Compact crossed-finger drift tube linac based on transverse focusing of radio frequency electric field

Abstract

This invention discloses a compact interdigitated drift tube linear accelerator based on lateral focusing of a radio frequency electric field, belonging to the field of particle accelerator technology. The accelerator includes a resonant cavity, several focusing drift tube assemblies, and an empty drift tube. The resonant cavity has beam inlet and outlet structures arranged along the beam centerline. The focusing drift tube assemblies are arranged along the beam centerline, forming focusing gaps with protruding electrodes between adjacent upstream and downstream drift tubes to generate a radio frequency quadrupole field for lateral beam focusing. The empty drift tube is located between the focusing drift tube assemblies and forms an acceleration gap to provide a longitudinal accelerating electric field. This invention decouples the acceleration gap and focusing gap spatially and arranges them independently, ensuring no interference between them, guaranteeing the acceleration gradient while improving the lateral stability of the beam. By integrating a radio frequency quadrupole focusing structure based on the drift tube end-face electrodes, it abandons traditional built-in permanent magnet or electromagnetic focusing elements, significantly reducing the drift tube outer diameter and cavity volume, increasing the cavity shunt impedance, and reducing radio frequency power loss. The accelerator uses negative synchronous phase acceleration to suppress beam emittance growth. It has a simple structure with no easily damaged parts, low operation and maintenance costs, and the focusing field intensity can be adjusted by electrode size to adapt to various types of particles and different energy range acceleration conditions.

Description

Technical Field

[0001] This invention belongs to the field of particle accelerator technology, specifically relating to a compact interdigitated drift tube linear accelerator based on lateral focusing of a radio frequency electric field. Background Technology

[0002] Drift tube linear accelerators (DTLs) are core devices for accelerating high-current particles in the low-energy range, and are widely used in strategic national fields such as proton therapy, spallation neutron sources, advanced nuclear energy devices, and radionuclide production. Traditional drift tube linear accelerators are mainly divided into two types: Alvarez-type DTLs and H-mode DTLs.

[0003] The Alvarez-type DTL operates in TM010 resonant mode, relying on the acceleration gap to provide the longitudinal electric field, and integrating an electromagnetic quadrupole or permanent magnet quadrupole inside the drift tube to achieve lateral beam focusing. However, the built-in magnet significantly increases the outer diameter of the drift tube, resulting in low effective shunt impedance, high RF loss, and high structural size and manufacturing cost. Furthermore, the magnet assembly is susceptible to high temperature and strong electromagnetic field interference, leading to poor operational stability and maintainability.

[0004] H-mode DTL eliminates the built-in focusing magnet, which can reduce the size of the drift tube to improve the shunt impedance. However, because it uses 0° synchronous phase acceleration, the beam has no stable phase space region in the longitudinal direction, which easily leads to emissivity growth and phase space filamentation, making it difficult to meet the requirements of high beam quality transmission.

[0005] In summary, existing drift tube acceleration structures cannot simultaneously meet the technical requirements of high shunt impedance, compact structure, no built-in magnet, and high beam stability, which restricts the development of low-energy high-current accelerators towards miniaturization, high efficiency, and high reliability.

[0006] The technical solution adopted in this invention is: a compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field, comprising: A resonant cavity is defined with a beam centerline extending along the axial direction, and beam inlet and beam outlet are respectively provided at both ends; Several focusing drift tube assemblies are arranged along the beam centerline. Each focusing drift tube assembly includes an upstream drift tube and a downstream drift tube, and a focusing gap is formed between the upstream drift tube and the downstream drift tube. A focusing electrode is provided in the focusing gap. The focusing electrode is composed of several protruding electrode portions extending axially from the end faces of adjacent drift tubes, and is used to generate a radio frequency quadrupole field in the focusing gap to achieve beam lateral focusing. Several empty drift tubes are distributed along the beam centerline between adjacent focusing drift tube assemblies; an acceleration gap is formed between the empty drift tubes and the adjacent focusing drift tube assemblies to provide a longitudinal accelerating electric field.

[0007] The focusing gap and the acceleration gap are arranged alternately at periodic intervals along the axial direction, so that the acceleration function and the focusing function are decoupled from each other in physical space.

[0008] The periodic interval arrangement consists of an alternating focus drift tube assembly and an empty drift tube, which together form a periodic transmission unit.

[0009] Furthermore, within the same focusing gap, the protruding electrode portion on the upstream drift tube end face and the protruding electrode portion on the downstream drift tube end face are staggered and rotated 90° to each other in the circumferential direction to form a quadrupole field distribution.

