RF Stations Research Articles

Coincident learning for unsupervised anomaly detection of scientific instruments

Abstract Anomaly detection is an important task for complex scientific experiments and other complex systems (e.g. industrial facilities, manufacturing), where failures in a sub-system can lead to lost data, poor performance, or even damage to components. While scientific facilities generate a wealth of data, labeled anomalies may be rare (or even nonexistent), and expensive to acquire. Unsupervised approaches are therefore common and typically search for anomalies either by distance or density of examples in the input feature space (or some associated low-dimensional representation). This paper presents a novel approach called coincident learning for anomaly detection (CoAD), which is specifically designed for multi-modal tasks and identifies anomalies based on coincident behavior across two different slices of the feature space. We define an unsupervised metric, F ^ β , out of analogy to the supervised classification F β statistic. CoAD uses F ^ β to train an anomaly detection algorithm on unlabeled data, based on the expectation that anomalous behavior in one feature slice is coincident with anomalous behavior in the other. The method is illustrated using a synthetic outlier data set and a MNIST-based image data set, and is compared to prior state-of-the-art on two real-world tasks: a metal milling data set and our motivating task of identifying RF station anomalies in a particle accelerator.

Status and upgrades of radio frequency system at Taiwan Photon Source

In order to meet the upcoming RF power requirements of TPS, a five-year upgrade project to construct a new RF station was completed in early 2022. This new RF station mainly consists of an SRF module, a 300-kW SSPA, an analog LLRF control system, RF transmission components, and accessary cooling systems. Test results of the SRF module and 300-kW SSPA met the expected performances. Also presented are the operation statistics of the two original RF stations and the ongoing upgrades. Meantime between RF failures during TPS user time reached 600 hours in 2022, while the construction and tests on DLLRF control system, power combination of klystron and SSPA, and 2-cell 1.5-GHz SRF module have all progressed well.

FAIR SIS100 accelerating RF system - Modeling and analysis of the coupled LLRF control loops

The SIS100 heavy ion synchrotron as core part of the Facility for Antiproton and Ion Research (FAIR) will be equipped with 14 accelerating RF stations in the first stage of realization. Each RF station consists of a tunable ferrite-loaded cavity powered by a tetrode amplifier. Further key components are a solid-state pre-amplifier, a power supply unit, and dedicated Low-Level Radio Frequency (LLRF) feedforward and feedback systems to control amplitude and phase of the cavity gap voltage as well as the resonance frequency. Each cavity has to provide up to 20 kV peak gap voltage in the frequency range from 1.1 to 3.2 MHz. While all components of the system have been successfully tested in the factory acceptance tests and transferred to the FAIR storage, the First-of-Series (FoS) RF station is still persistently operated at GSI to gain experience. For further insight into the LLRF part, especially the stability of the control loops, the inter-coupling of the three local control loops was analyzed with methods from control theory based on simplified but realistic models, which have been developed based on extensive measurements and analysis of the systems’ behavior. In this contribution, the modeling as well as the analysis of the coupling between the LLRF control loops are discussed and the results are presented in comparison with measurements on the FoS system.

Beam-based rf station fault identification at the SLAC Linac Coherent Light Source

Accelerators produce too many signals for a small operations team to monitor in real time. In addition, many of these signals are only interpretable by subject matter experts with years of experience. As a result, changes in accelerator performance can require time-intensive consultations with experts to identify the underlying problem. Herein, we focus on a particular anomaly detection task for radio-frequency (RF) stations at the Linac Coherent Light Source (LCLS). The existing RF station diagnostics are bandwidth limited, resulting in slow, unreliable signals. As a result, anomaly detection is currently a manual process. We propose a beam-based method, identifying changes in the accelerator status using shot-to-shot data from the beam position monitoring system; by comparing the beam-based anomalies to data from RF stations, we identify the source of the change. We find that our proposed method can be fully automated while identifying more events with fewer false positives than the RF station diagnostics alone. Our automated fault identification system has been used to create a new data set for investigating the interaction between the RF stations and accelerator performance.

