Low Level RF Research Articles

Dual-harmonic broadband low-level RF system for HIAF booster

Dual-harmonic broadband low-level RF system for HIAF booster

An MTCA-based LLRF prototype system of LINAC in Hefei Advanced light facility

The linear accelerator (LINAC) system currently under construction at the Hefei Advanced Light Facility (HALF) employs acceleration tubes to accelerate the electron beam to 2.2 GeV. The RF field within the acceleration tube is controlled by an MTCA.4-based low-level radio frequency (LLRF) prototype system to meet stringent phase stability requirements. This paper introduces the structure of the LLRF system, encompassing both its hardware and software components. The LLRF system uses the non-IQ-sampling method to suppress odd harmonics through digital filters, achieving a signal-to-noise ratio (SNR) of approximately 73 dB. Given that the power sources of most acceleration tubes operate in a saturated state, the radio-frequency (RF) control algorithm within the software is based on a separate amplitude open-loop and phase close-loop to maintain stable phase control during amplitude disturbances. The design of the RF control logic, including digital filters, the REF phase tracking method, the close-loop feedback controller, and the output pulse mode control algorithm, is detailed. Offline pulse-to-pulse test results demonstrate that it achieves a phase stability of 0.0176° rms with a solid-state amplifier, meeting the facility's requirements.

The design of LLRF system for STCF storage ring

The Super Tau Charm Facility (STCF) is a new generation of high brightness electron–positron collider planned by China, designed to achieve a center of mass collision energy is 2–7[Formula: see text]GeV and a peak brightness is 0.5–[Formula: see text][Formula: see text]cm[Formula: see text]s[Formula: see text]. The digital low-level RF control system (LLRF), as the essential equipment of the storage ring RF system, is usually responsible for maintaining the stability of the cavity field. This paper presents the design scheme of LLRF for cavity in STCF storage ring, which mainly includes the hardware structure and software design of the system. We perform simulation analysis on the STCF RF system to evaluate the influence of delay and control parameters on the stability of the system and the influence of control parameters on the suppression of two kinds of noise. Desktop testing on the existing prototype demonstrates that the adopted direct-sampling architecture introduces less phase drift compared to the down-conversion architecture. Under the direct-sampling architecture, non-IQ demodulation offers better SNR and stability in both amplitude and phase. Specifically, direct-sampling in non-IQ mode achieves open-loop RMS short-term stability of 0.019% for amplitude and 0.021∘ for phase. Long-term stability in the closed-loop setting measures 0.027% for amplitude and 0.024∘ for phase.

Calibration of Superconducting Radio-Frequency cavity forward and reflected channels based on stored energy dynamics

Modern superconducting radio-frequency linear accelerators require the cavity bandwidth and detuning to be within a specified range to maximize the efficiency of the machine. To correctly estimate these states during operation, the measured RF signals should be calibrated. Due to the finite isolation of the waveguide directional couplers, cross-coupling effects in the forward and reflected channels complicate the calibration of the RF signals in Low-Level RF control systems. Past work proposed a compensation method employing least-squares optimization. This method requires the directivity of the directional couplers to be much higher than one. Additionally, the algorithm requires a tuning parameter to optimize the calculated calibration. However, for some accelerating systems, finding an acceptable value for such a parameter is challenging and time-consuming. Other methods strongly depend on the forward-reflected ratio during the field decay phase. This, in some circumstances, may decrease the accuracy due to noise or systematic errors. In this paper, we present a way to overcome these limitations by performing a global nonlinear least square optimization constrained by energy conservation laws. The method is tested with L-band superconducting resonators at loaded quality factors of 4.6⋅106 and 2.8⋅107.

Transverse emittance growth due to rf noise in crab cavities: Theory, measurements, cure, and high luminosity LHC estimates

The High-Luminosity LHC (HL-LHC) upgrade with planned operation from 2029 onward has a goal of achieving a tenfold increase in the integrated number of recorded collisions thanks to a doubling of the intensity per bunch (2.2×1011 protons) and a reduction of β* (the β value in the two high luminosity detectors, namely ATLAS and CMS) to 15 cm. Such an increase in recorded collisions would significantly expedite new discoveries and exploration. Crab cavities are an important component of the HL-LHC upgrade and will contribute strongly to achieving an increase in the number of recorded collisions. However, noise injected through the crab cavity radio frequency (rf) system could cause significant transverse emittance growth and limit luminosity lifetime. We presented a theoretical formalism relating transverse emittance growth to rf noise in an earlier work. In this follow-up paper, we summarize measurements in the super-proton synchrotron (SPS) at CERN that validate the theory, we present estimates of the emittance growth rates using state-of-the-art rf and low-level rf (LLRF) technologies, and we set the rf noise specifications to achieve acceptable performance. A novel dedicated feedback system acting through the crab cavities to mitigate emittance growth will be required. In this work, we develop a theoretical formalism to evaluate the performance of such a feedback system in any collider, identify limiting components, present simulation results to validate these studies, and derive key design parameters for an HL-LHC implementation of such a feedback system. Published by the American Physical Society 2024

