Low Level RF System Research Articles

Design and development of the S-band high power test stand for multi-purpose applications at IHEP

A multipurpose S-band high-power test stand (HPTS) developed at the Institute of High Energy Physics (IHEP) of China is routinely used for radio-frequency (RF) conditioning of couplers and resonant structures, as well as other high-power experiments in the S-band frequency regime. The HPTS with a peak power of 230 MW was developed to meet requirements of various applications with multiple frequencies (2856 MHz/2998 MHz) and power levels (45 MW/50 MW/65 MW/80 MW) for different accelerator projects at the IHEP, e.g. the High Energy Photon Source (HEPS), Beijing Electron Positron Collider (BEPCII), Circular Electron Positron Collider (CEPC). This paper describes the design and implementation of the system in detail. The highlight of the HPTS is the impedance-matched design to accommodate different impedances, allowing a single pulse modulator to supply different types of klystron loads. The control system, developed under the Experimental Physics and Industrial Control System (EPICS), adopts a combination of Micro Telecommunications Computing Architecture (MTCA) based low-level RF system (LLRF) and programmable logic controller (PLC) based interlock unit controllers, and enables a real-time data acquisition and visualization. To ensure a high reliability and high-gradient RF power conditioning, the interlock unit integrates various protection features, like the short circuit load, high standing wave ratio, low vacuum, arcing detector, and abnormal dark current. It is worth mentioning that the entire conditioning procedure can be performed unattended automatically controlled. The test stand has been operated stably for more than 7,000 hours for many rounds of high-power RF conditioning with different power/frequency requirements, which indicates that the HPTS can provide a stable, reliable, and automatic high-power testing for the development of S-band related devices.

Application of a modified iterative learning control algorithm for superconducting radio-frequency cavities

Transient beam loading, which causes cavity gradient fluctuation, is becoming a major concern for the stable operation of the high current superconducting radio-frequency (SRF) accelerators. Iterative learning control (ILC) is an effective algorithm aiming to improve systems operated in repetitive mode. This ILC technique was successfully introduced to the low-level radio-frequency (LLRF) control in accelerators to compensate for the field fluctuation caused by repetitively pulsed beam. The modern LLRF system prefers to use the FPGA-based hardware platform to realize a real-time control framework. However, considering the algorithm complexity and the hardware cost, the ILC algorithm is usually implemented outside FPGA. This practice would decrease the real-time ability of the control system. In this paper, we present a modified ILC algorithm that can be implemented inside FPGA. The key idea of our method is to simplify the beam profile using a rectangular pulse. The method was demonstrated in the SRF cavities at Chinese Accelerator driven system Front-end demo SRF linac (CAFe). The experimental results in the CAFe beam-commissioning confirmed that the beam-induced gradient fluctuation is successfully suppressed.

Experimental Evaluation of Sub-Sampling IQ Detection for Low-Level RF Control in Particle Accelerator Systems.

The low-level radio frequency (LLRF) control system is one of the fundamental parts of a particle accelerator, ensuring the stability of the electro-magnetic (EM) field inside the resonant cavities. It leverages on the precise measurement of the field by in-phase/quadrature (IQ) detection of an RF probe signal from the cavities, usually performed using analogue downconversion. This approach requires a local oscillator (LO) and is subject to hardware non-idealities like mixer nonlinearity and long-term temperature drifts. In this work, we experimentally evaluate IQ detection by direct sampling for the LLRF system of the Polish free electron laser (PolFEL) now under development at the National Centre for Nuclear Research (NCBJ) in Poland. We study the impact of the sampling scheme and of the clock phase noise for a 1.3-GHz input sub-sampled by a 400-MSa/s analogue-to-digital converter (ADC), estimating amplitude and phase stability below 0.01% and nearly 0.01°, respectively. The results are in line with state-of-the-art implementations, and demonstrate the feasibility of direct sampling for GHz-range LLRF systems.