Development of the novel and superior adaptive beam-loading compensation system calculated in the frequency domain for the J-PARC Linac

The Japan Proton Accelerator Research Complex (J-PARC) is one of the highest intensity proton accelerator facilities in the world. Its accelerator consists of a 400-MeV linac (LINAC), a 3-GeV rapid-cycling synchrotron (RCS), and a 30-GeV main ring (MR) and its beam is provided for wide-ranging applications. The peak current of the beam at LINAC had been increased from 30 mA to the design value of 50 mA in 2018. Therefore, the performance of the beam-loading compensation implementing in the low-level radio frequency (LLRF) system has been more important to satisfy the stability of the accelerating gradient. Although the iterative learning control (ILC) scheme with adaptive ability was demonstrated, the practical use of ILC was not convenient for large accelerators such as the J-PARC LINAC with forty-nine RF stations. Therefore, we developed at novel and superior adaptive beam-loading compensation system that was calculated in the frequency domain. Compared to the scheme of the previous ILC, the newly developed system is useful because it does not require parameter adjustment using the beam. In the experimental results utilizing this system, the optimum tables of FF_BEAM were able to be obtained as a smaller number of iterations. The peak-to-peak stability of amplitude was achieved to ± 0.23% and the peak-to-peak stability of phase was improved to ± 0.036°. In addition, errors such as white noise did not accumulate for the table of FF_BEAM and this system solved the problem of the divergence when the number of iterations was increased. This implementation of the adaptive beam-loading compensation system calculated in the frequency domain will potentially contribute to the improvement of the J-PARC LINAC LLRF, and this system may lead to a stable user operation. This system is so simple in principle and effective that it could be applied to other linear accelerators with the pulsed beam.

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

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. The purpose is to improve the RF stabilities further and meet the requirements by the upgraded BEPCII project (BEPC3). A disturbance observer based (DOB) controller to suppress the noise was developed and tested with beam at BEPCII RF system. 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.

IFMIF-DONES RF System

The IFMIF-DONES accelerator cavities require around 5.7 MW of continuous RF power at 175 MHz to deliver a 125 mA and 40 MeV deuteron beam at the lithium target. The IFMIF-DONES RF System is composed of 56 RF Stations able to provide up to 200 kW RF power at 175 MHz in CW, delivering a grand total of 7.4 MW.Such a massive system requires an ambitious approach to be able to comply with the accelerator requirements with a high efficiency and a high availability. The newest advancements in the field of RF systems for accelerators, which are already implemented independently in some facilities, are integrated for the first time in a single system from the early design phases. The result of this approach is the RF Station, which is a totally independent and compact RF module containing all the components needed to feed, control, and protect the accelerator cavities. The amplification technology chosen is the Laterally-Diffused Metal-Oxide Semiconductor (LDMOS) transistor, providing a good efficiency and high robustness. The proposed RF power combination of up to 160 transistors is based on resonant cavity combiner, allowing a single-step combination that highly reduces the losses with respect to more traditional corporate combining schemes. In this proposal, the RF generation relies on a fully digital Low-Level RF (LLRF) providing accurate amplitude/phase control and high flexibility to implement all the required functions for a proper cavity operation. The RF Station has been designed maximizing the modularity and the reliability. In order to achieve the required modularity, the LDMOS transistors have been grouped in Amplifier Modules (AMDs) containing four of them but keeping independent outputs. Furthermore, the AC/DC power converters have also been grouped in modules feeding a common DC bus. The RF Station will have redundant modules to be able to operate properly in any condition, even with some of them failed.This innovative proposal for the IFMIF-DONES RF System design and detailed information on the RF Stations architecture are presented in this paper.