Design of a dual-mode transverse deflecting structure using neural network and multiobjective algorithms

Shanghai Synchrotron Radiation Facility/Shanghai Soft X-ray FEL Facility is currently developing an advanced variable polarization transverse deflecting structure TTDS (two-mode transverse deflecting structure) using a dual-mode rf structure concept. Driven by two different rf power sources, this novel TDS works using both the HEM11 and HEM12 modes for simultaneous vertical and horizontal deflections, consequently, it can provide a time-varying polarization of the electrical field at ultrafast speeds. It is capable of producing circular and elliptical polarizations as well as flexible vector combinations through amplitude and phase modulation from a low-level rf system. The work presented in this paper is focused on the analysis and design of the variable polarization TDS, consisting of dual-mode cells and two dual-mode couplers. Designing and optimizing for dual-mode design and optimization is complex; consequently, an advanced optimization procedure based on neural networks and multiobjective algorithms has been developed. This improves the accuracy and efficiency of the rf structure design process. Through iterations, the dual-mode cells in the final design are optimized for high impedance and other rf performance criteria for both the HEM11 and HEM12 modes. The two couplers for rf power input and output are also optimized. Based on the optimized design and rf sensitivity analysis, the mechanical design has been completed and is now ready for manufacture. Published by the American Physical Society 2024

The Goldstone Apple Valley Radio Telescope (GAVRT) Search for Extra Terrestrial Intelligence (SETI)

This paper reports the results from a student-led Search for Extraterrestrial Intelligence (SETI), also known as technosignatures, targeting the plane of the Milky Way as a part of the Goldstone Apple Valley Radio Telescope (GAVRT) collaboration between the Lewis Center for Educational Research (LCER) and the Jet Propulsion Laboratory. Students associated with LCER submit analytic reports of spectral data targeting specific regions of the Milky Way, identifying interference, noise, and Candidate signals potentially originating from intelligent sources. GAVRT-SETI's search is guided by the assumption that a narrow-band radio signal (<1.5 Hz) from a fixed location in the sky, occurring across multiple observation periods, is unlikely to be caused by instrument noise or by a natural source. Thus, we searched the reported data for similar signals occurring during different observation periods within the same region of sky. No such signals were found. However, our analysis of the frequency distribution of Candidates suggests that at least a few percent of the Candidates are associated with low-level radio-frequency interference.

New method to measure the unloaded quality factor of superconducting cavities

We describe a method that measures the unloaded quality factor Q0, the external quality factor QE, and the cavity detuning Δω with a recursive least-squares algorithm. It combines a large number of consecutive measurements to successively improve an estimate of fit parameters that asymptotically converges to the “real” values. Exploiting the large amount of data acquired by a digital low-level radio frequency system permits us to reach this asymptotic regime in a moderate time frame of seconds to minutes. Simulations show that the method works both for critically coupled and overcoupled cavities. A new calibration method addresses very tight tolerances of the method on system parameters. Published by the American Physical Society 2024

Retracted: Calibration of superconducting radio-frequency cavity forward and reflected channels based on stored energy dynamics

Modern superconducting radio-frequency linear accelerators require the cavity bandwidth and detuning to be within a specified range to maximize the efficiency of the machine. To correctly estimate these states during operation, the measured RF signals should be calibrated. Due to the finite isolation of the waveguide directional couplers, cross-coupling effects in the forward and reflected channels complicate the calibration of the RF signals in Low-Level RF control systems. Past work proposed a compensation method employing least-squares optimization. This method requires the directivity of the directional couplers to be much higher than one. Additionally, the algorithm requires a tuning parameter to optimize the calculated calibration. However, for some accelerating systems, finding an acceptable value for such a parameter is challenging and time-consuming. In this paper, we present a way to overcome these limitations by performing a nonlinear least square optimization constrained by energy conservation laws. The method is tested with L-band superconducting resonators at loaded quality factors of 4.6⋅106 and 2.8⋅107.

Development of a digital low-level RF system for the STCF

With the development of particle physics, higher requirements are put forward for the brightness and energy of the collider. A new collider — the Super Tau-Charm Facility (STCF) — is under study in China. To meet the high acquirement of the quality of the beam, a digital low-level RF (LLRF) system that achieves high stability plays a critical role as a part of the microwave system of Linacs. This paper introduces the design of a digital LLRF system that works in the S-band (2856 MHz) as required by Linacs of the STCF. The system architecture and hardware design as well as the control algorithms and software design are discussed and tested. The system includes a signal source, a frequency synthesis system and an IF signal processor. The phase noise of the signal source is as low as 20.3 fs, and the low-level IF signal processor adopts amplitude and phase control by a PI-type controller. They jointly realize ultra-low phase noise signal output. The table experiments show that the short-term amplitude and phase stability of the system is better than 0.01% and 0.01°, respectively. The beam experiments show that the closed-loop phase stability in the cavity is about 0.1°. The results prove that the LLRF system meets the requirement of the STCF.

