Method and system for liquid cooling of an insulated x-ray transparent target

Abstract

To provide a method and a system for liquid-cooling an insulated radiolucent target.SOLUTION: An x-ray source includes a target assembly including a target, an electron source for producing electrons that impinge on the target, and a flight tube assembly that separates the target assembly from the electron source and transports a coolant to the target assembly. The flight tube assembly includes a flight tube interface ring, a target cartridge tube, and an electrically insulating ring between the flight tube interface ring and the target cartridge tube.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Related Applications This application is related to U.S. patent application Ser. No. 17 / 238,785, filed on even date herewith, Attorney Docket No. 0002.0085US1 (2020ID00440), now U.S. Patent Application Publication No. ____, entitled "X-ray source with liquid cooled source coils," invented by Claus Flachenecker and Thomas A. Case, and to U.S. patent application Ser. No. 17 / 238,811, filed on even date herewith, Attorney Docket No. 0002.0087US1 (2020ID00446), now U.S. Patent Application Publication No. ____, entitled "Fiber-optic communication for embedded electronics in x-ray generator," invented by Claus Flachenecker.

[0002] All of the aforementioned applications are incorporated herein by reference in their entirety. [Background technology]

[0003] Background technology X-rays are widely used in microscopy due to their short wavelength and ability to penetrate objects. Typically, the best source of X-rays is a synchrotron, but these are expensive systems. Therefore, so-called tube or laboratory X-ray sources, in which a generated electron beam strikes a target, are often used. The resulting X-rays contain characteristic lines determined by the target's composition and a wide range of bremsstrahlung radiation.

[0004] X-ray microscope systems have several basic configurations. Some use a condenser to focus the X-rays onto the object being studied and / or an objective lens to image the X-rays after interaction with the object. The resolution and aberrations associated with these types of microscopes are typically determined by the spectral characteristics of the X-rays. Some microscope systems employ a projection configuration in which a small X-ray source spot is used, often with geometric magnification, to image the object.

[0005] Performance, and especially resolution, is affected by different factors. Because projection configurations lack aberrations, resolution is typically determined by the size of the X-ray source spot. Ideally, the X-ray source spot is a point spot. In practice, X-ray source spots are significantly larger. The source spot size is generally determined by the electron optics and their ability to focus the electron beam to a single point. The source spot size is typically about 50–200 micrometers (μm) with good electron optics; however, in other instances, when output is a more important figure of merit, the X-ray source spot size may be 1–5 millimeters (mm). For transmission target X-ray sources, spot sizes of several micrometers, such as 1–5 μm, are common. In fact, some transmission sources have spot sizes up to 150 nanometers (nm). In any case, the size of the X-ray source generally limits the resolution of the X-ray projection microscope.

[0006] Many microscopy applications often use transmission-target X-ray sources. In the basic configuration of an X-ray tube, thermal or field-emission electrons are generated in the cathode (filament) of a vacuum tube and accelerated to the anode (forming an electron beam that is shaped by different electrostatic and (electro)magnetic optical elements). For example, magnetic lenses often use a coil of copper wire inside an iron pole piece. Current passing through the coil generates a magnetic field within the bore of the pole piece. Electrostatic lenses use a charged dielectric to generate an electrostatic field. The electron beam then strikes the backside of a typically thin target; common target materials are tungsten, copper, and chromium. X-rays emitted from the front side of the target are then used to irradiate the object. Summary of the Invention [Problem to be solved by the invention]

[0007] Summary of the Invention During operation of a transmission (or reflection) target x-ray source, heat must be removed from the target, and because only a small percentage of the electron beam's energy is converted into x-rays, heat is excessive.

[0008] Heat removal from the target itself is typically accomplished by conduction toward a water- or air-cooled sink. The transmission target itself is also made of low-X-ray absorption (low-Z) materials such as diamond or beryllium, which are good thermal conductors. After transferring heat away from the X-ray burning spot, cheaper and more convenient materials such as copper are used to conduct heat to the cooling element.