LLRF Controller for High Current Cyclotron-Based BNCT System

China Institute of Atomic Energy (CIAE) is constructing a high current cyclotron-based boron neutron capture therapy (BNCT) system. The designed proton beam intensity of this cyclotron is 1 mA. The RF system of the cyclotron consists of two separate cavities, two 20-kW amplifiers, a 300-W amplifier for the buncher, and one low-level radio frequency (LLRF) system. The LLRF system controls the amplitudes and phases of the two independent cavities and the buncher. The previous analog–digital hybrid LLRF system in CIAE was designed for low beam loading applications. As during the beam commissioning and machine operation, the RF system requires a more powerful, flexible, and reliable real-time LLRF system, the LLRF group decides to design a new LLRF system for this application. At TRIUMF, a digital LLRF system was developed for the prebuncher of the ARIEL project. This state-of-the-art design is extended and utilized for the BNCT LLRF system. In which, the amplitude and phase of the two separate Dees, as well as the buncher for beam injection, will be regulated by a single field-programmable gate array (FPGA). For such a demanding control task, the design shows a promising future, both from real-time response and flexibility points of view. The design ideas, technology features, system structure, hardware and software development, and the desktop test will be presented.

MicroTCA.4-Based Low-Level RF for Continuous Wave Mode Operation at the ELBE Accelerator

At the ELBE Center for High-Power Radiation Sources, electrons are accelerated to an energy of up to 40 MeV in a superconducting linear accelerator that is operated in continuous wave (CW) mode. The acceleration is achieved using superconducting radio frequency (RF) cavities, which are driven by an analog low-level RF (aLLRF) system and solid-state-based RF amplifiers. The analog aLLRF system was transformed to a digital system based on the MicroTCA standard. It is in user operation since 2020. Here, the new digital system and its integration in the ELBE control system are described. Furthermore, the system is characterized by noise measurements and shows rms field stability of the digital LLRF (dLLRF) system of 0.01° in phase and 0.005% in amplitude. In addition, an algorithm for compensating long-term drifts is presented and characterized.

High-Level Software Interface to the LLRF System Developed for the European Spallation Source Facility

The European Spallation Source (ESS) accelerator is composed of superconducting elliptical cavities. When the facility is running, the cavities are fed with an electric field from klystrons. Parameters of this field are monitored and controlled by the low-level radio frequency (LLRF) system. Its main goal is to keep the amplitude and phase at a given set point. The LLRF system is also responsible for the reference clock distribution. During operation, the cavities are periodically experiencing strain caused by the Lorentz force, while the high gradient of the electric field is generated for the beam acceleration. Even small changes in the physical dimensions of a cavity shift its resonance frequency. This phenomenon, called detuning, causes significant power losses. It is actively compensated by the LLRF control system, which can physically tune lengths of the accelerating cavities with stepper motors (slow, coarse-grained control) and piezo elements (active compensation during operation). This article describes the implementation and tests of the software supporting various aspects of the LLRF system and cavities management. The Piezo Driver management and monitoring tool is dedicated to a piezo controller device. The LO distribution application is responsible for configuration of the local oscillator. The cavity simulator tool was designed to provide access to properties of the hardware device, emulating behavior of the elliptical cavities. IPMI Manager Software was implemented to monitor the state of MicroTCA.4 crates, which are a major part of the LLRF system architecture. All applications have been created using the experimental physics and industrial control system (EPICS) framework and built in the ESS EPICS Environment (E3).

Development of a 166.6 MHz Low-Level RF System by Direct Sampling for High Energy Photon Source

Development of a 166.6 MHz Low-Level RF System by Direct Sampling for High Energy Photon Source

A precision LLRF control system for UED

An ultrafast electron diffraction (UED) facility is located at Shanghai Jiao Tong University. The layout of the UED is divided into two modules. The accelerating module has one S-band photocathode RF gun, one C-band harmonic buncher, and one C-band deflector. The power-generating module has one S-band klystron and one C-band klystron. To meet the high-precision requirements, a MicroTCA.4-based low level radio frequency (LLRF) system is used to provide a stable RF field in the cavities. In this paper, we introduce the RF distributions, LLRF system, firmware and software of the feedback controller, and field stability of the RF gun; and present an analysis of the measurement results for UED.