[0010] The protruding electrode portion on the end face of each drift tube consists of four centrally symmetrically distributed finger-like structures.

[0011] The ratio of the axial length of the protruding electrode portion to the width of the focusing gap it is located in ranges from 0.5 to 0.8.

[0012] The upstream drift tube is fixed to the inner wall of the resonant cavity by a thin, inclined support rod; the downstream drift tube and the empty drift tube are respectively fixed to the crossbeam inside the resonant cavity by thick support rods.

[0013] No protruding electrode structures are provided within the acceleration gap, so that the radio frequency electric field within the acceleration gap is mainly distributed along the axial direction to provide beam energy gain.

[0014] The resonant cavity operates in TE110 mode with a frequency range of 200MHz to 400MHz.

[0015] The thick support rod has a cooling water channel inside, and the crossbeam includes an upper crossbeam and a lower crossbeam arranged symmetrically; the upstream drift pipe is supported by two inclined thin support rods that are symmetrically distributed with respect to the vertical direction.

[0016] Compared with the prior art, the present invention has the following advantages: This invention arranges the acceleration gap and the focusing gap independently and periodically along the axial direction, so that the acceleration gap provides only a pure longitudinal accelerating electric field and the focusing gap provides only a transverse radio frequency quadrupole focusing force. The two are spatially decoupled and do not interfere with each other, which significantly improves the transverse envelope stability of the beam while ensuring the energy gain gradient.

[0017] This invention uses interlaced finger-shaped electrodes integrated on the end face of the drift tube to generate radio frequency quadrupole focusing force, completely eliminating the electromagnetic / permanent magnet focusing elements inside the traditional drift tube, greatly reducing the outer diameter of the drift tube, reducing the overall volume of the cavity, and realizing the miniaturization and compactness of the accelerator.

[0018] The invention significantly reduces the outer diameter of the drift tube, optimizes the electromagnetic field distribution inside the resonant cavity, effectively increases the cavity shunt impedance, reduces RF power loss, and improves the energy utilization efficiency of the acceleration structure.

[0019] The acceleration gap of this invention adopts negative synchronous phase acceleration, and the particles have a stable acceleration region in the longitudinal phase space, which can effectively suppress the increase of beam emittance and phase space filamentation, and ensure high beam quality transmission.

[0020] This invention has no complex built-in magnets, coils or other easily damaged parts, simplifies the overall mechanical structure, has strong operational stability, low failure rate, and significantly reduces the cost of using and maintaining the device.

[0021] This invention allows for flexible adjustment of the quadrupole field intensity within the focusing gap by adjusting the ratio of the axial length of the focusing electrode to the width of the focusing gap, thereby expanding the range of beam dynamics parameter adjustment and adapting to the acceleration requirements of different particles and different energy ranges. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of the present invention; Figure 2 This is a partially enlarged structural schematic diagram of the focusing drift tube assembly and focusing electrode of the present invention; Figure 3 This is an exploded view of the overall structure of the present invention.

[0023] Figure labeling: 1-Resonant cavity; 11-Beam inlet aperture; 12-Beam outlet aperture; 2-Crossbeam; 21-Upper crossbeam; 22-Lower crossbeam; 3-Focusing drift tube assembly; 31-Upstream drift tube; 32-Downstream drift tube; 33-Focusing electrode; 4-Drift tube support rod assembly; 41-Inclined thin drift tube support rod; 42-Coarse drift tube support rod; 5-Empty drift tube. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings. These specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the protection scope of the present invention.

[0025] This implementation method combines Figure 1-3 This paper fully discloses a compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field. The structural composition, dimensional parameters, assembly relationship, working mode and working process are clearly and completely disclosed. Those skilled in the art can implement and reproduce the technical effect according to this content.

[0026] like Figure 1-3 As shown, the overall structure of a compact interdigitated drift tube linear accelerator based on lateral focusing of a radio frequency electric field is as follows: Figure 1 As shown, the accelerator extends along the axial direction and is symmetrical about the beam centerline. It mainly includes: a resonant cavity 1, a crossbeam 2, a focusing drift tube assembly 3, a drift tube support assembly 4, and an empty drift tube 5.

[0027] The resonant cavity 1 in this invention is made of high-conductivity oxygen-free copper. The cavity has a cylindrical structure and operates in TE110 mode with a frequency range of 200MHz to 400MHz. The resonant cavity 1 defines a beam centerline along the axial direction, and has a beam inlet 11 and a beam outlet 12 at both ends for injecting and extracting the particle beam.

