Design,fabrication,and cold test of an S-band high-gradient accelerating structure for compact proton therapy facility

An S-band high-gradient accelerating structure is designed for a proton therapy linear accelerator(linac)to accommodate the new development of compact,singleroom facilities and ultra-high dose rate(FLASH)radiotherapy.To optimize the design,an efficient optimization scheme is applied to improve the simulation efficiency.An S-band accelerating structure with 2856 MHz is designed with a low beta of 0.38,which is a difficult structure to achieve for a linac accelerating proton particles from 70 to 250 MeV,as a high gradient up to 50 MV/m is required.A special design involving a dual-feed coupler eliminates the dipole field effect.This paper presents all the details pertaining to the design,fabrication,and cold test results of the S-band high-gradient accelerating structure.

Radio frequency conditioning of an S-band accelerating structure prototype for compact proton therapy facility

The development of a high-gradient accelerating structure is underway to construct a compact proton linear accelerator for cancer treatment.Extensive experiments and numerous studies are being conducted to develop compact linear accelerators for proton therapy.Optimization of the electromagnetic and mechanical design has been performed to simplify the manufacturing process and reduce costs.A novel high-gradient structure with a low relativistic proton velocity(β),v/c=0.38,was designed,fabricated,and tested at high power.The first full-scale prototype was also successfully tested with high radio frequency(RF)power,a repetition rate of 50 Hz,and pulse length of 3μs to reach a high-gradient of 46 MV/m using a 50 MW S-band klystron power supply obtained from the Shanghai Soft X-ray Free Electron Laser Facility.This is the first high-power test in China,which is in line with the expected experimental goal.This study presents preliminary high-power testing of S-band standing wave accelerating structures with 11 cells.This work aims to verify the feasibility of using a high-gradient RF accelerating structure in compact proton therapy facilities.The cold test of the prototype cavity was completed in advance.Details of the high-power RF test setup,the process of RF conditioning,and the high-power results are described.

Fabrication,tuning,and high-gradient testing of an X-band traveling-wave accelerating structure for VIGAS

X-band high-gradient linear accelerators are a challenging and attractive technology for compact electron linear-accelerator facilities.The Very Compact Inverse Compton Scattering Gamma-ray Source(VIGAS)program at Tsinghua University will utilize X-band high-gradient accelerating structures to boost the electron beam from 50 to 350 MeV over a short distance.A constant-impedance traveling-wave structure consisting of 72 cells working in the 2π/3 mode was designed and fabricated for this project.Precise tuning and detailed measurements were successfully applied to the structure.After 180 h of conditioning in the Tsinghua high-power test stand,the structure reached a target gradient of 80 MV/m.The breakdown rate versus gradient of this structure was measured and analyzed.

Design, fabrication, and testing of low-group-velocity S-band traveling-wave accelerating structure

To implement the Tsinghua Thomson Scattering X-ray Source upgrade plan and the Very Compact Inverse Compton Scattering Gamma-ray Source (VIGAS) program, a new 1.5-m traveling-wave accelerating structure was designed to replace the old 3-m SLAC-type structure with the aim of increasing the accelerating gradient from15 to 30 MV/m. The new type of structure works in the 3π/4 mode with a comparatively low group velocity varying from 0.007c to 0.003c to increase the accelerating gradient at a given power. An elliptical iris was employed to reduce the surface field enhancement. The filling process of the low-group-velocity structure was analyzed using a circuit model. After fabrication, the structure was precisely tuned using the non-contact tuning method, followed by detailed low-power radiofrequency measurements. The structure was first installed and utilized at a beamline for the terahertz experiment at Tsinghua University. After 120 h of conditioning, it is now operating at a gradient of 24.2 MV/m and a 20.7-MW input power, with the klystron operating at its full power. It is expected to generate an accelerating gradient of 30 MV/m when the klystron power is upgraded to 30 MW in the near future.

RF-thermal-structural-RF coupled analysis on a travelling wave disk-loaded accelerating structure

The travelling wave （TW） disk-loaded accelerating structure is one of the key components in normal conducting （NC） linear accelerators, and has been studied for many years. In the design process, usually after the dimensions of each cell and the two couplers are finalized, the structure is fabricated and tuned, and then the whole structure RF characteristics are measured by using a vector network analyzer. Before fabrication, the whole structure characteristics （including RF, thermal and structural ones） are less simulated due to the limited capability of currently available computers. In this paper, we described a method for performing RF-thermal-structural-RF coupled analysis on a TW disk-loaded structure using only one PC. In order to validate our method, we first analyzed and compared our RF simulation results on the 3 m long BEPC Ⅱ structure with the corresponding experimental results, which shows very good consistency. Finally, the RF-thermal-structure-RF coupled analysis results on the 1.35 m long NSC KIPT linac accelerating structure are presented.

Accelerating structure design and fabrication for KIPT and PAL XFEL

ANL （Argonne National Laboratory） and the National Science Center ＂Kharkov Institute of Physics Technology＂ （NSC KIPT, Kharkov, Ukraine） jointly propose to design and build a 100 MeV/100 kW linear accelerator which will be used to drive the neutron source subcritical assembly. The linac has ahnost finished assembly in KIPT by a team from the Institute of High Energy Physics （IHEP, Beijing, China）. The design and measurement result of the accelerating system of the linac will be described in this paper.

