Radio-frequency system of the high energy photon source

Purpose High energy photon source is a 6 GeV diffraction-limited storage ring light source currently under construction in Beijing.A low-frequency fundamental radio-frequency(rf)system of 166.6 MHz was proposed to accommodate the accelerator physics design.Superconducting rf(srf)technologies were chosen for the storage ring rf accompanied by solid-state power amplifiers and digital low-level rf controls.The design of the rf system was completed,and the parameters are frozen.Elucidation of the rf design with key parameters is desired.Methods The requirements from the accelerator physics design will be presented followed by the detailed rf design.The logic behind the choice of key rf parameters is elaborated.The configuration of the entire rf system is presented.Results and conclusions The fundamental srf cavity of 166.6 MHz was designed to accelerate the ultrarelativistic electron beam.Heavy damping of higher-order modes in these cavities is required to avoid the coupled bunch instabilities.An active third harmonic srf of 499.8 MHz was adopted to realize the required rf gymnastics.Normal-conducting 5-cell cavities will be used for the booster rf.Solid-state amplifiers of 2.4 MW in total will be installed at HEPS to drive these cavities in the booster and the storage ring.A digital low-level rf system will be used to regulate rf field inside each cavity with high stabilities.The rf configuration during the commissioning and the operation scenarios are also presented.

Equilibrium electron beam parameters of the High Energy Photon Source

Purpose The physics design of the High Energy Photon Source(HEPS)was finished after many times of iteration.Hereby,the typical equilibrium electron beam parameters corresponding to the proposed two baseline operation modes in the baseline design of HEPS are presented.Methods To compute the equilibrium parameters of the electron beam,the lattice parameters,RF parameters,and the parameters of the insertion devices(IDs)were determined first.Furthermore,it is more precise to use the full-current electron beam parameters in the estimations of the performance of the synchrotron light.Therefore,not only the single-particle dynamics but also the current-dependent collective effects need to be considered in the computations of the full-current,equilibrium parameters of the electron beam.Both analytic computations and multi-particle tracking simulations were carried out.Results The full-current,equilibrium parameters of the electron beams in the HEPS storage ring are presented in this paper.Moreover,the main beam parameters in the injector(the booster and the LINAC),corresponding to the two baseline operation modes of the storage ring,are also presented.Conclusion The typical electron beam parameters corresponding to the two baseline operation modes are given in detail in this paper.

Modification and optimization of the storage ring lattice of the High Energy Photon Source

Purpose For the High Energy Photon Source(HEPS),a green-field fourth-generation storage ring light source,the prelimi-nary design report(PDR)was completed in 2018,when the accelerator physics design had been basically finished.During the subsequent hardware and engineering design of the HEPS storage ring based on the PDR design,a few problems and challenges emerged,calling for modifications of the lattice.Method In this paper,we will introduce the background and reasons for the modifications and present the linear optics and simulation results for the nonlinear performance of the modified lattice of the HEPS storage ring.Result and conclusion The modified lattice satisfies the requirements from hardware and engineering design.

ESRF-type lattice design and optimization for the High Energy Photon Source

A new generation of storage ring-based light sources,called diffraction-limited storage rings（DLSRs）,with emittance approaching the diffraction limit for multi-keV photons by means of multi-bend achromat lattices,has attracted extensive studies worldwide.Among various DLSR proposals,the hybrid multi-bend achromat concept developed at the European Synchrotron Radiation Facility（ESRF） predicts an effective way of minimizing the emittance while keeping the required chromatic sextupole strengths to an achievable level.For the High Energy Photon Source planned to be built in Beijing,an ESRF-type lattice design consisting of 48 hybrid seven-bend achromats is proposed to reach emittance as low as 60 pm-rad with a circumference of about 1296 m.Sufficient dynamic aperture,allowing vertical on-axis injection,and moderate momentum acceptance are achieved simultaneously for a promising ring performance.

Development of BPM feedthroughs for the High Energy Photon Source

Purpose Based on the requirements of the High Energy Photon Source(HEPS),the key components for strip-type and button-type beam position monitors(BPMs)and feedthroughs are designed and manufactured.Method The mechanical structure of the feedthroughs is designed according to the Computer Simulation Technology(CST)simulation results.Resultsandconclusions Small batches of feedthroughs are manufactured,and the structure is optimized based on mechanical,vacuum,electromagnetic,and other characteristic test results.The performance improved significantly owing to optimization through batch product testing,and quality control-related information from several manufacturers is obtained,which provide a foundation for the development of BPMs for HEPS.

Milestone progress of the HEPS booster commissioning

The high-energy photon source(HEPS)is the first fourth-generation synchrotron light source facility in China.The HEPS injector consists of a linear accelerator(Linac)and a full energy booster.The booster captures the electron beam from the Linac and increases its energy to the value required for the storage ring.The full-energy beam could be injected to the storage ring directly or after“high-energy accumulation.”On November 17,2023,the key booster parameters successfully reached their corresponding target values.These milestone results were achieved based on numerous contributions,including nearly a decade of physical design,years of equipment development and installation,and months of beam commissioning.As measured at the extraction energy of 6 GeV,the averaged beam current and emittance reached 8.57 mA with 5 bunches and 30.37 nm rad with a single-bunch charge of 5.58 nC,compared with the corresponding target values of 6.6 mA and 35 nm rad,respectively.This paper presents the physical design,equipment development,installation,and commissioning process of the HEPS booster.