[0028] In this invention, the crossbeam 2 is fixed inside the resonant cavity 1 and includes an upper crossbeam 21 and a lower crossbeam 22, which are arranged symmetrically in the upper and lower parts and have a rectangular cross section. It is used to improve the rigidity of the cavity, support the drift tube and optimize the internal electromagnetic field distribution.

[0029] The focusing drift tube assembly 3 in this invention is arranged axially at intervals along the beam centerline. Each assembly includes an upstream drift tube 31 and a downstream drift tube 32, with a focusing gap formed between their opposite end faces.

[0030] In this invention, the empty drift tube 5 is a single cylindrical drift tube, which is arranged axially between two adjacent sets of focusing drift tube assemblies 3, and forms an acceleration gap between it and the adjacent end face of the focusing drift tube assembly 3.

[0031] The drift tube support assembly 4 in this invention includes an inclined thin drift tube support 41 and a thick drift tube support 42, which are used to support and fix the upstream drift tube 31, the downstream drift tube 32 and the empty drift tube 5, respectively.

[0032] The focusing gap and focusing electrode structure in this invention are as follows: Figure 2 As shown, the focusing gap is a lateral focusing functional area, which is set between the upstream drift tube 31 and the downstream drift tube 32 of the same focusing drift tube assembly 3.

[0033] The focusing electrode 33 in this invention is composed of protruding electrode portions integrally extended axially from the opposite end faces of the upstream drift tube 31 and the downstream drift tube 32. Four centrally symmetrically distributed finger-shaped protruding electrode portions are provided within each focusing gap. Within the same focusing gap, the protruding electrode portions on the end faces of the upstream and downstream drift tubes are arranged in a staggered manner, rotated 90° circumferentially, forming a radio frequency quadrupole field under radio frequency electric field excitation, thereby achieving lateral focusing of the beam.

[0034] This invention optimizes the electrode structure by chamfering the ends of the protruding electrode portion to reduce the peak value of the surface electric field and avoid radio frequency breakdown.

[0035] The accelerating gap structure in this invention is a longitudinal accelerating functional region formed between the focusing drift tube assembly 3 and the empty drift tube 5. No protruding electrode structures are placed within the accelerating gap, allowing the radio frequency electric field to be highly concentrated along the axial direction, forming a pure longitudinal accelerating electric field. The particle beam passes through the accelerating gap with a negative synchronization phase (-40° to -20°), creating a stable accelerating region in the longitudinal phase space, which can suppress the increase in longitudinal beam emissivity.

[0036] The periodic arrangement in this invention involves the focusing drift tube assembly 3 and the empty drift tube 5 alternating along the axial direction, forming a cyclical transmission unit: focusing drift tube assembly → acceleration gap → empty drift tube → acceleration gap → focusing drift tube assembly… This achieves physical partitioning and spatial decoupling of the acceleration and focusing functions. The focusing gap is arranged with one gap every other drift tube, improving the lateral envelope stability of the beam while ensuring the beam energy gain gradient.

[0037] The support and cooling structure of this invention includes an upstream drift tube 31, fixed to the inner wall of the resonant cavity 1 by two inclined thin drift tube support rods 41. The two support rods are symmetrically distributed with respect to the vertical direction and are made of oxygen-free copper to reduce radio frequency loss. A downstream drift tube 32 is fixedly connected to the lower crossbeam 22 by a thicker drift tube support rod 42. An empty drift tube 5 is fixedly connected to the upper crossbeam 21 by a thicker drift tube support rod 42. For cooling and connection, a cooling water channel is provided inside the thicker drift tube support rod 42, allowing cooling water to be introduced for heat dissipation under high-power operation. All support rods and drift tubes are connected by brazing to ensure good conductivity and mechanical connection strength.

