Fundamental power couplers are utilized in SRF accelerators to transfer RF power from a source to the accelerating cavities. However, the issue of thermal heat load during high-power transmission in continuous wave (CW) mode operation poses a significant challenge for power couplers. To address this concern critical modifications have been implemented within the warm sections of the cERL injector prototype coupler which was previously tested for 30kW power level in CW mode operation. The modification includes implementation of active water cooling in the warm section of the coupler and material change from copper coated stainless steel to oxygen free copper for the inner conductor. As a result, the thermal load at the inner and outer conductor was effectively mitigated during high power transmission in CW mode. Prior to the modifications, the inner conductor of the warm section reached a maximum temperature of 183°C at 27 kW power in CW mode. However, with the modified inner conductor with water cooling, the temperature was a mere 25°C. Additionally, the overall coupler temperature of the modified coupler was significantly reduced due to the conduction cooling effect applied to other components. These results underscore the effectiveness of the implemented modifications and represent a highly effective approach for mitigating thermal load in critical coupler components.

The Bhabha Atomic Research Centre (BARC) of Department of Atomic Energy (DAE) has indigenously designed, developed and tested high efficiency compact 7 kW and 20 kW solid state amplifier (SSA) systems at 325 MHz. These SSAs will be used for both Indian accelerators and Proton Improvement Plan II (PIP-II) project of Fermilab, USA. The PIP-II accelerator requires two levels of RF power at 325 MHz for its single spoke resonator (SSR) section with 7 kW SSA for SSR1 with β of 0.22 and 20 kW SSA for SSR2with β of 0.47. Based on BARC design, eight 7 kW SSA systems were produced by Electronic Corporation of India (ECIL), DAE and deployed at PIP II injector test (PIP2IT) facility of Fermilab for beam acceleration. Performance evaluation of the 7 and 20 kW SSAs included, a detailed measurement survey of non-ionizing radiation at 325 MHz around SSA, validation of graceful degradation, measurement of mean time to replace etc. Enhancement accomplished in the SSA sub systems comprises of incorporation of inbuilt directional coupler in each 1 kW power amplifier (PA) module, a balanced input power divider, a 100W driver amplifier with heat pipe-heatsink and arrangement of three PA modules on single water cooled aluminum heat sink etc. This paper discusses all these performance evaluations and performance enhancements in detail for both 7 and 20 kW SSAs, which will be highly beneficial for reliable accelerator operation.

Abstract: Inductive Output Tube (IOT) is a vacuum electronic device used for generation of radio frequency power.. IOT based RF amplifiers are used in accelerator systems, industrial heating systems among other applications. It is compact in size and provides linear operation over its entire operating range with efficiency varying from60 to 70 percent. This paper proposes the conceptual design of an IOT operating at 325 MHz with an RF power of 100 kW at an efficiency of approximately 70%. The design of all the sub components of the IOT viz. the gridded electron gun, the input and output cavities, magnetic circuit, collector are discussed in this paper. The input cavity is a TM01 mode coaxial cavity while the output cavity is a TM01 mode re-entrant cavity. The magnetic circuit is designed to provide a brillouin focusing to the electron beam. Tthe simulation of the integrated model of IOT and studies of effect of the output gap and the R/Q of the output cavity on the efficiency and output power level are discussed and will be presented. Keywords: Accelerators, Amplifiers, Brillouin focusing, gridded electron gun, the input cavity, IOT, output cavity, R/Q

This work describes a C-band RF system for the SAPS (Southern Advanced Photon Source of China) test bench linear accelerator.SAPS' RF testing system comprises of a photocathode electron gun and a 2-metre-long equal gradient acceleration device.The klystron power source delivers energy to the photocathode electron gun and the travelling wave acceleration structure,respectively.Test the photocathode electron gun first,followed by the travelling wave acceleration structure.We investigated a short-pulse C-band spherical pulse compressor.The photocathode electron gun's preliminary high-power testing is now complete.

