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A Dielectric Disk Accelerator (DDA) is a metallic accelerating structure loaded with dielectric disks to increase coupling between cells, thus producing a high group velocity, while still maintaining a high shunt impedance. This is crucial for achieving high efficiency, high gradient acceleration in the short rf pulse acceleration regime. Recent research of these structures has produced traveling wave structures that are powered by very short (∼ 9 ns), very high power (400 MW) RF pulses using two beam acceleration at the Argonne Wakefield Accelerator Complex. In testing, these structures have withstood more than 320 MW of power and produced accelerating gradients of over 100 MV/m. A new standing wave DDA structure is being fabricated for testing on the Nextef2 test stand at KEK that will be powered by a more conventional, klystron power source. Simulation results of this structure show that at 50 MW of input power, the DDA produces a 457 MV/m gradient. It also has a large shunt impedance of 160 M Ω /m and an r/Q of 21.6 k Ω /m. Cold testing of this structure was conducted this summer showing a good match with the excited field but a significantly lower matching at the working frequency. [ABSTRACT FROM AUTHOR]

Klystrons are the predominant RF power sources utilized in large particle accelerators, and enhancing klystron efficiency is crucial for reducing the overall energy consumption of these accelerators. One optional approach to improve klystron efficiency is implementing the depressed collector technique, successfully applied in low-power microwave electronic vacuum devices such as TWT and TV communication klystrons. While the theoretical advancements in klystron efficiency are promising, the practical integration of these techniques, particularly in high-power klystrons, remains challenging. In this paper, a depressed collector design applied to high-power klystrons will be proposed, laying the foundation for improving the overall efficiency of the RF power source in the Circular Electron Positron Collider(CEPC). To verify the key technology of energy recovery for the klystron, we designed an energy recovery verification device composed of an electron gun and a collector. The one-stage depressed collector was theoretically analyzed, and the electrode parameters were determined through simulation design. Beam optics was simulated using DGUN and CST software, while thermal analysis of the collector was performed with ANSYS software. The structural design of the collector was optimized to meet electrical insulation and breakdown requirements. Additionally, secondary electron emission was considered, and its effect on reflux electrons was mitigated, achieving a collector efficiency simulation result of 88.2 %. • The depressed collector finds application in high-power CW klystrons. • Deionized water cooling allows the depressed collector to maintain efficient heat dissipation and performance at high operating voltages. • A ceramic cylinder enhances the insulation capability of the electrodes in the depressed collector. • The structure of the depressed collector is more compact, occupying a smaller volume. [ABSTRACT FROM AUTHOR]

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 quite large noise when a magnetron operates with the anode voltage above the threshold of self-excitation. Traditionally, an injection locking by a small signal is used for stabilization of magnetrons. In this case CW magnetrons with the injection-locked oscillations generate a high level of noise. This may preclude use of standard CW magnetrons in this operating mode in the Superconducting RF (SRF) accelerators. In this article we described a method developed 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 method is most suitable for powering high Q-factor cavities. [ABSTRACT FROM AUTHOR]

The klystron-based Compact Linear Collider (CLIC) was initially proposed with a low center-of-mass energy of 380 GeV, primarily due to potential cost-effectiveness. To enhance overall cost-efficiency, reliability, and stability, a novel RF module for klystron-based CLIC main linac has been designed and studied. This RF module utilizes two X-band klystrons to feed eight traveling wave accelerating structures, resulting in a beam energy increase of 138 MeV. A key innovation of the new RF module is the integration of double-height waveguides, which contributes to a 30% reduction in surface fields and a 40% decrease in RF loss. To meet the double-height requirement, a majority of the RF components responsible for transmitting the high RF power underwent a redesign. Two solutions, based on the choke mode flange and the L-shape waveguide, were proposed to facilitate the adjustment of the accelerating structures for beam-based alignment. Additionally, bent damping waveguides and HOM loads and a high order Magic-T, were designed for the accelerating structure named CLIC-K. The paper will comprehensively present and describe both the RF design and the integration of the klystron-based CLIC module. [ABSTRACT FROM AUTHOR]

A new electron accelerator has been designed and is now under construction at AIST, Tsukuba, Japan. The accelerator will be used to produce a pulsed neutron beam for materials analysis. The design maximizes the yield of slow neutrons with high pulse temporal resolution as described in a previous publication [K. Kino et al., Nuclear Inst. and Methods in Physics Research, A 927 (2019) 407–418]. The accelerator is composed of a thermal cathode type electron gun with S-Band buncher/accelerator cavity followed by three, 2.9 m long S-Band acceleration cavities. High power RF is supplied by three klystron units, with klystron No.1 supplying both the electron gun and the 1st cavity, and klystrons No.2 and No.3 connected to the 2nd and 3rd cavities respectively. The design parameters of the accelerator are; pulse rate: 100 Hz, pulse length: 10 μs, electron current: 250 mA, electron energy: 40 MeV. Operating at these parameters a maximum beam power of around 10 kW will be delivered to the water-cooled Ta target for neutron production. In this report we will describe the design and construction of the electron accelerator. [ABSTRACT FROM AUTHOR]