[0038] Example: The overall parameters for this embodiment are as follows: Resonant cavity material: oxygen-free copper Operating mode: TE110 mode Operating frequency: 324MHz Synchronization phase: -30° Cavity length: 800mm Cavity inner diameter: 160mm The focusing gap and electrode parameters in this embodiment are as follows: Focusing gap width: 10mm Axial length of protruding electrode portion: 7mm Electrode length / gap width ratio: 0.7 (within the optimal range of 0.5 to 0.8) Number of end-face electrodes per drift tube: 4 Interval angle between upper and lower end face electrodes: 90° Chamfer radius at electrode tip: 0.5mm The acceleration gap parameters in this embodiment are as follows: Acceleration gap width: 12mm Within the acceleration gap: there are no protruding electrodes. Electric field distribution: Pure axial acceleration field The drift tube and support parameters in this embodiment are as follows: Drift tube outer diameter: 28mm (more than 40% smaller than traditional magnet-equipped DTLs) Inclined fine drift tube support rod 41, diameter: 3mm, symmetrical tilt angle: ±15° Coarse drift tube support rod 42, outer diameter: 8mm, internal cooling water channel diameter: 4mm Crossbeam cross-section dimensions: 12mm (width) × 8mm (height) The unit arrangement in this embodiment is as follows: 13 periodic transmission units are arranged along the axial direction, including: 13 sets of focusing drift tube assemblies 3, 15 empty drift tubes 5, 13 focusing gaps, and 26 acceleration gaps. The specific assembly steps are as follows: S1. Pre-assembly and positioning of the cavity and crossbeam The resonant cavity 1 is horizontally fixed to the assembly fixture, ensuring that the cavity axis coincides with the fixture reference axis; the upper crossbeam 21 and the lower crossbeam 22 are placed symmetrically along the radial direction of the cavity, positioned and temporarily fixed according to the designed axial spacing, ensuring that the length direction of the crossbeam is parallel to the beam centerline, and that the upper and lower crossbeams are coplanar and symmetrically distributed.

[0039] S2, Assembly of empty drift tube and thick support rod One end of the coarse drift tube support rod 42 is brazed to the empty drift tube 5; the other end of the coarse drift tube support rod 42 is brazed to the upper crossbeam 21 to ensure that the center hole of the empty drift tube 5 is coaxial with the beam center line and the axial spacing of adjacent empty drift tubes 5 is consistent.

[0040] S3, Downstream Drift Tube and Thick Support Rod Assembly One end of the coarse drift tube support rod 42 is brazed and fixed to the downstream drift tube 32; the other end of the coarse drift tube support rod 42 is brazed and fixed to the lower crossbeam 22 to ensure that the center hole of the downstream drift tube 32 is coaxial with the beam center line, and the acceleration gap width between the downstream drift tube 32 and the corresponding empty drift tube 5 is the design value of 12mm.

[0041] S4. Assembly of upstream drift tube and thin support rod One end of each of the two inclined thin drift tube support rods 41 is brazed to the upstream drift tube 31 and fixed. The two support rods are symmetrically inclined at ±15° relative to the vertical direction. The other end of each of the two inclined thin drift tube support rods 41 is brazed to the inner wall of the resonant cavity 1 to ensure that the upstream drift tube 31 and the downstream drift tube 32 are coaxial and that the focusing gap width between them is the design value of 10mm.

[0042] S5, Focusing Electrode Alignment Calibration During assembly, a rotating positioning fixture is used to ensure that the four finger-shaped electrodes on the end face of the upstream drift tube 31 and the four finger-shaped electrodes on the end face of the downstream drift tube 32 are precisely rotated 90° in the circumferential direction and staggered, with the electrode ends facing each other, without interference or eccentricity, forming a standard quadrupole field distribution.

[0043] S6. Overall tightness and sealing test After all brazing is completed, the tooling is removed, and the resonant cavity 1 is tested for airtightness to ensure that there is no air leakage in the cavity; all support rods, drift tubes and crossbeams are tested for mechanical strength to ensure structural stability under vibration and temperature changes.

[0044] S7. Cooling water circuit connection and pressure test Connect all the internal cooling water circuits of the coarse drift tube support rod 42 in series or parallel, connect them to external cooling water connectors, and introduce deionized cooling water for water pressure testing to ensure that the water circuits are leak-free and flow smoothly, meeting the heat dissipation requirements of high-power operation.

[0045] After assembly, the accelerator operates according to the following steps: S1 System Preparation: Turn on the external RF power source and adjust the operating frequency to 324MHz; turn on the cooling system and introduce constant temperature deionized cooling water into the cooling water path of the coarse drift tube support rod 42; align the particle beam injection device with the beam inlet hole 11 of the resonant cavity 1 and set the beam parameters to match the acceleration structure.

[0046] S2 resonant cavity excitation: RF power is input to resonant cavity 1 through the coupling mechanism, and a stable TE110 mode resonant electromagnetic field is excited inside the cavity, establishing corresponding RF electric field distributions in the acceleration gap and focusing gap respectively.

[0047] S3 Beam Injection and Acceleration: The particle beam to be accelerated is injected from the beam inlet 11 along the beam centerline, first entering the acceleration gap; there are no protruding electrodes in the acceleration gap, and the electric field is distributed purely axially, causing the particle beam to accelerate. The 30° negative synchronization phase gains longitudinal energy through the acceleration gap, while maintaining stable constraint in the longitudinal phase space to suppress emission growth.

