Patent Grant
8339024
An electron beam emitter typically includes an electron gun or generator, positioned within a vacuum chamber for generating electrons. The generated electrons can exit the vacuum chamber in an electron beam through an electron beam transmission or exit window that is mounted to the vacuum chamber. The exit window commonly has a thin metallic exit window foil, which is supported by a metallic support plate or grid. The support plate has a series of holes which allow electrons to reach and pass through the exit window foil. The support plate dissipates heat from the exit window foil caused by electrons passing through the exit window foil. However, electrons that are instead intercepted by the support plate areas between the holes cause heating of the support plate, which can reduce the ability of the support plate to dissipate heat from the exit window foil.
The present invention can provide an exit window including an exit window foil, and a support grid contacting and supporting the exit window foil, in which the exit window foil can operate at lower temperatures than in the prior art. The support grid can have first and second grids, each having respective first and second grid portions that are positioned in alignment and thermally isolated from each other. The first and second grid portions can each have a series of apertures that are aligned for allowing the passage of a beam therethrough to reach and pass through the exit window foil. The second grid portion can contact the exit window foil. The first grid portion can mask the second grid portion and the exit window foil from heat caused by the beam striking the first grid portion.
In particular embodiments, the exit window can be in an electron beam emitter and the beam can be an electron beam. The thermal isolation of the first and second grid portions can provide the second grid portion with a lower temperature than the first grid portion during operation, and allow heat to be more readily conducted from the exit window foil. The first and second grid portions can be spaced apart from each other by a gap. In some embodiments, the first and second grid portions can be spaced apart by thermal insulating material. The first grid portion can provide thermal masking for the second grid portion by direct beam interception. An electrical source can be connected to at least one of the first and second grid portions for causing the deflection of the beam to reduce beam interception by the support grid. The second grid portion and the exit window foil can be formed of materials having substantially similar coefficients of thermal expansion. The second grid portion can have a grid surface on which the exit window foil is bonded continuously. The second grid portion can be contoured to provide additional surface area to mitigate affects of thermal expansion stretching or gathering of the exit window foil.
The present invention can also provide an electron beam emitter which can include a vacuum chamber, an electron generator positioned within the vacuum chamber for generating electrons, and an exit window mounted to the vacuum chamber for allowing passage of electrons out the vacuum chamber through the exit window in an electron beam. The exit window can have an exit window foil and a support grid contacting and supporting the exit window foil. The support grid can have first and second grids, each having respective first and second grid portions that are positioned in alignment and thermally isolated from each other. The first and second grid portions can each have a series of apertures that are aligned for allowing the passage of the electron beam therethrough to reach and pass through the exit window foil. The second grid portion can contact the exit window foil. The first grid portion can mask the second grid portion and the exit window foil from heat caused by the electron beam striking the first grid portion.
In particular embodiments, the thermal isolation of the first and second grid portions can provide the second grid portion with a lower temperature than the first grid portion during operation, and allow heat to be more readily conducted from the exit window foil. The first and second grid portions can be spaced apart from each other by a gap. In some embodiments, the first and second grid portions can be spaced apart by thermal insulating material. The first grid portion can provide thermal masking for the second grid portion by direct beam interception. An electrical source can be connected to at least one of the first and second grid portions for causing the deflection of the beam to reduce beam interception by the support grid. The second grid portion and the exit window foil can be formed of materials having substantially similar coefficients of thermal expansion. The second grid portion can have a grid surface on which the exit window foil can be bonded continuously. The second grid portion can be contoured to provide additional surface area to mitigate effects of the thermal expansion stretching or gathering of the exit window foil.
The present invention can also provide a method of reducing heat on an exit window foil of an exit window. The exit window foil can be contacted and supported with a support grid. The support grid can have first and second grids, each having respective first and second grid portions that are positioned in alignment and thermally isolated from each other. The first and second grid portions can each have a series of apertures that are aligned for allowing the passage of a beam therethrough to reach and pass through the exit window foil. The second grid portion can contact the first exit window foil. The first grid portion can mask the second grid portion and the exit window foil from heat caused by the beam striking the first grid portion.
In particular embodiments, the exit window can be in an electron beam emitter and can allow passage of an electron beam. Heat can be allowed to be more readily conducted from the exit window foil by providing the second grid portion with a lower temperature than the first grid portion during operation by the thermal isolation of the first and second grid portions. The first and second grid portions can be spaced apart from each other by a gap. In some embodiments, the first and second grid portions can be spaced apart from each other by thermal insulating material. The first grid portion can provide thermal masking for the second grid portion by direct beam interception. An electrical source can be connected to at least one of the first and second grid portions for causing deflection of the beam to reduce beam interception by the support grid. The second grid portion and exit window foil can be formed from the materials having substantially similar coefficients of thermal expansion. The exit window foil can be bonded continuously on a grid surface of the second grid portion. The second grid portion can be contoured to provide additional surface area to mitigate effects of thermal expansion stretching or gathering of the exit window foil.