• New high voltage power supply (PS) for a klystron has been developed. • This PS is used for the electron cyclotron heating (ECH). • 8.56 GHz-RF with the PS will be a promising ECH system in QUEST spherical tokamak. • An IGBT and a reactor can shut down the PS quickly to protect the klystron. • AC switching has been set up for the overload and overheat of the PS. A high voltage DC power supply for the cathode of the 8.56 GHz CW klystron has been set up for electron cyclotron heating (ECH) in steady state tokamak operation on QUEST spherical tokamak. The power supply is equipped with an IGBT array and a reactor for fast shutoff of the voltage in 10 μs, where the influx of electric energy at the short circuit is limited to 5 J. AC switches also have been installed in the three-phase power lines. High voltage relays are useful to save electric energy consumption. Fast three-phase AC switching by IGBT-stack is applicable to reduce the electric load of the components of the klystron power supply. [ABSTRACT FROM AUTHOR]

Abstract At Ibaraki Neutron Medical Research Center, an 8-MeV proton linear accelerator is being developed for iBNCT (Ibaraki - Boron Neutron Capture Therapy), with the aim of achieving the highest possible performance at a low cost. In this accelerator, there are two RF cavities driven by only one high-power klystron as the RF source. A digital low-level RF system has been successfully implemented for the iBNCT accelerator, with very good RF field regulation. An effective multipurpose and smart tuner controller was also developed. Finally, a novel auto-startup technology for two cavities with one RF source was successfully developed. The time required for the RF startup process is greatly reduced from the conventional time of 30 min, to less than 1 min. The good stability and fast response achieved by using this technology make it suitable for the medical use of iBNCT accelerators. This technology will be extremely useful in the development of similar accelerators. [ABSTRACT FROM AUTHOR]

The power required by each cavity of the ESS superconducting linac varies along the length of the accelerator, which results in a wide range of power levels to be provided by the RF power sources. For the medium and high beta sections of the linac, the selected power sources are klystrons with a nominal maximum output power of 1.5 MW. Operating these klystrons as efficiently as possible is among the goals of the ESS objectives to approach a carbon-neutral facility, but the ESS power profile presents specific challenges. This paper will discuss some of the solutions that will be implemented to increase the operational efficiency of the RF systems, focusing on klystron operation. • Energy efficiency is crucial for operating large particle accelerators. • Energy savings can be achieved by focusing on efficient operation of the klystrons, by optimizing output match and focusing. • The proposed optimization does not require large investment upfront and can also lead to increased klystron lifetime. [ABSTRACT FROM AUTHOR]

Highlights • Validation of 5-GHz 500-kW CW operation of the klystron which is relevant to the ITER LHCD system. • The weakest part was the RF window damage due to a thermal stress by the temperature gradient. • The accurate temperature of the test RF window was monitored using IR camera with a calibration. • The klystron operation at 500 kW for 1000s was successfully. • The klystron characteristics with reflective load. Abstract In this study, a klystron for the KSTAR lower hybrid current drive (LHCD) system was developed as a prototype for a 5 GHz, 500 kW CW operation that is aimed to meet the requirement of the ITER LHCD system as well. Before the 2012 KSTAR campaign, the prototype klystron was validated only for 350 kW CW operation. The weakest part of the klystron for 500 kW CW operation was the RF output window failure due to a thermal stress caused by the temperature gradient. The klystron was equipped with two RF windows, each of which should transmit 250 kW power. Prior to the full performance test of the klystron, the performance of the test RF windows that were identical to those mounted on the klystron was tested with half of the klystron's full power. The temperature increase in the test RF windows was monitored using an IR camera. The temperature difference, ΔT ce , between the center and edge of the test window at 250-kW 1000-s transmitting power was predicted to be 23 °C, which is smaller than the safe ΔT ce , which is 50 °C according to the manufacturer. Based on the result of the test window measurement, 500-kW 1000-s operation of the klystron was conducted successfully. The klystron output power characteristics depending on the phase of load reflection VSWR 1.4:1 were investigated. The klystron generated stable power on a load VSWR of 1.4:1 at various phases. The output power at the worst phase was 380 kW. Details of the IR-imaging setup are discussed and the results obtained are presented. [ABSTRACT FROM AUTHOR]

Abstract Suppression of beam losses is essential for successful operation of high-intensity linac. Historically, the values of the field amplitudes and phases of the side-coupled, 805-MHz LANSCE linac modules are maintained using a well-known delta-t tuning procedure. Transverse matching of the beam with accelerator is performed through adjustments of beam ellipses in 4D phase space with accelerator lattice using matching quadrupoles. Control of the beam-energy ramp along the length of a proton linear accelerator is required to improve tune of accelerator and decrease beam losses. Time-of-flight measurements of the H − beam energy are now being used to confirm and improve the overall control of the energy ramp along the linac. The time-of-flight method utilizes absolute measurements of beam energy using direct signals from beam at an oscilloscope, as well as the difference in RF phases measured as the beam passes installed delta-t pickup loops. A newly developed BPPM data acquisition system is used. Beam energy measurement along accelerator together with phase scans, klystrons output power control, and delta-t method, allow tuning of accelerator much more close to original design and reduce losses generated by linac. Details of the upgraded tuning procedure and results of tuning and operation are presented. [ABSTRACT FROM AUTHOR]

Abstract In the framework of the upgrade of the SPARC_LAB facility at INFN-LNF (Frascati, Italy), named EuPRAXIA@SPARC_LAB, a high gradient linac is foreseen. One of the most suitable options is to realize it in X-band. A preliminary design study of both accelerating structures and power distribution system has been performed. It is based on 0.5 m long traveling wave (TW) accelerating structures operating in the 2 π ∕ 3 mode and fed by klystrons and pulse compressor systems. The main parameters of the structures and linac are presented with the basic RF linac layout. [ABSTRACT FROM AUTHOR]

