Segmented core undulator magnet

Abstract

An undulator magnet for use with high energy electron beams is made from a series of segments stacked together, aligned on pole faces, and clamped together with threaded tensioning rods. Each of the segments has a length equal to exactly one-half an undulator period, and each of the segments is individually and separately machined from a flat steel plate and precision ground on an edge, with a contour machined to form a groove. The magnet is made by stacking the series of segments together; aligning the segments using a jig and precision surface; and clamping the segments together with threaded tensioning rods that pass through holes in the segments.

Description

[0001] SEGMENTED CORE UNDULATOR MAGNET

[0002] FIELD OF THE INVENTION

[0003] The present invention relates generally to undulator magnets used with high energy electron beams.

[0004] BACKGROUND OF THE INVENTION

[0005] Undulator magnet cores are used in superconducting undulators for synchrotron radiation sources and X-ray Free Electron Lasers (FELs). An undulator magnet is composed of a series of alternating dipole magnets 100 and 102, as shown schematically in Fig. 1, which act upon an electron beam 104 to undulate its trajectory and produce synchrotron radiation and x-rays 106. The undulator may have 100 or more magnetic periods, where one period length Au is defined as the length of one pair of dipoles of opposite polarity. In an X-ray FEL the period length Au of the undulator is around 2 cm, so that a typical 3 m long undulator magnet core will have 300 magnet pole pairs.

[0006] Fig. 2 shows an example of a planar superconducting undulator magnet comprising two monolithic iron cores 200 and 202 with a small gap in between where the electron beam passes. Pipes 204 and 206 for liquid helium cools the center of each core. Each core has a series of superconducting pancake coils. The two halves 200 and 202 are typically separated by a gap of several mm in height. To function effectively in a synchrotron or FEL the gap height needs to remain uniform to within 10-20 pm over the entire length of the magnet. This in turn requires that the machined magnet pole face surface must be flat within about 10 pm over the entire length of the magnet.

[0007] Fig. 3 shows a monolithic solid core undulator machined from a single billet of iron. The magnet dipoles 300, 302 are energized by current-carrying coils wound in grooves 304 on either side of the dipoles. The winding and current are reversed every one-half period, so the magnetic field in the gap alternates sinusoidally every period. The dimensions of these grooves must also be controlled with an accuracy of around 10-20 gm so that the conductor can be accurately positioned and layered in each groove so that the magnetic field has the same strength from one magnet pole to the next. This presents a challenge in the machining of present-day monolithic solid core undulators of the type shown in Fig. 3.

[0008] It is beneficial to best utilize tunnel length to make the undulator magnets up to 3 m long, which is the optimum spacing for quadrupole focusing magnets which occupy the inter- undulator gaps. However, present fabrication designs face great difficulty and expense in achieving the necessary mechanical tolerances for undulator magnets longer than about 1 m. A monolithic core with integrated poles is challenging to machine to the required 10 pm tolerances. Machining is limited to ~1.5 m long poles due to limits in the bed size of computer numerical control (CNC) machine, and tool wear machining the grooves in one setup. Thus, there exists a critical problem in the fabrication of long undulator magnet cores in attaining the required mechanical and alignment tolerances.

[0009] The machining process must achieve the required 20 pm mechanical tolerances on the period length between poles and ensure that the coil grooves, or pockets are uniform and smooth enough to contain the fragile superconducting wire. Commercially available CNC machine are limited to fabricating 1 - 1.5 m long cores, which is less than half the length required for an X-ray FEL.

[0010] The fabrication of an undulator core of the type shown in Fig. 3 is done on a CNC milling machine. There are two difficulties faced in this process once the core becomes longer than about 1 m. The first is that the bed of the CNC machine must be large enough to accommodate the full length of the core. Larger CNC machines are more expensive to procure and operate and may not even be commercially available at the desirable length of 3 m for an undulator core. The second difficulty is that ideally the grooves should all be machined on a single setup of the CNC so that all the grooves have the same reference. The problem is that for cores longer than about 1 m the cutting head becomes worn and must be changed out to avoid the grooves becoming rough and out of tolerance. Swapping out the cutting head is both difficult and expensive to restore the identical machining reference plane.