[0009] Nevertheless, especially for high-performance X-ray sources, heat conduction should be maximized. Generally, a shorter thermal path to the sink is desirable. Furthermore, conducting heat through other structural elements of the source (such as the magnetic lens yoke) should be avoided, as this would result in thermal drift of these components and therefore X-ray spot position drift or instability. Furthermore, adjusting the generated X-ray power also involves measuring the target current, which requires electrical insulation of the (thermal) target. [Means for solving the problem]

[0010] The approach involves several innovations. First, the X-ray target can be coupled to a small, cylindrical, electrically insulated, thermally conductive "cartridge." The "cartridge" includes two copper "cylinders," a flight tube connecting ring, and a target cartridge tube, bonded to either side of a thin, separating diamond insulator ring. This allows for excellent thermal conduction while maintaining electrical insulation.

[0011] Additionally, the cartridge or target assembly is attached to a special liquid-cooled flight tube assembly, which brings a liquid coolant, such as water, into close proximity with the cartridge. An example flight tube assembly incorporates a triple-walled tube with a diameter similar to that of the X-ray target. The inner wall acts as a barrier between the vacuum and the coolant, the outer wall acts as a barrier between the coolant and the ambient air, and an intermediate wall or baffle acts as a barrier between the inflowing and outflowing coolant. A manifold at the base of the flight tube assembly allows for the supply and return of the coolant. Thus, the coolant flows from the base to the distal end of the flight tube assembly, where the target "cartridge" is attached, minimizing the thermal path length between the X-ray target and the coolant. Preferably, the diameters of the flight tube assembly and target assembly are similar in size to the X-ray target itself, all small enough to fit inside an annular magnetic lens yoke. This therefore thermally and mechanically isolates the X-ray target element from the magnetic focusing lens and other steering and shaping units. Furthermore, the magnetic lens yoke or other parts of the steering unit do not need to be exposed to vacuum and can be aligned independently.

[0012] In general, according to one aspect, the invention features an x-ray source including a target assembly including a target, an electron emitter for generating electrons that impact the target, and a flight tube assembly that separates the target assembly from the electron source and delivers a coolant to the target assembly.

[0013] In the current embodiment, the target is a transmission target, but the principles can also be applied to reflective targets. In an embodiment, the source further comprises a flight tube manifold for supplying coolant to the flight tube assembly and receiving return coolant.

[0014] In one example, the flight tube assembly includes an inner flight tube, an outer flight tube, and a baffle between the inner and outer flight tubes for directing coolant to a distal end of the flight tube assembly.

[0015] In general, according to another aspect, the invention features a method for cooling a target of an x-ray source, the method including flowing coolant through a flight tube assembly to the target and flowing coolant returned from the target through the flight tube assembly.

[0016] In general, according to another aspect, the invention features a target assembly for an x-ray source, the assembly including a target, a cartridge tube for holding the target, an interface ring for connecting to a flight tube, and an insulating ring between the cartridge tube and the interface ring.

[0017] In general, according to another aspect, the invention features an x-ray source that includes a target assembly including a target, an electron source for generating electrons that impact the target, a high voltage generator for accelerating the electrons, a flight tube assembly separating the target assembly from the electron source, a magnetic focus lens, and a beam steering and shaping system nested between the magnetic focus lens and the flight tube assembly.

[0018] The beam steering and shaping system can include at least one steering / shaping unit between the flight tube assembly and the yoke of the magnetic focusing lens. The steering / shaping unit can include at least eight coils.

[0019] In general, according to another aspect, the invention features an x-ray source that includes a target assembly including a target, an electron source for generating an electron beam, a flight tube assembly separating the target assembly from the electron source, a magnetic focusing lens for focusing the electron beam on the target, and a beam steering and shaping system having separately controlled coils for steering and shaping the electron beam.

[0020] In general, according to another aspect, the invention features a method for operating an x-ray source that includes generating an electron beam and directing the beam at a target to generate x-rays, and in a standby mode, directing the electron beam away from the target to suppress x-ray generation at the target.