Abstract Due to difficulties associated with the conventional gas tube based modulators, solid-state switch based klystron modulators are gaining popularity in the scientific community. Research, design, and development of high voltage solid-state modulators for electron and proton accelerators are in progress at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore. A prototype of 100 kV, 20 A, 1 ms pulse duration Marx modulator has been designed and developed. The modulator has been constructed using 34 numbers of Insulated Gate Bipolar Transistors (IGBT), based 3 kV main modules for generation of a 100 kV base pulse. A droop correction scheme, consisting of 24 numbers of 1 kV corrector modules, has been adopted to compensate the droop in the output pulse. Before incorporating the droop compensation technique, the modulator had droop of 16% in the output voltage pulse. A microcontroller based control unit is used for the programmed staggered triggering of all the main modules and corrector modules. Fiber optic based control signal transmission was done to realize electrical isolation between high voltage circuits and control circuits. The modulator has been tested for 100 kV, 20 A, 1 ms pulse duration at 1 Hz, initially without droop compensation and later with the activation of the correction scheme for droop compensation. With the droop compensation scheme, less than 1% droop could be achieved in the output voltage pulse. Output voltage pulses of 1 ms pulse width, 1.4 μ s rise time and 9.7 μ s fall time at 100 kV have been achieved successfully. The present effort forms a strong basis for further development of this technology for higher average power as well as for replacing the earlier thyratron based modulators. [ABSTRACT FROM AUTHOR]

The life, career and scientific legacy of academician Vladimir Borisovich Braginsky are briefly reviewed and the contents of this special issue are shortly summarized. [ABSTRACT FROM AUTHOR]

A mechanism for driving current off-axis in high beta tokamaks using fast electromagnetic waves, called Helicons, will be experimentally tested in the DIII-D tokamak. This method is calculated to be more efficient than current drive using electron cyclotron waves or neutral beam injection, and it may be well suited to reactor-like configurations [1,2] . A low power (100 W) 476 MHz “combline” antenna, consisting of 12 inductively coupled, electrostatically shielded, modular resonators [3] , was recently installed in DIII-D. Initial operation showed that the plasma operating conditions were achieved under which helicon waves can be launched. Plasma operations also showed that the location of the antenna has not reduced the performance of, or introduced excessive impurities into, the discharges produced in DIII-D. The development of a high power (1 MW) Helicon system is underway. This antenna consists of 30 modules mounted on the inside of the outer wall of the vacuum vessel slightly above the midplane. Carbon tiles around the antenna protect the antenna from thermal plasma streaming along field lines. A 1.2 MW, 476 MHz klystron system will be transferred from the Stanford Linear Accelerator to DIII-D to provide the RF input power to the antenna. A description of the design and fabrication of high power antenna and the RF feeds, the klystron and RF distribution systems, and their installation will be presented. [ABSTRACT FROM AUTHOR]

Circulators are used in high power microwave systems to protect the vacuum source against reflection. The Lower Hybrid Current Drive (LHCD) system of SST-1 tokamak commissioned at IPR, Gandhinagar in India comprises of four high power circulators to protect klystrons (supplying 500 kW CW each at 3.7 GHz) which power the system. This paper presents the design of a Differential Phase Shift Circulator (DPSC) capable of handling 500 kW CW power at 3.7 GHz so that four circulators can be used to protect the four available klystrons. As the DPSC is composed by three main components, viz., magic tee, ferrite phase shifter and 3 dB hybrid coupler, the designing of each of the proposed components is described. The design of these components is carried out factoring various multiphysics aspects of RF, heating due to high CW power and magnetic field requirement of the ferrite phase shifter. The primary objective of this paper is to present the complete RF, magnetic and thermal design of a high CW power circulator. All the simulations have been carried out in COMSOL Multiphysics. The designed circulator exhibits an insertion loss of 0.13 dB with a worst case VSWR of 1.08:1. The total length of the circulator is 3 m. [ABSTRACT FROM AUTHOR]

The Line type modulators have been widely used to generate high voltage rectangular pulses to power the klystron for high power RF generation. In Line type modulator, the Pulse Forming Network (PFN) which is a cascade combination of lumped capacitors and inductors is used to store the electrical energy. The charged PFN is then discharged into a klystron by firing a high voltage Thyratron switch. This discharge generates a high voltage rectangular pulse across the klystron electrodes. The amplitude and phase of Klystron's RF output is governed by the high voltage pulse amplitude. The undesired RF amplitude and phase stability issues arises at the klystron's output due to inter-pulse and during the pulse amplitude variations. To reduce inter-pulse voltage variations, the PFN is required to be charged at the same voltage after every discharge cycle. At present, the combination of widely used resonant charging and deQing method is used to regulate the pulse to pulse PFN voltage variations but the charging transformer's leakage inductance puts an upper bound on the regulation achievable by this method. Here we have developed few insights of the deQing process and devised a new compensation method to compensate this undesired effect of charging transformer's leakage inductance on the pulse to pulse PFN voltage stability. This compensation is accomplished by the controlled partial discharging of the split PFN capacitor using a low voltage MOSFET switch. Theoretically, very high values of pulse to pulse voltage stability may be achieved using this method. This method may be used in deQing based existing modulators or in new modulators, to increase the pulse to pulse voltage stability, without having a very tight bound on charging transformer's leakage inductance. Given a stable charging power supply, this method may be used to further enhance the inter-pulse voltage stability of modulators which employ the direct charging, after replacing the direct charging with the resonant charging. [ABSTRACT FROM AUTHOR]

