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Passive magnetic channel is a kind of beam focusing elements in a cyclotron. It consists of several soft iron bars that are magnetized by the main field in a cyclotron. In this paper, we proposed a topology optimization method to design the pole shape in passive magnetic channel, this method does not require any fixed geometry pattern or initial design. The nonlinear static magnetic finite-element analysis model is used to calculate the objective magnetic field function. Persuade iron material with variable density is used to describe the iron distribution during the iteration. Method of Moving Asymptotes (MMA) is used to optimize the control variable of iron density distribution on magnetic channel cross-section. In three numerical examples, magnetic channels for a 250 MeV superconducting cyclotron is provided, where the design goal is to provide the given magnetic field gradient and bending angle. The relationship between the design goal and the pole shape pattern is discussed. It reveals that magnetic channel pattern with only 2 iron bars is possible for some design goals.

Preserving the phase space quality has been of crucial importance to high-brightness electron beam transport. Any driving source of beam performance limitations in such a transport system must be carefully examined in order to preserve the beam phase space quality. It is found that in the low beam energy, high bunch charge regime, space charge effect on the charged particle beam evolution can be complicated. It may be insufficient to analyze the beam properties through the envelope or lower order moments for the mere bulk of the beam. In this work we will study the space-charge-dominated, low-energy, high-brightness electron beam dynamics based on the slice beam matrix method. The slice beam matrix method, extended from the existing bulk beam analysis, is applicable to an arbitrary longitudinal bunch distribution and can thus take the nonlinear RF curvature into account. The semi-analytical sliced beam space charge model is constructed and assumes an axisymmetric beam in the presence of perfect conducting cylindrical pipe with circular cross section. We present in this paper the theoretical formulation, starting from single-particle dynamics, space charge field calculation, followed by the moment description of a collection of particles, based on beam sigma matrix and longitudinal slice decomposition. We prove the equivalence between the beam matrix method and the envelope formalism in the presence of space charge effect, in both the transverse and longitudinal dimensions. Two examples are then demonstrated, one for a unit cell of superconducting radio-frequency (RF) cavity and the other for a 1.5-cell RF gun system. The semi-analytical results from beam matrix calculation are discussed and compared with particle tracking simulation using ASTRA. We expect that the analysis based on the slice beam matrix method shall serve as an efficient and advanced tool to further investigate the low-energy high-brightness electron beam dynamics.

An Ultrafast Electron Diffraction (UED) based on an RF photocathode electron gun has the advantage of producing MeV relativistic probing electron beams, which can maintain a high time resolution of ~100 fs while keeping more electrons to improve the S/R ratio of the image. However, the jitter of driving RF power in the electron gun between pulse to pulse has an indispensable impact on the electron energy stability leading to the Time of Flight (ToF) jitter, which creates asynchronization between the pump laser and the probing electron worsening the time resolution. To stabilize the beam energy to the designed value 3 MeV and reduce the ToF jitter further, we propose controlling the electron energy based on an energy spectrometer directly. An electron spectrometer based on a C-type dipole is being designed to achieve high energy resolution. This paper will introduce the design of the energy spectrometer, and particle tracking is implemented to demonstrate the feasibility of the design.

A cyclotron-based proton therapy facility (PTF) is being developed at Huazhong University of Science and Technology (HUST). An energy degrader driven by a stepper motor is used to regulate the proton beam energy to the range of 70-240 MeV for clinical requirements. However, the interactions between protons and materials in the energy degrader lead to severe ionizing radiation, which will damage the stepper motor nearby, consequently deteriorating the beam quality. Shielding made of composite materials must be used to block the gamma rays and neutron effectively. For a given shield design, the stepper motor’s maximum radiation exposure does not always correspond to the severest beam loss. FLUKA simulation results show that the stepper motor will suffer the maximum radiation dose rate of 5.62 Gy/h at 190 MeV of output beam energy. A composite shielding material of Pb and polyethylene is chosen to shield ionizing radiation. Because of the installation space restriction, the total shielding thickness is 14 cm. When the Pb thickness is 5.5 cm and the polyethylene thickness is 8.5 cm, the motor’s suffered dose rate achieves the minimum, which is only 1/4 times the case without shielding.

Microbunching instability (MBI) has been one of the most challenging issues in designs of high-brightness beam transport lines for single-pass or recirculating accelerators. Although the intrabeam scattering (IBS) has long been studied in lepton or hadron storage rings as a slow diffusion process or in high-intensity proton linear accelerators as one mechanism for the beam halo, the effects of IBS on single-pass or recirculating electron accelerators have drawn attention only in the recent two decades due to emergence of linac-based or energy-recovery-linac-based fourth-generation light sources, which require high-quality electron beams during the beam transport. In this paper we develop a theoretical formulation of microbunching instability in the presence of IBS for single-pass or recirculation accelerators. To quantify MBI with inclusion of IBS, we start from the Vlasov-Fokker-Planck (VFP) equation, combining both collective interactions and incoherent IBS effects. The linearized VFP equation and the corresponding friction and diffusion coefficients are derived. The evolutions of the resultant density and energy modulations are formulated as a set of coupled integral equations. The theoretical formulation is then applied to a recirculating beamline design. The results from the semianalytical calculation are compared and show good agreement with massive particle tracking simulations.