Performance of the continuous ions suppression TPC prorotype for circular collider

The circumference of CEPC (Circular Electron Positron Collider) is 100 km, with two interaction points available for exploring different detector design scenarios and technologies. Two common RF stations are deployed for the Higgs operation, which result in 286 beam bunches evenly distributed over a half ring. While for W and Z operations, independent RF cavities are used, 5220 and 10900 bunches are spreading in equal distance over the full ring, respectively. Therefore, the bunch spacing are about 500 ns, 50 ns and 30 ns for Higgs, W and Z operations, respectively. Aimed to the CDR (Concept Design Report) and TDR (Technical Design Report) of the CEPC project, the feasibility study of TPC (Time Projection Chamber) tracker detector was initiated for the purpose to identify feasible technology options and to gain expertise to build the detector units which meet the basic requirements of the CEPC detector design. The TPC detector at the proposed circular collider will have to be operated continuously and the backflow of ions (IBF (Ions Back Flow)) must be minimized without the open/close time of a gating device technology. The gain of the selection detector module can be achieved up to about 5000 without any obvious discharge behaviour. The currents on the anode and drift cathode were measured precisely with an electrometer. The experimental results showed that IBF can be reduced to less than 0.1% at the specific gain. All update IBF results have been measured using 266 nm UV (Ultra Violet) light, 55Fe and X-ray tube. The future R&D has been mentioned to meet the high luminosity at Z.

The automatic frequency control based on artificial intelligence for compact particle accelerator.

In this work, an advantage actor critic (A2C) based intelligent automatic frequency control (AFC) system was developed for X-band linear accelerator (LINAC). A2C is one type of reinforcement learning, which indicates how software agents should perform actions in an environment. In this paper, the A2C based AFC algorithm and its environment design, simulation result, and controller hardware and software processes are described. The objective of our design is to match LINAC and magnetron frequency, and it is implemented via reward shaping using comparison with the reflected power in the adjacent time. The simulation with the A2C algorithm was implemented in two modes, namely, periodic and White Gaussian Noise (WGN) waves, for analysis with temperature and random disturbance, respectively. In order to create artificial disturbance in the experiment, the magnetron shaft was shifted randomly every 0.5 s by using WGN with 18° angle of the step motor. The standard deviation of the reflected power was 5.63 kW, and the average power was 130.9 kW. To obtain maximum reward at the beginning of the A2C training, the adjacent frequency is needed. The measured average reflected power and standard deviation were 122.8 kW and 1.75 kW, respectively, after 2000 iterations. The results show that the reflected power and standard deviation of the AFC with A2C were lower compared to open-loop with artificial disturbance. The RF station of a medical X-band LINAC was used as the test bench, and the performance was confirmed by the results of an experiment conducted at Sungkyunkwan University in Korea.

RF System for the NICA Collider

The new NICA (Nuclotron-based Ion Collider fAсility) accelerator complex being built at the Joint Institute for Nuclear Research (Dubna, Russia) is intended for experiments with 197Au79+ gold nuclei at energies in the range of 1–4.5 GeV/n. The RF system of the collider should provide the capture of bunches to be injected from the nuclotron, the accumulation of the necessary number of particles, their acceleration (if necessary), and the formation of bunches. For this, in each ring of the collider, three types of accelerating stations are used: one barrier station and two harmonic stations. The barrier station accumulates the particles and can used to accelerate them up to the energy of the experiment. The harmonic RF stations operating at the 22th and 66th harmonics of the revolution frequency are used for the formation of bunches. This paper describes the operating modes of the RF system and the design features of the RF stations.

Beam-based optimization of SwissFEL low-level RF system

SwissFEL is a free electron laser (FEL) under commissioning at the Paul Scherrer Institut (PSI) in Switzerland. Digital low-level RF (LLRF) systems are used in SwissFEL to control more than 30 RF stations with either standing wave cavities (e.g. RF gun) or travelling wave structures working at different frequencies. After conditioned to desired power levels, the RF stations need to be setup for beam operation and the LLRF parameters need to be optimized for maximizing the beam stability. Several beam-based algorithms were developed to facilitate the setup, calibration and optimization of the SwissFEL RF stations for beam operation. The algorithms were implemented as automation procedures in the framework of Experimental Physics and Industrial Control System (EPICS) and were used in commissioning and daily operation of SwissFEL. In this paper, the algorithms and the implementation will be introduced together with the test results during the commissioning of SwissFEL.