Efforts aimed at developing klystron parameters have made significant progress in recent years. However, the ultimate parameter list of connected pulse compressors (PCs) has been given insufficient attention. We propose to develop a new high efficiency, high power gain pulse compressor based on the use of a dielectric storage resonator (100% dielectric filling factor) that is operated at a cryogenic temperature (77K). It is well known that, at cryogenic temperatures, a copper cavity can gain a much higher Q factor. However, at cryogenic temperatures, the RF loss tangent of some dielectric materials also decreases substantially (tan~10-9 for Sapphire at 10 K). This inspires our effort to develop dielectric resonators for PCs with an intrinsic quality factor, Q0, that is several orders of magnitude higher than the Q0 for all metallic resonators at room temperature, and at least twice as high as for cryogenic copper cavities. In addition, the dielectric storage cavity can make the PC system more compact and lower their cost. The concern for multipactor occurring on the dielectric surfaces can be successfully addressed by special RF design and coatings like the DLC (diamond-like carbon) coating. We anticipate improving the parameters of the well-known SLED and SLED-II PCs. We consider both a passive PC (switched with a fast change of the klystron’s phase) as well as an active PC (which requires a fast RF switch).

The power coupler is one of the most important components for superconducting cavities. Different from the normal conducting cavity, the superconducting cavity has to keep an ultra-high cleanliness environment for operation. As the vacuum barrier, power couplers are welded by many different materials and maybe the gas source since they are installed to the cavities after vertical test, therefore, they should be high power conditioned before operation. Generally speaking, test bench equipment with two power couplers is often designed to improve the high conditioning efficiency. In this paper, different types of test benches are compared according to simulation and the cylindrical quarter-wavelength cavity is chosen. Besides, the detailed electromagnetic and mechanical design of the test bench is presented; to verify machining accuracy, two test pieces are also designed to measure the transmission of the test bench. In addition, to meet the high power conditioning of different power couplers, the test bench is optimized to have a capacity of 300 kW CW forward power. Finally, limited by the output power of klystron, the test bench with a pair of couplers is high power conditioned to a standing power level of 500 kW with a repetition rate of 25 Hz and a pulse width of 1.2 ms.

The China Spallation Neutron Source (CSNS) project is now operating stably at the CSNS campus and the upgrade work (CSNS-Ⅱ) has already started in 2023, meanwhile, the preliminary research work on the south advance photon source (SAPS) project is in progress. More than six types of accelerator cavities: radio frequency quadrupole (RFQ), drift tube linac (DTL), double spoke superconducting cavities, elliptical superconducting cavities, Debuncher and C band traveling wave structure, and so on in these projects, requiring corresponding different fundamental power couplers (FPCs). These FPCs are divided into waveguide and coaxial types. Different coaxial FPCs are chosen for the superconducting cavities and RFQ, while waveguide FPCs are chosen for the DTL, Debuncher, and traveling wave structure as they need a high peak power. In this paper, we will review the FPCs development at the CSNS campus. The basis for selection, design considerations, operating or testing results, etc. will be all described in this paper.

The China Spallation Neutron Source Upgrade Project (CSNS-Ⅱ) will use two debuncher cavities to supplement the beam energy at the end of the linear accelerator. The PI mode structure operating at room temperature is chosen, and each debuncher cavity is equipped with an online adjustable waveguide coupler. The main body of the coupler is the WR1500 waveguide, and a hole on the narrow wall of the waveguide is opened to achieve the coupling between the cavity and the waveguide. Meanwhile, every coupler contains a removable waveguide window. In this paper, we will detail describe the electromagnetic, cooling and mechanical design of the coupler. Finally, the coupler is high-power conditioned to 1 MW with a duty factor of 2.25%, and the coupler factor of it can be online adjusted between 0.6~3 without arc event.

The 805 MHz RF power plant at Los Alamos Neutron Science Center (LANSCE) is powered by 44 86kV 1.25 MW klystrons which generate the required RF to produce 800MeV proton beam. These 805 MHz klystrons are of the modulated-anode type and are specially engineered for a long pulse duration of 1.475 ms pulse and 120 Hz repetition rate with a 15% duty factor. In this paper we will talk about the original design of these klystrons, provide calculations and simulation results for the original design parameters, and then talk about the changes that need to be incorporated in this style of tubes to convert them into the newer style hard pulsed diode type of design. The proposed gun design will be discussed and how the design change pertains to the 805 MHz system performance improvement for the LANSCE SCCL.

