The present invention relates generally to linear accelerator designs and associated features, systems and methods of use, design and manufacture.
Conventional linear particle accelerators typically have low RF to beam efficiency, with only about 20-30% of the RF power being used for beam acceleration while the rest gets deposited as heat in the copper structure. These conventional designs are either geared towards producing high gradients and involve cavities coupled to the neighbors through the beam tunnel (see for example, ((1) C. Karzmark, C. Nunan, and E. Tanabe, Medical Electron Accelerators. Chap. 3, ISBN: 97 071054102 (McGraw-Hill, Incorporated, Health Professions Division, 1993); (2) T. Wangler, Rf Linear Accelerators. Chap. 4 (Wiley, New York, 2008); (3) T. Maury and C. Wu, Handbook Of Accelerator Physics And Engineering, 3rd printing (World Scientific Publishing Company, Singapore, 1999)) or avoid this coupling issue by using bi-periodic or side-coupled cavities but are unable to handle high gradients (see for example, (4) E. A. Knapp, Linear Accelerator Conference, MURA-714, p. 31. (1964); (5) E. A. Knapp, B. C. Knapp, and J. M. Potter, Standing wave high energy linear accelerator structures, Rev. Sci. Instrum. 39, 979 (1968)). The coupling of adjacent cells in the first case limits the ability to optimize the cavity shapes. Side-coupled and bi-periodic linear accelerators are better suited for cavity optimization, but suffer from RF losses in these coupler cavities. These cavities are considered idle cavities with no energy stored in them, but this is a myth because poynting vector theory dictates that power has to flow through these side cavities to supply power to the accelerating cavities that are separated from the structure's feed waveguide by many of these side-coupled cavities. The basic concept of distributed coupling addresses some of these issues. Distributed coupling allows for feeding of each accelerator cell independently using a periodic feeding RF network. (see for example, 1) U.S. Pat. No. 9,386,682; (2) Sami Tantawi, Mamdouh Nasr, Zenghai Li, Cecile Limborg, and Philipp Borchard, “Design and demonstration of a distributed-coupling linear accelerator structure,” Phys. Rev. Accel. Beams 23, 092001 Pub. 10 Sep. 2020; (3) Peter G. Maxim, Sami G. Tantawi, Billy W. Loo Jr., “PHASER: A platform for clinical translation of FLASH cancer radiotherapy,” Radiotherapy and Oncology, Volume 139, October 2019). However, this approach presents challenges that, thus far, have placed practical limitations on the scale and efficiencies of such accelerators.
Thus, there exits a need for improved LINAC designs having improved efficiency and that are relatively compact. There is further needs for such LINAC designs with streamlined power supply requirements. There is additionally a need for such LINAC designs that can be optimized for a wide range of capabilties in a practical manner in regard to both design and manufacturing.
The present invention pertains to a high-efficiency distributed-coupling linear accelerator, in particular, linear accelerators having multiple cavities with distributed-couplings to a pair of RF waveguide manifolds.
In one aspect, the invention pertains to a linear accelerator comprising a Y-coupler RF waveguide combined with vacuum ports and RF windows.
In some embodiments, the linear accelerator includes a single RF feed that is common to both manifolds. The single RF feed can include a Y-coupler RF waveguide that splits the RF input to each of the pair of manifolds along an intermediate portion thereof, which in turn feeds each cavity through distributed-couplings. In some embodiments, the multiple cavities include a buncher and capture cavity section and an accelerating section with accelerating cavities, and the single RF feed is common to the entire accelerator structure. In some embodiments, each of the cavities of the entire structure are optimized by application of a scattering matrix iteratively applied to the design of each cavity. In some embodiments, the accelerator design is such that the accelerator body includes all cavities, manifolds and junction-couplings and can be formed by a CNC machine. Similarly, the RF power supply assembly can include a Y-coupler that is formed by a CNC machine with minimal parts.