Efficient model for low-energy transverse beam dynamics in a nine-cell 1.3 GHz cavity

FLASH and the European XFEL are SASE-FEL user facilities, at which superconducting TESLA cavities are operated in a pulsed mode to accelerate long bunch-trains. Several cavities are powered by one klystron. While the low-level rf system is able to stabilize the vector sum of the accelerating gradient of one rf station sufficiently, the rf parameters of individual cavities vary within the bunch-train. In correlation with misalignments, intrabunch-train trajectory variations are induced. An efficient model is developed to describe the effect at low beam energy, using numerically adjusted transfer matrices and discrete coupler kick coefficients, respectively. Comparison with start-to-end tracking and dedicated experiments at the FLASH injector will be shown. The short computation time of the derived model allows for comprehensive numerical studies on the impact of misalignments and variable rf parameters on the transverse intra-bunch-train beam stability at the injector module. Results from both, statistical multibunch performance studies and the deduction of misalignments from multibunch experiments are presented.

Fundamental cavity impedance and longitudinal coupled-bunch instabilities at the High Luminosity Large Hadron Collider

The interaction between beam dynamics and the radio frequency (rf) station in circular colliders is complex and can lead to longitudinal coupled-bunch instabilities at high beam currents. The excitation of the cavity higher order modes is traditionally damped using passive devices. But the wakefield developed at the cavity fundamental frequency falls in the frequency range of the rf power system and can, in theory, be compensated by modulating the generator drive. Such a regulation is the responsibility of the low-level rf (llrf) system that measures the cavity field (or beam current) and generates the rf power drive. The Large Hadron Collider (LHC) rf was designed for the nominal LHC parameter of 0.55 A DC beam current. At 7 TeV the synchrotron radiation damping time is 13 hours. Damping of the instability growth rates due to the cavity fundamental (400.789 MHz) can only come from the synchrotron tune spread (Landau damping) and will be very small (time constant in the order of 0.1 s). In this work, the ability of the present llrf compensation to prevent coupled-bunch instabilities with the planned high luminosity LHC (HiLumi LHC) doubling of the beam current to 1.1 A DC is investigated. The paper conclusions are based on the measured performances of the present llrf system. Models of the rf and llrf systems were developed at the LHC start-up. Following comparisons with measurements, the system was parametrized using these models. The parametric model then provides a more realistic estimation of the instability growth rates than an ideal model of the rf blocks. With this modeling approach, the key rf settings can be varied around their set value allowing for a sensitivity analysis (growth rate sensitivity to rf and llrf parameters). Finally, preliminary measurements from the LHC at 0.44 A DC are presented to support the conclusions of this work.

RF Control Optimization and Automation for Normal Conducting Linear Accelerators

Normal conducting linear accelerators are widely used in many free-electron laser (FEL) machines all over the world. They normally consist of multiple RF stations either with standing wave cavities (e.g., RF Gun) or with traveling wave accelerating structures working at different frequencies. Low-level radio frequency (LLRF) systems are used to measure the RF fields in cavities or structures and correct the fluctuations in RF fields with pulse-to-pulse feedback controllers. To facilitate the operations of multiple RF stations, the LLRF system should also provide algorithms and procedures to automate the setup, calibration, and optimization of the RF systems. In this paper, several typical algorithms and procedures will be described, such as calibrating the digital-to-analog converters offset to reduce the RF leakage from vector modulator, calibrating the RF signal group delay, and flattening the intrapulse phase distribution with an adaptive feed forward. The algorithms and procedures have been implemented as an LLRF high-level applications (HLAs) software for SwissFEL, which is an FEL machine under commissioning at Paul Scherrer Institut, Switzerland. The architecture of the LLRF HLA software will be introduced, and the test results at SwissFEL will also be described.