A 250MeV superconducting cyclotron (SCC250) for proton therapy is being constructed at Huazhong university of science and technology (HUST). A well-designed magnetic field and an excellent beam extraction are critical for obtaining the desired beam in the compact cyclotron. However, the high magnetic field brings about great challenges to achieve isochronism and high extraction efficiency. Poor field isochronism will cause severe particle acceleration problems. Besides, the enormous beam loss due to the small turn separation at the extraction region may lead to the damage of the superconducting system. This paper focuses on improving the field isochronism by cutting patches at different radii of the magnetic pole, and increasing the turn separation by adjusting the amplitude and phase of the first harmonic field. The resonances tune and the acceleration process are also optimized to acquire a high-quality beam. Calculations show that the error accuracy AB/B is smaller than 3 × 10-4, which meets the requirement of beam acceleration. And the turn separation can reach 7mm, which reduces the beam loss to an acceptable level.

With the development of electromagnetic metrology, the issue of electric current quantization has been paid more attention but still confronts challenges. By recalling the previous proposal of introducing the theory and technique in electron-accelerators to the current quantization, the design of a prototype for electron-beam tuning is conducted based on both theoretical analysis and multi-particle calculations. Besides, an overall scheme of the equipment is illustrated explicitly, which is based on an existing electron-beam source and an additional tuning tube with adjustable slit combinations. Furthermore, corresponding beam dynamics simulations are performed precisely to verify the feasibility of the beam tuning equipment and demonstrate the influence factors on the stability of the generated current. Although it is an entirely new idea, simulation results show that the designed prototype is sufficient to provide preliminary experimental verification for further theoretical and engineering researches.

Free electron laser Terahertz source (FEL-THz) has significant advantages, such as wide and continuously adjustable spectrums. A compact terahertz source is desired to reduce the cost and space but still provide high-quality terahertz beams. However, the space constraints requires all the magnets should be downscaled as well. In this paper, a combined-function corrector is designed, capable of providing vertical and horizontal correction at the same time. Poisson and Opera-3D have been used to design and optimize the magnet. The vertical correction is mainly performed by shimming and trimming the magnetic pole head, and the horizontal correction is mainly performed by adjusting the current coil. As a result, the good field range of the vertical field and the horizontal field reach the expected value, and the relative error of the integrated field meets the requirements of beam correction.

The solenoid lens is the only transverse focusing device in the ultrafast electron diffraction (UED) system that is being developed at HUST, which -control the beam spot size and compensate the beam transverse emittance. This paper focuses on the parameter optimization of the multi-coil solenoid lens MeV-UED to obtain a desired magnetic field, especially for the shape of the pole shoe, the axial and radial dimensions of the solenoid. Two finite element analysis (FEA) software Opera and Parmela are combined to optimize the electromagnetic field. Results show that after parameter optimization, the transverse beam emittance can be reduced by 60.6%, and the beam spot size is reduced by 70%.

The study of proton therapy equipment has earned more and more attention in China in recent years. A superconducting cyclotron based proton therapy facility is being developed at Huazhong University of Science and Technology (HUST). Since the proton beam is extracted out of the compact, high energy cyclotron by electrostatic deflectors followed by a series of magnetic channels, the fierce interaction between the proton beam and the deflector septum is the main cause of beam loss, leading to unwanted excess radiation and heat. Excess Radiation effect results in activation or damage to superconducting materials and high risk of quench of superconducting magnets , while thermal effect involves temperature rise and deformation of the septum, both of these phenomena put extraordinary demands on high efficiency extraction of the cyclotron. This paper introduces the design of the electrostatic deflector, including the structure optimization and the material selections. Moreover, in order to minimize the damage to the deflector destruction caused by the proton beam loss, numerical simulations of radiation effect and thermal effect have been conducted by applying Monte Carlo Geant4 code and computational fluid dynamics (CFD) on ANSYS respectively. The energy depositions of the proton beam in diverse septum materials are compared. Meanwhile, the energy and angular distributions of secondary particles are obtained. The maximum temperatures and deformations of the septum in various cases of thickness and material are investigated. The results based on these analyses will give us valuable implications for the development of a superconducting cyclotron.