[0011] SUMMARY OF THE INVENTION

[0012] According to embodiments of the present invention, the above limitations are overcome by machining the core in short, half-period long segments that are stacked, aligned, and clamped together to make magnet cores that can be arbitrarily long and still reach the desired mechanical and alignment tolerances. The machining of the short segments is greatly simplified compared to that of the older, solid cores and can be done with greater precision and significantly lower cost.

[0013] Herein is disclosed an undulator magnet core which can be fabricated longer than 1 m with high precision required for desired applications. Also disclosed are methods for fabricating with high precision an undulator magnet core longer than 1 m.

[0014] An undulator magnet core according to embodiments of the invention is composed of short sections joined together, which overcomes the difficulty in machining long, monolithic cores. The optimum length of the segment is equal to half the magnet period length of the undulator core. Each magnet half period contains one magnet pole. Choosing this length for the segment means that instead of machining a deep, precision groove into the core we can machine a simpler, precision contour around the edges of a flat, parallel steel plate. When these parallel plate, machined segments are stacked together and suitably aligned they form an assembled structure with the same magnetic properties as the monolithic core. A key difference is that there is no limit on the number of segments that can be stacked together so that the final undulator magnet can be made to any length, which is not the case for the monolithic core.

[0015] All the ancillary features of the magnet design such as the cooling channels for liquid helium, holes for tensioning rods, and attachments for coil spacers and coil turnaround posts can all be machined into each segment with the necessary precision.

[0016] In one aspect, the invention provides an undulator magnet for use with high energy electron beams, the undulator magnet comprising a series of segments stacked together, aligned on pole faces, and clamped together with threaded tensioning rods, wherein each of the segments has a length equal to exactly one-half an undulator period, wherein each segment is individually and separately machined from a flat steel plate and precision ground on an edge. Preferably, each of the segments has a contour to form a groove in the undulator magnet. Preferably, each of the segments has holes adapted for the threaded tensioning rods.

[0017] In another aspect, the invention provides a method for fabricating an undulator magnet for use with high energy electron beams, the method comprising: individually and separately machining a series of segments from flat steel plates, wherein each of the segments has a thickness equal to exactly one-half an undulator period; stacking the series of segments together; aligning the segments using a jig and precision surface; and clamping the segments together with threaded tensioning rods. Preferably, individually and separately machining the series of segments comprises machining a contour to form a groove in the undulator magnet.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. 1 is a schematic diagram of a conventional undulator formed by a series of alternating magnetic poles which create an undulation in the electron beam trajectory causing it to emit radiation. In an X-ray FEL the period length,u, of the undulator is around 2 cm so that a typical 3 m long undulator magnet core will contain 300 magnet pole pairs.

[0020] Fig. 2 is a perspective view of a planar superconducting undulator magnet formed using two iron cores with a small gap between them, where the electron beam passes. A pipe for liquid helium cools the center of each core. Each core has a series of superconducting pancake coils.

[0021] Fig. 3 is a perspective view of a monolithic magnet core for a conventional superconducting undulator magnet, which is typically machined from a single billet of magnet iron. Precision grooves must be machined into the iron billet to form the pockets in which the superconducting coils are wound.

[0022] Figs. 4A, 4B, 4C are a front view and two side views, respectively, of a segmented core design for the superconducting undulator, according to an embodiment of the invention, in which the conventional solid iron billet is replaced with a stack of precision machined plates, each a half period long, and which are aligned on a flat surface and clamped with long threaded rods to form an undulator magnet core without any restriction on length.

[0023] Figs. 5A, 5B, 5C, 5D are front, side, back, and bottom views, respectively, of an individual segment making up the magnet core. Each segment is precision machined with reference to the reference surface of the magnet pole piece. Additional features, such as the bolt holes, and cooling channels, are machined in the same setup.

[0024] Fig. 6 is a perspective view of a magnetic core showing how individual segments are stacked, aligned, clamped in a fixture. Threaded rods are inserted and correctly tensioned to lock the segments in place.

