In the era of the High-Luminosity Large Hadron Collider (HL-LHC), the main RF system will be limited in voltage and power on the injection plateau due to strong beam loading. At the same time, significant start-of ramp losses, that are originating from capture and flat bottom losses, are expected and can severely impact machine availability or even prevent the beam from reaching the collision energy. In this contribution, we present the recent experience with high-intensity beams during operation and dedicated measurements to give an update on the estimated RF voltage reach for HL-LHC beam parameters. Projections for beam losses at capture, along the flat bottom, and at the start of the ramp are calculated, taking into account also the effect of intra-beam scattering. We discuss in detail the mitigation measures put in place, such as high-efficiency klystrons, the revision of beam loss monitor thresholds at the start of the ramp, and automatic working point optimization.

The International Muon Collider Collaboration (IMCC) has been formed following the 2020 European Strategy for Particle Physics Update, with the goal of studying the feasibility of a muon collider at a centre of mass energy of around 10 TeV. One of the most challenging sections of a muon collider is the initial cooling before acceleration, due to the necessity to apply intense magnetic and electric fields to reduce the 6D emittance of the muon beam by 5 orders of magnitude in a very short time, to cope with the limited lifetime of muons (2.2 μs at rest). The IMCC proposes to build a Demonstrator to prove that all the involved technologies (RF, magnets, absorbers, beam instrumentation) can be built at the required specifications, and integrated in order to limit the length of the cooling sections to an acceptable value. Several options are being considered in different laboratories within the collaboration. This paper describes a possible implementation at CERN, in the existing TT7 tunnel.

This talk will report on the status of commissioning of the Cathodes And Radio-frequency Interactions in Extremes (CARIE) C-band high gradient photoinjector test facility and other high-gradient C-band research activities at Los Alamos National Laboratory (LANL). The construction of CARIE began in October of 2022. CARIE is powered by a 50 MW 5.712 GHz Canon klystron and will house a high gradient copper RF photoinjector with a high quantum-efficiency cathode and produce an ultra-bright 250 pC electron beam accelerated to the energy of 7 MeV. The klystron was received, installed, and conditioned in 2024. The output of the klystron is connected to a circulator that was conditioned to operate for up to 12 MW of power. The WR187 waveguide line brings the power from the circulator into a concrete vault. The test RF injector is made of copper and does not have cathode plugs. It will be commissioned to validate operation of the CARIE facility in Spring of 2025. The second injector that will accommodate cathode plugs is in fabrication. The designs of the photoinjector and the beamline, and status of the high-power testing of the injector and other C-band components will be presented.

The Taiwan Photon Source (TPS) is a third-generation synchrotron light source located in Taiwan. Currently, it operates with two RF stations, each capable of delivering 300 kW of RF power. As the number of beamlines at TPS increases, more insertion devices will be installed, necessitating additional RF power. Presently, each RF station provides approximately 250 kW of power. To maintain operational margin, increasing the RF power available per station is a critical task. To address this, we have implemented a heterogeneous power combination method, where the power from solid-state power amplifiers is combined to raise the available RF power per station to 375 kW. This report describes the power combination methodology employed at one of the RF stations, high-power testing results, and the outcomes of long-term operation under combined power conditions. Future plans for power combination are also discussed in this paper.

High Energy Sources R&D group at Varex Imaging has developed several Accelerator Beam Centerline (ABC) and Linear Accelerator (linac) designs in the past 8 years. Here we present a summary of our recent progress. M9V linac, featuring our new ABC, is being developed to further improve characteristics of 9 MeV accelerator. The new ABC is shorter than the standard 9 MeV linac, and the focusing solenoid is completely removed. The overall system design increases dose rate and reduces the weight and complexity. In addition, our new version of K15, called K15V or V15, is being redesigned with a hybrid Standing Wave (SW) and Traveling Wave (TW) reverse feed configuration, protected by US patent. We expect it to produce significantly higher dose rate of up to 40000 R/min at 1 m. The first SW section of this linac may be used separately in 9 MeV system we called V9, which is also expected to deliver higher dose rate of up to 20000 R/min while substantially reducing neutron yield compared to 15 MeV machine. We have also tested a new concept implemented on 6 MeV linac, which permitted reducing the electron beam focal spot size to 350±150 µm without utilization of any magnetic systems.