[0021] These and other features of the invention, including various novel details of construction and combination of parts, as well as other advantages, will be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the specific methods and devices embodying the invention are shown by way of illustration and not as limitations of the invention. The principles and features of this invention can be employed in various and numerous embodiments without departing from the scope of the invention.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. [Brief explanation of the drawings]

[0023] [Figure 1]1 is a schematic cross-sectional view of an X-ray source according to the present invention; [Figure 2] FIG. 1 is a scale perspective view showing the flight tube assembly 400, the manifold 150, and the flight tube aperture assembly 142. [Figure 3] 1 is a perspective exploded view of the flight tube assembly and its interface to the manifold 150. The flight tube assembly 400 is shown exploded. [Figure 4] FIG. 10 is a cross-sectional, scale elevation view showing the distal end of the flight tube assembly 400 and its interface with the target assembly 500. [Figure 5] FIG. 5 is a scale front cross-sectional view of the target assembly 500. [Figure 6] FIG. 6 is a cross-sectional side view showing the beam steering and shaping system 600 and magnetic focusing lens 700 surrounding the flight tube assembly 400. [Figure 7] FIG. 6 is a schematic diagram illustrating the beam steering driver 224 that provides individual coil control for the beam steering and shaping system 600. DETAILED DESCRIPTION OF THE INVENTION

[0024] MODE FOR CARRYING OUT THE INVENTION The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0025] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Also, all conjunctions used should be understood in the most inclusive sense possible. Accordingly, the word "or" should be understood to have the definition of a logical "or" rather than a logical "exclusive or" definition, unless the context clearly requires otherwise. Furthermore, the singular forms and the articles "a," "an," and "the" are intended to include the plural unless otherwise stated. It will be further understood that the terms "includes," "comprises," and / or "including," as used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Furthermore, when an element, including a component or subsystem, is referred to and / or shown as being connected or coupled to another element, it will be understood that it may be directly connected or coupled to the other element, or that intervening elements may be present.

[0026] Although terms such as "first" and "second" are used herein to describe various elements, it should be understood that these elements are not limited by these terms. These terms are used only to distinguish one element from another. Thus, an element described below could be referred to as a second element, and similarly, the second element could be referred to as a first element without departing from the teachings of the present invention.

[0027] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by those skilled in the art to which this invention belongs. Terms such as those defined in commonly used dictionaries should be interpreted to have a meaning consistent with their meaning in the context of the relevant art, and it will be further understood that they should not be interpreted in an idealized or overly formal sense unless expressly defined as such in this specification.

[0028] FIG. 1 is a schematic cross-sectional view of an x-ray source 100 constructed in accordance with the principles of the present invention. The illustrated embodiment is a "transmission target" source: the electron beam B strikes the target in the target assembly 500, and the x-rays X emitted from the opposite side of the target are used to illuminate the object. However, many aspects of the following innovations are equally applicable to other x-ray tube source configurations, including side windows, rotating anodes, and metal jet anodes.

[0029] Generally, the x-ray source comprises a vacuum vessel 112 and an oil vessel 114 disposed within the vacuum vessel. Preferably, the vacuum vessel 112 is metal, such as aluminum or stainless steel, for its vacuum strength. The oil vessel 114 is preferably constructed from a non-conductive material, such as a ceramic, e.g., sintered alumina, that provides electrical insulation to prevent arcing to the high voltage components contained therein.

[0030] A vacuum generator 118 is used to draw and / or maintain a vacuum on the vacuum vessel 112. In one example, an ion pump is used.

[0031] A heat exchanger 119 is located inside the oil vessel. To this end, a plate heat exchanger can be used to remove thermal energy (heat) from the oil and pass it on to a coolant, such as water, which circulates through the exchanger. Some embodiments further use a submerged pump 121 to circulate the oil within the oil vessel 114. In a preferred embodiment, a circulator 152 is used to force the coolant through the heat exchanger 119 to carry the heat away from the oil.

[0032] Generally, the vacuum vessel 112 defines a volumetric vacuum region through which the electron beam B propagates from the electron emitter 126 (filament or cathode), typically located near the distal end of the oil vessel 114, to the target held by the target assembly 500. The vacuum region also preferably surrounds at least a portion of the oil vessel, which houses high voltage components and provides high voltage insulation.

[0033] The system controller 200 is located outside both vessels 112, 114. It contains the main controller and data interfaces to external devices. It also contains a power supply for connecting to the mains power supply. It also controls the vacuum generator 118 and the circulator 152.

[0034] The high voltage generator 116 is disposed within the oil vessel 114. Its base is proximal to the oil vessel 114, allowing it to receive power from the system controller 200. The high voltage generator 116 is immersed in oil contained in the oil vessel 114 for thermal control and high voltage insulation. The oil is needed primarily to keep the generator 116 relatively small; however, the generator 116 can also be potted. Moving distally, the high voltage generator 116 is further electrically insulated from the environment by the oil and surrounding vacuum of the vacuum vessel 112.