To study the atomization characteristics of a liquid–centered gas–liquid pintle injector under periodic operating conditions, the gas–liquid interaction, breakup dynamics and spray morphology of the pintle element are simulated based on the three–dimensional volume of fluid to discrete particle model (VOF to DPM) and octree adaptive mesh refinement (AMR) when the liquid jet velocity changes according to sinusoidal law. The simulation results show that: (1) The fragmentation mechanism of liquid jets under periodic operating conditions can be divided into column breakup caused by Rayleigh–Taylor (R–T) instability and surface breakup caused by Kelvin–Helmholtz (K–H) instability. (2) Under specific operating conditions, the "flow interruption" phenomenon occurs. When the variation amplitude of liquid jet velocity increases from 3.125 m/s to 12.5 m/s, the occurrence time of "flow interruption" decreases by 10.46%, the spread angle increases by 36.79%, and the dimensionless length of liquid jet before "interruption point" decreases by 59.89%. (3) Additional disturbance waves occur due to the klystron effect. The gas–liquid interaction increases at the location where liquid flow accumulates. When the variation frequency of liquid jet velocity increases from 8 kHz to 16 kHz, the reduction amplitude of λ 1 , λ 2 and λ 3 is all around 50%. [ABSTRACT FROM AUTHOR]

In order to achieve steady state and high performance plasma in EAST, a new lower hybrid current drive system at a frequency of 4.6 GHz has been built. The system is composed of 24 continuous wave (CW) klystron amplifiers to generate 6MW/CW microwave, 24 standard rectangle waveguide transmission lines with water-cooling plate, a multi-junction grill composed of 576 active (in groups of 8) and 84 passive sub-waveguides arranged in 12 rows and 6 columns, and four sets of high voltage power supplies. The power value and the spectrum of the launched microwave from the antenna can be controlled by the low-power microwave circuits in front of the klystrons. The new LHCD system has been applied to the experiments on EAST tokmak since 2014, and the obtained results suggest that it is effective to couple the wave into plasma and drive plasma current. [ABSTRACT FROM AUTHOR]

C-band RF acceleration is a crucial technology for the compact Free Electron Laser (FEL) facility at the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences. A project focusing on C-band RF acceleration technology was launched in 2008, based on high-gradient accelerating structures powered by klystron and pulse compressor units. The target accelerating gradient is 40 MV/m or higher. Recently one prototype of C-band RF unit, consisting of a 1.8 m accelerating structure and a klystron with a TE0115 mode pulse compressor, has been tested with high-power and electron beam. Stable operation at 40 MV/m was demonstrated and, 50 MV/m approached by the end of the test. This paper introduces the C-band R&D program at SINAP and presents the experiment results of high-power and beam tests. [ABSTRACT FROM AUTHOR]

The RF system of the TPS booster ring was upgraded in 2018 with a digital low-level RF control system. Energy-saving operation in the top-up mode was also applied to the RF system at TPS booster ring. An unexpected phase jump occurred, however, which led to a failure, ramping the waveform of the gap voltage of the RF cavity. This large phase drift was caused by modulation of the klystron cathode current for energy-saving operation; sometimes it would trigger an interlock of the RF system. A large phase drift (from 82° to −4°) occurred while the cathode current was pulling high or low. The digital PID controller could not afford such large phase compensation. A phase-drift-compensation loop was implemented in the FPGA, operated with 1 kHz of a clock, which can provide ±1°/ms phase compensation. Through a combination of a PID controller and a phase-drift-compensation loop, the phase disturbance became stabilized. The maximum phase error of the gap voltage during energy-saving operation decreased from ±3.5° to ±0.7°. After applying the phase-drift-compensation loop, the cathode current of klystron can be set lower than before in an energy-saving period, which enables saving 60% power consumption compared with full-power operation. [ABSTRACT FROM AUTHOR]

In this paper, we study spontaneous emission of radiation by relativistic electrons in a gyro-klystron. The scheme consists of two solenoid sections separated by a dispersive section. In the dispersive section the electrons are made non resonant with the radiation. The dispersive section transforms a small change of the velocity into changes of the phases of the electrons.This leads to enhanced radiation as compared to a conventional gyrotron type device driven by cyclotron maser interaction. It is shown that the klystron modulated spectrum depends on the dispersive field strength, finite perpendicular velocity component and length of the solenoids but do not depend on the axial magnetic field strength. The analysis is further extended to include the combined effects of the undulator aided gyrotron klystron radiation. [ABSTRACT FROM AUTHOR]

We report the development and implementation of four FPGA-based predistortion-type klystron linearization algorithms. Klystron linearization is essential for the realization of ILC, since it is required to operate the klystrons 7% in power below their saturation. The work presented was performed in international collaborations at the Fermi National Accelerator Laboratory (FNAL), USA and the Deutsches Elektronen Synchrotron (DESY), Germany. With the newly developed algorithms, the generation of correction factors on the FPGA was improved compared to past algorithms, avoiding quantization and decreasing memory requirements. At FNAL, three algorithms were tested at the Advanced Superconducting Test Accelerator (ASTA), demonstrating a successful implementation for one algorithm and a proof of principle for two algorithms. The functionality of the algorithm implemented at DESY was demonstrated successfully in a simulation. Besides this, a proof of principle of an FPGA-based klystron and cavity simulator implemented at the High Energy Accelerator Research Organization (KEK), Japan was demonstrated. Its purpose is to allow the development and test of digital LLRF control systems including klystron linearization algorithms when no actual klystron and cavity are available. [ABSTRACT FROM AUTHOR]