The present invention can also provide a method of reducing heat in an exit window foil of an exit window on an electron beam emitter. The electron beam emitter can have a vacuum chamber, and an electron generator positioned within the vacuum chamber for generating electrons. The exit window can be mounted to the vacuum chamber for allowing passage of electrons out the vacuum chamber through the exit window in an electron beam. The exit window foil can be contacted and supported with a support grid. The support grid can have first and second grids, each having respective first and second grid portions that are positioned in alignment and thermally isolated from each other. The first and second grid portions can each have a series of apertures that are aligned for allowing the passage of the electron beam therethrough to reach and pass through the exit window foil. The second grid portion can contact the exit window foil. The first grid portion can mask the second grid portion and the exit window foil from heat caused by the electron beam striking the first grid portion.
In particular embodiments, heat can be allowed to be more readily conducted from the exit window foil by providing the second grid portion with a lower temperature than the first grid portion during operation by the thermal isolation of the first and second grid portions. The first and second grid portions can be spaced apart from each other by a gap. In some embodiments, the first and second grid portions can be spaced apart from each other by thermal insulating material. The first grid portion can provide thermal masking for the second grid portion by direct beam interception. An electrical source can be connected to at least one of the first and second grid portions for causing deflection of the beam to reduce beam interception by the support grid. The second grid portion and the exit window foil can be formed from materials having substantially similar coefficients of thermal expansion. The exit window foil can be continuously bonded on a grid surface of the second grid portion. The second grid portion can be contoured to provide additional surface area to mitigate effects of thermal expansion stretching or gathering of the exit window foil.
The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.
A description of example embodiments of the invention follows.
For example, a 150 keV 10 mA (1500 W) beam that passes through a 70% transparent grid 10 will dissipate 450 W (150 keV*10 mA*30%/100%=450 W) directly on the grid 10. The remaining 1050 W of beam power is incident on the exit window foil 12, which may transmit ˜96.4% of the beam energy for a 7 micron thick titanium foil. Thus 1050 W*0.964=1012 W of beam power is transmitted through the exit window foil 12 and about 38 W is dissipated in the exit window foil 12. The grid 10 must remove the total heat load of 488 W, of which the exit window foil 12 heat load in only about 8%. The units used are as follows: keV=kilo electron volts, mA=milliamperes, W=watts, C=degrees celsius and cm=centimeter.
In this example, the full heat load creates an elevated temperature in the grid 10, which must also remove the heat load from the exit window foil 12. For an example grid 10 (copper, 25 cm long by 0.6 cm thick, 70% transparent, a 5 cm path to a water cooled heat sink, and a line heat load of 488 W for simplicity), the peak temperature difference between the center and edge of the grid would be about 140 deg. C. The increased temperature of the foil at the center may lead to mechanical failure, oxidation, and fatigue failure. Thermal loads on the grid 10 and the exit window foil 12 may result in thermal expansion. If the grid 10 and the exit window foil 12 undergo thermal expansion at differing amounts, exit window foil 12 may have compromised mechanical performance and result in loss of vacuum integrity.
