Low Level RF Research Articles

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.

Radiofrequency Fields and Calcium Movements Into and Out of Cells.

The recent rollout of 5G telecommunications systems has spawned a renewed call to re-examine the possibility of so-called "non-thermal" harmful effects of radiofrequency (RF) radiation. The possibility of calcium being affected by low-level RF has been the subject of research for nearly 50 years and there have been recent suggestions that voltage-gated calcium channels (VGCCs) are "extraordinarily sensitive" to ambient RF fields. This article examines the feasibility of particularly modulated RF coupling to gating mechanisms in VGCCs and also reviews studies from the literature from the last 50 years for consistency of outcome. We conclude that the currents induced by fields at the ICNIRP guideline limits are many orders of magnitude below those needed to affect gating, and there would need to be a biological mechanism for detection and rectification of the extremely-low-frequency (ELF) modulations, which has not been demonstrated. Overall, experimental studies have not validated that RF affects Ca2+ transport into or out of cells.

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.

Effects of radiofrequency field exposure on proteotoxic-induced and heat-induced HSF1 response in live cells using the bioluminescence resonance energy transfer technique.

As of today, only acute effects of RF fields have been confirmed to represent a potential health hazard and they are attributed to non-specific heating (≥ 1°C) under high-level exposure. Yet, the possibility that environmental RF impact living matter in the absence of temperature elevation needs further investigation. Since HSF1 is both a thermosensor and the master regulator of heat-shock stress response in eukaryotes, it remains to assess HSF1 activation in live cells under exposure to low-level RF signals. We thus measured basal, temperature-induced, and chemically induced HSF1 trimerization, a mandatory step on the cascade of HSF1 activation, under RF exposure to continuous wave (CW), Global System for Mobile (GSM), and Wi-Fi-modulated 1800MHz signals, using a bioluminescence resonance energy transfer technique (BRET) probe. Our results show that, as expected, HSF1 is heat-activated by acute exposure of transiently transfected HEK293T cells to a CW RF field at a specific absorption rate of 24W/kg for 30min. However, we found no evidence of HSF1 activation under the same RF exposure condition when the cell culture medium temperature was fixed. We also found no experimental evidence that, at a fixed temperature, chronic RF exposure for 24h at a SAR of 1.5 and 6W/kg altered the potency or the maximal capability of the proteasome inhibitor MG132 to activate HSF1, whatever signal used. We only found that RF exposure to CW signals (1.5 and 6W/kg) and GSM signals (1.5W/kg) for 24h marginally decreased basal HSF1 activity.

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 and Performance Test of the FPGA-Based Digital LLRF for RAON Accelerator

An ion accelerator, RAON is under construction in Daejeon, Korea, by the Rare Isotope Science Project (RISP) in the Institute of Basic Science (IBS). In this accelerator various ions, such as uranium, proton, carbon, calcium, etc. are planned to be accelerated by using four kinds of superconducting cavities. The four kinds of cavities are the quarter wave resonator (QWR), half wave resonator (HWR), and two kinds of single spoke resonators (SSRs) with different betas. Recently, the upgrade and the performance tests with newly developed field-programmable gate array (FPGA)-based digital low level radio frequency (LLRF) and superconducting cavities were conducted at the superconducting radio frequency (SRF) facility. In this paper, the status and the test result of the RAON LLRF will be described.

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.

Design of an RF Reference Distribution System for the RISP Linac

The heavy-ion accelerator of the Rare Isotope Science Project (RISP) in Korea has been developed. The RF reference distribution system must deliver phase reference signals to all low-level RF (LLRF) systems and BPM systems with low phase noise and low phase drift. The frequencies of the RISP linac are 81.25 MHz, 162.5 MHz and 325 MHz, and 30 LLRF systems and 60 BPMs are used for SCL3, and 210 LLRF systems and 60 BPMs are used for SCL2. A signal with a frequency of 81.25 MHz is chosen as the reference frequency signal, and a 1-5/8″ rigid coaxial line is installed with temperature control. This paper address aspects of the design for the RF reference distribution system, such as the reference frequency, phase noise on master oscillator, phase stability, temperature influence, and reference line attenuation.