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.

New digital low-level RF controls based on the Red Pitaya STEMlab for the TLS Linac system

The Linac system at Taiwan Light Source (TLS) has been in operation for almost a quarter of a century and requires upgrades to improve its reliability. To achieve this, some components of the control system have been replaced with new digital low-level RF control units that use emerging technologies. A new unit is based on the open-source hardware platform which is named “Red Pitaya STEMlab” and offers a compact size and low power consumption. The unit features DAC (Digital-to-Analog Converter) blocks for downloading arbitrary waveforms with external trigger play and ADC (Analog-to-Digital Converter) blocks for waveform acquisition, enabling the development of real-time diagnostic toolkits. The new low-level RF control interface has been fully integrated into the existing EPICS software framework for system integration. The new digital low-level RF control system supports I/Q (in-phase/quadra-ture-phase) data with online amplitude and phase set-tings, and a waveform digitizer for inspecting low-level RF signals from the klystron modulator. Specific graphical applications have been designed and integrated into the existing operation interfaces. The system has been successfully achieved during routine operations. This paper describes the details of these efforts.

Low-Level Radio-Frequency system integrated with feed-forward control and vector modulation

To provide a more accurate and stable Radio-Frequency (RF) signal in conditioning and processing test progress, it is necessary to design a Low-Level Radio-Frequency (LLRF) control system which can provide high precision RF driving signal based on meeting the amplitude and phase stabilization requirement. Through Feed-Forward operation, accurate phase adjustment and amplitude adjustment are realized inside the pulse, to realize the precision and automation of phase-inversion, amplitude stabilization, phase stabilization, and wave-form adaptation matching. An LLRF System integrated with feed-forward control and vector modulation output was designed and built, and the long-term working stability of the LLRF system was verified during a new 50MW S-band Klystron conditioning progress.

Tuner Loop Based on FPGA for Petra Cavity at TPS Booster Ring

The low-level radio frequency system of the Taiwan Photon Source, TPS, booster ring was replaced by the digital low-level RF system, DLLRF, in 2018. After that, the phase drift compensation loop for energy-saving operation and the tuner loop were also implemented in the DLLRF system sequentially. We used Altera-DE3 to build the core of DLLRF and handle the high-speed ADC/DAC procedure for RF signal sampling. As we con-sider the response time of the tuner, we choose an-other low-cost board, Altera-DE0-Nano, to develop the tuner loop for 5-Cell-Petra-Cavity. It has an eight-channel 12-bits-ADC, 150 ksps, ADC128S022, to detect the positions of two tuners and two transmitted powers sampled from two cells of the 5-Cell-Petra-Cavity for a power balance function. The phase information of for-ward power and cavity gap voltage will come from the Altera-DE3 to tell the tuner loop in the Altera-DE0-Nano whether the cavity is on resonance or not. The tuner loop controls the cavity to work not only at the resonance frequency but also with a balanced electric field distribution. This study presents the tuner loop’s architecture, including the locking resonance frequency and field balance functions. The performance of the field balance function is observed by the archive data of the two tuners positions and two transmit power signals.

Coupled bunch mode zero correction within the orbit feedback bandwidth

The fast orbit feedback (FOFB) bandwidth for Advanced photon source upgrade (APS-U) will be DC-1 kHz and the synchrotron frequency will be between 100-560 Hz. This frequency overlap places coupled bunch mode 0 (CBM0) induced horizontal orbit motion inside the orbit feedback bandwidth, potentially affecting our ability to achieve beam stability goals. Longitudinal feedback kicker is not strong enough to damp CBM0 oscillations. We developed new beam-based feedback method to suppress CBM0 oscillations with low level RF phase as actuator. It uses existent FOFB framework with no changes to the feedback algorithm. Effectiveness of this method is verified using present APS operations lattice where synchrotron frequency is outside orbit feedback bandwidth. In the present work, low alpha lattice is created to emulate APS-U setting where synchrotron frequency is inside the orbit feedback bandwidth. Experiments with this lattice successfully demonstrated CBM0 correction within orbit feedback bandwidth. Combined operation of orbit feedback and CBM0 correction is stable, and CBM0 oscillations are damped. We achieved better orbit motion suppression and corrector drive efforts are reduced as well.