Design studies on an s-band hybrid accelerating structure

Background In an electron linac,the standard bunching system is generally composed of a standing-wave(SW)pre-buncher(PB),a travelling-wave(TW)buncher and a standard TW accelerating structure.Purpose To keep the beam performance similar to the standard bunching system but reduce the cost as much as possible,the hybrid accelerating structure(HAS)was proposed by integrating the PB,the B and the accelerating structure together.Methods The RF design of the HAS was conducted by 2D code SUPERFISH,which is used to determine the dimensions of the SW and TW cells,and 3D code HFSS,which is used to optimize the input and output RF coupler cells.Results The on-axis electric field distribution simulated by HFSS can fully meet the dynamic requirement.The phase jump in SW section is 180◦while the phase advance in TW section is 120°.Conclusion The RF design of the HAS is completed,and the engineering design of the prototype is being conducted.

Structure and material study of dielectric laser accelerators based on the inverse Cherenkov effect

Dielectric laser accelerators(DLAs)are considered promising candidates for on-chip particle accelerators that can achieve high acceleration gradients.This study explores various combinations of dielectric materials and accelerated structures based on the inverse Cherenkov effect.The designs utilize conventional processing methods and laser parameters currently in use.We optimize the structural model to enhance the gradient of acceleration and the electron energy gain.To achieve higher acceleration gradients and energy gains,the selection of materials and structures should be based on the initial electron energy.Furthermore,we observed that the variation of the acceleration gradient of the material is different at different initial electron energies.These findings suggest that on-chip accelerators are feasible with the help of these structures and materials.

Structure study of a dielectric laser accelerator with discrete translational symmetry

The dielectric laser accelerator(DLA) is a promising technology for achieving high-gradient acceleration in a compact design. Its advantages include ease of cascading and an energy gain per unit distance which can exceed that of conventional accelerators by two orders of magnitude. This paper establishes rules for efficient particle acceleration using dielectric structures based on basic equations, proposes a design principle for DLA structures with clear physical images and verifies the accuracy of the corresponding formula for energy gain. DLA structures with different specifications, materials and geometric shapes are constructed, and the achievable acceleration gradient is calculated. Our results demonstrate that effective acceleration can be achieved when the electric field sensed by particles in the acceleration cavity has zero frequency,which provides a powerful method for designing such devices. Furthermore, we demonstrate that the simplified formula for calculating energy gain presented in this paper can accurately determine the energy gain of particles during the design of acceleration structures using a dielectric accelerator.

Design,fabrication,and testing of an X-band 9-MeV standing-wave electron linear accelerator

In this study,an X-band standing-wave biperiodic linear accelerator was developed for medical radiotherapy that can accel-erate electrons to 9 MeV using a 2.4-MW klystron.The structure works atπ/2 mode and adopts magnetic coupling between cavities,generating the appropriate adjacent mode separation of 10 MHz.The accelerator is less than 600-mm long and constitutes four bunching cells and 29 normal cells.Geometry optimizations,full-scale radiofrequency(RF)simulations,and beam dynamics calculations were performed.The accelerator was fabricated and examined using a low-power RF test.The cold test results showed a good agreement with the simulation and actual measurement results.In the high-power RF test,the output beam current,energy spectrum,capture ratio,and spot size at the accelerator exit were measured.With the input power of 2.4 MW,the pulse current was 100 mA,and the output spot root-mean-square radius was approximately 0.5 mm.The output kinetic energy was 9.04 MeV with the spectral FWHM of 3.5%,demonstrating the good performance of this accelerator.

Robust vision-based displacement measurement and acceleration estimation using RANSAC and Kalman filter

Computer vision(CV)-based techniques have been widely used in the field of structural health monitoring(SHM)owing to ease of installation and cost-effectiveness for displacement measurement.This paper introduces computer vision based method for robust displacement measurement under occlusion by incorporating random sample consensus(RANSAC).The proposed method uses the Kanade-Lucas-Tomasi(KLT)tracker to extract feature points for tracking,and these feature points are filtered through RANSAC to remove points that are noisy or occluded.With the filtered feature points,the proposed method incorporates Kalman filter to estimate acceleration from velocity and displacement extracted by the KLT.For validation,numerical simulation and experimental validation are conducted.In the simulation,performance of the proposed RANSAC filtering was validated to extract correct displacement out of group of displacements that includes dummy displacement with noise or bias.In the experiment,both RANSAC filtering and acceleration measurement were validated by partially occluding the target for tracking attached on the structure.The results demonstrated that the proposed method successfully measures displacement and estimates acceleration as compared to a reference displacement sensor and accelerometer,even under occluded conditions.

