A niobium prototype and two test unjacketed 325 MHz coaxial half-wave cavities with beta=0.21were designed, fabricated, and tested at low RF fields. Electromagnetic properties wereinvestigated using a vector network analyzer in the frequency domain and an RF generator,directional couplers, and power detectors in the time domain (both continuous wave and pulsemode). We analyzed and compared the response of the low-beta cavity in both normal andsuperconducting states. We studied the relationship between the resonator parameters andcoupling devices to optimize the design of the RF power coupler and field probe antenna,aiming to achieve accurate cavity characterization. We proposed a tunable power coupler designthat allows for varying the coupling coefficient when operating in a vacuum at liquid heliumtemperature. The typical responses of the cavity in case of under-, critical-, and over-couplingconditions obtained within a single cool-down are presented and analyzed. We introduced asimplified method based on decay measurements with cavity port switching for effective cavitycharacterization at low RF amplitudes. This method aligns well with standardQ-factormeasurement protocols and can expedite experimental investigations with small RF signals. TheQ0(Eacc)curve for half-wave resonators at ultra-low amplitudes is analyzed and discussed.Experiments with coaxial half-wave cavities showed an almost constantQ0at RF fields rangingfrom 1x10-4MVm-1to 1x10-1MVm-1. Additionally, a significant increase in the qualityfactor was observed after 120 degrees C baking. The presented results are discussed in the context ofpotential low-field applications of superconductive cavities.

A prototype of the 325-MHz niobium half-wave coaxial resonator ( β = 0.21 ) is developed, built, and tested at low microwave field amplitudes. Electromagnetic properties of the prototype in the superconducting state are investigated in the continuous wave regime and the damping regime using a highly stable radio-frequency (RF) generator and power detectors. The experimental data on the resonator’s response in the superconducting state are used to calculate its most important characteristics—the intrinsic Q value and the accelerating field. The experimentally measured Q value of the prototype is Q_0 = (3.5 ± 0.1) ×10^8 at input powers of up to +20 dBm.

The creation of accelerators for the inspection of large objects during their transportation across state borders is an important task. To obtain high-quality images of the inspected object internal composition, the accelerator must be able to do a dual switch of beam energy from pulse to pulse [1]. A number of works are devoted to solving this problem, describing methods and real facilities with dual energy switch [2–5]. The energy switch is achieved by a stepwise change in the input power of the accelerating section and/or the magnitude of the accelerated current. In order to provide the necessary energy and beam current in high and low energy modes, the accelerating section parameters must be carefully tuned. This article compares two power supply schemes of a two-section linac with energy switch due to disconnection of one of the sections using circulators and bridges.

An S-band linear accelerator (linac) equipped with a radio-frequency (RF) photogun with an operating frequency of 2.8 GHz is under development in the National Research Center “Kurchatov Institute.” This linac will be used as a top-up injector for the storage ring of the SILA Ultimate Source of Synchrotron Radiation (USSR) and as a driver of high-brightness electron bunches for the free-electron laser (FEL). A pilot RF photogun for this linac injector is currently under development in the National Research Nuclear University “Moscow Engineering Physics Institute.” The optimization of geometric parameters and the modeling of electrodynamic characteristics have been performed in an RF gun based on a 3.6-cell π-mode standing wave accelerating structure. Beam dynamics has been studied. In addition, the results from the analysis of thermal processes will be reported. The accelerating cavity design will also be presented.

The results of the development of an accelerating structure and an input coupler device device for a compact linear electron accelerator designed to operate as part of a radiation therapy unit are presented. The optimized geometry and electrodynamic characteristics of the structure are obtained.

This paper considers H-type cavity power coupling loop design issues and provides the results of the modeling. The dependence of the coupling coefficient on the loop geometry is established.

The article discusses the design of power coupling device for IH-type resonators for a new linear ion accelerator under development at the National Research Nuclear University MEPhI. The influence of mesh partitioning parameters on the accuracy of calculating the electric field intensity on the coupling loop surface is studied. The effect of the radius of curvature of the loop ends on the electric field intensity is also investigated. The electric field intensity values on the coupling loop surface are determined at the operating power level.

This paper considers power supply issues arising in hybrid accelerating structures containing both standing and traveling wave sections in operation and power filling regimes and provides recommendations on power feeding design. Calculations of the 10 MeV electron linac efficiency in wide beam current range are presented. Magnetron stability issues are discussed.

The results of designing the accelerating structure of a compact linear electron accelerator for a radiation-therapy facility are presented. The optimized geometry and electrodynamic characteristics of the structure are obtained.

The paper presents the results of calculating the electrodynamic characteristics (EDCs) of a biperiodic accelerating structure with internal coupling cells, which is intended for the linear accelerator of the injection complex of the fourth-generation specialized synchrotron radiation source for the SILA project. The ways to limit the accelerating field in the accelerating structure by electric and magnetic fields maintained on the surface of the cavity in the S, C, and X bands are considered.