Effects of low-level RF fields reveal complex pattern of magnetic input to the avian magnetic compass

The avian magnetic compass can be disrupted by weak narrow-band and broadband radio-frequency (RF) fields in the lower MHz range. However, it is unclear whether disruption of the magnetic compass results from the elimination of the perception pattern produced by the magnetic field or from qualitative changes that make the pattern unrecognizable. We show that zebra finches trained in a 4-arm maze to orient relative to the magnetic field are disoriented when tested in the presence of low-level (~ 10 nT) Larmor-frequency RF fields. However, they are able to orient when tested in such RF fields if trained under this condition, indicating that the RF field alters, but does not eliminate, the magnetic input. Larmor-frequency RF fields of higher intensities, with or without harmonics, dramatically alter the magnetic compass response. In contrast, exposure to broadband RF fields in training, in testing, or in both training and testing eliminates magnetic compass information. These findings demonstrate that low-level RF fields at intensities found in many laboratory and field experiments may have very different effects on the perception of the magnetic field in birds, depending on the type and intensity of the RF field, and the birds’ familiarity with the RF-generated pattern.

Additive Manufacturing of an IH-Type Linac Structure from Stainless Steel and Pure Copper

Additive manufacturing (AM) of metals has the potential to provide significant benefits for the construction of future particle accelerators. The combination of low cost manufacturing of complex geometries in combination with efficiency gains from improved linac design enabled by AM may be one way towards future cost-effective green accelerator facilities. As a proof of concept, we present a high-efficiency Zeff=280 MΩ/m, 433.632 MHz IH-DTL cavity based on an AM design. In this case, the complex internal drift tube structures with internal cooling channels have been produced from 1.4404 stainless steel and from pure copper using AM. The prototype cavity, as well as stainless steel AM parts have been electroplated with copper. We present results from successful vacuum tests, low level RF measurements of the cavity, as well as the status of preparations for high-power RF tests with a 30 kW pulsed power amplifier.

Amplitude and Phase Control of RF Pulse Using IQ Modulator to Improve Electron Beam Quality

A test-Accelerator as Coherent Terahertz Source (t-ACTS) has been under development at Tohoku University, in which an intense coherent terahertz radiation is generated from the short electron bunches. Velocity bunching scheme in a traveling wave accelerating structure is employed to generate the short electron bunches. The in-phase and quadrature (IQ) modulator and demodulator were installed to the low-level RF systems of t-ACTS linac to control and measure the amplitude and phase of RF power. The amplitude and phase of the RF power applied to an RF electron gun cavities and the accelerating structure are controlled to produce the electron bunches with a uniform and small momentum spread suitable for the velocity bunching. By installing the feed-forward control system using IQ modulators for the beam conditioning, we have successfully generated flat RF pulses and improved beam quality, including the energy spectrum of the beam. The details of feed-forward control system of the amplitude and phase using the IQ modulator and the beam experiments are presented in this paper.

Feedforward resonance control for the European X-ray free electron laser high duty cycle upgrade

The High Duty Cycle (HDC) upgrade is a proposed improvement to the existing European X-ray Free Electron Laser (EuXFEL) to extend the pulsed RF duty factor from the actual value of around 1% to more than 5% up to Continuous Wave (CW). To implement this upgrade, the loaded quality factor (QL) of the superconducting cavities will increase by more than one order of magnitude. This will result in shrinking the cavity bandwidth to values as low as a few Hertz. Since the Lorentz Force Detuning (LFD) experienced during the accelerating field buildup is of hundreds of Hertz, the Low-Level RF (LLRF) system has to accurately track and control the cavity resonance frequency to obtain the desired accelerating gradient. Moreover, ponderomotive instabilities have to be suppressed to achieve stability during beam acceleration. Since LFD is a repetitive disturbance in cavity frequency, the correction to its effects can be implemented as a feedforward compensation on the piezoelectric tuners of the cavity. Initial results on the simulation of feedforward resonance control in the HDC regime are discussed in this proceeding.

Evaluation of inactivation of bovine coronavirus by low-level radiofrequency irradiation

Inactivation of influenza A virus by radiofrequency (RF) energy exposure at levels near Institute of Electrical and Electronics Engineers (IEEE) safety thresholds has been reported. The authors hypothesized that this inactivation was through a structure-resonant energy transfer mechanism. If this hypothesis is confirmed, such a technology could be used to prevent transmission of virus in occupied public spaces where RF irradiation of surfaces could be performed at scale. The present study aims to both replicate and expand the previous work by investigating the neutralization of bovine coronavirus (BCoV), a surrogate of SARS-CoV-2, by RF radiation in 6–12 GHz range. Results showed an appreciable reduction in BCoV infectivity (up to 77%) due to RF exposure to certain frequencies, but failed to generate enough reduction to be considered clinically significant.