[0035] The high voltage generator 116 in the present example generates a negative 20-160 kV acceleration voltage and powers the gun controller 300, which controls, among other things, the electron source (emitter or filament). The high voltage generator biases the entire gun controller to this large negative voltage so that the generated electrons accelerate toward the less negative voltage and ground.

[0036] The inner vessel 120 is located distal to the distal end of the high voltage generator 116. The inner vessel 120 is immersed in oil in the oil vessel 114. In the current embodiment, the inner vessel is preferably constructed from a metal such as aluminum or mild steel. It is also filled with oil, which aids in the transfer of heat from the electronics, as well as from the source coil, as will be described below.

[0037] The gun controller 300 is housed within the inner vessel 120, which also functions as a Faraday cage to electrically protect the controller 300. It drives the electron emitters and provides controls for the electron emitters, beam generation, modulation, and steering.

[0038] An electron emitter, e.g., a filament 126, is held in a filament mount 124. In the current example, the electron emitter 126 includes a lanthanum hexaboride (LaB6) crystal and a carbon heater rod, which protrudes into the vacuum of the vacuum vessel to function as a thermionic source or electron emitter (cathode). Other configurations are possible, such as W, CeB6, HfC, and carbon nanotube filaments.

[0039] The vacuum feedthrough 122 provides an electrical connection between the gun controller 300 within the inner vessel 120 and the outer wall of the oil vessel 114 through the oil contained in the oil vessel 114 .

[0040] A suppressor electrode or Wehnelt cap 127 is attached distally to the filament mount 124 and covers the filament 126. Electrons emitted from the filament 126 pass through a central aperture in the suppressor electrode 127, the voltage of which is controlled by the gun controller 300.

[0041] A protective field cap 138 has a general bell shape and extends over the electron emitter 126 and its filament mount 124, wrapping back around the distal end of the oil container 114. Its distal end carries a first or extractor anode 140. The voltage and cap of the first anode are controlled by the gun controller 300 to accelerate the emitted electrons through a central aperture 141 of the first anode 140 into beam B. Thus, during operation, the electron beam passes through the central aperture 141 of the first anode 140.

[0042] However, the first anode is not required, and the system can also be designed without this first anode and rely on other means to accelerate the electrons.

[0043] Beam B is directed through an aperture in a flight tube aperture assembly 142 in the distal wall of the vacuum vessel 112. This flight tube aperture assembly acts as a second anode and is currently held at ground potential 143. Therefore, the gun controller is biased to a large negative voltage, causing the electrons to be further accelerated in the gap between the first anode 140 and the flight tube assembly 142.

[0044] In other embodiments, the flight tube aperture assembly 142 is electrically isolated from the vacuum vessel 112 using an insulating gasket such as diamond. A voltage generator is also added to provide a controlled potential to the flight tube aperture assembly. In this configuration, the system controller 200 also controls the voltage of this second anode to provide additional control, such as additional acceleration, to the electron beam B. The flight tube assembly 400 extends the vacuum to the target assembly 500 and its target. The flight tube manifold 150 provides liquid cooling to the target assembly using a coolant, such as water, from a circulator 152 through the flight tube assembly walls.

[0045] Located along the flight tube assembly 400 is a flight tube beam steering and shaping system 600 for adjusting the electron beam and directing the beam to any desired location on the target. This is accomplished by the flight tube assembly 400 and the beam steering and shaping system 600, which directs the electron beam B to the desired angle and location through a magnetic focus lens 700. As the target is consumed during operation, the beam steering typically positions the spot at different locations on the target.

[0046] Additionally, a magnetic focusing lens 700 is positioned along the flight tube assembly 400 to focus the beam B onto the target.

[0047] Preferably, both the flight tube beam steering and shaping system 600 and the magnetic focusing lens 700 are cooled by a coolant circulated from the circulator 152 and controlled by the system controller 200 .

[0048] A pair of source coils 132N, 132S (before and after the image plane), not shown, and 132E, 132W and their respective cores 134N, 134S, 134E, and 134W, not shown, are integrated with the oil container 114, the gun controller inner container 120, and the protective field cap 138. The coils are located outside the vacuum of the vacuum container. In one example, they can be located on the outer wall of the vacuum container exposed to the ambient atmosphere. In the example shown, the source coils 132N, 132S, 132E, and 132W are located within the oil container and are therefore efficiently cooled by the contained oil, although the coils could alternatively be potted.