In the PAL-XFEL system, an X-ray free electron laser facility, 51 modulator power supplies in total have been operated with thyratron tubes as the high voltage pulse switch devices in order to drive an X-band linearize and 50 S-band klystrons for a beam energy of 10 GeV. PAL-XFEL requires beam energy stability of less than 0.02% and very tight control of the klystron RF phase jitter. The modulator output pulse amplitude stability is directly related to the RF phase jitter. There are several factors to satisfy stability and reliability for the PAL-XFEL modulator. The largest sources of pulse-to-pulse instability are a current charging power supply (CCPS) for PFN charging, thyratron switch parts, and a klystron focusing magnet power supply. This paper describes how to deal with the failures of these devices and the debugging results.

The Variable Energy Gamma (VEGA) system is under implementation in Bucharest-Magurele (Romania) as one of the major components in the project of Extreme Light Infrastructure Nuclear Physics (ELI-NP). Photon beams will be resulting from the Inverse Compton Scattering of laser photons off relativistic electrons. VEGA is dedicated for photonuclear research both in applied and fundamental physics and will be open for worldwide users. The RF and synch system has to ensure stable, synchronized and coherent operation of all RF pulsed devices. It plays a crucial role in the overall performance of the particle accelerator and ultimately the beam quality. This paper presents the LLRF and synch system for the VEGA electron linac. It is based on commercial S-band Libera products from Instrumentation Technologies (Slovenia). Experience with its operation, tests results of key performance parameters and the current operational status together with plans for future upgrade are presented. Likewise the HLRF system for the RF photocathode gun and the TW cavities, based on klystrons followed by SLEDs and hybrid power dividers, is described here. Also the phase tuning of the RF cavities is discussed.

The Canadian Light Source (CLS), a 3^rd generation synchrotron light source, has operated the CESR-B type superconducting radio frequency cavity since 2005. We report on 20 years of operating experience of the facility with this type of accelerating cavity.

The long term sustainability of future accelerators is now a crucial problem for our community. Many groups and collaborations are actively working in this area (e.g. European projects included IFAST and iSAS, RUEDI (STFC) has recently published a case study for the project lifecycle, Centre of Excellence in Sustainable Accelerators is now being vigorously pursued in the UK with CERN backing, European LDG working group, etc). This talk will review the wider community efforts and highlight where good progress is being made and where future efforts are planned or required.

In ultra-low-emittance synchrotron light sources, the bunch-lengthening technique is useful to mitigate harmful effects due to the intrabeam scattering. The perfomacne of the bunch lengthening can be degraded by the transient beam loading (TBL) effect induced in the cavities. To mitigate the TBL effect, we proposed a TBL compensation technique using a wide-band longitudinal kicker cavity. In this presentation, we report the result of the experimental demonstration of the TBL compensation performed at the KEK PF 2.5 GeV ring. In this experiment, the fill pattern of the electron bunches were customized to enlarge the phase variation of electron bunches induced by the TBL effect. The fundamental cavities and newly developed digital low-level RF (DLLRF) system were used for the experiment. The DLLRF enables the TBL compensation by an arbitrary feedforward pattern of the cavity voltage modulation that is synchronized with the revolution frequency. Although the bandwidth of the fundamental cavity is limited, the variation of the cavity voltage and bunch phase induced by the TBL effect was reasonably mitigated by applying sinusoidal wave modulation of the cavity voltage.

We have been developing a compact pulse power supply with output pulse waveform specifications of 75kV/40A/50us/25Hz. This power supply is used to drive klystron for muon linac, which requires high stability and reliability. Next-generation power semiconductor SiC-MOSFETs with excellent characteristics of ultra-high breakdown voltage and low loss at 13kV, which were realized through the technological development of wide bandgap semiconductor devices, are used. Combining this SiC-MOSFET with the MARX circuit will realize a more compact pulse power supply with lower loss than conventional ones. In addition, it can be applied to portable accelerators in the future. In this presentation, the circuit design of the MARX power supply will be reported.