Design and preliminary test of the LLRF in C band high-gradient test facility for SXFEL

This paper describes the design and preliminary test of the low-level radio frequency(LLRF)part of the C band high-gradient test facility for the Shanghai Soft X-ray Free-Electron Laser(SXFEL)-Linear Accelerator(LINAC).Before installation,the accelerating structures should be tested and conditioned.During the conditioning process,breakdown detection is needed to protect the accelerating structures and klystron from damage.The PCI extensions for instrumentation-based LLRF system and auto-conditioning algorithm are designed and applied in the LLRF part of the C band high-gradient test facility.Three C band accelerating structures and 1 pulse compressor have completed conditioning and were installed in the SXFEL-LINAC.

Analysis of Numerical Simulation Results of LIPS-200 Lifetime Experiments

Accelerator grid structural and electron backstreaming failures are the most important factors affecting the ion thruster＇s lifetime.During the thruster＇s operation,Charge Exchange Xenon（CEX） ions are generated from collisions between plasma and neutral atoms.Those CEX ions grid＇s barrel and wall frequently,which cause the failures of the grid system.In order to validate whether the 20 cm Lanzhou Ion Propulsion System（LIPS-200） satisfies China＇s communication satellite platform＇s application requirement for North-South Station Keeping（NSSK）,this study analyzed the measured depth of the pit/groove on the accelerator grid＇s wall and aperture diameter＇s variation and estimated the operating lifetime of the ion thruster.Different from the previous method,in this paper,the experimental results after the 5500 h of accumulated operation of the LIPS-200 ion thruster are presented firstly.Then,based on these results,theoretical analysis and numerical calculations were firstly performed to predict the on-orbit lifetime of LIPS-200.The results obtained were more accurate to calculate the reliability and analyze the failure modes of the ion thruster.The results indicated that the predicted lifetime of LIPS-200＇s was about 13218.1 h which could satisfy the required lifetime requirement of 11000 h very well.

Gradient Structured Copper by Rotationally Accelerated Shot Peening

A new technology-rotationally accelerated shot peening（RASP）, was developed to prepare gradient structured materials. By using centrifugal acceleration principle and large steel balls, the RASP technology can produce much higher impact energy compared to conventional shot peening. As a proof-of-concept demonstration, the RASP was utilized to refine the surface layer in pure copper（Cu） with an average grain size of 85 nm. The grain size increases largely from surface downwards the bulk, forming an800 ？m thick gradient-structured surface layer and consequently a micro-hardness gradient. The difference between the RASP technology and other established techniques in preparing gradient structured materials is discussed. The RASP technology exhibits a promoting future for large-scale manufacturing of gradient materials.

Particle Measurement Sensor for in situ determination of phase structure of fluidized bed

Based on three-dimensional （3D） acceleration sensing, an intelligent particle spy capable of detecting, transferring, and storing data, is proposed under the name of Particle Measurement Sensor （PMS）. A prototype 60-mm-dia PMS was tested to track its freefall in terms of velocity and displacement, and served as a particle spy in a fluidized bed delivering the in situ acceleration information it detects. With increasing superficial gas velocity in the fluidized bed, the acceleration felt by PMS was observed to increase. The variance of the signals, which reflect the fluctuation, increased at first, reaching a maximum at the gas velocity （Uc） which marks the transition from bubbling to turbulent fluidization. Through probability density distribution （PDD） analysis, the PDD peak can be divided into the emulsion phase peak and the bubble phase peak. The average acceleration of emulsion and bubble phase increased, while the variance of both phases reached a maximum at Uc, at the same time. However, the difference between the variances of two phases reached the maximum at Uc. Findings of this study indicate that PMS can record independent in situ information. Further, it can provide other in situ measurements when equipped with additional multi-functional sensors.

BEM for wave interaction with structures and low storage accelerated methods for large scale computation

The boundary element method（BEM） is a main method for analyzing the interactions between the waves and the marine structures. As with the BEM, a set of linear equations are generated with a full matrix, the required calculations and storage increase rapidly with the increase of the structure scale. Thus, an accelerated method with a low storage is desirable for the wave interaction with a very large structure. A systematic review is given in this paper for the BEM for solving the problem of the wave interaction with a large scale structure. Various integral equations are derived based on different Green functions, the advantages and disadvantages of different discretization schemes of the integral equations by the constant panels, the higher order elements, and the spline functions are discussed. For the higher order element discretization method, the special concerns are given to the numerical calculations of the single-layer potential, the double layer potential and the solid angle coefficients. For a large scale computation problem such as the wave interaction with a very large structure or a large number of bodies, the BEMs with the FMM and p FFT accelerations are discussed, respectively, including the principles of the FMM and the p FFT, and their implementations in various integral equations with different Green functions. Finally, some potential applications of the acceleration methods for problems with large scale computations in the ocean and coastal engineering are introduced.

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.

Design studies on the ERL-FEL test facility at IHEP, Beijing

A proposed compact ERL test facility at IHEP, Beijing, is presented in this paper, and includes the design parameters, the essential lattice, and the key components features, such as the photocathode DC gun and the CW superconducting accelerating structures. Some important beam physics issues such as the space charge effect, the coherent synchrotron radiation （CSR） effect and the beam break-up （BBU） effect are briefly described with the simulation results.