The driving engine of the superconducting accelerator of the European X-ray free electron laser (XFEL) is a set of 27 radio frequency (RF) stations. Each of the underground RF stations consists of a ...multibeam horizontal klystron that can provide up to 10 MW of power at 1.3 GHz. Klystrons are sensitive devices with a limited lifetime and a high mean time between failures. In real operation, the lifetime of the tube can be significantly reduced because of failures. The special fast protection klystron lifetime management (KLM) system has been developed to minimize the influence of service conditions on the lifetime of klystrons. The main task of this system is to detect all events which can destroy the tube as quickly as possible, and switch off the driving RF signal or the high voltage. Detection of events is based on a comparison of the value of the real signal obtained at the system output with the value estimated on the basis of a high-power RF amplifier model and input signals. The KLM system has been realized in field-programmable gate array (FPGA) and implemented in XFEL. Implementation is based on the standard low-level RF micro telecommunications computing architecture (MTCA.4 or xTCA). The main part of the paper focuses on an estimation of the klystron model and the implementation of KLM in FPGA. The results of the performance of the KLM system will also be presented.
Vacuum-tube amplifiers are the most widespread type of radio frequency (RF) sources used to produce high-power signals needed for beam acceleration in superconducting cavities. At Deutsches ...Elektronen-Synchrotron (DESY), megawatt-rated klystrons are used to produce millisecond-long RF shots for pulsed operation in particle accelerators. In contrast, inductive output tubes (IOTs) are used to provide a continuous RF signal for continuous-wave (CW) operation. In both cases, the amplifiers suffer from amplitude-dependent nonlinearity between the driving and generated signals. This nonlinearity complicates the setup operations of the low-level RF (LLRF) system and makes it harder to regulate the accelerating field. Therefore, a way to linearize the amplifier is highly valuable. This article covers the design, implementation, and test of a field-programmable gate array (FPGA)-based predistortion linearization unit. The first results on the performance of this component in linearizing the amplifiers of running CW and pulsed superconducting accelerators are presented. Such a component uses programmable interpolating lookup tables (LUT) that are addressed using the squared value of the requested signal amplitude. This component only adds 64-ns latency to the RF control system without relying on any vendor-dependent FPGA component. Other benefits of using programmable interpolating LUT are low usage of FPGA resources and flexibility in terms of the type of amplifier to be corrected. The benefits of using this linearizer for klystrons and IOTs are presented and quantified.
Low-level radio frequency (LLRF) systems have been designed to regulate the accelerator field in the cavity; these systems have been used in the free electron laser (FLASH) and European X-ray ...free-electron laser (E-XFEL). However, the reliable operation of these cavities is often hindered by two primary sources of noise and disturbances: Lorentz force detuning (LFD) and mechanical vibrations, commonly known as microphonics. This article presents an innovative solution in the form of a narrowband active noise controller (NANC) designed to compensate for the narrowband mechanical noise generated by certain supporting machines, such as vacuum pumps and helium pressure vibrations. To identify the adaptive filter coefficients in the NANC method, a least mean squares (LMS) algorithm is put forward. Furthermore, a variable step size (VSS) method is proposed to estimate the adaptive filter coefficients based on changes in microphonics, ultimately compensating for their effects on the cryomodule. An accelerometer with an SPI interface and some transmission boards are manufactured and mounted at the cryomodule test bench (CMTB) to measure the microphonics and transfer them via Ethernet cable from the cryomodule side to the LLRF crate side. Several locations had been selected to find the optimal location for installing the accelerometer. The proposed NANC method is characterized by low computational complexity, stability, and high tracking ability. By addressing the challenges associated with noise and disturbances in cavity operation, this research contributes to the enhanced performance and reliability of LLRF systems in particle accelerators.