In one aspect, the invention pertains to a linear accelerator comprising: a body defining: a plurality of cavities along a beamline extending between an input and an output; a pair of distribution waveguide manifolds; a sequence of feed arms connecting the manifolds to the plurality of cavities; wherein the distribution waveguide manifolds are connected such that alternating pairs of cell sections are connected to opposite distribution waveguide manifolds; and a single RF power feed common to both of the pair of distribution waveguide manifolds and the plurality of cavities. In some embodiments, the single RF power feed comprises a Y-coupler RF waveguide. In some embodiments, the Y-coupler RF waveguide comprises a main body that splits into two arms that each extends to an RF port of a corresponding manifold of the pair of waveguide manifolds. In some embodiments, the Y-coupler RF waveguide comprise an RF window for a single RF input. Optionally, the Y-coupler is designed so as to be machinable on a CNC machine. In some embodiments, each of the plurality of cavities is optimized. In some embodiments, a design of each of the plurality of cavities is individually optimized by application of a scattering matrix.
In some embodiments, each of the plurality of cavities is individually optimized by adjusting their lengths and shapes for an electron speed at a location of the respective cavity along the beamline. In some embodiments, the plurality of cavities includes one or more cavities defined for a buncher and capture section so that the length is optimized to match the beam bunch arrival time with the RF phase for the varying sub-speed of light beam velocities along these sections. In some embodiments, the buncher and capture section comprises at least one cavity configured for both buncher and capture functions with varying lengths (periods) to accommodate the slowly varying beam speeds as they approach the speed of light.
In some embodiments, the plurality of cavities includes multiple subsequent cavities along the beamline defined as an accelerating section. In some embodiments, the common RF feed powers both the buncher and capture section and the accelerating section. In some embodiments, the sequence of feed arms are defined as a plurality of T-cell junctions. In some embodiments, a geometry of each of the plurality of T-cell junctions coupling the manifolds to the plurality of cells is optimized such that the dimensions thereof differ along a length of the beamline to appropriately supply the power in the correct phase for each cavity. In some embodiments, a geometry of each of the plurality of T-cells coupling the manifolds to the plurality of cells are designed so as to be machinable on a CNC machine. In some embodiments, each waveguide manifold comprise a plurality of irises and Miter bends to allow for equal distribution of power with minimum losses in the forward direction.
In another aspect, the invention pertains to a method of designing a high-efficiency distributed linear accelerator. Such methods can comprise: determining geometries of a linear accelerator body to be defined from a conductive metal block, the body having a plurality of cavities aligned along a central beamline and a pair of manifold waveguides on opposite sides of the central beamline for distribution of power supply to the plurality of cavities from a single common RF feed; and determining geometries of a plurality of waveguide coupling junctions between the pair of manifolds and each of the cavities in order to transmit RF from the pair of manifolds to the plurality of cavities. In some embodiments, a respective cavity is optimized individually by application of a scattering matrix, and this same optimization approach is iteratively to each other cavity of the plurality such that the geometries of each of the plurality of cavities is optimized for an electron speed associated with a given location along the beamline.
In some embodiments, due to the differing geometries of the plurality of cavities, a distance between the plurality of cavities and the respective manifolds coupled thereto differs along the beamline. In such embodiments, the methods can further comprise: determining geometries of the plurality of waveguide coupling junctions so as to include a serpentine portion such that a length of a waveguide channel for each of the plurality of waveguide coupling junctions is consistent along the beamline. In some embodiments, the methods can further comprise: determining a geometry of a Y-coupler RF waveguide for the single RF feed to supply RF power to both manifolds and the plurality of channels of the entire linear accelerator.
In yet another aspect, the invention pertains to a method of forming a high-efficiency distributed linear accelerator. Such methods can comprise: fabricating a linear accelerator body from a conductive metal, such as copper, the body having a plurality of cavities aligned along a central beamline and a pair of manifolds on opposite sides of the central beamline for distribution of power supply to the plurality of cavities, wherein the linear accelerator body is defined by upper and lower halves; fabricating a plurality of waveguide coupling junctions between the pair of manifolds and each of the cavities in order to transmit the supply RF power from a single common RF power feed coupled to the pair of manifolds to the plurality of cavities, wherein each of the cavities is optimized individually by application of a scattering matrix such that the geometries of each of the plurality of cavities is optimized for an electron speed associated with a given location along the beamline; and fabricating a Y-coupler RF waveguide for coupling the single RF power feed to the pair of manifolds.
In some embodiments, one or both of: the plurality of cavities, the pair of manifolds and the plurality of RF waveguide coupling junctions are fabricated by a 3-axis CNC machine in two blocks of conductive metal defining the upper and lower halves; and the Y-coupler RF supply is formed by a 3-axis CNC machine, optionally within three or less parts.