Proton linac-based therapy facility for ultra-high dose rate (FLASH) treatment

As an advanced treatment method in the past five years, ultra-high dose rate (FLASH) radiotherapy as a breakthrough and milestone in radiotherapy development has been verified to be much less harmful to healthy tissues in different experiments. FLASH treatments require an instantaneous dose rate as high as hundreds of grays per second to complete the treatment in less than 100 ms. Current proton therapy facilities with the spread-out of the Bragg peak formed by different energy layers, to our knowledge, cannot easily achieve an adequate dose rate for FLASH treatments because the energy layer switch or gantry rotation of current facilities requires a few seconds, which is relatively long. A new design for a therapy facility based on a proton linear accelerator (linac) for FLASH treatment is proposed herein. It is designed under two criteria: no mechanical motion and no magnetic field variation. The new therapy facility can achieve an ultra-high dose rate of up to 300 Gy/s; however, it delivers an instantaneous dose of 30 Gy within 100 ms to complete a typical FLASH treatment. The design includes a compact proton linac with permanent magnets, a fast beam kicker in both azimuth and elevation angles, a fixed gantry with a static superconducting coil to steer proton bunches with all energy, a fast beam scanner using radio-frequency (RF) deflectors, and a fast low-level RF system. All relevant principles and conceptual proposals are presented herein.

Long term stability of 1.3 GHz high power RF amplifier

Turkish accelerator and radiation laboratory (TARLA) is an accelerator based oscillator mode free electronlaser (FEL) facility under construction. The acceleration beamline includes a fundamental buncher (FB) in order to confine the electron beam into a compressed bunch. In this study, we demonstrate the commissioning tests of 500 W RF amplifier powering FB and present the phase stability dependence on the pressure and temperature of the water cooling line under long time run conditions.As the disturbing effects break down the synchronization between the particle and RF power during acceleration, the reasons disquieting the stability of the RF amplifier are investigated. One reason which causes disturbance in the stability is the variation in the phase of the output power. Phase variation of the output power of the RF amplifier which was measured as $\sim$10 deg/$^{\circ}$C in average initially, which was beyond permitted variation levels by the controller electronics, namely the low level RF system (LLRF), is decreased to less than $\sim$0.45 deg/$^{\circ}$C by incorporating a phase shifter. In addition, this phase variation reduction is accompanied with a reflected power at the output of the RF amplifier decline from 4.3 W (~36.33 dBm) down to 3 mW (~4.7 dBm) by utilizing a circulator, which is also important and desirable in terms of protection of the electronics in the long term run. As ensuring phase stability in the output power is very important, this work may serve as a guide since the documentation regarding the output power dependence of the water cooling line for long term run is very limited.

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

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 $$^\circ $$ (rms error) for the phase. Adopting a quarter-wave geometry with $$\beta =1$$ , the 166.6-MHz cavity has two intrinsic mechanical modes at $$\sim $$ 100 Hz 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. A digital low-level RF system previously in-house developed was connected to a 166.6-MHz dressed cavity at room temperature in the laboratory. Piezo-tuners were 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. 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.

Application of a drift compensation low-level radio frequency system based on time-multiplexing pick-up/reference signals.

The high accuracy, low drift low-level radio frequency (LLRF) system is essential for the long-term stability of the accelerator RF and the acquirement of low emittance, high intensity electron beams. A time-multiplexing pick-up/reference signal based LLRF system is proposed to deal with the component temperature related phase drift and has been deployed and applied at the Xi'an Gamma-ray Light Source (XGLS) injector. The long term dual-receiver out-of-loop stability experiments with a continuous wave laser based phase reference distribution system (PRDS) show that the LLRF system can achieve ∼40 fs Root-Mean-Square (rms) phase accuracy and 51 fs/52 fs peak-peak drift (in 7 days/17 h with the high power RF system, respectively) while the reference phase varies both ∼30 ps. An ∼4 h beam-based experiment has also been conducted to evaluate the overall performance of the whole XGLS timing and synchronization system, which shows that the PRDS, LLRF system, high power RF system, and laser oscillator laser-RF synchronization system can keep long-term phase stability.