Development of a high gradient rf system using a nanocrystalline soft magnetic alloy

The future high intensity upgrade project of the J-PARC (Japan Proton Accelerator Research Complex) MR (Main Ring) includes developments of high gradient rf cavities and magnet power supplies for high repetition rate. The scenario describing the cavity replacements is reported. By the replacement plan, the total acceleration voltage will be almost doubled, while the number of rf stations remains the same. The key issue is the development of a high gradient rf system using high impedance magnetic alloy, FT3L. The FT3L is produced by the transverse magnetic field annealing although the present cavity for the J-PARC adopts the magnetic alloy, FT3M, which is annealed without magnetic field. After the test production using a large spectrometer magnet in 2011, a dedicated production system for the FT3L cores was assembled in 2012. This setup demonstrated that we can produce material with 2 times higher ${\ensuremath{\mu}}_{p}^{\ensuremath{'}}\mathrm{Qf}$ product compared to the cores used for present cavities. In this summer, the production system was moved to the company from J-PARC and is used for mass production of 280 FT3L cores for the J-PARC MR. The cores produced in the first test production are already used for standard machine operation. The operation experience shows that the power loss in the cores was reduced significantly as expected.

Radio frequency noise effects on the CERN Large Hadron Collider beam diffusion

Radio frequency (rf) accelerating system noise can have a detrimental impact on the Large Hadron Collider (LHC) performance through longitudinal motion and longitudinal emittance growth. A theoretical formalism has been developed to relate the beam and rf station dynamics with the bunch length growth [T. Mastorides et al., Phys. Rev. ST Accel. Beams 13, 102801 (2010)]. Measurements were conducted at LHC to determine the performance limiting rf components and validate the formalism through studies of the beam diffusion dependence on rf noise. As a result, a noise threshold was established for acceptable performance which provides the foundation for beam diffusion estimates for higher energies and intensities. Measurements were also conducted to determine the low level rf noise spectrum and its major contributions, as well as to validate models and simulations of this system.

Interfaces and Communication Protocols in ATCA-Based LLRF Control Systems

Linear accelerators driving Free Electron Lasers (FELs), such as the Free Electron Laser in Hamburg (FLASH) or the X-ray Free Electron Laser (XFEL), require sophisticated Low Level Radio Frequency (LLRF) control systems. The controller of the LLRF system should stabilize the phase and amplitude of the field in accelerating modules below 0.02% of the amplitude and 0.01 degree for phase tolerances to produce an ultra stable electron beam that meets the required conditions for Self-Amplified Spontaneous Emission (SASE). Since the LLRF system for the XFEL must be in operation for the next 20 years, it should be reliable, reproducible and upgradeable. Having in mind all requirements of the LLRF control system, the Advanced Telecommunications Computing Architecture (ATCA) has been chosen to build a prototype of the LLRF system for the FLASH accelerator that is able to supervise 32 cavities of one RF station. The LLRF controller takes advantage of features offered by the ATCA standard. The LLRF system consists of a few ATCA carrier blades, Rear Transition Modules (RTM) and several Advanced Mezzanine Cards (AMCs) that provide all necessary digital and analog hardware components. The distributed hardware of the LLRF system requires a number of communication links that should provide different latencies, bandwidths and protocols. The paper presents the general view of the ATC A-based LLRF system, discusses requirements and proposes an application for various interfaces and protocols in the distributed LLRF control system.