In some embodiments, the linear accelerator is configured such that the connection between the T-junction and the cavity is achieved through a narrow, folded waveguide section with a very specific length that achieves any or all of the following goals: (1) preventing a choke condition on the T-junction when the cavity is tuned to off resonance due to manufacturing error or breakdown events; (2) the length of this folded waveguide supplies a constant phase from the manifold to the cavity (3) use of folding of the waveguide as an added degree of freedom to keep the total length between the manifold and the cavity independent from the location of the exit of the manifold to the entrance of the cavity; these distances between the exit of the manifold and the entrance to the cavities change because of the changing periods of the cavities and the periods of the T-junctions.
In some embodiments, the first cavity of the linear accelerator can be fed independently at much lower power and with varying phase to bunch the initial beam and therefore affect the captured electrons from a DC gun. This is done by a separate waveguide coupling that goes on top of the linear accelerator with a separate feed. In some embodiments, feeding of the first cavity can be done with a tap-off from one of the manifolds so that it also externally couples the power to the top of the linear accelerator and the connection between that tap-off and the cavity can be done through a variable phase shifter and variable attenuator to adjust the power to the first cavity. In some embodiments, power can also be supplied to the first cavity from an individual phase-locked amplifier or oscillator. In some embodiments, the power can also be supplied to the first cavity by tapping off from the main power to the linear accelerator.
In some embodiments, not only can the first cavity can be powered independently as mentioned previously, but this can also be done for a set of the initial cavities such that the capture is not affected by the main power supply to the linear accelerator. In some embodiments, the topology of such a linear accelerator as described would allow varying the main power to the linear accelerator to result in a variable energy linear accelerator. In some embodiments, the initial set of cavities can be powered by their own Y-coupler and two manifolds as a sub-linear-accelerator section of the main linear accelerator thus allowing also for a variable current by changing the capture through a variation of power and phase between the first cavity and the remaining initial set of cavities in addition to the variability of the energy in the embodiments described previously. In some embodiments, the current to the main linear accelerator can be typically modified using only the buncher cavity for a varying-dose linear accelerator by changing the phase and the amplitude of the first cavity.
It is appreciated the innovations described herein make it possible to produce a distributed-linear accelerator on a large scale with high yield with advantages in terms of compactness, weight and RF power needed, and further allows for ease in both design and manufacture.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The present invention pertains to high-efficiency distributed-coupling linear accelerators, in particular, linear accelerators having multiple cavities with distributed-couplings to a pair of manifolds. This topology of distributed coupling allows for the coupling between cells to have a wide range of acceptable values, including being negligibly coupled. The accelerator cells can then be individually optimized to either achieve the highest shunt impedance and/or provide high field gradients by modifying the field along the cell walls (see for example, (1) U.S. Pat. No. 9,386,682; (2) Sami Tantawi, Mamdouh Nasr, Zenghai Li, Cecile Limborg, and Philipp Borchard, “Design and demonstration of a distributed-coupling linear accelerator structure,” Phys. Rev. Accel. Beams 23, 092001 Pub. 10 Sep. 2020; (3) Peter G. Maxim, Sami G. Tantawi, Billy W. Loo Jr., “PHASER: A platform for clinical translation of FLASH cancer radiotherapy,” Radiotherapy and Oncology, Volume 139, October 2019). Thus far, the design adopted for distributed coupling linear accelerators uses a π-phase shift between adjacent cavities to achieve maximum isolation between cavities. This allows for the RF to be fed through two sets of manifolds. The manifold, in the present case, can be further simplified by supplying the power from the manifold to one cavity every λg/2 resulting in a two-cavity alternating tap-off design as shown. The existing designs for conventional linear accelerators has a dedicated buncher section with its own RF feed and a speed-of-light (SOL) section with identical cavities and the manifold for supplying RF power to these cavities. In the present case, the buncher and accelerating cavities can be unified into one compact and efficient structure with one common RF feed. This approach is greatly desirable for productizing such linear accelerators, especially if it is a standalone device starting from a low voltage gun. The concepts of distributed couplings as proposed however, can raise certain design challenges, which can be address by utilizing various novel layout approaches and design features as discussed in further detail below.
