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.

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.

New RF trip diagnostic system of BEPCII

Background The upgrade project of the Beijing Electron Positron Collider(BEPCII)is a high-current electron–positron collider with beam intensity close to 1A.The RF system plays an important role in providing energy for the beam,compressing the beam length and maintaining beam lifetime.Once the RF system shuts down the RF power for various reasons,the accelerator will stop working.Purpose In order to determine which reason causes the RF trip,a new RF trip diagnostic system was built based on the NI PXI system.Method The new RF trip diagnostic system is used to collect the data of the forward power,reflected power,cavity voltage,and beam current in a period of time before and after the RF trip event happens.The system uses LabVIEW as the control software,which conducts I/Q sampling of down-converted RF signals directly,then calculates the amplitude and phase of each RF signal,and plots the change curve.The data and figures are stored in the disk and MySQL database.The change curve can be replotted by the data browser which developed by the Python or PHP language.Results Through the analysis of the RF trip data and figures that produced by the new RF trip system,it can be clear whether the RF trip is caused by the RF system,so as to locate the reason of the RF trip more quickly and improve the reliability and stability of the machine operation.

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.

Improving the RF stabilities of BEPCII by a disturbance observer based controller

Background A digital low level radio frequency(DLLRF)system has been developed to replace the old analog LLRF and operated stably at BEPCII east RF station in the past two years.The RF stabilities required for BEPCII are±1%in magnitude and±1 degree in phase.These are satisfied both by the new DLLRF and the old analog LLRF.Yet,the DLLRF did not improve the RF stabilities so much as expected,especially when the beam current was high in collision mode.Purpose The purpose is to improve the RF stabilities further and meet the requirements by the upgraded BEPCII project(BEPC3).Method A disturbance observer based(DOB)controller to suppress the noise was developed and tested with beam at BEPCII RF system.Results The DOB controller works and the RF stabilities with beam may be improved from±1%to less than±0.5%in magnitude and from±1 degree to less than±0.5 degrees in phase.