Los Alamos Neutron Science Center uses a coupled-cavity linac (CCL) to accelerate H- beam from 100 to 800 MeV. This was the first CCL put into operation (1972) and is powered by forty-four 1.25 MW 805 MHz klystrons developed in the same era. A new initiative is underway to develop a replacement RF amplifier that fits in place of one klystron with HV modulator tank, and is functionally equivalent or better in RF performance. Conventional LDMOS transistors based on silicon have reduced power above 500 MHz, and are also limited in peak power by the maximum drain voltage (50-65 volts). Changing wireless infrastructure is causing leading manufacturers to introduce and discontinue products within a decade. Long term operation of LANSCE requires continuity of product availability. We have chosen leading-edge high voltage Gallium Nitride (GaN) on Silicon Carbide transistors to be able to reduce the number of active devices and the complexity of power combing. GaN has inherent higher temperature and voltage capability. We are testing devices for 3.6 kW of saturated power at 100 volts, and improvements are underway. Combining technology is also under study as part of the overall system.

CW magnetrons, initially developed for industrial RF heaters, were suggested to power RF cavities of superconducting accelerators due to their higher efficiency and lower cost than traditionally used klystrons, IOTs or solid-state amplifiers. RF amplifiers driven by a master oscillator serve as coherent RF sources. CW magnetrons are regenerative RF generators with a huge regenerative gain. This causes regenerative instability with a large noise when a magnetron operates with the anode voltage above the threshold of self-excitation. Traditionally for stabilization of magnetrons is used injection locking by a quite small signal. Then the magnetron except the injection locked oscillations may generate noise. This may preclude use of standard CW magnetrons in some SRF accelerators. Recently we developed briefly described below a mode for forced RF generation of CW magnetrons when the magnetron startup is provided by the injected forcing signal and the regenerative noise is suppressed. The mode is most suitable for powering high Q-factor SRF cavities.

The RAON facility, under the Institute for Basic Science (IBS) in Daejeon, is an advanced accelerator complex designed for research involving rare isotopes. RAON uses different types of cavities to accelerate various ions. The 81.25 MHz RF superconducting Radio Frequency Quadrupole (RFQ) cavity plays a key role in the initial acceleration of the ion beam. Supplying RF power efficiently to this RFQ cavity requires a total of 150 kW of RF power from Solid State Power Amplifiers (SSPAs). To fulfill this requirement, the RF group initially developed a 20 kW SSPA. The developed 20 kW SSPA showed good performance in frequency stability, power amplification efficiency, and thermal management. Based on these good performance results, several 20 kW SSPAs were combined to make two 80 kW SSPAs, meeting the RF power requirements for the RFQ cavity. In this paper, we present the development process and performance results of the 80 kW RF SSPAs.

Construction of a heavy ion accelerator facility to support various scientific studies is underway. The heavy ion accelerator facility is largely comprised of SCL3 for low-energy acceleration and SCL2 for high-energy acceleration. SCL3 consists of 22 quarter wave resonators (QWR) with a superconducting acceleration cavity frequency of 81.25 MHz and 102 half wave resonators (HWR) with a frequency of 162.5 MHz, and SCL3 consists of 213 single spoke resonators (SSR) with a frequency of 325 MHz. A low-energy superconducting linear accelerator consisting of an injector, QWR, and HWR was successfully commissioned. SCL3 superconducting accelerator tube can supply up to 4kW of RF power to the acceleration cavity using a solid-state power amplifier (SSPA) based on LDMOS (Lateral Double-Diffused Metal Oxide Semiconductor). The basic principle of the solid-state power amplifier applied to the acceleration cavity of 81.25 MHz and 162.5 MHz is the same, with differences in the location and quantity of components such as circulator and RF combiner. The main components of SSPA are the main transistor, a bidirectional coupler for RF input power monitoring, an attenuator, a limiter to prevent over-input, an ultra-short MMIC, a driving amplifier, a 4-way input power divider, a 4-way output power combiner, a circulator, and a dummy load.

The performances and failure cases of the power couplers of the IFMIF/EVEDA RFQ and ESS DTLs have been analyzed with dedicated high-power test campaigns and multipacting simulation methods. The paper presents test and simulation methodology, results, and inputs for the next activities.