The X-LAB has been commissioned at the University of Melbourne. A key project within this laboratory involves rehoming half of the CERN high-gradient X-band test stand, XBOX3, now known as Mel-BOX. This initiative aims to validate the performance of high-gradient traveling wave accelerating structures operating at a frequency of 12 GHz. Mel-BOX is employed to evaluate the performance of these accelerating structures under high-power pulsed RF conditions. Two TD24 high-gradient structures, previously conditioned at CERN, were reconditioned at X-LAB after being shipped and stored for five years. Additional components have also been tested, including a compact pillbox-type RF window with traveling waves in ceramic, SLED-I type pulse compressors with a novel piston design, and high-power loads fabricated via 3D titanium printing and 1-meter-long stainless steel. As with XBOX3, Mel-BOX utilizes the combined power of two high-average-power klystron units to feed two test slots at a repetition rate of up to 400 Hz. Additionally, there are plans to leverage this technology as a foundation for developing compact accelerators for medical and university applications.

For years, Instrumentation Technologies and ScandiNova have developed advanced products to optimize RF performances in LINAC applications. In 2024, the companies began integrating the Libera LLRF system into ScandiNova modulators during assembly. This innovation enables the modulators to offer enhanced operational flexibility and improved performances. This paper will focus on mechanical integration and performance results. The integrated system enables real-time monitoring of critical signals such as drive power to the RF amplifier and klystron, as well as forward and reflected klystron power. Performance metrics include amplitude stability <0.01% RMS and phase stability <0.01° RMS. Experimental results are presented using a ScandiNova modulator with an Sband klystron and a standard Sband Libera LLRF. Pulse-to-pulse stability measurements demonstrate consistency between conventional electrical methods and RF-based methods, achieving stability in the 10 ppm range. Electromagnetic compatibility tests confirm that the modulators do not interfere with the LLRF system. Additionally, new tools are introduced to identify components with the greatest impact on phase stability.

We have developed a new SLED-type RF pulse compressor for powering ultra-high gradient X-band photoinjectors with pulse lengths shorter than 10 ns. Klystrons capable of generating these short pulses at multi-MW levels are non-existent. However, RF pulse compression is an alternative technique used to increase klystron output peak power at the cost of pulse length. Over the years, we have developed numerous pulse compression systems, including super-compact SLEDs for X-band transverse deflectors at SLAC’s LCLS and LCLS-II. Our new compact pulse compressor uses spherical cavities with axially-symmetric TE modes which have no electric field on the cavity surface. This allows our new SLED to potentially achieve higher peak RF power compared to the LCLS-II SLEDs. We present the design of this SLED composed of two spherical cavities and a waveguide hybrid with TE01 circular waveguide ports. During high power test this SLED produced peak RF power up to 317 MW.

The RF window acts as a barrier between the vacuum and air, gas, or water while allowing RF power to pass through with minimal loss. Resonant modes (called "ghost modes") can occur within the ceramic disk of a window. The frequencies of these modes depend on the material and size of the ceramic. Ceramic disk dimensions must be carefully optimized to minimize reflections and avoid ghost mode resonances within the operating bandwidth. In this paper we present the design of an input window used in an X-band klystron. The dimensions of the window and ceramic disk are optimized to minimize insertion and reflection losses while preventing ghost mode resonances in the operating bandwidth. In addition to this, we ensure that the maximum electric field at the window surface is kept low to reduce the probability of RF breakdowns. Analytical analysis, numerical simulations and experimental measurements of the ghost modes of ceramic disks were carried out. The measured ghost mode frequency was used to evaluate the ceramic dielectric constant. In this article we present simulated and measured results.

The Low Energy Accelerator Development Facility * is located at the site of the Brookhaven National Laboratory (Upton, NY, USA) and is aimed to run a program specially targeting new collaborations for user-driven research. The facility has two fully radiation-shielded bunkers (153 and 77 sq. m) to where a range of electrical, cooling and RF capabilities are presently being introduced. The facility runs also the Ultrafast Electron Diffraction (UED) Facility. The first shielded bunker will support the deployment of a demonstrator for the Electron Cyclotron Resonance Accelerator ** (eCRA). The deployment is expected to start in April of 2025. At the UED Facility beamline updates are now going into place for a NASA Jet Propulsion Laboratory *** electron irradiator beamline for Single Event Effects (SEE) testing; the capability for UED/UEM testing will be expanded; and the deployment of a new stable solid-state modulator and klystron is in progress. The presented article provides further details.