[0025] Fig. 7 is a perspective view of the magnet core after it has been removed from the assembly fixture and taken to a Coordinate Measuring Machine for verification that the alignment of the magnet pole pieces meets the required tolerances.

[0026] Fig. 8 is a view of the segments with coil spacers and coil turnaround posts. After the segments have been assembled and clamped coil spacers made of nonferrous insulating material are attached to the top of each segment to accurately position the superconducting wire during the coil winding process and to prevent the coil from moving during operation. A dowel pin is inserted at the top of each segment to act as a turnaround post so that the superconducting coil can be wound in the opposite direction to its neighbor.

[0027] DETAILED DESCRIPTION

[0028] According to embodiments of the present invention, instead of machining grooves into a single monolithic solid core, as is the traditional fabrication method, an undulator magnet core is assembled from individually machined segments that are exactly one-half undulator period in length. This segmented core design replaces the monolithic magnet core with a series of segments, each one half a magnet period long. When clamped together, these segments form the undulator magnet core. Figs. 4A, 4B show front and side views, respectively, of single half-period magnetic core segment, according to an embodiment of the invention. The machining of the short segments is greatly simplified compared to that of the older, solid cores and can be done with greater precision and significantly lower cost. As shown in the top view of Fig. 4A, the segment has a rectangular shape with rounded corners. A central hole 404 forms a segment of a pipe for liquid helium to flow through the center of the core. Four holes 406 are provided for tensioning rods to pass through the segments. As shown in Fig. 4B, the segments is machined with a larger portion 400 and slightly narrower contour portion 402 which forms the gap for wires when the core is assembled. Fig. 4C illustrates how multiple segments are stacked, aligned, and clamped to form a single long magnetic core that can be arbitrarily long and still reach the desired mechanical and alignment tolerances. The segments are stacked together, aligned on their pole faces, and then clamped together with long threaded rods 410, 412. The clamping of the plates is done on a precision surface 414 such as a granite bench or a precision tool v-block which ensures that magnet pole faces are all aligned and flat with respect to the precision surface. Each segment plate has 4 bolt holes 406 through which long threaded rods 410, 412 are passed through to clamp the structure.

[0029] Each of the segments is individually CNC-machined from a section of steel plate rather than from a solid bar as in the old method of fabrication. The CNC machining process for the plate is much simpler and can be done on a smaller machine that only needs the bottom pole plate surface as the precision reference plane. A contour 402 is machined on the plate rather than a groove so the machine tooling issues are simplified since there are no interior corners that are degraded when the tool bit becomes worn. The groove into which the coil is wound is formed when adjacent plate segments are bolted together.

[0030] Front, side, back, and bottom views of one half period dipole segment are shown in Figs. 5A, 5B, 5C, 5D, respectively. The drawing shows a central borehole 500 in the segment through which a liquid helium cooling pipe can be passed to operate the undulator at superconducting temperatures. Four holes 502a, 502b, 502c, 502d are provided for tensioning rods to pass through. A reference surface portion 504 of the segment extends slightly from the smaller portion 506 to create a gap for guiding wires. The individual magnet segments are preferably fabricated from inexpensive plates of magnet iron. The contour cut is performed on a commercially available precision CNC machine. In one example, the segment is machined from a flat plate, approximately 10 cm x 5 cm x 1 cm. The central hole is approximately 2.5 cm in diameter and the four tensioning rod holes are approximately 1 cm in diameter. Fig. 6 illustrates precision assembly of a magnet core 600 using a jig fixture to achieve alignment and flatness specification. The jig fixture includes a precision surface 602, a side alignment bar 604 with corresponding bolt 606, and a top alignment bar 608 with corresponding bolt 610. The segments are stacked and clamped together in the fixture, the threaded rods are inserted, and the segments are bolted together. A key advantage of the segmented design is that the overall length of the undulator core can be made arbitrarily long to suit the needs of the synchrotron radiation source or FEL and is not limited by the machining process employed in the solid core design.

