desired isotope ion beam has an energy of about 100V when the desired isotope ion beam reaches the target.

9. The system of claim 1 , wherein the first polarity is positive.

10. The system of claim 1 , wherein the target is configured to collect the ionized material as a film on the target.

11. 2. The system of claim 1, further comprising a magnetic analyzer between the one or more extraction electrodes and the mass resolving slit or aperture, the magnetic analyzer configured to generate a magnetic field configured to cause separation of the desired isotope from other isotopes of the ion beam.

12. The system of claim 1 , wherein the ions comprise ytterbium ions.

13. The system of claim 1 , wherein the target comprises a crystal structure.

14. The system of claim 1 , wherein the target comprises a carbon fiber material.

15. 1. A method for collecting ions, comprising: providing extraction energy to an ion beam, the ion beam including ions having a first polarity; positioning the target such that the ion beam is incident on the target; maintaining the target at a voltage having the first polarity such that the voltage causes a decrease in the extraction energy as the ion beam approaches the target; collecting the ions on the target; A method comprising:

16. The method of claim 15 further comprising removing the collected ions from the target as structured ionized material.

17. The method of claim 15 , wherein the voltage and the extraction energy differ by an amount corresponding to a thermal energy of the ions.

18. 16. The method of claim 15, wherein the step of supplying extraction energy to the ion beam comprises accelerating the ions out of the ion source using an electrode held at an extraction voltage, the extraction voltage having the first polarity and being greater than the voltage of the target.

19. The method of claim 15 , wherein the extraction energy is greater than the voltage.

20. The method of claim 15 , wherein the ions comprise ytterbium atoms.

21. A method according to claim 15, performed using a system according to any one of claims 1 to 14.

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