[0049] More generally, the oil can be replaced with potting material or other high voltage compatible cooling materials such as Fr-77 by Sigma Aldrich, Sf6-Novec 4710 by 3M, or C3F7CN.

[0050] More specifically, two source coils 132N, 132S are generally positioned above and below the filament 126. Two additional source coils 132E, 132W are positioned on the other two axes, below and above the plane of the drawing, respectively. North and south pole pieces 130N, 130S extend from cores 134N and 134S of the source coils 132N, 132S, respectively, wrap around the inside of a protective field cap 138, and converge above and below, respectively, a central aperture 141 of the first anode 140. Similarly, east pole piece 130E and west pole piece 130W (in the other two axes below and above the plane of the drawing, respectively) extend from cores 134E and 134W of source coils 132E, 132W, wrap around the inner sides of protective field cap 138, and converge to the left and right of central port 141, respectively, thus forming a magnetic circuit surrounding the emitter in vacuum.

[0051] Pole pieces 130N, 130S, 130E, and 130W can be mechanically connected to virtually anything within the emitter region. Thus, although they are carried by protective field caps in the illustrated embodiment, they need not be directly connected. However, in the present example, pole pieces 130N, 130S, 130E, and 130W are connected to protective caps that are electrically at the potential of first anode 140.

[0052] An annular ring-shaped yoke 135 is disposed proximal to the cores 134N, 134S, 134E and 134W and is fabricated as part of the container 120 to improve the magnetic circuit. Indeed, in the current embodiment, the distal end of the inner container 120 is soft iron, thus completing the magnetic circuit by directing the magnetic flux between the cores.

[0053] In a preferred embodiment, the magnetic circuit for the source coils 132N, 132S, 132E, and 132W is further improved by magnetizable or ferromagnetic wall plugs 136N, 136S, 136E, and 136W. These wall plugs are inserted into holes formed in the oil reservoir 114 opposite the distal ends of the respective cores 134N, 134S, 134E, and 134W. This improves the magnetic flux through the circuit. Specifically, the plugs minimize the gap between the coil cores 134N, 134S, 134E, and 134W and the respective pole pieces 130N, 130S, 130E, and 130W.

[0054] In some cases, plugs 136N, 136S, 136E, 136W are inserted into pre-drilled holes in ceramic oil reservoir 114. Alternatively, the same can be accomplished by welding nickel-cobalt iron alloy or soft iron plugs into pre-drilled holes in stainless steel vacuum chamber 112. Other combinations are possible.

[0055] In the current embodiment, source coils 132N, 132S, 132E, 132W are driven and operated in a current-controlled mode by gun controller 300. Feedback is obtained indirectly by measuring the amount of beam passing through the "anode aperture" and onto the target by system controller 200, which provides this information to the gun controller. The source coils are controlled by gun controller 300, which steers the electron beam near its source, specifically, steering the beam within the gap between filament 126 and first anode 140, steering the beam as it is initially accelerated.

[0056] FIG. 2 shows the flight tube assembly 400 and its interface to the manifold 150, as well as the flight tube aperture assembly 142.

[0057] The general diameters of flight tube assembly 400 and target assembly 500 are sized to fit inside the yokes of flight tube beam steering and shaping system 600 and magnetic focusing lens 700, which therefore thermally and mechanically isolates the x-ray target from magnetic focusing lens 700 and beam steering and shaping system 600.

[0058] More specifically, the flight tube aperture assembly 142 is mounted proximally to the disk-shaped flight tube manifold 150. The central aperture 143 of the flight tube aperture assembly 142 provides a port for the electron beam to enter the flight tube assembly 400. When at ground potential, the flight tube aperture assembly 142 also functions as a second anode to further accelerate the electron beam between the first anode 140 and the origin of the flight tube.

[0059] The proximal side of manifold 150 is secured to the distal side of the vacuum vessel via a set of alignment pins 158, providing a vacuum seal. Coolant is supplied to manifold 150 by circulator 152 via coolant supply tube 154 that connects around the periphery of manifold 150. Similarly, coolant return tube 156 also connects along the periphery of the manifold to return coolant to circulator 152 (FIG. 1). Flight tube assembly 400 protrudes axially from manifold 150, with flight tube assembly 400 capped by target assembly 500 (not shown).