Referring to
Referring to
The first 16 and second 18 grids can be mounted or stacked together axially along axis A such that the apertures 16a and 18a, and solid material regions 16b and 18b, are aligned with each other generally longitudinally or axially in the direction of axis A, or in the direction or the electron beam 14, while at the same time the first 16c and second 18c grid portions are thermally isolated from each other. The thermal isolation of the first 16c and second 18c grid portions can be achieved by spacing the first 16c and second 18c grid portions apart from each other by a gap G, such as a vacuum gap, within the vacuum chamber 32. Since the first 16c and second 18c grid portions are separated by a vacuum gap G, very little heat is transmitted across the gap G between the grid portions 16c and 18c. In the embodiment shown in
The apertures 16a and 18a can progressively angle outwardly moving towards the outer perimeter 16d and 18d towards the ends 15a of exit window 15. Apertures 16a and 18a near the central axis A (
With the apertures 16a and 18a of the first 16c and second 18c grid portions being aligned, the first grid portion 16c of the first grid 16 can act as a mask for the second grid portion 18c of the second grid 18. Electrons e− that are not aligned with apertures 16a and 18a can be blocked or intercepted by the solid material regions 16b of the first grid portion 16c, while electrons e− that are aligned with apertures 16a and 18a can pass through and out the exit window foil 12. Substantially all electrons e− or energy passing through the apertures 16a of the first grid portion 16c can pass through the apertures 18a of the second grid portion 18c. Consequently, the first grid portion 16c of the first grid 16 can act as an electron beam and/or a heat mask or shield for the second grid portion 18c of the second grid 18 due to the alignment of apertures 16a and 18a, and the thermal isolation of the first grid portion 16c from the second grid portion 18c. The first grid portion 16c of the first grid 16 is subject to the heat load of direct electron e− interception, and this heat load is thermally isolated from the second grid portion 18c of the second grid 18. Therefore, the second grid portion 18c and second grid 18 can act as a heat sink primarily for the heat generated in or dissipated into the exit window foil 12 by electrons e− passing through the exit window foil 12. Since the majority of the heat or thermal load absorbed by the exit window 15 is absorbed by the first grid portion 16c and first grid 16, and is isolated from the second grid portion 18c, the exit window foil 12 of exit window 15 can be at lower temperatures at equivalent power levels when electron beam emitter 30 is operated in comparison to the exit window 9 of
In comparison with the power example previously discussed for exit window 9 of
Referring to
Referring to
In the various embodiments, the upper or outer grid (such as 18 or 20) that is in contact with the exit window foil 12, can be made of material with a similar or the same coefficient or thermal expansion (CTE), or the same material, as the foil material of the exit window foil 12. Such materials can be metallic or nonmetallic and can include: beryllium, boron, carbon, magnesium, aluminum, silicon, titanium, copper, molybdenum, silver, tungsten, gold and combinations thereof, materials such as tungsten copper (fabricated by powder metallurgy) and silicon carbide, aluminum nitride, beryllium oxide (ceramics).
The masking first, inner, or lower grid 16 can be made of a lower Z material so as to minimize x-rays created from electrons e− intercepted by grid 16. Such materials can be metallic or nonmetallic and can include the upper grid materials listed above. In some embodiments, the grids can be made of the same materials, such as copper, as described in a previous example. The first grid 16 can also be plated or coated with low Z materials, such as beryllium, boron, carbon, aluminum, silicon, or compounds containing these. Although an example of a thickness of 0.3 cm has been previously described for the grids, this dimension can be varied for one or all grids. In some embodiments, the entire grid structure can be made of micromachined silicon (or other material) with a transmissive window layer deposited or bonded to it. The first 16 and second 18 or additional grids can be brazed or welded together at the outer perimeters or joined by other suitable methods.
The exit window foil 12 can be metallic or nonmetallic, and can be made of beryllium, boron, carbon or carbon based material such as a polymer, magnesium, aluminum, silicon, or titanium, combinations thereof, or oxides, nitrides, or carbides of these materials. The grid materials and exit window foil 12 materials can be selected so as to match coefficients of thermal expansion, or can have the same materials, so that the grid and exit window foil 12 can expand at similar rates providing for more thermally robust exit window foil which can prevent wrinkles in the exit window foil 12. For example, the exit window foil 12 and the outer grid surface 15c can both be titanium, or other suitable materials. Depending on the design, in some embodiments, the CTEs can be different. The exit window foil 12 can be a multilayer structure that includes various coatings for purposes such as corrosion protection or thermal conductivity. The coatings may include the previously listed foil materials, but also materials well known to be corrosion resistant such as gold and platinum. Embodiments of the exit window foil 12 can have thicknesses which can range from about 4-13 micrometers thick, but in some cases, can be thicker.
Bonding the exit window foil 12 to the upper or outer grid (such as 18 or 20), can be accomplished through diffusion bonding, brazing, soldering, cementing, welding (e.g. laser welding), or other hermetic attachment techniques. This can be done as a separate process at the time of electron beam emitter vacuum processing, or may he done independently. The benefits of bonding the exit window foil 12 to the upper grid independently can include allowing the initial vacuum integrity to be tested prior to processing the entire emitter 30, emitter 30 processing time can be shorter, and exit windows 15 can be manufactured in a batch process, and more efficiently.
The bonding of the exit window foil 12 to the grid (such as 18 or 20), can be done as a perimeter type of bond in order to make a vacuum seal. In addition, the exit window foil can be bonded continuously across the upper or outer grid surface 15c which can improve heat transfer between the exit window foil 12 and the grid, as well as thermal expansion effects. For a perimeter type of bond, the pressure due to atmosphere on one side and vacuum on the other pushes the exit window foil against the grid (such as 18 or 20), and provides some degree of contact for heat transfer. With a continuous surface bond, there is essentially no thermal impedance between the two materials and therefore can provide improved heat transfer. This can allow the exit window foil 12 to operate at a lower temperature for the same power level versus a foil bonded at the perimeter only. The bonding may be accomplished by means of diffusion, by welding, brazing, soldering or other bonding methods.