Superconducting cavities are responsible for beam acceleration in superconducting linear accelerators. Challenging cavity control specifications are necessary to reduce radio frequency (RF) costs and ...to maximize the availability of the accelerator. Cavity detuning and bandwidth are two critical parameters to monitor when operating particle accelerators. Cavity detuning is strongly related to the power required to generate the desired accelerating gradient. Cavity bandwidth is related to the cavity RF losses. A sudden increase in bandwidth can indicate the presence of a quench or multipacting event. Therefore, calculating these parameters in real time in the low-level RF (LLRF) system is highly desirable. A real-time estimation of the bandwidth allows for a faster response of the machine protection system in the case of quench events, whereas the estimation of cavity detuning can be used to drive piezoelectric tuner-based resonance control algorithms. In this article, a new field programmable gate array (FPGA)-based estimation component is presented. Such a component is designed to be used either in continuous wave (CW) or pulsed operation mode with loaded quality factors between <inline-formula> <tex-math notation="LaTeX">10^{6} </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">10^{8} </tex-math></inline-formula>. Results of this component with free-electron LASer in Hamburg (FLASH), European X-ray free electron laser (EuXFEL), cryo module test bench (CMTB), and electron linac for beams with high brilliance and low emittance (ELBE) are presented.
Modern digital low level radio frequency (RF) control systems used to stabilize the accelerating field in facilities, such as free electron laser in Hamburg or the European X-ray free electron laser, ...are based on the field programmable gate array (FPGA) technology. Presently, these accelerator facilities are operated with pulsed RF. In the future, these facilities will operate with the continuous wave (CW), which requires significant modifications on the real-time feedbacks realized within the FPGA. For example, higher loaded quality factor of the superconducting RF cavities operated in the CW mode requires sophisticated resonance control methods. However, iterative learning techniques widely used for machines operated in pulsed mode are not applicable for the CW. In addition, the mechanical characteristic of the cavities now have a much more important impact on the choice of the feedback scheme. To overcome the limitations of classical proportional-integral controllers, a novel real-time adaptive feed-forward algorithm is implemented in the FPGA. Also, the high power RF amplifier, which is an inductive output tube (IOT) for CW operation instead of a klystron for the pulsed mode, has a major impact on the design and implementation of the firmware for regulation. In this paper, we report on our successful approach to control the multicavity vector sum with an ultrahigh precision (amplitude error <;0.01% rms and phase stability <;0.02° rms), using a single IOT source and the individual resonance control through piezoactuators. Performance measurements of the proposed solution were conducted at the cryomodule test bench facility.
Stable and reproducible generation of a photon beam at Free Electron Lasers (FELs) necessitates a low energy spread of the electron beam. This spread is disturbed by various factors. A low level ...radio frequency (LLRF) control system stabilizes the RF field inside accelerating modules, as amplitude and phase fluctuations are the main bunch-to-bunch energy spread sources. This paper describes the architecture of the LLRF control system based on the Micro-Telecommunications Computing Architecture (MTCA.4) platform developed by the PCI Industrial Computer Manufacturers Group (PICMG). This architecture offers manageability, reliability, and scalability which are crucial for high energy physics experiments. The hardware modules such as digitizers, down-converters (DWC), or vector modulators (VM) have been adopted from multi-channel, vector sum based systems and optimized. The field detection scheme utilizes the analog frequency shifting to an intermediate frequency (IF). Analog-to-digital conversion and digital complex frequency down-conversion are applied. The firmware has been optimized for minimal latency. The software communicates with the firmware using a PCIe bus, reads waveforms for diagnostic purposes, and abstracts hardware and firmware settings for the paneling system. Distinctive characteristics of the CW mode of operation affecting the LLRF system are discussed. The system design process and the important parameter value selection criteria are presented. The noise contribution of hardware subcomponents and other limitations of the field detection are analyzed. The digital signal processing path is split into atomic operations and examined. Interfacing with other systems is discussed. Proposals for future developments are given.
An increasing number of unhardened commercial-off-the-shelf embedded devices are deployed under harsh operating conditions and in highly-dependable systems. Due to the mechanisms of hardware ...degradation that affect these devices, ageing detection and monitoring are crucial to prevent critical failures. In this paper, we empirically study the propagation delay of 298 naturally-aged FPGA devices that are deployed in the European XFEL particle accelerator. Based on in-field measurements, we find that operational devices show significantly slower switching frequencies than unused chips, and that increased gamma and neutron radiation doses correlate with increased hardware degradation. Furthermore, we demonstrate the feasibility of developing machine learning models that estimate the switching frequencies of the devices based on historical and environmental data.