Superconducting radio-frequency cavity fault classification using machine learning at Jefferson Laboratory

We report on the development of machine learning models for classifying C100 superconducting radio-frequency (SRF) cavity faults in the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab. CEBAF is a continuous-wave recirculating linac utilizing 418 SRF cavities to accelerate electrons up to 12 GeV through 5-passes. Of these, 96 cavities (12 cryomodules) are designed with a digital low-level RF system configured such that a cavity fault triggers waveform recordings of 17 RF signals for each of the 8 cavities in the cryomodule. Subject matter experts (SME) are able to analyze the collected time-series data and identify which of the eight cavities faulted first and classify the type of fault. This information is used to find trends and strategically deploy mitigations to problematic cryomodules. However manually labeling the data is laborious and time-consuming. By leveraging machine learning, near real-time (rather than post-mortem) identification of the offending cavity and classification of the fault type has been implemented. We discuss performance of the ML models during a recent physics run. Results show the cavity identification and fault classification models have accuracies of 84.9% and 78.2%, respectively.

A new beam synchronous processing architecture with a fixed frequency processing clock. Application to transient beam loading compensation in the CERN SPS machine

New projects being implemented or planned at CERN, such as the LHC Injectors Upgrade (LIU), the High-Luminosity LHC (HL-LHC) or the Future Circular Collider (FCC), motivate the change of several architectural paradigms in the Low Level RF systems (LLRF). In the upgraded Super Proton Synchrotron (SPS) LLRF, the distribution of RF, the revolution frequency, settings and synchronization timings will rely on a deterministic link, the White Rabbit (WR), exploiting a distributed topology. The digital electronics (FPGA) will now use a fixed frequency clock extracted locally in each node from this data stream. The paper presents a new solution for algorithms that treat beam-induced perturbations as they must solve the challenge to tune their processing to the instantaneous revolution frequency, Beam Synchronous Processing (BSP). We use dynamic resampling of uniform sampled data as tuning element between a sweeping spectrum and the static processing implemented with fixed frequency clocks. The paper applies the novel Beam Synchronous Processing method to transient beam loading compensation. It presents a One Turn delay FeedBack (OTFB) for the SPS based on a cascade of two variable ratio resamplers.

P.445 Dose-response relationships of antipsychotic drugs for major depressive disorder: a systematic review and meta-analysis

New projects being implemented or planned at CERN, such as the LHC Injectors Upgrade (LIU), the High-Luminosity LHC (HL-LHC) or the Future Circular Collider (FCC), motivate the change of several architectural paradigms in the Low Level RF systems (LLRF). In the upgraded Super Proton Synchrotron (SPS) LLRF, the distribution of RF, the revolution frequency, settings and synchronization timings will rely on a deterministic link, the White Rabbit (WR), exploiting a distributed topology. The digital electronics (FPGA) will now use a fixed frequency clock extracted locally in each node from this data stream. The paper presents a new solution for algorithms that treat beam-induced perturbations as they must solve the challenge to tune their processing to the instantaneous revolution frequency, Beam Synchronous Processing (BSP). We use dynamic resampling of uniform sampled data as tuning element between a sweeping spectrum and the static processing implemented with fixed frequency clocks. The paper applies the novel Beam Synchronous Processing method to transient beam loading compensation. It presents a One Turn delay FeedBack (OTFB) for the SPS based on a cascade of two variable ratio resamplers.

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.

Low level Radio Frequency studies for the Compact Linear Collider Damping Rings

The Compact Linear Collider (CLIC) Damping Rings (DRs) need to generate ultra-low emittance bunches to achieve high luminosity in CLIC. Strong wiggler magnets are required to significantly increase the energy loss per turn. A high total voltage Radio Frequency (RF) system is needed to compensate these losses. The resulting strong beam loading transients affect the bunch position and length. On the other hand, in order to maintain the luminosity loss below 1%, the bunch position has to be regulated within ±1∘ at 2 GHz (±400μm) at the DR extraction. These conflicting specifications lead to a challenging design for the Low-Level RF (LLRF) system. In this work, simulations of the LLRF system are presented and validated using theoretical expressions. They are then used to evaluate various potential LLRF architectures, and to estimate and compare their performance to the demanding specifications on bunch longitudinal position in the CLIC DRs.