A SCC-250 MeV cyclotron, producing a 250 MeV proton beam, is under development in Huazhong University of Science and Technology (HUST) for proton therapy. The magnetic flux density, as a function of radius, decreases rapidly in the beam extraction region, which increases the radial beam size continuously along the extraction orbit. In this paper, an extraction channel inside the SCC-250 MeV is designed to control the beam size using passive magnetic channels. An equivalent lumped parameter method is used to establish the model of the extraction channel in the complex fringe magnetic field of the main magnet. Then, the extraction channel is designed using the lattice design software MADX. The beam envelopes are verified using particle tracing method. The maximum radial size of 6.8 mm and axial size of 4.3 mm meet the requirements of the extraction from the SCC-250 MeV.

An analytical model of an RF phasing method using arbitrary phase scanning for bunch length measurement is reported. We set up a statistical model instead of a linear chirp approximation to analyze the energy modulation process. It is found that, assuming a short bunch ( σ φ ∕ 2 π → 0 ) and small relative energy spread ( σ γ ∕ γ r → 0 ), the energy spread ( Y = σ γ 2 ) at the exit of the traveling wave linac has a parabolic relationship with the cosine value of the injection phase ( X = cos φ r | z = 0 ), i.e., Y = A X 2 + B X + C . Analogous to quadrupole strength scanning for emittance measurement, this phase scanning method can be used to obtain the bunch length by measuring the energy spread at different injection phases. The injection phases can be randomly chosen, which is significantly different from the commonly used zero-phasing method. Further, the systematic error of the reported method, such as the influence of the space charge effect, is analyzed. This technique will be especially useful at low energies when the beam quality is dramatically degraded and is hard to measure using the zero-phasing method.

A proton therapy facility with two 360° gantry treatment rooms and one fixed beamline is under development in the Huazhong University of Science and Technology, which is based on the isochronous superconducting cyclotron scheme. This paper presents the beam optics and main magnetic elements including dipoles, quadrupoles, and fast scanning magnets of the gantry beamline. In addition, the energy selection system will be introduced, which enables fast energy modulation of proton beam between 70 and 240 MeV.

Purpose/objective(s) Online proton range/dose verification based on measurements of proton-induced positron emitters is a promising strategy for quality assurance in proton therapy. Because of the nonlinear correlation between the dose distribution and the activity distribution of positron emitters in addition to the presence of noise, machine learning approaches were proposed to establish their relationship. Materials/methods Simulations were carried out with a spot-scanning proton system using GATE-8.0 and Geant4-10.3 toolkit with a computed tomography (CT)-based patient phantom. The one-dimensional (1D) distributions of positron emitters and radiation dose were obtained. A feedforward neural network classification model comprising two hidden layers, was developed to estimate whether the range is within a preset threshold. A recurrent neural network (RNN) regression model comprising three layers and ten neurons in each hidden layer was developed to estimate dose distribution. The performance was quantitatively studied in terms of mean squared error (MSE) and mean absolute error (MAE) under different signal-to-noise ratio (SNR) values. Results The feasibility of proton range and dose verification using the proposed neural network framework was demonstrated. The feedforward NN model achieves high classification accuracy close to 100% for individual classes without bias. The RNN model is able to accurately predict the 1D dose distribution for different energies and irradiation positions. When the SNR of the input activity profiles is above 4, the framework is able to predict with an MAE of ~0.60 mm and an MSE of ~0.066. Moreover, the model demonstrates a good capability of generalization. Conclusions The RNN model is found to be effective in identifying the relationship between the distributions of dose and positron emitters. The machine learning-based framework and RNN models may be a useful tool to allow for accurate online range and dose verification based on proton-induced positron emitters.

A thermionic gun usually suffers from the long beam tail problem, and this is also the case in the compact terahertz free-electron laser facility in the Huazhong University of Science and Technology. Here we propose to use a radio frequency (RF) deflector operated in the dipole mode as a beam chopper to cut off the unnecessary beam tail. The RF deflector, followed by a scraper and a scintillation screen, will be installed in the drift space between the gun and the linac. The RF deflector system can be operated in two configurations: (1) a beam chopper, with the scraper pushed in, to prevent the tail particles from entering the linac; (2) a bunch length diagnostic instrument, with the scintillation screen pushed in, to measure the longitudinal profile from the electron gun. The systematic design of the RF deflector is presented, and the performance on beam chopping and bunch length measurement is investigated.