The Accelerator Test Facility* (ATF) is the DoE Office of Science User Facility aimed to provide users with a high brightness electron beam, near-infrared (NIR), and long-wave infrared (LWIR) laser beams. The unique capabilities at the ATF include the possibility to combine the electron beam with synchronized high-power laser beams. It is planned to upgrade the facility to have enhanced capabilities. They will include: an increased electron beam energy from the present 65-70 MeV to 110-120 MeV; a reduced by a factor of about 10 phase jitter; and an improved - to femtoseconds’ scale - time synchronization between the electron beam and the laser beams. To accomplish these tasks, the ATF will design and deploy a new High Level RF System, a new Low Level RF System, and a new Time Distribution System. In addition, the ATF will change the Power Plant for the quadrupole and correction magnets to increase operations’ reliability. It is expected that the planning stage will be completed in about 3 years, and the actual hardware deployment will be finished after that in the next 2 years. Different upgrade options are being investigated now and are described in the presented article.

The Karlsruhe Institute of Technology operates the accelerator test facility Karlsruhe Research Accelerator, which also provides synchrotron radiation at 2.5 GeV. Roughly one third of the wall-plug power is used for cooling. Optimizing the infrastructure for cooling has a huge impact on the overall sustainability. To reduce the environmental impact a thermal well system was installed. It reduces the base heat load by eliminating one of three 500 kW cooling units. This paper describes the challenges, such as iron-manganese rich groundwater, and their solution for our 1 MW passive cooling system. The average energy consumption of 28 kW for the thermal well infrastructure is compensated by a new 540 kWp solar power plant. This paper elaborates on the commissioning of the wells and shows the first results of this overall sustainable cooling concept.

KEK Accelerator Test Facility (ATF) conducts beam instrumentation R&D for International Linear Collider (ILC) project. ATF includes 1.3 GeV normal conductivity S-band Linac and Damping Ring (DR). There are 9 S-band pulsed klystrons at Linac, which supply High-Power RF to accelerate electron beam up to 1.3 GeV, 1 CW klystron at DR. The electron beam is generated by a photocathode irradiation by a laser pulse. The laser pulse generation is synchronized with the accelerating fields by the laser system oscillator Piezo feedback. These Linac, DR High-Power RF field and laser pulse arrival time jitter and/or drift define the stability of the electron beam parameters, such as average energy, energy spread (RMS), emittance, bunch charge etc. This study demonstrates KEK ATF Linac and DR High-Power RF field phase and amplitude, as well as the laser system oscillator laser pulse arrival stability measurement results. Also, FPGA board based digital Low-Level RF phase&amplitude feedback system is described in this report.

The KEK e-/e+ LINAC delivers the beams to four storage rings with the top-up injections by switching the beam mode in 50 Hz repetition rate. The beam charge, energy, and number of bunches (one or two) are different for each ring. Therefore, the RF timing and phase are adjusted to each beam mode independently. To stabilize the RF phase drifts caused by the klystron high voltage, the cooling water and accelerating structure temperature, the RF phase feedback was introduced. The correction phase quantity is obtained by feedback calculation using non-injection mode without beam acceleration, and the value is added to set phase value in each mode. As a result, the RF phase in each beam mode has been stabilized.

In various of particle accelerator designs, amplitude and phase modulation methods are commonly applied to shape the RF pulses for implementing pulse compressors or compensating for the fluctuations introduced by the high-power RF components and beam loading effects. The phase modulations are typically implemented with additional phase shifters that requires drive or control electronics. With our recent next generation LLRF (NG-LLRF) platform developed based on the direct RF sampling technology of RF system-on-chip (RFSoC) devices, the RF pulse shaping can be realized without the analogue phase shifters, which can significantly simplify the system architecture. We performed a range of high-power experiments in C-band for evaluating the RF pulse shaping capabilities of the NG-LLRF system at different stages of the RF circuits. In this paper, the high-power characterization results with the Cool Copper Collider structure driven by RF pulses with different modulation schemes will be described. With the pulse modulation and demodulation completely implemented in digital domain, the RF pulse shaping schemes can be rapidly adapted for X-band structures just by adding analogue mixers.