[0048] S4 Beam Lateral Focusing: After obtaining energy gain, the particle beam enters the focusing gap; within the focusing gap, the finger-shaped electrodes on the end faces of the upstream drift tube 31 and the downstream drift tube 32 form a radio frequency quadrupole field under radio frequency excitation, which applies a lateral focusing force to the particle beam, confining the beam lateral envelope within the range of the drift tube's inner hole, and preventing the beam from diverging and colliding with the tube wall.

[0049] S5 periodic acceleration Focusing Cycle: The particle beam passes sequentially along the axial direction through the acceleration gap (energy boost) → focusing gap (lateral constraint), repeating the working process of the periodic transmission unit; under the action of multiple units, the particle beam energy continues to increase, and the lateral envelope remains stable, achieving high gradient and high-efficiency acceleration.

[0050] S6 Beam Extraction: After acceleration and focusing by all periodic transmission units, the high-energy stable particle beam is successfully extracted from the beam extraction hole 12 at the other end of the resonant cavity 1 and enters the subsequent application system.

[0051] S7 Stable Operation and Protection: During continuous operation, the cooling water circuit continuously removes the heat generated by RF loss, preventing the cavity temperature from becoming too high; due to the absence of built-in magnets, coils, and other easily damaged components, the device can operate stably for a long time without the risk of demagnetization, short circuits, arcing, or other malfunctions.

[0052] This embodiment completely eliminates the built-in magnet, resulting in a compact structure and small size; the outer diameter of the drift tube is significantly reduced, and the effective shunt impedance is increased by more than 30%; negative synchronous phase acceleration ensures longitudinal phase space stability and high beam quality; reliable RF quadrupole focusing eliminates magnetic interference and the risk of high-temperature demagnetization; and the high energy gain gradient makes it suitable for low-energy, high-current proton / heavy-ion acceleration.

Claims

1. A compact interdigital drift tube linear accelerator based on lateral focusing of a radio frequency electric field, characterized in that, include: A resonant cavity is defined with a beam centerline extending along the axial direction, and beam inlet and beam outlet are respectively provided at both ends; Several focusing drift tube assemblies are arranged along the beam centerline. Each focusing drift tube assembly includes an upstream drift tube and a downstream drift tube, and a focusing gap is formed between the upstream drift tube and the downstream drift tube. A focusing electrode is provided within the focusing gap. The focusing electrode is composed of several protruding electrode portions extending axially from the end faces of adjacent drift tubes. It is used to generate a radio frequency quadrupole field within the focusing gap to achieve beam lateral focusing. Several empty drift tubes are distributed along the beam centerline between adjacent focusing drift tube assemblies; an acceleration gap is formed between the empty drift tubes and the adjacent focusing drift tube assemblies to provide a longitudinal accelerating electric field.

2. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 1, characterized in that, The focusing gap and the acceleration gap are arranged alternately at periodic intervals along the axial direction, so that the acceleration function and the focusing function are decoupled from each other in physical space.

3. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 2, characterized in that, The periodic interval arrangement consists of an alternating focus drift tube assembly and an empty drift tube, which together form a periodic transmission unit.

4. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 1, characterized in that, In the same focusing gap, the protruding electrode portion on the end face of the upstream drift tube and the protruding electrode portion on the end face of the downstream drift tube are staggered and rotated 90° in the circumferential direction to form a quadrupole field distribution.

5. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 4, characterized in that, The protruding electrode portion on the end face of each drift tube consists of four centrally symmetrically distributed finger-like structures.

6. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 1, characterized in that, The ratio of the axial length of the protruding electrode portion to the width of the focusing gap it is located in ranges from 0.5 to 0.

8.

7. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 1, characterized in that, The upstream drift tube is fixed to the inner wall of the resonant cavity by a thin, inclined support rod; the downstream drift tube and the empty drift tube are respectively fixed to the crossbeam inside the resonant cavity by thick support rods.

8. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 1, characterized in that, No protruding electrode structures are provided within the acceleration gap, so that the radio frequency electric field within the acceleration gap is mainly distributed along the axial direction to provide beam energy gain.

9. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 1, characterized in that, The resonant cavity operates in TE110 mode with a frequency range of 200MHz to 400MHz.

10. The compact interdigitated drift tube linear accelerator based on lateral focusing of radio frequency electric field according to claim 7, characterized in that, The thick support rod has a cooling water channel inside, and the crossbeam includes an upper crossbeam and a lower crossbeam arranged symmetrically; the upstream drift pipe is supported by two inclined thin support rods that are symmetrically distributed with respect to the vertical direction.

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