Preliminary study on detection and cleaning of parasitic bunches

To establish a nuclear resonant scattering beamline at the High Energy Photon Source(HEPS),it is essential to develop tools for detection and cleaning of parasitic bunches,for meeting the stringent demands on bunch purity.To this end,a novel time-correlated single-photon counting system was implemented at the electron storage ring of the Beijing Electron–Positron Collider II(BEPCII).The purity deterioration process over a week-long operation was recorded by the system.In this study,the mechanism of impurity growth was analyzed by numerical methods and validated on measurements.The agreement between the experimental results and the cal-culation was fairly good.Two main sources of parasitic bunches,pre-accelerators and the Touschek scattering were confirmed.A bunch-cleaning technique,based on a sinu-soidal signal mixed with a pseudo-square wave,was also developed and implemented,and its capability to improve the bunch purity to the level of 10–7 was experimentally demonstrated.We present the experimental setup,princi-ple,and measurement results of a system for detection and cleaning of parasitic bunches.

Design and fabrication of button-style beam position monitors for the HEPS synchrotron light facility

At the High Energy Photon Source (HEPS),a high orbital stability of typically 10% of the beam size and angular divergence must be achieved.The beam size at the insertion devices is 10μm horizontally and 1μm vertically,which implies that the beam orbit must be stabilized to the sub-micrometer level.This results in stringent tolerance and quality control requirements for the series production of beam position monitor (BPM) pickups.In this study,analytical formulas were used and CST simulations were performed to analyze the effects of the mechanical tolerances of BPM pickups on beam position measurement.The results of electromagnetic?eld simulations revealed how various mechanical errors,such as button size and location accuracy,as well as the related button capacitance,exert different in?uences on the beam position measurement.The performance of an actual BPM pickup was measured,along with an assessment of the error on the beam position measurement.Additionally,a wake?eld analysis,including an investigation of trapped resonant modes and related thermal deformation,was conducted.

Using a pre-kicker to ensure safe extractions from the HEPS storage ring

The High Energy Photon Source(HEPS)is a 6 GeV diffraction-limited storage ring light source under construction.The swap-out injection is adopted with the depleted bunch recycled via high-energy accumulation in the booster.The extremely high beam energy density of the bunches with an ultra-low emittance(about 30 pm horizontally and 3 pm vertically)and high bunch charges(from 1.33 to 14.4 nC)extracted from the storage ring could cause hazardous damage to the extraction Lambertson magnet in case of extraction kicker failure.To this end,we proposed the use of a pre-kicker to spoil the bunches prior to extraction,significantly reducing the maximum beam energy density down to within a safe region while still maintaining highly efficient extractions.The main parameters of the pre-kicker are simulated and discussed.

Development of a 500-MHz waveguide directional coupler with high directivity for HEPS

Purpose In order to achieve a high-precision measurement of the incident and the reflected power,a WR1800 rectangular waveguide directional coupler with high directivity and high power level has been in-house developed.Multiple couplers will be installed in the 500-MHz high-power radio-frequency transmission lines delivering 200-kW continuous-wave power for the High Energy Photon Source(HEPS).Methods The directional coupler adopts the design scheme of primary and secondary transmission lines and coaxial coupling-head structure.The shape and dimensional parameters of the coupling head were carefully optimized by using microwave simulation codes.An optimum directivity of 64 dB was achieved in simulations.Results and conclusions A prototype coupler was subsequently manufactured,and its directivity was measured to be 48.2 dB following a rigorous calibration procedure,largely exceeding the design goal and the commercial product.The coupler was then connected to an existing 500-MHz klystron system,and a high-power test with short-circuit termination was conducted.The high directivity of the coupler was confirmed up to 200 kW.During the 6 hours of power aging with continuous-wave 200 kW in a standing-wave setup,no performance degradation was observed on the coupler.The coupler temperature was measured to be 20◦C above the ambient environment.The design requirements were comfortably fulfilled.These constitute thefirst in-house development of a large-size waveguide directional coupler with high directivity and high power level for HEPS.The design,fabrication,and performance tests of the directional coupler are presented.

Development of a high-performance RF front end for HEPS 166.6 MHz low-level RF system

Purpose Digital low-level radio frequency(LLRF)system has been proposed for 166.6 MHz superconducting cavities at High Energy Photon Source.The RF field inside the cavities has to be controlled better than 0.03%(rms)in amplitude and 0.03°(rms)in phase.A RF front end system is required to transform the RF signal from the cavity to IF signal before inputting into the digital signal processing(DSP)board,and up-convert the IF signal back to RF to drive the power amplifier.Methods Connectorized off-the-shelf microwave components were used to realize the RF front end system.The local oscillator generation and distribution,choices of main components and design of down-/up-conversion channels have been elaborated in detail with a focus on minimizing nonlinearity and signal interferences among channels with optimized signal distribution loss.Results and conclusions The RF front end has been incorporated with the existing DSP board and tested on a warm 166.6 MHz cavity in the laboratory.Excellent channel isolations and good linearities were achieved on the RF front end system.The RF field inside the cavity was controlled with a residual noise of 0.004%(rms)in amplitude and 0.002°(rms)in phase,well below the HEPS specifications.The sensitivity to ambient environment changes have also been studied and presented in this paper.This demonstrates a first high-performance 166.6 MHz RF front end system and provides valuable insights into HEPS LLRF system development in the future.