The 60 kW CW AR RF HPA is critical major equipment in new RF system for ALS-U project at LBNL and so it has been designed & built with a modular redundant topology having large array of 96 RF final PA modules (each delivering ~ 700 W RF output) that are combined in parallel, and large 30 DC PS modules (each ~ 5 kW DC power) operating in parallel for achieving very high reliability (MTBF ~ 135,000 hours) & availability (~ 99.997 %) of RF HPA which is essential for continuous 24/7 beam operations. The redundancy design to modules failures is such that in the event upto 10% failures of RF PA modules and simultaneously upto 15 % failures of DC PS modules the HPA still can generate minimum 48 kW CW RF output that is needed for full beam power and so RF power headroom of 12 kW is built in. The operating power levels & temperatures of all components in HPA are well below to their maximum ratings for high reliability. The MBTF values of subsystems in HPA has been estimated based on components with high failures rates. The reliability probabilities having exponential distribution parameterized on failure rate were determined and the binomial distribution used for modules having redundancy. This paper presents such redundancy design analysis of HPA to such modules failures to achieve such minimum output power. Also the Availability (~99.997%) and the Reliability (MTBF ~ 135,000 hours) Estimation analysis of the overall HPA with such redundancy to modules failures is presented.

Modern CW or pulsed Superconducting RF (SRF) accelerators require efficient RF sources controllable in phase and power with a reduced cost. Therefore, utilization of the high-power CW magnetrons as RF sources in SRF accelerator projects was proposed in a number of works. But typically, the CW magnetrons are designed as RF sources for industrial heating, and the lifetime of the tubes is not the first priority as it is required for high-energy accelerators. The high-power industrial CW magnetrons use the cathodes made of pure tungsten. The emission properties of the tungsten cathodes are not deteriorated much by electron and ion bombardments, but the latter causes sputtering of the cathode in the magnetron crossed fields. The sputtered cathode material covers the magnetron interior. That lead to sparks and discharges limiting magnetrons lifetime. We considered an analysis of magnetron failure modes vs. output power, developed a model of ionization of the residual gas in the magnetrons interaction space and simulated the spattering of the cathode in 100 kW CW magnetrons to estimate the life expectancy. Basing on results we proposed ways to increase the CW magnetrons longevity for SRF accelerators.

CW magnetrons, initially developed for industrial RF heaters, were suggested to power RF cavities of superconducting accelerators due to their higher efficiency and lower cost than traditionally used klystrons, IOTs or solid-state amplifiers. RF amplifiers driven by a master oscillator serve as coherent RF sources. CW magnetrons are regenerative RF generators with a huge regenerative gain. This causes regenerative instability with a large noise when a magnetron operates with the anode voltage above the threshold of self-excitation. Traditionally for stabilization of magnetrons is used injection locking by a quite small signal. Then the magnetron except the injection locked oscillations may generate noise. This may preclude use of standard CW magnetrons in some SRF accelerators. Recently we developed briefly described below a mode for forced RF generation of CW magnetrons when the magnetron startup is provided by the injected forcing signal and the regenerative noise is suppressed. The mode is most suitable for powering high Q-factor SRF cavities.

Novel hadron radiotherapy accelerator-based systems require a fast-imaging capability, synchronized with the hadron beam, to allow positioning and treating the tumor practically at the same time. Such systems must operate at high repetition rates (~1,000 pulses per second) to provide reasonable treatment times. Currently, Argonne and RadiaBeam are collaborating on a high-gradient carbon therapy linac project, ACCIL, based on 40 MV/m S-band accelerating structures. In order to operate at repetition rates, the structures must be powered by the 5 MW klystrons. However, high gradient operation requires quadruple of this power. Therefore, we developed a compact S-band RF pulse compressor based on an E-plane polarizer and a spherical cavity operating at 2856 MHz. It incorporates features such as a cut-off circular port opposite to the circular waveguide to facilitate vacuum pumping, along with cooling channels distributed around the cavity and polarizer to manage the thermal loads. The RF pulse compressor is expected to generate a flat 18 MW 300 ns flat-top RF pulse with a 62% efficiency. We will present the mechanical design and fabrication status of the device.

