Low Level RF Research Articles

Modeling the interaction of a heavily beam loaded SRF cavity with its low-level RF feedback loops

A superconducting radio frequency (SRF) cavity provides superior stability to power high intensity light sources and can suppress coupled-bunch instabilities due to its smaller impedance for higher order modes. Because of these features, SRF cavities are commonly used for modern light sources, such as the TLS, CLS, DLS, SSRF, PLS-II, TPS, and NSLS-II, with an aggressive approach to operate the light sources at high beam currents. However, operating a SRF cavity at high beam currents may result with unacceptable stability problems of the low level RF (LLRF) system, due to drifts of the cavity resonant frequency caused by unexpected perturbations from the environment. As the feedback loop gets out of control, the cavity voltage may start to oscillate with a current-dependent characteristic frequency. Such situations can cause beam abort due to the activation of the interlock protection system, i.e. false alarm of quench detection. This malfunction of the light source reduces the reliability of SRF operation. Understanding this unstable mechanism to prevent its appearance becomes a primary task in the pursuit of highly reliable SRF operation. In this paper, a Pedersen model, including the response of the LLRF system, was used to simulate the beam-cavity interaction of a SRF cavity under heavy beam loading. Causes for the onset of instability at high beam current will be discussed as well as remedies to assure the design of a stable LLRF system.

Real-time cavity simulator-based low-level radio-frequency test bench and applications for accelerators

Real-time cavity simulator-based low-level radio-frequency test bench and applications for accelerators

Fully Digital and White Rabbit-Synchronized Low-Level RF System for LIPAc

The International Fusion Materials Irradiation Facility (IFMIF) is an international project to study and qualify candidate materials for the construction of a future fusion reactor. One of the objectives of the IFMIF-Engineering Validation and Engineering Design Activity Project is to build a linear prototype accelerator (LIPAc) to validate the final IFMIF accelerator concept. LIPAc, which is currently under construction in Rokkasho (Japan), will generate a 9-MeV deuteron beam of 125-mA current with 100% duty cycle. CIEMAT (Spain) is in charge of providing the RF power system, including the low-level radio frequency (LLRF) system. Most of the developed LLRF systems are not completely digital, as they use analog front ends for intermediate frequency conversion before or after digitalization. However, the LIPAc LLRF system is a fully digital system: no analog frequency conversion is performed, the radio frequency (RF) signals are directly digitally synthesized and sampled by means of high-speed digital-to-analog converters and analog-to-digital converters. This is a clear advantage in terms of flexibility, reliability, reconfigurability, cost, and response time, as all the signal processing is performed in the digital domain. The other main advantages and novelties are the use of White Rabbit (WR) for timing synchronization and master oscillator distribution (distributed RF over WR). The LIPAc LLRF system is the first LLRF based on WR, and it has been designed and fabricated using the most advanced technology. This paper presents the detailed description of the LIPAc LLRF system and its advantages, performance evaluation, and verification.

Development of sub-100 femtosecond timing and synchronization system.

The precise timing and synchronization system is an essential part for the ultra-fast electron and X-ray sources based on the photocathode injector where strict synchronization among RF, laser, and beams are required. In this paper, we present an integrated sub-100 femtosecond timing and synchronization system developed and demonstrated recently in Tsinghua University based on the collaboration with Lawrence Berkeley National Lab. The timing and synchronization system includes the fiber-based CW carrier phase reference distribution system for delivering stabilized RF phase reference to multiple receiver clients, the Low Level RF (LLRF) control system to monitor and generate the phase and amplitude controllable pulse RF signal, and the laser-RF synchronization system for high precision synchronization between optical and RF signals. Each subsystem is characterized by its blocking structure and is also expansible. A novel asymmetric calibration sideband signal method was proposed for eliminating the non-linear distortion in the optical synchronization process. According to offline and online tests, the system can deliver a stable signal to each client and suppress the drift and jitter of the RF signal for the accelerator and the laser oscillator to less than 100 fs RMS (∼0.1° in 2856 MHz frequency). Moreover, a demo system with a LLRF client and a laser-RF synchronization client is deployed and operated successfully at the Tsinghua Thomson scattering X-ray source. The beam-based jitter measurement experiments have been conducted to evaluate the overall performance of the system, and the jitter sources are discussed.

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.

Control and performance improvements of a pulse compressor in use for testing accelerating structures at high power

New developments relating to compact X-band, SLED-I type pulse compressors being developed at CERN for testing high gradient structures are described. Pulse compressors of interest take rf pulses from one or more high power klystrons with duration typically >1.5 μs and deliver up to 5 times the input power for a shorter duration <250 ns. Time domain models for pulse compressor operation with low level rf (LLRF) control have been developed. Input drive amplitude and phase for each pulse is evolved with a control algorithm from the pulse compressor output for previous pulses. The goal is to deliver precise amplitude for pulses to test stands and precise amplitude and phase for pulses to accelerator systems. Control algorithms have been developed and validated experimentally.