The driving engine of the superconducting accelerator of the European X-ray Free-Electron Laser (XFEL) are 27 Radio Frequency (RF) stations. Each of an underground RF station consists from multi-beam ...horizontal klystron which can provide up to 10MW of power at 1.3GHz. Klystrons are sensitive devices with limited lifetime and high mean time between failures. In the real operation the lifetime of the tube can be thoroughly reduced by failures. To minimize the influence of service conditions to the klystrons lifetime the special fast protection system named as Klystron Lifetime Management System (KLM) has been developed. The main task of this system is to detect all events which can destroy the tube as quickly as possible and switch off driving RF signal or HV. Detection of events is based on comparison of model of high power RF amplifier with real signals. Implementation is done in Field Programmable Gate Array (FPGA). For the XFEL implementation of KLM is based on the standard Low Level RF (LLRF) Micro Tele-communications Computing Architecture (MTCA.4 or xTCA). This article focuses on the klystron model estimation and implementation of KLM in FPGA. Results of the system implemented on MTCA.4 architecture will be presented in the end.
The recent introduction of advanced hardware architectures such as the Micro Telecommunications Computing Architecture (MTCA) caused a change in the approach to implementation of control schemes in ...many fields. It required the development to move away from traditional programming languages (C/C++) to hardware description languages (VHDL, Verilog), which are used in FPGA development. With MATLAB/Simulink it is possible to describe complex systems with block diagrams and simulate their behavior. Those diagrams are then used by the HDL experts, to implement exactly the required functionality in hardware. Both the porting of existing applications and adaptation of new ones requires a lot of development time from them. To solve this, Xilinx System Generator, a toolbox for MATLAB/Simulink, allows rapid prototyping of those block diagrams using hardware modelling. It is still up to the firmware developer to merge this structure with the hardware-dependent HDL project. This prevents the application engineer from quickly verifying the proposed schemes in real hardware. The framework described in this article overcomes these challenges, offering a hardware-independent library of components that can be used in Simulink/System Generator models. The components are subsequently translated into VHDL entities and integrated with a pre-prepared VHDL project template. Furthermore, the entire implementation process is run in the background, giving the user an almost one-click path from control scheme modelling and simulation to bit-file generation. This approach allows the control theory engineers to quickly develop new schemes and test them in real hardware environment. The applications may range from simple data logging or signal generation ones to very advanced controllers. Taking advantage of the Simulink simulation capabilities and user-friendly hardware implementation routines, the framework significantly decreases the development time of FPGA-based applications.
Modern digital low level radio frequency (LLRF) control systems used to stabilize the accelerating field in facilities such as Free Electron Laser in Hamburg (FLASH) or European X-Ray Free Electron ...Laser (E-XFEL) are based on the Field Programmable Gate Array (FPGA) technology. Presently these accelerator facilities are operated with pulsed RF. In future, these facilities should be operated with continuous wave (CW) which requires significant modifications on the real-time feedbacks realized within the FPGA. For example, higher loaded quality factor of the cavities when operated in a CW mode requires sophisticated resonance control methods. However, iterative learning techniques widely used for machines operated in pulsed mode are not applicable for CW. In addition, the mechanical characteristic of the cavities have now a much more important impact on the choice of the feedback scheme. To overcome the limitations of classical PI-controllers novel realtime adaptive feed forward algorithm is implemented in the FPGA. Also, the high power RF amplifier which is an inductive output tube (IOT) for continuous wave operation instead of a klystron for the pulsed mode has major impact on the design and implementation of the firmware for regulation. In this paper, we report on our successful approach to control multi-cavities with ultra-high precision (dA/A<;0.01%, dphi<;0.02 deg) using a single IOT source and individual resonance control through piezo actuators. Performance measurements of the proposed solution were conducted at Cryo Module Test Bench (CMTB) facility.
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