The 19 MeV electron linear accelerator ELSA at CEA DAM has been in operation for 30 years. A renovation of the RF system was necessary to improve the reliability of the linac. The second part of the renovation deals with the 144 MHz RF amplifier, supplying power to the photo-injector. The former tetrode based amplifier has been replaced by a 1.6 MW Solid State Power Amplifier delivered by Ampegon company. One of the challenges was to design a compact amplifier to keep the same footprint. This paper presents the amplifier, the tests and the commissioning.

The delivery of high RF power---from hundreds of kW to MW---by klystrons, is linked with a high overall energy consumption. A research programme led by CERN in collaboration with the industry is being conducted to understand what limits klystron efficiency and how to develop high-efficiency klystrons. As a result of this program, two first prototypes of X-band (11.994 GHz) high-efficiency klystrons have been successfully designed and manufactured in collaboration with Canon Electron Tubes and Devices. The first results look promising, revealing a remarkable ~60% efficiency, and validating the proposed HE klystron technology. A comprehensive characterisation campaign has been conducted at CERN to verify and demonstrate these results. The methodology for the HEK tubes characterisation is based in two independent measurements: a RF power measurement, and a calorimetric methodology ---less subject to calibration inaccuracies. We describe the setups, principle of the calorimetry methodology, and we discuss the feasibility and precision of the results.

The CSNS-II superconducting Linac accelerator includes 20 sets of 324 MHz superconducting spoke cavities and 24 sets of 648 MHz superconducting Ellipsoidal cavities. The beam energy at the end of the superconducting Linac accelerator reaches 300 MeV. The 324 MHz solid-state power source supplies RF power to superconducting Spoke cavity, while the 648 MHz klystron power source supplies RF power to superconducting Ellipsoid cavity. The RF pulse width of the 648 MHz klystron is 1.2 ms, the repetitive frequency is 50 Hz, and the peak power is 800 kW. The 1.5 ms long pulse solid-state modulator provides high voltage pulse for the klystron, and each modulator is equipped with four klystrons.

After the discovery of Higgs boson at LHC, Chinese scientists have planned to build a “Great Collider”, that is a next-generation multinational particle accelerator research facility proposed as a circular electron positron collider (CEPC) and a super proton–proton collider (SPPC). The main component of the CEPC accelerator complex is the Collider ring, which has a circumference of 100 kilometers and the CEPC Booster and Collider rings will be located on the inner side of the tunnel. The Linac is built on the ground level. It raises the electron and positron beam energy up to 30 GeV. The CEPC Linac is a type of linear accelerator that uses normal conducting RF technology and operates at two different frequencies, S-band (2860 MHz) and C-band (5720 MHz). To achieve compactness in the Linac, the baseline design also uses klystrons operating at the C-band frequency (5720 MHz). A 80 MW pulsed-power RF source is required to power four accelerating structures. Institute of High Energy Physics (IHEP) is developing high power pulsed klystron of frequency 5720 MHz having output power of 80 MW. The design of 5720 MHz (80 MW) klystron for CEPC Linac is completed and manufacture is also started.

Fundamental power couplers are utilized in SRF accelerators to transfer RF power from a source to the accelerating cavities. However, the issue of thermal heat load during high-power transmission in continuous wave (CW) mode operation poses a significant challenge for power couplers. To address this concern critical modifications have been implemented within the warm sections of the cERL injector prototype coupler which was previously tested for 30kW power level in CW mode operation. The modification includes implementation of active water cooling in the warm section of the coupler and material change from copper coated stainless steel to oxygen free copper for the inner conductor. As a result, the thermal load at the inner and outer conductor was effectively mitigated during high power transmission in CW mode. Prior to the modifications, the inner conductor of the warm section reached a maximum temperature of 183°C at 27 kW power in CW mode. However, with the modified inner conductor with water cooling, the temperature was a mere 25°C. Additionally, the overall coupler temperature of the modified coupler was significantly reduced due to the conduction cooling effect applied to other components. These results underscore the effectiveness of the implemented modifications and represent a highly effective approach for mitigating thermal load in critical coupler components.