[0031] Fig. 7 shows a final, clamped magnetic core. The structure is taken to a coordinate measuring machine (CMM) to measure the final flatness and straightness of the assembled magnet poles.

[0032] As shown in Fig. 8, the assembled magnet core has flat machined segments 800 with nonferrous coil spacers 802 attached to each segment 800. The coil spacer serves to constrain the coil windings at the top and sides of the segment against the magnetic forces when operating. Nonferrous, insulating material such as G10 is used for the coil spacer to not perturb the magnetic field and to minimize the risk of shorting the superconducting wire in the coil. The coil spacers are attached to the iron segment with pins, screws, or glue. Also shown are coil turnaround posts 804.

[0033] Fabrication and Assembly of a Segmented Core Undulator Magnet

[0034] In the following we outline a method for constructing an undulator magnet core from precision-machined segments, according to an embodiment of the invention. This method overcomes critical problems in the fabrication of long monolithic undulator magnet cores by attaining required mechanical and alignment tolerances previously difficult or expensive to achieve with monolithic designs. This method allows for the creation of undulator magnets that can be arbitrarily long (e.g., up to 3 meters), as opposed to the previous practical limit of about 1 meter due to machining difficulties. I. Fabrication of Individual Core Segments

[0035] The segmented core undulator magnet is built from a series of individual segments, each precisely manufactured to specific dimensions and features.

[0036] 1. Material Selection: Begin with a flat steel plate, such as 1006 Steel. This contrasts with the conventional methods which use a long solid bar to be machined into a single monolithic core.

[0037] 2. Segment Dimensions: Each segment is fabricated to be exactly one-half undulator period in length. For a prototype, segments are approximately 10x5x1 cm, with a nominal thickness of 10.5 mm and a tight tolerance of +.00 mm / -.02 mm. The tolerance specification is designed to allow post-assembly correction of cumulative length errors using fine, precision shims.

[0038] 3. Precision Machining:

[0039] ° CNC Machining: Each segment is individually and separately CNC machined from the flat steel plate. This process is significantly simpler and allows for greater precision and lower cost compared to machining solid cores.

[0040] ° Utilizing Smaller Machines: Compared with conventional methods, the machining of the individual segments can be performed on smaller, less expensive CNC machines that only require the bottom pole plate surface as the precision reference plane. This eliminates the need for large, costly, and potentially commercially unavailable CNC machines required for long monolithic cores.

[0041] ° Contour Machining: Instead of machining grooves into a solid core, a contour is machined on the plate. This is an advantageous feature, as it simplifies machine tooling issues by eliminating interior corners that degrade when the tool bit becomes worn. The groove for the coil is formed when adjacent plate segments are bolted together, rather than being pre-machined into a solid piece.

[0042] ° Bolt Holes: Machine four bolt holes through each segment plate. These holes will accommodate long threaded rods used for clamping the assembled structure.

[0043] ° Central Borehole: For superconducting applications, a central borehole is machined through the segment. This borehole allows for the passage of a liquid helium cooling pipe, enabling the undulator to operate at superconducting temperatures. ° Integration of Features: Incorporate features such as coil turn-around posts and GIO coil spacers as specified in the design drawings. These are crucial for the subsequent coil winding process. These features are used as part of the coil winding process for superconducting magnets. The entire length of the superconducting undulator magnet is wound with one continuous thread of the superconducting wire, without splices or joins, since these would be points of potential failure. A coil winding machine is used to wind the coil in layers starting at the bottom of the groove, winding the turns neatly side-by-side, and then layer upon layer. When the last turn of the top layer is reached for one coil it is necessary to change the direction of winding in the adjacent groove by making a half-turn around a smooth post placed near the center of the segment on the top, opposite side of the magnet poles. The superconducting wire is then positioned in the bottom of this adjacent groove and winding commences in the desired opposite direction for this coil. The process is repeated for each coil using these coil turn-around posts.