The use of THz-driven split-ring resonator (SRR) for temporal characterization of sub-ps electron pulse has been proposed for a few years and was experimentally demonstrated recently. The dependence of the streaking performance on the dimensions of SRR is to be further discussed here. For SRRs with simple structures, the resonance frequency can be derived through an equivalent LC circuit, which reveals the quantitative dependence of the resonance frequency on its geometric dimensions. Besides, the geometric dimensions of SRR determine its streaking region and field enhancement factor, which have a substantial impact on the streaking process and the temporal resolution. We analyze the existing equivalent LC circuit model for SRRs and propose an analytical beam dynamics model to describe the streaking process of electron pulse. Formulas for the streaking angle, the post-streaking beam divergence, the spot size on the detector and the temporal resolution of SRR are derived. The analytical results are compared with the particle tracking simulation to discuss its applicability. The restriction of the bunch length measurement due to the space charge effect are discussed as a complement to the analytical model. This analytical model provides a link between the temporal resolution and the geometric dimensions of SRR, which can be utilized for performance evaluation and structural optimization.

Recent research indicates that ultrafast electron diffraction and microscopy (UED/M) have unprecedented potential in probing ultrafast dynamic processes, especially in organic and biological materials. However, reaching the required brightness while maintaining high spatiotemporal resolution requires new design of electron source. In order to produce ultrashort electron beam with extreme high brightness, a 1.4-cell RF gun is being developed to reach higher acceleration gradient near the photocathode and thus suppress the space charge effect in the low energy region. Simulation of the 1.4-cell RF photocathode gun shows considerable improvement in bunch length, emittance and energy spread, which all lead to better temporal and spatial resolution comparing to traditional 1.6-cell RF photocathode gun. The results demonstrate the feasibility of sub-ps temporal resolution with normalized emittance less than 0.1 πmm·mrad while maintaining 1 pC electron pulse.

Recently, the development of a 250 MeV cyclotron for advanced cancer therapy has been carried out by Huazhong University of Science and Technology(HUST) . It has four sector magnet and RF cavity which resonance frequency is 74.69 MHz. The internal ion source was adopted and the central region was designed to accommodate the starting beam. In this paper, the design of the central region to optimize the initial circumstances for H¬+ beam were described. The electric and magnetic field distribution were designed by electrostatic and magnetic solver in OPERA-3D TOSCA. The beam characteristics including the beam orbit, motion of the center of orbit, energy gain was investigated for central region was simulated by means of computer code Z3CYCLONE., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The magnetic septa have been developed for the slow beam extraction from the 50 GeV Proton Synchrotron to the Hadron Experimental Hall at J-PARC (Japan Proton Accelerator Research Complex). The magnetic septa consist of two thin magnetic septa, four medium thick magnetic septa and four thick magnetic septa. The typical operating current is 3000 A and the total kick angle is 77 mrad with the 30 GeV proton beam. All parts of the thin and of the medium thick septum magnets are made of inorganic materials to resist high radiation environment. The positions of the thin and medium thick septa can be aligned remotely in the horizontal range of ±5 mm, which enables us to minimize the beam loss at the magnetic septum section. The septa were installed in the synchrotron ring in December, 2008, after a test at KEK (High Energy Accelerator Research Organization), and were successfully operated in the beam time for the slow beam extraction in January and February, 2009, which resulted in the first 30 GeV slow extracted beam delivery to the Hadron Experimental Hall.

A large-aperture thin-septum magnet is employed in the slow-extraction system of J-PARC 50 GeV ring. This magnet is excited with currents as high as 5000 A to produce a field of 0.114 T across the high-aperture gap (55 mm). Due to the high-current density, the power dissipation in the septum conductor creates a serious thermal problem. Since the septum is edge cooled by 2 brazed-cooling tubes, a high temperature rise in the middle of septum with respect to the edge part is unavoidable. Consequently, the current density distribution is nonuniform, which inevitably destroys the gap-field uniformity and increases the leakage field. In this paper, the leakage fields affected by temperature distribution are studied in several cases.

Hefei synchrotron radiation Light Source (HLS) presently has three kickers for electron injection. It is a lattice-dependent system, and different lattices will form different shapes of bump orbits. In the upgrading of HLS, a compact, lattice-independent injection system will be adopted. This paper looks into the possibilities of using quadrupole-magnet trim coils to produce assistant DC injection bumps and then to decrease the required amplitude of kicker bump. Some calculation results of injection schemes are discussed.

An eddy current septum has been studied for the 50 GeV main ring injection in the JPARC. Due to the high beam intensity, very large aperture is required to accommodate the large size injected beam. A large aperture magnet leads to significant end field and considerable eddy current losses, which degrades magnet performance and creates thermal problems. This paper discusses the eddy current effects on the field distribution and studies the methods for the eddy current effects compensation.

Hefei Light Source (HLS) is a low energy Synchrotron Radiation (SR) facility. The interval of SR pulses is in the range 5/spl sim/220 ns. Some experiments need very long interval SR pulses with a strict time structure that HLS cannot provide. To meet the needs of these experiments, a scheme of local vertical bump is studied. Physical calculations and numerical simulations demonstrate the theoretical possibility, and a prototype fast kicker system has been built to demonstrate the technical possibility.