[0060] This construction of the flight tube assembly 400 provides a continuous tubular pathway from the flight tube aperture assembly 142 to the target assembly 500, extending through the beam steering and shaping system 600 and the magnetic focus lens 700. This connection is not hindered by components such as O-rings that can be damaged by high temperatures. Instead, the flight tube assembly 400 is a continuous metal assembly constructed from high-temperature resistant joints formed by welding, brazing, and / or soldering. Additionally, the manifold 150 is attached using a copper gasket that can be baked to high temperatures.

[0061] Figure 3 shows the flight tube assembly in exploded view. Flight tube assembly 400 generally comprises a triple-layered metal tube structure. The inner bore of inner flight tube 414 communicates with the vacuum of the vacuum vessel through flight tube aperture 143, thus providing a vacuum path to the target in target assembly 500 located at the distal end of inner flight tube 414.

[0062] A tubular cooling baffle 412 is concentrically disposed between the inner flight tube 414 and the outer flight tube 410. Generally, the outer periphery of the proximal end of the cooling baffle 412 is in communication with the coolant return tube 156 by the manifold 150, while the inner periphery of the proximal end of the cooling baffle is in communication with the coolant supply tube 154 by the manifold 150.

[0063] The outer flight tube 410, tubular cooling baffle 412, and inner flight tube 414 are each constructed from a non-ferromagnetic metal, currently copper due to its excellent thermal properties and ease with which it can be brazed and soldered.

[0064] 4 is a cross-sectional view showing the distal end of flight tube assembly 400 and its interface with target assembly 500. It shows how supply channel 402 is formed between the outer wall of inner flight tube 414 and the inner wall of cooling baffle 412, and return channel 404 is formed between the outer wall of cooling baffle 412 and the inner wall of outer flight tube 410. An annular spacer 416 spaces the distal end of baffle 412 from the inner wall of outer tube 410. The spacer includes an axial hole that allows coolant to flow through return channel 404.

[0065] However, in other examples, the flow can be reversed with channel 402 being the return channel and 404 being the supply channel.

[0066] In either case, direct liquid cooling of the flight tube assembly 400 ensures that its temperature remains stable over long periods of operation, avoiding thermally induced dimensional variations and preventing fluctuations in the electron beam focal position.

[0067] The flight tube end cap 420 is integral with or brazed to the inner flight tube 414. The end cap 420 is also preferably copper and seals against the distal ends of the outer flight tube 410 and inner flight tube 414. More specifically, the distal end of the outer flight tube 410 has an annular shoulder 422. The outer periphery of the flight tube end cap 420 fits into this shoulder, forming a fluid-tight seal against the outer flight tube 410. At the same time, the inner flight tube 414 forms a vacuum seal with the proximal face of the flight tube end cap 420. Furthermore, the inner bore of the inner flight tube 414 is aligned with the end cap 420 and an electron beam port 426 extending through its neck 424.

[0068] The cooling baffle 412 terminates distally before the ends of the outer flight tube 410 and the inner flight tube 414. This defines a gap 418 between the distal end 412E of the cooling baffle 412 and the proximal face of the flight tube end cap 420 to provide fluid communication between the supply channel 402 and the return channel 404.

[0069] This construction also affects vacuum quality: because the vacuum region is defined by the inner bore of the inner flight tube 414, which extends to the flight tube end cap 420, and the entire flight tube assembly 400, which is constructed from metal components, the entire assembly can be fired to burn off impurities that would otherwise outgas and compromise the vacuum.

[0070] Another feature of this design is the close mechanical connection between the flight tube aperture assembly 142 and the manifold 150. The flight tube aperture assembly 142 is prone to heating during operation. Electrons that avoid the central aperture that defines the beam heat the flight tube aperture assembly 142 itself. This, in turn, heats the aperture assembly 142. However, this heat generated within the flight tube aperture assembly 142 is transferred to the manifold 150, where it is removed by the coolant flowing through the manifold.

[0071] In addition, heat is generated even in standby operation mode. According to a first option, the source coils 132N, 132S, 132E, and 132W are controlled to direct the beam into the body of the flight tube aperture assembly 142 so that the beam avoids the aperture 143. The flight tube aperture assembly thus functions as a beam dump. Another option is to use the beam steering and shaping system 600 to bend the beam onto the inner wall of the inner flight tube 414 of the flight tube assembly 400, thereby acting as a beam dump.