For CEPC LINAC, the first 80 MW class klystron at C-band (5720 MHz) has been proposed to accelerate a high-current beam at an acceleration gradient of 45 MV/m. This paper discusses the design and simulation of beam optics, beam envelope, drift tube, and solenoid of the klystron. Space charge beam current of 390 A at an acceleration potential of 410 kV is obtained with average cathode loading of less than 6.0 A/cm2. The maximum surface electric field at the beam optics and high voltage ceramic seal have been evaluated less than 22 kV/mm and 4.50 kV/mm, respectively. The electron beam with an average beam radius of 5.40 mm has successfully been transported to the interaction cavity with beam ripple rate around 4.0% with low scalloping factor and a fill factor of 0.670. 2-D, DGUN & POISSON simulations results are validated by the 3-D CST simulation and found in agreement. [ABSTRACT FROM AUTHOR]

Power consumption is one of the major concerns for a large-scale high-energy accelerator such as the Future Circular Linear Collider (FCC). We address this problem in this article by describing the concept and the results of numerical modelling of an efficient 1 MW, 800 MHz two-stage CW klystron having a depressed collector. The klystron has a compact design. The numerical simulations predict an efficiency of 92%. [ABSTRACT FROM AUTHOR]

Contemporary light sources operate in a top-up mode to maintain their photon intensity quasi-constant so as to improve significantly the thermal stability of the photon beam and to maximize ultimately the average photon flux at a designed maximum operational beam current. Operating in a top-up mode requires frequent beam injection from the synchrotron booster to the storage ring of the light source, but the injection intervals occupy only a tiny portion of the operational time of the integrated machine. To maintain a high operational reliability, the booster RF system practically operates necessarily under injection conditions around the clock and consumes full electric power whether during top-up injection or not. How to decrease the power consumption of the booster RF system during its stand-by time but not to sacrifice the reliability and availability of the RF system is obviously of fundamental interest for routine operation of the light source in a top-up mode. Here, an energy-efficient operation of a booster RF system adaptive to top-up operation of a light source is proposed that has been developed, realized and integrated into the booster RF system of the Taiwan Light Source (TLS), and routinely operated since the end of year 2008. The klystron cathode current and RF gap voltage of the booster׳s accelerating RF cavity are both periodically modulated to adapt the injection rhythm during top-up operation, which results in decreased consumption of electric power of the booster RF system by more than 78%. The impact on the reliability and availability of the booster RF system has been carefully monitored during the past five operational years, delivering more than 5000 h scheduled user beam time per year. The booster RF system retains its excellent reliability and availability as previously. Neither a decrease of the service time nor an induced reliability issue from the klystron or any high-power high-voltage component of the transmitter has been experienced. We conclude that our energy-efficient operation of the booster RF system is harmless for the operation of the light source but decreases significantly the consumption of electric power. Here we report our approach and operational results in detail. [ABSTRACT FROM AUTHOR]

We developed a solid-state switch with static induction thyristors for the klystron modulator of the L-band electron linear accelerator (linac) at the Institute of Scientific and Industrial Research, Osaka University. This switch is designed to have maximum specifications of a holding voltage of 25 kV and a current of 6 kA at the repetition frequency of 10 Hz for forced air cooling. The turn-on time of the switch was measured with a matched resistor to be 270 ns, which is sufficiently fast for the klystron modulator. The switch is retrofitted in the modulator to generate 1.3 GHz RF pulses with durations of either 4 or 8 μs using a 30 MW klystron, and the linac is successfully operated under maximum conditions. This finding demonstrates that the switch can be used as a high-power switch for the modulator. Pulse-to-pulse variations of the klystron voltage are measured to be less than 0.015%, and those of RF power and phase are lower than 0.15% and 0.1°, respectively. These values are significantly smaller than those obtained with a thyratron; hence, the stability of the main RF system is improved. The solid-state switch has been used in normal operation of the linac for more than a year without any serious trouble. Thus, we confirmed the switch’s robustness and long-term reliability. [ABSTRACT FROM AUTHOR]

We report the development and implementation of four FPGA-based predistortion-type klystron linearization algorithms. Klystron linearization is essential for the realization of ILC, since it is required to operate the klystrons 7% in power below their saturation. The work presented was performed in international collaborations at the Fermi National Accelerator Laboratory (FNAL), USA and the Deutsches Elektronen Synchrotron (DESY), Germany. With the newly developed algorithms, the generation of correction factors on the FPGA was improved compared to past algorithms, avoiding quantization and decreasing memory requirements. At FNAL, three algorithms were tested at the Advanced Superconducting Test Accelerator (ASTA), demonstrating a successful implementation for one algorithm and a proof of principle for two algorithms. The functionality of the algorithm implemented at DESY was demonstrated successfully in a simulation. Besides this, a proof of principle of an FPGA-based klystron and cavity simulator implemented at the High Energy Accelerator Research Organization (KEK), Japan, was demonstrated. Its purpose is to allow the development and test of digital LLRF control systems including klystron linearization algorithms when no actual klystron and cavity are available. [ABSTRACT FROM AUTHOR]