Results on FPGA-Based High-Power Tube Amplifier Linearization at DESY

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.

Digital self-excited vertical test system of superconducting cavity

Vertical test is an important method for characterizing the performance of superconducting cavities. We designed an superconducting cavity vertical test system based on digital self-excited algorithm and the technology of low-level radio frequency, which could improve the vertical test efficiency of superconducting cavities. The RF front-end and clock distribution system of the vertical test system adopts the second up-and-down conversion scheme. To some extent, the working frequency of the digital self-excited loop of the vertical test system can be set flexibly, and the working bandwidth of the test system is increased. The test results of the pass-band frequency of the 1.3 GHz 9-cell superconducting cavity show that the vertical test system has strong frequency resolution (＜800 kHz) to ensure the smooth progress of the multi-cell superconducting cavity pass-band test.

Robustness issues of timing and synchronization for free electron lasers

Free electron lasers (FEL) require strict time relations for the electron bunch and RF field interaction, which must be precise and deterministic in time. This is guaranteed by the timing and synchronization systems that should be robust under the situations like a power cycle in the master oscillator, timing master, reference frequency distribution devices or low-level radio frequency (LLRF) devices. After the power cycles, the time relations should be kept or be capable to recover quickly to improve the availability of the FEL machine. This article focuses on the robustness of the timing and synchronization systems, such as the time uncertainty of the RF pulse related with the trigger, the phase uncertainty of frequency dividers after power cycles and the race condition between the trigger and clock. The possible solutions to achieve a robust design of the timing and synchronization systems are also discussed.

Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR

ABSTRACTA new upper limit on the 21 cm signal power spectrum at a redshift of z ≈ 9.1 is presented, based on 141 h of data obtained with the Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally weighted power spectrum inference. Previously seen ‘excess power’ due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25–0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-σ upper limit of $\Delta ^2_{21} \lt (73)^2\, \mathrm{mK^2}$ at $k = 0.075\, \mathrm{h\, cMpc^{-1}}$ is found, an improvement by a factor ≈8 in power compared to the previously reported upper limit. The remaining excess power could be due to residual foreground emission from sources or diffuse emission far away from the phase centre, polarization leakage, chromatic calibration errors, ionosphere, or low-level radiofrequency interference. We discuss future improvements to the signal processing chain that can further reduce or even eliminate these causes of excess power.

Multi-port cavity model and low-level RF systems design for VHF gun

Very high frequency (VHF) photocathode guns have excellent performance and are being increasingly selected as electron sources for high-repetition-rate X-ray free-electron lasers. As a highly loaded quality factor cavity, the VHF gun requires high stability in the amplitude and phase of the cavity field. However, the gun is microwave powered by two solid-state power sources through two separate power couplers. The input difference between the two power couplers will influence the stability of the cavity field. To systematically study this influence and obtain measurement formulae, a multi-port VHF gun LCR circuit model is built and analyzed. During the warm-up condition, the cavity structure will be deformed due to the large-scale change in the cavity temperature. Then, the deformation will result in cavity resonant frequency changes. To prevent the mechanic tuner from suffering damages due to the frequent and long-distance movement for correcting the cavity resonant frequency, a self-excited loop (SEL) control system is considered for changing the loop phase and make the loop frequency follow the resonant frequency. In this study, a steady-state model of the VHF gun cavity is built for obtaining the optimal input coupler coefficient and the stability requirement of the forward voltage. Then, the generator-driven resonator and SEL control system, which combine with the VHF multi-port modeling, are modeled and simulated. The simulated results show that the SEL system can perfectly operate in the process of condition and warm-up.