[0044] Since the coils are wound in several layers the coils are constrained at the sides to stop them moving under magnetic forces. On the side closest to the electron beam the coils are constrained by the protruding pole pieces. However, on the side opposite the electron beam we use non-ferrous, insulating coil spacers to perform this function so that they do not perturb the magnetic field. We use GIO for this purpose since it can be readily machined and retains its properties at cryogenic operating temperatures. Other materials with similar properties could also be used.

[0045] II. Assembly of the Segmented Core

[0046] Once individual segments are fabricated, they are precisely assembled to form the complete undulator core.

[0047] 1. Stacking: The machined segments are stacked together. For a 0.5-meter prototype, this involved stacking 48 segments, resulting in a nominal length of 504 mm.

[0048] 2. Precision Alignment: The segments are aligned on their pole faces. This critical step is performed on a precision surface, such as a granite bench or a precision tool v-block. This ensures that all magnet pole faces are aligned and remain flat with respect to the precision surface within stringent tolerances (e.g., aiming for sub-10 pm RMS flatness). 3. Clamping: The aligned segments are then clamped together using long threaded rods that pass through the previously machined bolt holes. Titanium rods are preferred to provide the necessary compressive force, maintaining the structural integrity of the assembly when cooled to cryogenic operating temperatures.

[0049] 4. Coil Pocket Formation: As the segments are clamped, the grooves into which the coils will be wound are effectively formed by the precise contours machined into the adjacent segments.

[0050] 5. Coil Pocket Insulation: Prior to coil winding, the coil pockets are lined with Kapton insulation to ensure electrical isolation.

[0051] 6. Integration of Cooling System: For superconducting operation, the liquid helium cooling pipe is passed through the central boreholes of the assembled segments.

[0052] III. Post- Assembly and Testing Considerations

[0053] • Mechanical Integrity: The assembled core's mechanical tolerances, including parallelism of the stack ends, are verified to ensure they meet the stringent requirements for undulator performance (e.g., 0.0104 mm parallelism for a 0.5 m length).

[0054] • Cryogenic Performance: Before winding the assembled magnet core with superconducting wire we cool the assembly in a liquid nitrogen bath, wait until thermal equilibrium is reached and then warm it up again to room temperature. Mechanical tolerances of the assembled core are checked again on a precision Coordinate Measuring Machine (CMM) to confirm that no misalignments took place as result of thermal contraction during cool down.

[0055] • Winding the superconducting coils: The superconducting wire is wound in the coil pockets, or grooves, using a conventional coil winding machine at room temperature. This design of magnet core can be used with any type of superconducting wire including Nb:Ti, Nb:Sn, and newer "room temperature" superconductors that are wound as tape rather than wire.

[0056] Applications of the present invention include the commercial development of superconducting undulator magnets for synchrotron radiation sources and x-ray FELs. Research facilities such as LCLS at the SLAC National Accelerator Laboratory have future plans to build out their FEL beamlines which could utilize hundreds of these magnets at one laboratory. More research laboratories around the world are following suit. An even larger commercial application awaits in the development of VUV lithography machines for next-generation chip manufacture which will utilize the same undulator technology in use at research laboratories.

Claims

CLAIMS1. An undulator magnet for use with high energy electron beams, the undulator magnet comprising a series of segments stacked together, aligned on pole faces, and clamped together with threaded tensioning rods, wherein each of the segments has a length equal to exactly one-half an undulator period, wherein each segment is individually and separately machined from a flat steel plate and precision ground on an edge.

2. The undulator magnet of claim 1 wherein each of the segments has a contour to form a groove in the undulator magnet.

3. The undulator magnet of claim 1 wherein each of the segments has holes adapted for the threaded tensioning rods.

4. A method for fabricating an undulator magnet for use with high energy electron beams, the method comprising: individually and separately machining a series of segments from flat steel plates, wherein each of the segments has a thickness equal to exactly one-half an undulator period; stacking the series of segments together; aligning the segments using a jig and precision surface; and clamping the segments together with threaded tensioning rods.

5. The method for fabricating an undulator magnet of claim 4 wherein individually and separately machining the series of segments comprises machining a contour to form a groove in the undulator magnet.

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