Wojciech Cichalewski

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.

The European Spallation Source (ESS) is a neutron source powered by a 5 MW, 2 GeV superconducting proton linac which is currently being constructed in Lund. Due to high beam current, just a few microseconds of operation with a misaligned beam can melt the accelerating cavity wall. Therefore, a Beam Loss Monitor (BLM) system is necessary to protect hardware and personnel from the consequences of beam loss. The BLM’s main function is to inhibit beam production when excessively high beam losses are detected. Besides protection functionality, the system is anticipated to provide the means to monitor the beam losses during all modes of operation with the objective of preventing excessive machine activation. This paper concentrates on implementation for ionization chamber Beam Loss Monitor (ICBLM) and Neutron Sensitive Beam Loss Monitor (nBLM) systems. The required microsecond reaction time has been achieved due to the application of an FPGA. A single detector module was installed and successfully tested at LINAC4 at CERN, in the section where conditions are similar to the ones expected in the ESS linac. The summary of these tests is also included.

This paper proposes an optimal variable step size least mean square algorithm of the active noise controller (ANC). The ANC compensates for microphonics effects in the cavities. Microphonics is a prominent vibration source in the resonant frequency when resonators operate in continuous wave mode. The microphonics increase the required RF power reserve. IT also causes the amplitude and phase-jitter of the accelerated electron beam. Therefore, it is profitable to control and identify microphonics. Convergence rate, tacking ability, and low computational complexity are essential parameters for the performance of adaptive algorithms. The proposed method presents a high convergence rate and low computational complexity in the shortest period. The proposed method is implemented on the SIS8300-KU board. The top module occupies the FPGA resources of 4.69 LUT, 3.46 FF, and 2.29 DSP.

In the European Spallation Source (ESS) accel-erator, which is currently being built in Lund, Sweden, the typical hardware platform for fast acquisition devices is Micro Telecommunications Computing Architecture.4 (MTCA.4). A major part of the LLRF control system devices are designed as MTCA.4 modules - Advanced Mezzanine Carrier (AMCs) or Rear Transition Modules (RTMs). The main feature of the mentioned standard is its high reliability and maintainability. Those aspects are crucial in terms of keeping the accelerator in its operational state. Thanks to the management layer in MTCA.4 which is based on Intelligent Platform Management Interface (IPMI) protocol it is possible to control and monitor modules state. The paper presents the softwares development process sup-porting MTCA.4 chassis management. It shows the evolution of the proposed tool (in two phases), and how it has been modified and improved thanks to users feedback. It explains why we started our work with an EPICS application with a single Input/Output Controller (IOC) and why our final solution is composed of several separate modules. Finally, the paper describes tests and activities performed to confirm the proper functioning of the software. It also discusses areas of possible improvements and encountered issues.

The European Spallation Source (ESS), currently under construction in Lund, Sweden, will be a neutron source based on a partly superconducting linac. The ESS linac will be accelerating protons to 2 GeV with a peak current of 62.5 mA and ultimately delivering a 5 MW beam to a rotating tungsten target for neutron production. For a successful tuning and operation of a linac, a beam loss monitoring (BLM) system is required. BLM systems are designed to protect the machine from beam-induced damage and unnecessary activation of the components. This paper focuses on one of the BLM systems to be deployed at the ESS linac, namely the neutron sensitive BLM (nBLM). An overview of the ESS nBLM system design will be presented. In addition to this, results of the tests performed with the full nBLM data acquisition chain will be discussed. These tests represent the first evaluation of the system prototype in a realistic environment. They served as an input to initial study of the procedure for neutron detection algorithm configuration discussed in this contribution as well.

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).

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 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 EXFEL specifications of 0.01% for the field amplitude and 0.01° for the phase stability is 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 8 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 high Ql is also presented and discussed.

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 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 paper covers the design, implementation, and test of an 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 Look Up 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 FPGA 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 inductive output tubes are presented and quantified.