Prototype Control System for Compensation of Superconducting Cavities Detuning Using Piezoelectric Actuators

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Pulsed operation of high gradient superconducting radio frequency (SCRF) cavities results in dynamic Lorentz force detuning (LFD) approaching or exceeding the bandwidth of the cavity of order of a few hundreds of Hz. The resulting modulation of the resonance frequency of the cavity is leading to a perturbation of the amplitude and phase of the accelerating field, which can be controlled only at the expense of RF power. Presently, at various labs, a piezoelectric fast tuner based on an active compensation scheme for the resonance frequency control of the cavity is under study. The tests already performed in the Free Electron Laser in Hamburg (FLASH), proved the possibility of Lorentz force detuning compensation by the means of the piezo element excited with the single period of sine wave prior to the RF pulse. The X-Ray Free Electron Laser (X-FEL) accelerator, which is now under development in Deutsche Elektronen—Synchrotron (DESY), will consists of around 800 cavities with a fast tuner fixture including the actuator/sensor configuration. Therefore, it is necessary to design a distributed control system which would be able to supervise around 25 RF stations, each one comprised of 32 cavities. The Advanced Telecomunications Computing Architecture (ATCA) was chosen to design, develop, and build a Low Level Radio Frequency (LLRF) controller for X-FEL. The prototype control system for Lorentz force detuning compensation was designed and developed. The control applications applied in the system were fitted to the main framework of interfaces and communication protocols proposed for the ATCA-based LLRF control system. The paper presents the general view of a designed control system and shows the first experimental results from the tests carried out in FLASH facility. Moreover, the possibilities for integration of the piezo control system to the ATCA standards are discussed. </para>

Electromagnetic Analysis of Propagation and Scattering Fields in Dielectric Elliptic Cylinder on Planar Ground

Radio wave technologies using electromagnetic waves of microwaves and ultra-high frequencies have been rapidly developed for high bit rate wireless communications and RFID application in out-door and in-door spaces. Signal detection evaluation is very important factor in these systems including several objects along propagation paths of urban streets and country suburb, and, in houses and building. In order to estimate scattering and interference flelds in many environments, typical useful mod- els for scattering objects are dielectric elliptic cylinders on the planar ground. Field intensity distributions of electromagnetic waves in these circumstances are studied by Mathieu function expansion with addition theorems. Incident and scattering flelds are expanded by Mathieu func- tion series and coe-cients of series are derived by electromagnetic fleld continuity equations of boundary conditions on the elliptic cylinder and planar surface. Re∞ection and scattered flelds by elliptic cylinders on planar ground are shown by these series co- e-cients, and also flelds in shadow regions between dielectric elliptic cylinders and planar ground. Field intensities in the elliptic cylinder including resonance cases for particular frequencies are evaluated. Re∞ection and scattered flelds are compared with computer simulation results de- rived by numerical calculation of the FDTD method. Based on these fleld calculations optimum system design of WiMAX wireless communications and RFID systems in out-door and in-door space regions are investigated. Positions and locations of radiating and receiving antenna for RF stations and RF readers may be decided from fleld results of this theory.

Analysis of longitudinal beam dynamics behavior and rf system operative limits at high-beam currents in storage rings

A dynamics simulation model is used to estimate limits of performance of the Positron-Electron Project (PEP-II). The simulation captures the dynamics and technical limitations of the Low Level Radio Frequency (LLRF) system, the high-power RF components and the low-order mode coupled bunch longitudinal beam dynamics. Simulation results showing the effect of non-linearities on the LLRF loops, and studies of the effectiveness of technical component upgrades are reported, as well as a comparison of these results with PEP-II measurements. These studies have led to the estimation of limits and determining factors in the maximum stored current that the Low Energy Ring/High Energy Ring (LER/HER) can achieve, based on system stability for different RF station configurations and upgrades. In particular, the feasibility of the PEP-II plans to achieve the final goal in luminosity, which required an increase of the beam currents to 4A for LER and 2.2A for HER, is studied. These currents are challenging in part because they would push the longitudinal low-order beam mode stability to the limit, and the klystron forward power past a level of satisfactory margin. An acceptable margin is defined in this paper, which in turn determines the corresponding klystron forward power limitation.