[0072] This standby mode is useful for preventing x-ray generation at the target while still keeping the components associated with generating the electron beam operational. When either of these two options is employed, the heat generated is efficiently removed by the coolant. When the flight tube aperture assembly 142 is used as a beam dump, the heat in the flight tube aperture assembly 142 is transferred to the manifold 150 and removed by the coolant flowing through the manifold. When the flight tube assembly 400 is used as a beam dump, the heat is removed by the coolant flowing within the flight tube assembly. Therefore, active cooling of either beam dump allows the standby mode to be maintained indefinitely.

[0073] FIG. 5 is a cross-sectional, scale elevation view of a target assembly 500. The target assembly 500 includes a flight tube interface ring 510 having a proximal face 511 that seals against the distal side of the flight tube end cap 420. An inner bore 516 of the flight tube interface ring 510 is in vacuum communication with the electron beam port 426 in the end cap 420 (FIG. 3).

[0074] The target cartridge tube 520 includes a cartridge tube throat 526 that protrudes into the inner bore 516 of the flight tube interface ring 510. An annular cartridge tube saddle 524 protrudes outward from the throat 526 and forms a seat against the distal end of the flight tube interface ring 510. An electrically insulating ring 530, constructed of a non-conductive material such as diamond, is disposed between the cartridge tube saddle 524 and the distal end of the flight tube interface ring 510, providing a seat for the insulating ring 530 and electrically insulating the target cartridge tube 520. At the same time, the diamond insulating ring 530 is thermally conductive, forming a good thermal connection between the target 552 and the cooling reservoir / heat sink.

[0075] The distal end of the target cartridge tube 520 is characterized by a cartridge tube port 522. The target 550 is concentrically sealed within the port 522.

[0076] Generally, the target comprises a substrate layer 554 that provides the bulk of the target, and a target metal layer 552 on the distal side of the target that is used to generate x-rays when struck by electron beam B along optical axis AO. Often, metal layer 552 is W, Cu, Cr, Fe, or Ag, or an alloy containing these.

[0077] FIG. 6 is a cross-sectional side view showing the flight tube assembly 400, the target assembly, the beam steering and shaping system 600, and the magnetic focusing lens 700 surrounding the flight tube assembly 400.

[0078] With respect to the manifold 150, a supply port 155 is also formed through the body of the manifold and couples a supply line 154 to the inner periphery of the proximal end of the cooling baffle 412 so that coolant is supplied to the flight tube assembly 400. A return port 157 is also formed through the body of the manifold and couples a return line 156 so that coolant is received from the outer periphery of the proximal end of the cooling baffle 412. A beam steering and shaping system 600 surrounds the proximal end of the flight tube assembly 400 but is distal to the manifold 150. In the current embodiment, the beam steering and shaping system 600 comprises a first steering / shaping unit 610 and a second steering / shaping unit 620. The steering / shaping units are annular structures with the flight tube assembly 400 passing concentrically through each unit 610, 620.

[0079] Preferably, each of the units 610, 620 comprises an octapole configuration, i.e., eight coils equally spaced at 45 degree increments around the flight tube assembly 400. Currently, the magnetic circuit is facilitated by an outer ferromagnetic ring 612, 622 for each of the units. Furthermore, each coil preferably has a ferromagnetic core, although in some embodiments the coils have an air core.

[0080] The first steering / shaping unit 610 and the second steering / shaping unit 620 are controlled by the system controller 200 to optically condition the electron beam B and control the position of the beam's impingement on the target. In particular, these two units enable the system controller 200 to correct high-order aberrations (i.e., astigmatism) in the cross-sectional profile of the electron beam B.

[0081] A magnetic focusing lens 700 surrounds the beam steering and shaping system 600. More specifically, a focusing coil 710 overlaps at least a portion of the first steering / shaping unit 610 and all of the second steering / shaping unit 620 in the distal direction.

[0082] Focus coil 710 is wound around focus yoke 720. Generally, focus yoke 720 has a generally "U" shaped profile that is circularly symmetric about an axis defined by electron beam B.

[0083] More specifically, the yoke outer body 722 also functions as a heat sink. The outer periphery of the yoke outer body is penetrated by an annular yoke cooling duct 725. A coolant, such as water, is circulated through the cooling duct 725 to remove heat.