Digital low level rf control system with four different intermediate frequencies for the International Linear Collider

A field programmable gate array--based digital low level rf (LLRF) control system will be used in the International Linear Collider (ILC) in order to satisfy the rf stability requirements. The digital LLRF control system with four different intermediate frequencies has been developed to decrease the required number of analog-to-digital converters in this system. The proof of concept of this technique was demonstrated at the Superconducting RF Test Facility in the High Energy Accelerator Research Organization, Japan. The amplitude and phase stability has fulfilled the ILC requirements.

The AD and ELENA orbit, trajectory and intensity measurement systems

This paper describes the new Antiproton Decelerator (AD) orbit measurement system and the Extra Low ENergy Antiproton ring (ELENA) orbit, trajectory and intensity measurement system. The AD machine at European Organization for Nuclear Research (CERN) is presently being used to decelerate antiprotons from 3.57 GeV/c to 100 MeV/c for matter vs anti-matter comparative studies. The ELENA machine, presently under commissioning, has been designed to provide an extra deceleration stage down to 13.7 MeV/c. The AD orbit system is based on 32 horizontal and 27 vertical electrostatic Beam Position Monitor (BPM) fitted with existing low noise front-end amplifiers while the ELENA system consists of 24 \\gls{BPM}s equipped with new low-noise head amplifiers. In both systems the front-end amplifiers generate a difference (delta) and a sum (sigma) signal which are sent to the digital acquisition system, placed tens of meters away from the AD or ELENA rings, where they are digitized and further processed. The beam position is calculated by dividing the difference signal by the sum signal either using directly the raw digitized data for measuring the turn-by-turn trajectory in the ELENA system or after down-mixing the signals to baseband for the orbit measurement in both machines. The digitized sigma signal will be used in the ELENA system to calculate the bunched beam intensity and the Schottky parameters with coasting beam after passing through different signal processing chain. The digital acquisition arrangement for both systems is based on the same hardware, also used in the ELENA Low Level Radio Frequency (LLRF) system, which follows the VME Switched Serial (VXS) enhancement of the Versa Module Eurocard 64x extension (VME64x) standard and includes VITA 57 standard Field Programmable Gate Array Mezzanine Card (FMC). The digital acquisition Field Programmable Gate Array (FPGA) and Digital Signal Processor (DSP) firmware shares many common functionalities with the LLRF system but has been tailored for this measurement application in particular. Specific control and acquisition software has been developed for these systems. Both systems are installed in AD and ELENA. The AD orbit system currently measures the orbit in AD while the ELENA system is being used in the commissioning of the ELENA ring.

A 166.6 MHz superconducting rf system for the HEPS storage ring

A superconducting 166.6 MHz quarter-wave beta=1 cavity was recently proposed for the High Energy Photon Source (HEPS), a 6 GeV kilometer-scale light source. Four 166.6 MHz cavities will be used for main acceleration in the newly planned on-axis beam injection scheme realized by a double-frequency RF system. The fundamental frequency, 166.6 MHz, was dictated by the fast injection kicker technology and the preference of using 499.8 MHz SC RF cavity as the third harmonic. Each 166.6 MHz cavity will be operated at 4.2 K providing 1.2 MV accelerating voltage and 150 kW of power to the electron beam. The input coupler will use single-window coaxial type graded up to 200 kW CW power. Each cavity will be equipped with a 200 kW solid-state amplifier and digital low-level RF system. This paper describes the 166.6 MHz RF system with a focus on the design and optimization of the RF cavity and its ancillaries, the LLRF system and the status of the solid-state amplifiers.

A Model-Based Fast Protection System for High-Power RF Tube Amplifiers Used at the European XFEL Accelerator

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.

Validation of the superconducting 3.9 GHz cavity package for the European X-ray Free Electron Laser

A full test of the cavity package concept under realistic operating condition was a necessary step before the assembly of the European XFEL (EXFEL) 3.9 GHz superconducting system and its installation in the accelerator. One cavity, equipped with magnetic shielding, power coupler and frequency tuner has been tested in a specially designed single cavity cryostat in one of the test benches of the DESY Accelerator Module Test Facility (AMTF). The cavity was operated at high pulsed power up to an accelerating field of $24\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, above the quench accelerating field of $21\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ achieved during the continuous wave (CW) vertical qualification test and with a large margin with respect to the EXFEL maximum operating specification of $15\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ for the 3.9 GHz system. All subsystems under test---coupler, tuner, waveguide tuners, low level radio-frequency (LLRF) system---were qualified to their design performances.