Active cancellation of power supply ripple effects in continuous wave superconducting radio frequency cavities

Digital low-level radio-frequency (LLRF) systems are used at the compact energy recovery linac (cERL) test facility to stabilize the accelerating field inside the radio frequency (RF) cavities. Proportional and integral feedback controllers are implemented in the LLRF system to suppress the various disturbances in the cavities. At cERL, the typical disturbances include microphonics detuning and high-voltage power supply (HVPS) ripples. These two types of disturbances are reflected in the sum of sinusoidal fluctuations in the RF phase. Recently, a real-time narrow-band active noise control (ANC) approach which aims to reject the microphonics detuning of less than several dozen Hz, was proved to be effective in cavity resonance control. We extend this ANC method to the LLRF field control to suppress the RF ripples caused by the HVPS. In this paper, we present our experience of applying the ANC algorithm to the LLRF system of the cERL facility at KEK. We confirmed that high frequency ripples of up to 20 kHz can be well compensated by the ANC method during the cERL beam-commissioning.

Development of a high-performance RF front end for HEPS 166.6 MHz low-level RF system

Digital low-level radio frequency (LLRF) system has been proposed for 166.6 MHz superconducting cavities at High Energy Photon Source. The RF field inside the cavities has to be controlled better than 0.03% (rms) in amplitude and $$0.03^\circ $$ (rms) in phase. A RF front end system is required to transform the RF signal from the cavity to IF signal before inputting into the digital signal processing (DSP) board, and up-convert the IF signal back to RF to drive the power amplifier. Connectorized off-the-shelf microwave components were used to realize the RF front end system. The local oscillator generation and distribution, choices of main components and design of down-/up-conversion channels have been elaborated in detail with a focus on minimizing nonlinearity and signal interferences among channels with optimized signal distribution loss. The RF front end has been incorporated with the existing DSP board and tested on a warm 166.6 MHz cavity in the laboratory. Excellent channel isolations and good linearities were achieved on the RF front end system. The RF field inside the cavity was controlled with a residual noise of 0.004% (rms) in amplitude and $$0.002^\circ $$ (rms) in phase, well below the HEPS specifications. The sensitivity to ambient environment changes have also been studied and presented in this paper. This demonstrates a first high-performance 166.6 MHz RF front end system and provides valuable insights into HEPS LLRF system development in the future.

Low-Level Radiofrequency Exposure Does Not Induce Changes in MSC Biology: An in vitro Study for the Prevention of NIR-Related Damage.

BackgroundThe ubiquitous diffusion of radiofrequency (RF) radiation across human living environments has attracted the attention of scientists. Though the adverse health effects of RF exposure remain debatable, it has been reported that the interaction of such radiation with biological macromolecular structures can be deleterious for stem cells, inducing impairment of their main functions involving self-renewal and differentiation.PurposeThe purpose of this study was to determine whether exposure to RF of 169 megahertz (MHz) that is part of very high radiofrequency (VHF) range 30–300 MHz, could cause damage to stem cells by inducing senescence and loss of regenerative and DNA repair capacity.MethodsThe study was conducted on mesenchymal stromal cells (MSCs) containing a subpopulation of stem cells. The MSCs were exposed to RFs of 169 MHz administered via an open meter 2G “Smart Meter” for different durations of time.ResultWe did not observe modifications in MSC biology as a result of the RF exposure conducted in our experiments.ConclusionWe concluded that MSCs are insensitive to RF radiation exposure at 169 MHz for various time intervals, including longer durations.

The preliminary long-term slow drift calibration study in low- level RF system

The phase drift of the RF signal in the low-level radio frequency (LLRF) system is observed in the long-term operation, which limits the performance and stability of the LLRF system. The long-term drift was reproduced in the lab. Its effect and sources of error were explored in the simple LLRF46 board and the simplest LLRF system. It is founded that the temperature will significantly lead to the phase distortion of the two signal channels, although with the same electron device. The distortion will finally cause the long-term drift with temperature floating. A fixed phase calibration signal (CAL signal) is applied to deal with the signal channels difference. The preliminary tests were conducted and the results were analysed.