Abstract: Klystrons are key components of particle accelerators since they generate the radio-frequency (RF) signals required to produce high energy particle beams. They are amplifiers characterised by nonlinear input/output dynamics and are affected by external disturbances. In order to regulate the output of the klystron to the desired level, a Low Level RF (LLRF) control system is usually employed. This paper proposes three different control methods for the design a LLRF system of a klystron: classical Proportional-Integral control, Iterative Learning Control and Model Predictive Control. Theoretical aspects of the presented control algorithms are reviewed and commented. The resulting LLRF system has been implemented on a FPGA board mounted in a PXIe chassis. Experimental results are shown to compare the algorithms and to provide an evidence of their capability to fulfil the control specifications. [Copyright &y& Elsevier]

Abstract: The RF power for the superconducting 1.3GHz CW LINAC “ELBE” has been doubled from less than 10kW to 20kW per cavity. In January 2012 the four 10kW klystrons used to drive the four superconducting cavities of the LINAC have been replaced by pairs of 10kW solid state power amplifiers (SSPA). ELBE is now worldwide the first 1.3GHz CW LINAC equipped with solid state RF power amplifiers. This technical note details on this project. [Copyright &y& Elsevier]

Abstract: As a part of the scheduled expansion of the Alcator C-Mod lower hybrid current drive (LHCD) system from 12 to 16 klystrons to accommodate installation of a second LH antenna, the active control system (ACS) is being redesigned to accommodate the additional klystrons. Digitizers and output modules will be cPCI modules provided by D-tAcq Solutions. The real-time application will run on a standard PC server running Linux. Initially, the new ACS system will be designed to control 8 klystrons on the second LH antenna and the existing ACS will control the remaining 8 klystrons on the existing LH antenna. Experience gained operating the existing LHCD system has given us insight into the design of a more robust, compact, efficient and simple system for the new ACS. The design upgrade will be patterned on the digital plasma control system (DPCS [1]) in use on C-Mod. [Copyright &y& Elsevier]

Abstract: Indiana University is operating a Low Energy Neutron Source which provides cold neutrons for material research and neutron physics and MeV energy for the neutron radiation effects studies. Neutrons are being produced by a 13 MeV proton beam incident on a Beryllium target. The LENS Proton Delivery System (PDS) is routinely operating at 13 MeV and 25mA at 1.8% duty factor. The RF system, consisting of three Litton 5773 klystron RF tubes at 425MHz and 1 MW each, power the AccSys Technology PL-13 LINAC. The proton beam delivers up to 6 kilowatts of power to the Beryllium target. Details of the beam spreading system, target cooling system, and accelerator operations will be discussed. [Copyright &y& Elsevier]

Abstract: The X-band high gradient studies at Nextef, the 100MW X-Band test station in KEK operated since 2008, is reviewed. Recent high power test results of TD18#2, an 18-cell CLIC prototype structure with HOM damping slots, are given. The measured breakdown rate is ∼10−5/pulse/m under the operation at 100MV/m gradient with 250ns rf pulse width. The rate is higher by one- to two-order compared with that of the T18 structure, which is a disk loaded structure without the HOM slots. The recent performance of the station is also reviewed, especially that of PPM klystrons is discussed. An overview of our on-going programs such as the pulse compression system of Nextef as well as the future plans of X-band study is given. [Copyright &y& Elsevier]

Abstract: The current activities and future perspectives involving the use of X-band technology at the Elettra Laboratory are presented. These are mainly related to the development of the FERMI@Elettra FEL project, a fourth generation light source facility, now under commissioning in Trieste. The facility, based on a seed scheme, requires sub-ps electron bunches (∼1kA), with very high optical qualities, at the entrance of the undulator chain. This is achieved by compressing the beam with two magnetic chicanes. A large effort has been undertaken to develop an X-band accelerating structure, to be installed downstream the first bunch compressor, before the magnetic chicane. In addition, we are also evaluating the possibility of extending the FERMI FEL wavelength capabilities beyond the foreseen limit of 4nm, by increasing the linac energy from 1.5GeV to 2.4–2.5GeV, using the X-band technology. [Copyright &y& Elsevier]

Abstract: Traditionally, the pulse-to-pulse charging and switching time for a thyratron-switched line-type modulator has limited the repetition rate of normal-conducting high-energy linacs; however the recent development of solid-state modulators employing IGBT switches has allowed the repetition rate of a high-power modulator to be increased by an order of magnitude. When combined with the high accelerating gradients that can be achieved in an X-band accelerating structure, this enables a relatively compact linear accelerator to be constructed that can operate with a pulse repetition rate in excess of 1kHz. A study has been undertaken on the issues arising from the operation of an X-band linac in this mode, including a survey of availability and limitations of modulators, klystrons and structures. Close attention has been paid to the dissipation of power in critical parts of the high-power RF chain, both on average and transiently during the high-power RF pulse. [Copyright &y& Elsevier]

Abstract: A 5GHz LHCD system is being designed for current drive and profile modification necessary for AT mode and steady-state operation of the KSTAR tokamak. A prototype 500kW CW klystron operating at 5GHz was developed for the steady-state RF source. In this klystron, a multi-cell cavity is introduced to reduce cavity voltage and ohmic power loss. The klystron is designed with a triode system for optimization of gain, efficiency and beam control. The high voltage for the cathode is turned by using a thyristor switching system at the low voltage transformer unit. For anode voltage control, a mod-anode voltage divider system is used which utilize the parallel-circuit of the FET switch and Zener diodes. The RF output power of the klystron was 300kW for 800s and 450kW for 20s. The maximal temperature at collector top surface was 83°C and power loss at the tube body did not exceed 10kW, the interlock level for the protection of the klystron. Detailed results of the klystron system test and commissioning are presented. [Copyright &y& Elsevier]