[0084] The yoke bridge 724 is an annular portion that extends inward from the yoke outer body 722 at the proximal end of the magnetic focusing lens 700 .

[0085] A yoke inner body 726 originates at the inner end of the yoke bridge 724 and protrudes distally. It originates on the beam steering and shaping system 600 and then converges toward the outside of the flight tube assembly 400 as it moves distally toward the target assembly 500. In this manner, the yoke bridge 724 guides the magnetic field generated by the large focus coil 710 and directs that field into beam B near the target assembly 500. A pole tip 728 of the focus yoke 720 represents the end of the yoke inner body 726 that terminates just proximal to the target assembly 500.

[0086] Yoke cap 730 closes the magnetic circuit. Yoke cap 730 is annular in shape, wrapping concentrically around target assembly 500. At its outer edge, the cap mates with the distal end of yoke outer body 722 and projects inward from there toward target assembly 500 and the distal end of pole tip 728. A gap 732 is present between the distal end of pole tip 728 and the inner end of yoke cap 730. Gap 732 ensures that magnetic flux through the yoke and cap leaks into the path of electron beam B, where it strikes the target and focuses the beam.

[0087] FIG. 7 is a schematic diagram illustrating the beam steering driver 224 and a beam steering and shaping system 600.

[0088] Control of the first steering / shaping unit 610 and the second steering / shaping unit 620 is provided by the digital processor 210 of the system controller 200. This has individual control of each of the eight coils 1 through 8 in each of the two units 610, 620. More specifically, the beam steering coil driver 224 comprises two banks of eight coil drivers. These coil drivers drive the system controller 200, allowing it to individually control the current in each coil in each of the two units. This level of control allows the beam B to be steered and shaped.

[0089] In the current embodiment, a calibration memory 630 is added to a printed circuit board 632 on which the 16 coils are mounted. The memory 630 is read and programmed from the control board. It stores the mapping between the different drivers and coils and the polarity of those coils.

[0090] While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

1. 1. An X-ray source comprising: a target assembly including a target; an electron source for generating electrons that impact the target; a high voltage generator for accelerating the electrons; a flight tube assembly separating the target assembly from the electron source and delivering a coolant to the target assembly; The flight tube assembly an inner flight tube for providing a vacuum and directing the electrons to the target; an outer flight tube; a baffle concentrically disposed between the inner and outer flight tubes for directing coolant to a distal end of the flight tube assembly.

2. The source of claim 1 , further comprising a flight tube manifold for supplying coolant to the flight tube assembly and receiving return coolant.

3. The source described in claim 1, wherein the target assembly includes a transparent target at a distal end of the target assembly.

4. The target assembly comprises: Flight tube interface ring; a target cartridge tube; 10. The source of claim 1, further comprising an electrical isolation ring between the flight tube interface ring and the target cartridge tube.

5. The source of claim 4 , wherein the target is attached to the mouth of the target cartridge tube.

6. The source of claim 1 , further comprising a spacer between the outer flight tube and the baffle.

7. The source of claim 1 , wherein the flight tube assembly extends through a beam steering system and a magnetic focusing lens.

8. The source of claim 1 , wherein the target is a transmission target.

9. A method for cooling a target of an X-ray source according to claim 1, comprising: flowing the coolant through the flight tube assembly to the proximal end of the target assembly, including the target at the distal end of the target assembly, while applying a vacuum to the proximal end of the target; and flowing the coolant returned from the target assembly through the flight tube assembly.

10. The method of claim 9 , further comprising separating the coolant flowing to the target assembly from the coolant flowing from the target assembly with a baffle.

11. The method of claim 9 further comprising flowing the coolant through a target steering system and a magnetic lens.

12. The method of claim 9 , further comprising electrically isolating the flight tube assembly from a target assembly that holds the target.

13. 1. A target assembly for an x-ray source, comprising: The target, a cartridge tube holding the target; an interface ring for connecting to a flight tube; an insulating ring between the cartridge tube and the interface ring.

14. 14. The target assembly of claim 13, wherein the insulating ring is electrically insulating and thermally conductive.

15. The target assembly of claim 13 , wherein the insulating ring is made of diamond.

16. The target assembly of claim 13 , wherein the flight tube assembly of the flight tube transports a coolant.

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