For distributed-coupling linear accelerators, the design of the structure, including the RF distribution manifold and cascaded T-junctions is such that the power in the feeding lines has minimal influence on the cavity cells. This is achieved when the RF wave reaches minimal standing-wave ratio (SWR) within the feeding lines to the cavities. Also, the cavities are independent of each other in this it-mode operation. This results in the two nearly isolated systems: the RF feeding networks and the individual cavities, allowing for their independent optimization. Advantageously, a cell-by-cell optimization can be performed to extend this topology to the bunching and capture section, during which the speed of the electrons changes very rapidly. In one aspect, the choice of the cavity dimensions and their locations along the beam path as well as the manifold and serpentine feeder line dimensions are optimized for maximum RF to beam efficiency. In some embodiments, this approach allows the accelerator to best captured without focusing magnets and minimum sensitivity to dimensional tolerances. In some embodiments, the beam tunnel, especially in the initial section, is optimized and modified for maximizing the capture of the electrons.
In one aspect, the linear accelerator design herein has modified existing distributed coupling-based linac topology and designing methodology. This modified approach allows for optimized cell shapes for efficiency (e.g. high shunt impedance), and improved gradient handling capabilities. For a 10 MeV, 300 mA linear accelerator, the design utilized a genetic optimization algorithm that generates the highest possible gradients. In this embodiment, it was configured to produce cavity shapes with shunt impedance of ˜180 MΩ/m. In another aspect, the design is such that it can be easily machined. For example, in assembling the linear accelerator from these cavities, a minimum wall thickness as is practical is maintained between structures, to maintain mechanical integrity during machining, and the reentrant features were designed to obey machining rules for a conventional 3-axis milling machines. In some embodiments, the first buncher cavity was additionally designed and simulated in conjunction with the electron gun with its location optimized for maximum electron capture.
In another aspect, optimal cavity-type and locations have been selected through an iterative process to maximize the electron capture along the length of the linear accelerator. In particular, it is desirable to maintain the it-mode character of the structure while allowing the cavities fields to be synchronous with the electron bunches despite their fast varying speed. To keep the phase advance between cavities constant, the distance between cavities has to change from small values to larger values as the speed of the electrons increases to reach light speed. Then, the distances between cavities could be maintained at half of the free-space wavelength. As mentioned above, the cavities have been optimized with different lengths to allow this design methodology. Consequently, in such embodiments, the RF feeding network must have a varying distance between the tap-off points to follow the cavity locations. This can be accomplished by varying the dimensions of the manifold feeding network to change the guided wavelength along the manifold, and hence the distance between tap-offs is maintained at half of the guided wavelength while the physical distance is varying to accommodate the cavity locations. Thus, the individual sections between tap-off points are also optimized along the beamline.
The optimal design of the manifold junction requires achieving a minimal standing wave within the manifold waveguide. To this end, each three-port network representing the manifold with a feed point must have a precise scattering matrix representation. This mathematical representation could be achieved by adding features to the junction such that the manifold exerts minimal influence on the cavity. This allows for the cavity coupling to be adjusted separately.
In another aspect, the exemplary system has been designed with a single RF feed for the whole system, including the capture and bunching sections. This is a considerable advancement over conventional designs requiring multiple separate power feeds for different sections. In this embodiment, each RF manifold block connected to each linear accelerator cavity has been individually optimized. The RF manifold block includes a T-cell/junction 11 and waveguide portion 10b extending between junctions, as shown in
In some embodiments, the whole manifold is constructed with this design methodology. Additionally, a Y-coupler can be added at the center of the manifold so that the linear accelerator is fed from a single input waveguide. An electric field model of this design is shown in
where n is the number of cavities feed by a single manifold. This would guarantee attaining a minimal VSWR along the manifold. To achieve this matrix, one can modify the shape of the waveguide around the manifold by use of one or more features. One feature can include a protrusion on the wall of the waveguide opposite to the wall with the feed of the junction. Another feature can include widening of the feed from a narrow portion to a wider portion. It is appreciated that various other features could be realized in a similar manner. This approach can be used to design and scale the linear accelerator to accommodate any desired capability, for example to design linear accelerators from 1 MeV to 1 GeV and beyond.