Abstract: In the framework of the EFDA task HCD-08-03-01, the ITER lower hybrid current drive (LHCD) system design has been reviewed. The system aims to generate 24MW of RF power at 5GHz, of which 20MW would be coupled to the plasmas. The present state of the art does not allow envisaging a unitary output of the klystrons exceeding 500kW, so the project is based on 48klystron units, leaving some margin when the transmission lines losses are taken into account. A high voltage power supply (HVPS), required to operate the klystrons, is proposed. A single HVPS would be used to feed and operate four klystrons in parallel configuration. Based on the above considerations, it is proposed to design and develop twelve HVPS, based on pulse step modulator (PSM) technology, each rated for 90kV/90A. This paper describes in details, the typical electrical requirements and the conceptual design of the proposed HVPS for the ITER LHCD system. [Copyright &y& Elsevier]

Abstract: To develop Continuous Wave (CW) high power klystrons for fusion experiments, calorimetric matched loads absorbing the RF power are necessary. To test and adjust the parameters of the new klystrons TH2103C able to produce 700kW/CW at a frequency of 3.7GHz for upgrading the RF power available in Tore Supra LHCD transmitter, SPINNER GmbH has successfully developed, in collaboration with the CEA, and manufactured a water load capable to absorb the RF power with a Voltage Standing Wave Ratio (VSWR)<1.05. Crucial part of the water load is the ceramic window cooled with demineralised water to absorb the RF power. Klystrons working at different frequencies like 4.6GHz and 5GHz are being developed for CW experiments. The water load could be adapted for these frequencies. The paper describes the specifications, the development and the high RF power tests to validate the dummy load. [Copyright &y& Elsevier]

Abstract: We present the development of a compact narrow-band Solid-State Power Amplifier (SSPA). We foresee a promising application of solid-state amplifiers specifically in accelerators for new generation synchrotron light sources. Such a new technology has reached a competitive price/performance ratio and expected lifetime in comparison with klystron and IOT amplifiers. The increasing number of synchrotron light sources using 500MHz as base frequency justifies the effort in the development of the proposed amplifier. Two different techniques are also proposed to improve the control and performance of these new distributed amplification systems which we call, respectively, complete distributed system and forced compression. [Copyright &y& Elsevier]

Abstract: In this paper we analyze the effect of beam energy spread on cascade optical klystron undulator radiation. The analysis suggests that stronger dispersion strength needs a better beam quality in both single stage and cascade optical klystron undulator radiation. [Copyright &y& Elsevier]

Abstract: A steady-state lower hybrid current drive (LHCD) system is under development for advanced tokamak experiments of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The KSTAR 5GHz steady-state LHCD system is being designed to couple an input power of 2MW for 300s generated by four 5GHz klystrons. For the development of this system, there are two critical issues. One is the development of a 5GHz CW klystron for the RF source of the system. The other is the design of a steady-state LH launcher with active water cooling. In this paper, the current status of the development and design for the KSTAR steady-state LHCD system is described. For the LHCD system, aiming at a basic experimental study of 5GHz LH wave propagation and operational experience with an LHCD system, the installation of an initial LHCD system with a capacity of 0.5MW for 2s is scheduled in 2010 using a 5GHz prototype klystron and an un-cooled 1MW launcher. The design and progress for the initial LHCD system are also presented. [Copyright &y& Elsevier]

Abstract: To study the performance of the ELBE RF-couplers until their limits a 1.3GHz resonant ring driven by a 10kW CW klystron (VKL7811St, CPI) has been built to operate a RF-coupler and RF-window test bench. The ring allows tests with RF power up to 100kW in continuous wave (CW) mode and up to 200kW in pulsed mode. Details of the resonant ring, the test bench design and results of the ELBE RF-coupler tests will be presented. [Copyright &y& Elsevier]

Abstract: One of the main Tore Supra (TS) objectives is to produce long and performing non-inductive discharges whose studies are crucial for the next step. The Lower Hybrid (LH) system is routinely used on TS to provide such discharges. The CIMES project will improve the TS LH power injection facilities leading to a transmitter power of 10MW CW. A new klystron, TH2103C has been developed at Thales Electron Devices since November 2001. The specified RF output power is 620kW CW on plasma (Voltage Standing Wave Ratio (VSWR)=1.4) and 700kW CW on matched load. Six klystrons have been successively manufactured and tested to define the good design. The last one was successfully tested on both TED and CEA test beds. It shows adequate margins of its design towards specifications. These margins ensure high reliability needed for long pulse operation (LPO) foreseen in the frame of the CIMES project. The procedure used to test the klystron and the safety interlocks carried out to avoid klystron defaults are detailed. The results obtained with prototype are presented and analyzed. [Copyright &y& Elsevier]

