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.

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.

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.

Medium-temperature baking of 1.3 GHz superconducting radio frequency single-cell cavity

Background A reliable and repeatable post-processing technology of improving the performance of 1.3 GHz superconducting radio frequency(SRF)cavities is one of the critical technologies for the ILC and XFEL and ERL projects.Methods Three 1.3 GHz single-cell cavities were fabricated and received a baking in temperature 330℃,while the interior of the cavity stayed in ultra-high vacuum(UHV).The cavities were also vertical-tested after electropolishing(EP)with 120℃48-h baking and with nitrogen doping separately for a comparison.Results The Q_(0) of 1.3 GHz single cavity after medium-temperature baking can be 2-3×10^(10) in the accelerating gradient range of 2-35 MV/m in the 2 K vertical test in IHEP.Meanwhile,the outer surface oxidation of niobium cavity caused by baking will decrease the performance of the SRF cavity.Conclusions Medium-temperature(250-400℃)baking on the 1.3 GHz single-cell cavity will improve its Q_(0) in 2 K vertical test compared with EP followed by 120℃48-h baking baseline and reach a similar level of nitrogen doping,and the quench field will lower to a typical range of 20-30 MV/m.Meanwhile,the cavity performance is sensitive to the baking time and temperature,which indicates that a tremendous improvement can be made on the current treatment.