(−15.5, −12.1)
(−15.5, −13.2)
(−15.5, −13.2)
(−15.5, −13.3)
(−15.6, −13.5)
(−15.5, −13.1)
(−15.5, −13.3)
(−15.5, −13.3)
(−15.5, −7.32)
(−15.6, −57.9)
(−15.6, −13.4)
(−15.6, −13.4)
(−15.6, −13.4)
(−15.6, −13.4)
(−15.6, −13.8)
(−15.6, −13.6)
In yet another aspect, the linear accelerator beam dynamics were simulated using full 3D particle tracking techniques, allowing for simulating non-linear and space charge effects of charged particles dynamics in electromagnetic fields. The electromagnetic fields for several different cavity types were generated as described in the cavity section. An iterative method was used to select the cavity type and the distance between successive cavities. As the beam propagates along the linear accelerator length, particles are lost. The loss rate is higher in the first few cavities and becomes minimal after that. Nonetheless, this causes beam loading variation from cavity to cavity. Accordingly, the external coupling of the cavities needs to be adjusted. The appropriate coupling coefficient for each cavity can be calculated separately. Then, not only the cavity type is varied along the linac, but also the geometry of the coupling aperture is varied from cavity to cavity. This is an extremely tedious process, however, automating this process allows for a sufficiently speedy design cycle to be practical.
In still another aspect, the previous sections have described the process of designing and tuning the cavities and the manifold T-junction for optimal beam propagation. Accordingly, methods have been developed that optimize the joining of the manifold T-junctions to the cavities while maintaining the phase relation and the isolation between the two structures. In some embodiments, the shape of the connecting channel is selected to satisfy a few conditions: (1) it must seamlessly connect with the openings of both the cavity and the T-junction; (2) the feature needs to be easily machinable on conventional CNC machines; and (3) it must have minimum lateral width so that the linac is narrow and hence becomes compact and light weight. In some embodiments, these conditions are met by use of a serpentine structure, such as that shown in
In yet another aspect, the particle acceptance and forward propagation requires the maximum capture of the charged particles at each stage especially in the bunching section. The dimensions of the beam tunnel that allows for the particles to travel forward need to be optimized for minimizing interaction with the wall and maximizing the interaction with the cavity fields. Typically, a single tunnel diameter is used throughout the interaction length of the linear accelerator. It was found that changing the diameter of the tunnel in sections of the linear accelerator, especially in the buncher region, significantly increases the efficiency of the linear accelerator. This feature is another fundamental departure from conventional linear accelerator design.
The methods, systems, and devices discussed above are examples. It is appreciated that each of the above aspects could be incorporated into a linear accelerator design to impart certain advantages described herein in isolation or in combination with any other design feature discussed herein. Various configurations can omit, substitute, or add various procedures or components as appropriate. For instance, in alternative configurations, the methods can be performed in an order different from that described, and/or various stages can be added, omitted, and/or combined. Also, features described with respect to certain configurations can be combined in various other configurations. Different aspects and elements of the configurations can be combined in a similar manner. Also, technology evolves and some of the elements as described are provided as non-limiting examples and thus do not limit the scope of the disclosure or claims.
Specific details are given in the description to provide a thorough understanding of exemplary configurations (including implementations). However, configurations can be practiced without these specific details. For example, well-known circuits, processes, algorithms, structures, and techniques have been shown without unnecessary detail in order to avoid obscuring the configurations. This description provides exemplary configurations that do not limit the scope, applicability, or configurations of the claims. Rather, the preceding description of the configurations will provide those skilled in the art with an enabling description for implementing described techniques.
Also, configurations can be described as a process or method. Although the various steps can be described as a sequential process, some of the operations can be performed in parallel or concurrently. Furthermore, examples of the methods can be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware, or microcode, the program code or code segments to perform the necessary tasks can be stored in a non-transitory computer-readable medium such as a storage medium. Processors can perform any or all steps.
Having described several exemplary configurations, various modifications, alternative constructions, and equivalents can be used without departing from the spirit of the disclosure. The above elements can be components of a larger system, wherein other rules can take precedence over or modify the application of the invention. Accordingly, the above description does not bound the scope of the claims. All patents, patent applications, and other publications cited in this application are incorporated by reference in their entirety for all purposes.
This application is a Non-Provisional of and claims the benefit of priority of U.S. Provisional Application No. 63/344,322 filed May 20, 2022, which is incorporated herein by reference in its entirety.
This invention was made with government support under Grant DE-SC0017771 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
Number | Date | Country | |
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63344322 | May 2022 | US |