Abstract: It is important to accurately adjust the amplitude and the phase of RF power sources for a high intensity proton linac. J-PARC (Japan proton accelerator research complex) linac is one of those high-intensity linacs. J-PARC linac has 30 SDTL (separate-type drift tube linac) tanks to accelerate the negative hydrogen ions from 50 to 181MeV. Two neighboring SDTL tanks are driven by one klystron, where the phase and the amplitude of these two tanks are controlled in terms of the vector sum. The target-value of the vector sum control should be determined with a beam-based tuning for each klystron. During the beam commissioning, the RF tuning has been performed with a phase scan method introducing a concept of phase signature matching. In the tuning, the output beam energy from the SDTL module is monitored while scanning the RF phase. Comparing the obtained phase dependence of the output beam energy with those from a numerical model, the target-values for the low-level RF control system has been tuned within the required accuracy of 1° in phase and 1% in amplitude. The same tuning procedure has successfully been applied to the RF tuning of buncher and debuncher cavities. [Copyright &y& Elsevier]

Abstract: The proposed RF distribution scheme for the two 15km long ILC LINACs uses one klystron to feed 26 superconducting RF cavities operating at 1.3GHz. For a precise control of the vector sum of the signals coming from the SC cavities, the control system needs a high-performance, low-cost, reliable and modular multichannel receiver. At Fermilab we developed a 96-channel, 1.3GHz analog/digital receiver for the ILC LINAC LLRF control system. In this paper we present a balanced design approach to the specifications of each receiver section, the design choices made to fulfill the goals and a description of the prototyped system. The design is tested by measuring standard performance parameters, such as noise figure, linearity and temperature sensitivity. Measurements show that the design meets the specifications and it is comparable to other similar systems developed at other laboratories, in terms of performance. [Copyright &y& Elsevier]

Abstract: We examine here the effects of plasmas and beam slippage on the gain of an optical klystron (OK) free electron laser (FEL) device under the influence of a weak guiding magnetic field and background plasma. The beam energy spread decreases with background plasma density and increases with beam plasma density . The maximum gain () scales proportional to . The beam slippage phase Δψ scales directly to five power of relativistic factor and one half power to . [Copyright &y& Elsevier]

Abstract: Klystrons and triodes have been the accepted choice for particle accelerators because they produce high power RF and offer high gain (60dB) with efficiencies of ∼50%. Although fairly new to the market, inductive output tubes (IOTs) have become available at L-band frequencies and have maintained their high efficiency. The development of superconducting RF at the L-band frequency allows IOTs to become the choice for future accelerator programs. Due to the operational nature of SRF technology in energy recovery mode, there is no longer the requirement for large amounts of RF power from single sources. This report reviews some of the developments in RF power sources suitable for energy recovery linacs (ERLs). [Copyright &y& Elsevier]

Abstract: One of the main Tore Supra objectives is to produce long and high performance discharges, studies which are crucial for the next step. In the frame of the CIMES project, associated with the manufacture of an ITER relevant PAM launcher, a new klystron is under development at Thales Electron Devices. The upgrade will lead to an installed power of 12 MW in the lower hybrid transmitter. Each of the 16 tubes will work at 3.7GHz, 76kV, 22A with an efficiency up to 45%, providing 700kW on a VSWR up to 1.4, during 1000s. Two factory prototypes have been manufactured and tested on the Thales test bed. The first one is installed on Tore Supra Lower Hybrid test bed. Up to now, 770kW at 75kW, 22A (50% duty cycle) have been achieved on matched load. A prototype, derived from these two factory prototypes, will reach full performance in few months. The tube''s parameters are described in this paper. They are compared to those of the TH2103 presently used in the transmitter and their choices are detailed and explained. The results obtained with prototypes as well as the technological difficulties are presented and analysed. [Copyright &y& Elsevier]

Far-infrared free electron lasers (FELs) with central wavelengths of 11.6 and ∼70 μm were obtained by using an S-band RF linac (LEENA) at electron energies of 14.4 and ∼5.5 MeV, respectively. Small-signal gain and cavity loss were measured to be 8.2% and 3.3%, respectively, at a peak current of 4.4 A, an electron energy of 15 MeV, and a central wavelength of 10 μm. The pulsewidth of the FEL signal could be lengthened by tailoring the waveform of klystron drive voltage, since uniformity of electron energy in a macropulse was improved. A tungsten needle photo-RF gun was investigated using the field-dependence of QE for a third-harmonics Nd:YAG laser (wavelength of 355 nm). Numerical calculations of electron beam performances in a needle photo-RF-gun were made. A tungsten needle photocathode, with a tip radius of 10 μm, was introduced into the RF-gun cavity of LEENA. [Copyright &y& Elsevier]

The short-wavelength free electron laser (FEL) requires a stable electron beam with long pulse duration, narrow energy spread, and high current density. The pulse forming network (PFN) for the klystron pulse modulator at LEBRA was designed to realize pulse flatness error within 0.05%. The PFN consists of 30 capacitors and adjustable inductors. A PFN adjustment was performed to suppress changes of the RF phase difference between the input and the output of the klystron. The flatness of the pulse voltage applied to the klystron has been adjusted to an error within 0.06% as a result of the phase flatness error within 0.3° achieved by the PFN adjustment. Furthermore, the time dependence of the energy spectrum in the pulse duration has also been suppressed. [Copyright &y& Elsevier]

We are pursing operational stability of the RF linac with a method of system analysis. The RF phase and the power for the pre-buncher have been measured, together with environmental parameters including temperatures of the main components, the room temperatures and the AC line voltage. The measured data have been analyzed with the autoregressive model, and the feedback structure of the system has been derived from noise contribution ratios and impulse responses analysis. It was found that the AC line voltage and the temperatures of the klystron room significantly affected operational stability of the linac. [Copyright &y& Elsevier]