Optimization of dynamic aperture by using non-dominated sorting genetic algorithm-Ⅱ in a diffraction-limited storage ring with solenoids for generating round beam

Purpose Round beam,i.e.,with equal horizontal and vertical emittance,is preferable than a horizontally flat one for some beamline applications in Diffraction-limited storage rings(DLSRs),for the purposes of reducing the number of photons getting discarded and better phase space match between photon and electron beam.Conventional methods of obtaining round beam inescapably results in a reduction of dynamic aperture(DA).In order to recover the DA as much as possible for improving the injection efficiency,the DA optimization by using Non-dominated sorting genetic algorithm-Ⅱ(NSGA-Ⅱ)to generate round beam,particularly to one of the designed lattice of the High Energy Photon Source(HEPS)storage ring,are presented.Method According to the general unconstrained model of NSGA-Ⅱ,we modified the standard model by using parallel computing to optimize round beam lattices with errors,especially for a strong coupling,such as solenoid scheme.Results and conclusion The results of numerical tracking verify the correction of the theory framework of solenoids with fringe fields and demonstrates the feasibility on the HEPS storage ring with errors to operate in round beam mode after optimizing DA.

Optimizing the lattice design of a diffraction-limited storage ring with a rational combination of particle swarm and genetic algorithms

In the lattice design of a diffraction-limited storage ring（DLSR） consisting of compact multi-bend achromats（MBAs）, it is challenging to simultaneously achieve an ultralow emittance and a satisfactory nonlinear performance, due to extremely large nonlinearities and limited tuning ranges of the element parameters. Nevertheless, in this paper we show that the potential of a DLSR design can be explored with a successive and iterative implementation of the multi-objective particle swarm optimization（MOPSO） and multi-objective genetic algorithm（MOGA）. For the High Energy Photon Source, a planned kilometer-scale DLSR, optimizations indicate that it is feasible to attain a natural emittance of about 50 pm·rad, and simultaneously realize a sufficient ring acceptance for on-axis longitudinal injection, by using a hybrid MBA lattice. In particular, this study demonstrates that a rational combination of the MOPSO and MOGA is more effective than either of them alone, in approaching the true global optima of an explorative multi-objective problem with many optimizing variables and local optima.

Improving nonlinear performance of the HEPS baseline design with a genetic algorithm

A baseline design for the High Energy Photon Source has been proposed, with a natural emittance of 60 pm.rad within a circumference of about 1.3 kilometers. Nevertheless, the nonlinear performance of the design needs further improvements to increase both the dynamic aperture and the momentum acceptance. In this study, genetic optimization of the linear optics is performed, so as to find all the possible solutions with weaker sextupoles and hence weaker nonlinearities, while keeping the emittance at the same level as the baseline design. The solutions obtained enable us to explore the dependence of nonlinear dynamics on the working point. The result indicates that with the same layout, it is feasible to obtain much better nonlinear performance with a delicate tuning of the magnetic field strengths and a wise choice of the working point.

Active microphonics noise suppression based on DOB control in 166.6-MHz superconducting cavities for HEPS

Purpose Superconducting 166.6-MHz cavities will be used to accelerate electron beams in high-energy photon source(HEPS).The radio-frequency(RF)fields inside these cavities have to be controlled better than 0.03%(rms error)for the amplitude and 0.03◦(rms error)for the phase.Adopting a quarter-wave geometry withβ=1,the 166.6-MHz cavity has two intrinsic mechanical modes at∼100Hz observed in both simulations and cryogenic tests.If coupled to external vibrations,these microphonics modes shall stress the existing proportional–integral(PI)feedback controller and inevitably deteriorate the field stabilities.Therefore,additional noise suppression may be required.Methods Adigital low-level RF system previously in-house developedwas connected to a 166.6-MHz dressed cavity at room temperature in the laboratory.Piezo-tunerswere used to“knock”on the cavity at various frequencies to excite cavity vibrations,and microphonics spectrum was subsequently measured.A disturbance observer(DOB)-based algorithm was adopted and integrated into the existing feedback controller.The performance of PI controller,DOB controller and a combination of PI and DOB controller was compared.The limitation of the DOB controller was also examined.Results and conclusions The PI controller was proved to be insufficient in suppressing large cavity microphonics during the tests.By adding the DOB controller,the excellent field stabilities can be restored.Optimized loop parameters were obtained.The simple first-order filter was adequate thanks to the robustness of the DOB controller.This constitutes a first laboratory demonstration of the active microphonics noise suppression in the 166.6-MHz RF cavity for HEPS.