Adaptive Control of Low-Level Radio Frequency Signals Based on In-Phase and Quadrature Components

This research work presents a low-level radio frequency (RF) control method based on the in-phase, $I$ , and quadrature, $Q$ , components of the RF signal. The proposed method uses only the main four arithmetic operations, i.e., addition, subtraction, multiplication, and division, which makes this control method suitable for implementation on the field-programmable gate array. The control scheme is adaptive in the sense that it estimates the system response on-the-fly, and therefore, it is robust against changes in the loop phase and/or gain during the operation.

Laser–RF synchronization based on digital phase detector

Laser oscillator synchronization with RF reference signal is ultra-important for a modern light source based on the accelerator. For Tsinghua Thomson scattering X-ray source, we have synchronized the mode-locked laser oscillator to RF reference signal with 48.2 fs RMS relative jitter. Both fundamental and harmonic signals derived from photo diode detection are used for laser–RF synchronization in our scheme. The fundamental signal is for coarse laser–RF synchronization and multiple laser oscillator synchronization. The harmonic signal is for high precise phase locking. The digital phase detector is implemented in the synchronization scheme for less noise, replacing the mixing to DC phase detection scheme. The digital processing algorithm for synchronization is commonly used in low-level RF control field. In order to test the phase locking loop logic without damaging the real laser oscillator, a laser oscillator emulator was developed for phase locking. This paper will report the laser–RF synchronization scheme and its performance. The laser oscillator emulator system will also be introduced here.

Superconducting Cavity Control and Model Identification Based on Active Disturbance Rejection Control

Superconducting cavities are widely used in linear accelerator facilities all over the world. It is challenging for the low level RF (LLRF) system to stabilize the RF field inside the cavities working in pulsed mode with heavy beam loading and strong Lorenz force detuning. Feedback loops are implemented in LLRF system which should have fast responses to the disturbances on cavities. To enhance the feedback performance, active compensation of Lorenz force detuning with Piezo tuners and the compensation of beam loading with feed forward control are strongly needed. They all require the knowledge of cavity parameters, such as the quality factor, time varying detuning and the amplitude and phase of the beam. A disturbance observer based on the active disturbance rejection control (ADRC) was designed for superconducting cavities counting the Lorenz force detuning and beam loading as disturbances to the cavities. The ADRC disturbance observer can be integrated into LLRF feedback loops to speed up the responses to disturbances and the outputs of the observer are interpreted furtherly to identify the cavity parameters. This paper presents the algorithm and architecture of the ADRC disturbance observer, the simulation of the LLRF feedback loop with disturbance rejection and the cavity parameter identification applied on the archived data of superconducting cavities in operation.

Traffic Decorrelation Techniques for Countering a Global Eavesdropper in WSNs

We address the problem of preventing the inference of contextual information in event-driven wireless sensor networks (WSNs). The problem is considered under a global eavesdropper who analyzes low-level RF transmission attributes, such as the number of transmitted packets, inter-packet times, and traffic directionality, to infer event location, its occurrence time, and the sink location. We devise a general traffic analysis method for inferring contextual information by correlating transmission times with eavesdropping locations. Our analysis shows that most existing countermeasures either fail to provide adequate protection, or incur high communication and delay overheads. To mitigate the impact of eavesdropping, we propose resource-efficient traffic normalization schemes. In comparison to the state-of-the-art, our methods reduce the communication overhead by more than 50 percent, and the end-to-end delay by more than 30 percent. To do so, we partition the WSN to minimum connected dominating sets that operate in a round-robin fashion. This allows us to reduce the number of traffic sources active at a given time, while providing routing paths to any node in the WSN. We further reduce packet delay by loosely coordinating packet relaying, without revealing the traffic directionality.

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.

General DSP firmware for cyclotron low level RF system

General DSP firmware for cyclotron low level RF system

Updates to the Low-Level RF Architecture for Fermilab

Updates to the Low-Level RF Architecture for Fermilab

An S-band solid-state radio frequency power amplifier used at Shanghai soft X-ray FEL facility

In this paper, we present the general design methods and parameter measurements of a 1-kW solid-state radio frequency (RF) power amplifier at 2856 MHz, for the soft X-ray free electron laser facility. Three-stage amplification with a 4-way combination is used. An RF switch module is integrated with the solid-state RF power amplifier to convert the continuous wave (CW) signal into pulse signal, with adjustable pulse width. The power gain is measured at 57.7 dB at 60 dBm output. The RF phase noise, which is measured by the low-level RF system, is <0.015 degree (RMS), while the pulse frontier jitter is <5 ns.