Energy-frontier accelerators provide powerful tools performing high precision measurements confirming the fundamentals of the physics and broadening new research horizons. Such machines are either driven by circular or linear accelerators. The circular machines, having the centre-of-mass (CM) energy values reaching 200 GeV (for leptons) and above, experience beam energy loss and quality dilution, for example, due to synchrotron radiation, limiting the overall CM energy achievable and requiring a constant energy top-up to compensate the loss and the beam quality dilution. Linear colliders overcome these limitations, while the finite capabilities of generating high average current beams limits the luminosity. This is partially compensated by the quality of the colliding beams. In this work, we suggest a novel design of circular-linear accelerator based on the merging of the “non-emitting”, low-energy storage rings and energy recovery linear accelerators. We suggest using the recently considered dual-axis asymmetric cavities to enable the operation of such a system, and in particular the energy recovery from spent, high-intensity beams. The machine considered, under the scope of the SNOWMASS-2021 initiative, can be potentially used to reach ultimate energy frontiers in high-energy physics as well as to drive next generation light sources. The merging of circular and linear systems, and applications of dual axes cavities, should allow the maintaining of high beam quality, high luminosity, and high energy efficiency, while offering a flexible energy management and opening clear opportunity for reducing the running cost. We note that the numbers shown in the paper are for illustration purpose and can be improved further.

The paper presents the results of the development of short five-gap IH-type accelerating structures with a fixed length of accelerating gaps, operated at 80 MHz and relative velocity of 0.06с and 0.1с. It has considered the issues of selecting the optimal length of accelerating gaps, the drift tube geometry, and the drift tube suspension system, taking into account limits on the magnitude of high-frequency losses in the cavity walls and the axis-field distribution, at which the field value in the extreme accelerating gaps is at least 50% of the field value in the central gaps.

The results of calculating the threshold voltage levels causing multipactor discharge in single-gap buncher cavities at 80 MHz are presented in the article. Two types of gap geometry with an extension from 18 to 25 mm and with an extension from 18 to 80 mm are considered. The results of modeling a multipactor discharge using the MultP-M program are presented.

Nowadays design of accelerating structures and traveling wave mode converter coupler cells in particular is almost excursively done using 2.5D and 3D codes based on finite elements method. These methods are extremely versatile and precise but require a lot of computational power. This paper describes mode converter coupler matching method using both finite element and equivalent circuit methods. Analytical calculation using equivalent circuit method provides an initial coupler parameter set for the finite element method calculations, accelerating further parameter conversion and reducing overall calculation time.

Preliminary results on accelerating cavities for USSR-4 facility (also known as SYLA SYnchrotron and free-electron LAser) project are presented. This facility is under development by collaboration hosted by National Research Center "Kurchatov Institute". SYLA is synchrotron radiation facility based on injector linac and 6 GeV storage ring. Beam energy loss in storage ring is to be compensated by several modified pillbox cavities. Cavity geometry features, its operation frequency choice and induced HOM parameters are discussed. HOM damping technique using corrugated cylindrical waveguides were studied. Longitudinal impedance values of HOM are presented for initial accelerating cavity and structure with waveguides. INTRODUCTION 6 GeV Ultimate Source of Synchrotron Radiation is planned to be built in Russia. Storage ring scheme is based on the ESRF-EBS design [1-3]. For sources of synchrotron radiation, large values of beam currents are required. Excitation of Higher order modes (HOM) can lead to multibunch instabilities, emittance growth, beam breakup, etc. [4,5]. The general view of the accelerating cavity with an operating frequency of 357 MHz is shown in Fig. 1. Figure 1: General view of an accelerating cavity at 357 MHz. For an accelerating cavity the electrodynamic characteristics (EDC) [6] of the fundamental mode (Table 1) and HOM were carried out [7]. Table 1: EDC of Accelerating Cavity EDC Values f, MHz 357 Q, ×103 41 Rsh eff, MΩ 10 The frequency dependence of the longitudinal shunt impedance for monopole HOMs is shown in Fig. 2. Figure 2: Frequency dependence of the longitudinal shunt impedance for monopole modes for an accelerating cavity at 357 MHz. Graph – result of wakefield simulations, dots – results of eigenmode simulations. From Fig. 2 one could see that that the shunt impedance of parasitic HOMs accelerating cavity can reach values of up to 106 Ohm. One of the requirements for EBS-ESRF cavity design was to ensure unconditional beam stability up to currents of 1000 mA. For EBS-ESRF longitudinal coupled bunch instability threshold at 200 mA is RHOM∙fHOM = 16 kW∙GHz (Fig. 3) [4]. Figure 3: Threshold for longitudinal impedance necessary for unconditional beam stability for 200, 500 and 1000 mA beam current. From Fig. 2 and 3 we can see that it is necessary to significantly decrease the shunt impedance values of HOMs. To reduce these values, it is proposed to add HOM couplers to the system. ________________________________________ † yvshashkov@mephi.ru 27th Russian Particle Acc. Conf. RuPAC2021, Alushta, Russia JACoW Publishing ISBN: 978-3-95450-240-0 ISSN: 2673-5539 doi:10.18429/JACoW-RuPAC2021-WEPSC13 Accelerating structures and powerful radio engineering WEPSC13 367 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I