Upgrade of low level RF system based on Micro Telecom Computing Architecture (MTCA) for HLS-II LINAC

The performance of Hefei Light Source II (HLS-II) has improved a lot after major maintenance and reconstruction. To further provide continuous and stable light, the RF system of the HLS-II LINAC needs to be upgraded for top-off mode. It is required that the RF power source have long-time stability and reliability, but the old analog low level RF system(LLRF) can’t meet the requirement. Hence a digital low level RF control system based on Micro Telecom Computing Architecture(MTCA) is designed and implemented to control the amplitude and phase of the RF power source. This system is composed of digital board cards based on FPGA, RF board cards, MTCA chassis and a frequency synthesis system. It works at 2856 MHz of S band, with phase and amplitude stability up to 0.2° and 0.04% respectively, which meets the top-off mode requirement of 0.25° RMS phase jitter of the digital low-level RF system in the HLS-II LINAC.

Continuous Wave Operation of Superconducting Accelerating Cavities With High Loaded Quality Factor

Close to 780 superconducting 1.3-GHz accelerating cavities made of bulk niobium have been installed in the European X-ray free electron laser (E-XFEL) linear accelerator. The linac can operate at a mean gradient (Eacc) of ca. 23 MV/m in the nominal short pulse (SP) operation mode with 1.4-ms pulses and a repetition rate of 10 Hz. An R&D program at Deutsches Elektronen-Synchrotron (DESY) is ongoing since 2011 on the feasibility of a continuous wave (CW) upgrade of the E-XFEL accelerator. In this publication, a modification of the low-level radio frequency (LLRF) control system to the CW mode is presented. Currently, the control system successfully stabilizes accelerating field for the SP mode. The demanding E-XFEL specifications of 0.01% for the field amplitude and 0.01° for the phase stability are required to keep the photon beam quality for all experimental stations. The proposed modification has been implemented and tested to verify the system versatility for SP and CW operation modes. The tests were conducted for a prototype E-XFEL cryomodule XM-3. As for the series cryomodules, this prototype contains eight superconducting 1.3-GHz cavities. The results presented here confirm that the same LLRF system can be used with minor software modifications for either operation mode (CW or SP). However, the CW mode requires a more complex RF-power management and more precise control, especially when cavities operate with very high loaded quality factor (Ql) of the order of 6E7. The achieved stability for such highQl is also presented and discussed.

Design and performance of the LLRF control system for CSNS linac

The China Spallation Neutron Source (CSNS) linac is designed with beam energy of 80 MeV and a peak current of 15 mA in the first phase. It consists of RFQ, two bunchers of medium-energy beam transmission line, four DTL tanks and one debuncher of linac-to-ring beam transmission line. Correspondingly, eight online RF power sources are used to power these accelerators. In order to stabilize the amplitude, phase and resonant frequency of the RF accelerating field, and minimize beam loss, we have established digital low-level RF (LLRF) control system. The LLRF system includes RF reference line, analog module (AM), clock distribution module, digital control module (DCM), high-power protection module, timing and RF interlock module and so on. The DCM is mainly responsible for the stability of the RF field amplitude and phase, and RF interlock module can quickly cut off the RF drive in case of arc in the RF transmission system, VSWR over threshold or cavity vacuum fault and so on. During beam commissioning, all of eight online units of LLRF control system were operating stably and reliably. The amplitude and phase variations of the linac fields have been achieved about ± 0.4% and ± 0.15° with 10-mA beam loading, much better than the design requirements of ± 1% in amplitude and ± 1° in phase. With the help of this system, we achieved stable operation under different beam loads. Also, many important progresses have been achieved in the LLRF control system for a more convenient operation and a higher stability performance. This article describes the design and implementation of the LLRF for CSNS linac.