The grid structure and exit window 12 may be attached to the rest of the vacuum enclosure or connecting structures by various methods including welding, brazing, soldering, bolted wire seal or conflat joint, or other hermetic bonding methods. The grids of the exit window 15 can be diffusion bonded together, and can be done at the same time or different time that the exit window foil 12 is bonded to the upper grid (such as 18 or 20). The first grid or grids may alternatively be integral to the emitter 30 structure and the final grid supporting the exit window foil 12 may be attached to it, for example, by soldering. The apertures 16a, 18a or 20a may be in the form of holes or slots that are aligned to the beam trajectories, such as depicted in
In some embodiments, the exit window foil 12 can be titanium, the intermediate, upper or outer grid (such as 18 or 20) copper or tungsten, and the first grid 16 copper. Although copper and titanium have different CTEs, they are often used together due to copper's high thermal conductivity and titanium's corrosion resistance. In hermetically sealed emitters, such as in some embodiments of emitter 30, the use of hermetically sealed joints gives rise to additional complexity, as the coefficients of thermal expansion, CTE, of adjacent materials in some embodiments may differ considerably. For example, the CTE of copper is on the order of 10 um/m/C greater than titanium. Hermetically sealed electron beam emitters typically require a bake out at elevated temperature to reduce outgassing of constituent materials such that, once sealed, a good working vacuum can be maintained. If the exit window structure is fabricated by permanently joining a metal exit window foil 12 membrane to a grid (such as 18) with a different CTE, the vacuum bake out can cause wrinkles to be formed. By way of example, consider titanium (Ti) foil bonded to a copper (Cu) grid. If the hermetically sealed joint is made while the materials are substantially at room temperature, elevating the temperature of the structure for bake out can cause the exit window foil 12 to be stretched beyond its elastic limit by the strain imposed by the grid by virtue of its larger CTE. When returned to room temperature, the excess foil which results from this plastic deformation can gather into wrinkles across the surface.
Wrinkling of the exit window foil 12 in an electron transparent membrane can present several problems. The electron beam typically intercepts the exit window normal to its travel direction. If a wrinkle is present, the beam strike is more oblique, and therefore intercepts an increased effective thickness of foil. This can lead to preferential energy absorption and heat load. Note also that a portion of the foil is separated from the heat sinking grid which can exacerbate the heat rise. On the atmospheric side, wrinkles can disrupt and degrade convective cooling as well. The local stiffening of the foil caused by the wrinkle can act as a stress riser and lead to low cycle fatigue failure.
In the present invention, CTE mismatch problems can be mitigated by diffusion bonding the exit window foil 12 to the grid surface 15c of the grid (such as 18 or 20) in a substantially continuous manner across the surface of the grid. In this way, the macroscopic wrinkles and the attendant problems described above can be eliminated.
A titanium (Ti) exit window foil 12 can be diffusion bonded to the outer grid surface 15c of a grid (such as 18 or 20) by applying pressure at elevated temperature under vacuum (
In a continuous or full face bond 15e of an exit window foil 12, the free span of foil between attachment points is reduced significantly in comparison to an exit window bonded only at its perimeter. Since the foil that is used is typically fabricated by cold rolling, pre-existing microscopic defects are common. In a perimeter bond of an exit window foil, by stretching the foil from its perimeter, the strain is borne by the “weakest” areas of foil (the areas with highest defect density, local thinning, or inclusions). In the present invention, by bonding continuously over the grid surface 15c, the free span of foil is limited to the much smaller area defined by the holes or slots (i.e., the windowlettes), such strain concentration is restricted or minimized.
In addition, with a continuous full face bond 15e, the thermal impedance at the foil/grid interface is reduced. In a conventional window, the foil is typically brought into contact with the grid by the ambient pressure outside the vacuum vessel. Since the physical contact between foil and grid occurs in vacuum, significant thermal impedance can be created by small voids. In the present invention, by diffusion bonding the exit window foil 12 directly to the grid, surface 15c, the two materials are brought into intimate contact, eliminating the small voids created by imperfect geometry.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
The above examples have been described for electron beams, but can also apply to ion beams, x-rays, and optical beams that rely on vacuum windows. In addition, features of the various exit windows described can be omitted or combined, or have different configurations. In some embodiments, the apertures in the grids and insulating member can have shapes other than slots, for example, can be round. Furthermore, the exit window 15 can have other shapes, such as a generally round shape. It is also understood that the electron beam emitters and exit windows in the present invention can include other suitable shapes, configurations or dimension than those shown or described.
This application claims the benefit of U.S. Provisional Application No. 61/226,925, filed on Jul. 20, 2009. The entire teachings of the above application are incorporated herein by reference.
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