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.

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.

Nowadays design of accelerating structures is almost exclusively done using 2.5D and 3D codes based on finite elements method. Equivalent circuit method is frequently considered limited and inconvenient to use in real-life projects. However, low resource requirements make this method attractive for wide range sweep calculations.This paper describes LinacCalc application — a user friendly accelerating structure simulation software based on the equivalent circuit method. It allows calculating characteristics of the accelerating sections with large number of cells in short time on machines with moderate amount of computational power. Core application modules are validated by comparing calculation results with ones obtained by a conventional finite element method based programs.

Since 2015 the superconducting (SC) linac-injector development for Nuclotron NICA (JINR, Dubna, Russia) is carried out by the collaboration of JINR, NRNU MEPhI, INP BSU, PTI NASB. This new SC linac is to accelerate protons up to 20 MeV and light ions to 7.5 MeV/u with possible energy upgrade up to 50 MeV for proton beam. This paper reports the current status of the development and manufacturing of superconducting accelerating cavities for a new linear accelerator of the injection complex of the Nuclotron-NICA project.

Now available for the first time in English translation, this important book contains extensive material relating to the electrodynamic characteristics of linear accelerators, and gives a good overview of the fundamentals of accelerating cavity design. The authors describe the experimental methods and measurement techniques essential in this area of research, and provide comprehensive data about the electrodynamic characteristics of resonant structures, which are widely used in charged particle accelerators and microwave devices. Single cavities and coupling chains, excited in electrical and magnetic modes, are described numerically and analyzed in detail. The book also provides a valuable description of the perturbation method, which is illustrated using a unique collection of data.

The developed and currently manufactured debuncher for the injector part of the accelerator facility NICA is designed to reduce the energy spread by a factor of up to ten in the ion bunches with Z/A=(0.33-1) at the output of the LU-20 linac. The debuncher includes a Split-Ring cavity, a vacuum system, a solid-state RF amplifier and an RF controller. The main design parameters are described.

The 800 MHz superconducting cavities with grooved beam pipes were suggested as one of the harmonic cavities design options for High Luminosity LHC project. Cavity simulations were carried out and scaled aluminium prototype having operational mode frequency of 2400 MHz was manufactured for testing the results of simulations. The experimental measurements of transverse shunt impedance with error estimation for higher order modes TM110 and TE111 for S-band elliptical cavity were done. The experiments using dielectric and metallic spherical beads and with ring probe were carried out. The Q-factor measurements for two-cell structure and array of two cells were carried out.

The project for a new injector linac for the Nuclotron–NICA accelerator tandem has been under development since 2015. The linac will accelerate protons and light ions to energies of 25 MeV and ∼7.5 MeV/nucleon, respectively. To fulfill the project, technology for fabricating superconducting RF cavities needs to be developed. The current status of the project, including the linac design, the required parameters of RF cavities, and our simulations of beam dynamics, is presented. Technological restrictions on the RF cavity parameters are discussed.

A pulsed 55-MeV race-track microtron that was developed and constructed jointly at the Skobeltsyn Institute of Nuclear Physics, Moscow State University and the Lebedev Physical Institute with the participation of Moscow Engineering Physics Institute is described. The results of calculations of the beam dynamics and the main elements of the accelerator are presented, their design is described, and the results of their measurements and adjustment are presented. The technique and results of the accelerator commissioning are described.

Electron linear accelerators with an energy of 10 MeV are widely used for industrial purposes. This article presents the electron dynamics calculations and the design of linac with a standing wave (SW) buncher based on the biperiodic accelerating structure and a constant impedance backward traveling wave (BTW) after it. In such accelerator, all unused RF power coming out from BTW section is used in SW section to improve the linac efficiency. Thus, no RF load is needed. Also, a beam is experiencing an RF focusing in the SW buncher. Solenoid focusing field influence on the beam dynamics in the TW section was studied. INTRODUCTION Electron linear accelerators to the fixed 10 MeV energy are in demand for the industrial purposes. For example, for the sterilization of medical supplies, food, cosmetics etc. [1]. One of the first choices the developer is faced – it is the choice between SW or TW operating regimes. Both options have their own advantages, disadvantages, and special issues. TW is suitable for the acceleration of high electron currents. In the meantime, SW buncher is much shorter than TW buncher and doesn’t require additional focusing fields [2]. The way to combine advantages of both SW and TW structures is a hybrid linac [3], where the beam is bunching in the biperiodic accelerating SW structure (BPS) [4] and continuing to accelerate in TW structure based on the reliable diaphragm loaded structure technology. ACCELERATOR SCHEME We propose the hybrid structure (Fig.1), where the unused for the acceleration in BTW RF power goes not to the load but, via the rectangular waveguide, to the BPS buncher (Fig.2). Figure 1: Hybrid linac scheme. Before the drift tube – BPS, after – BTW. In the operating regime, power reflection from BPS, tuned to the optimal overcoupling [5], is equal to zero, thus accelerating section is operating in the TW regime. Accelerator operates at 2856 MHz frequency. Figure 2: BTW and BPS connection. ACCELERATOR GEOMETRY Accelerating Section Accelerating section for the relativistic particles is made from the disk-loaded waveguide (DLW) with an additional magnetic coupling. Magnetic coupling is designed to be higher than electric coupling, because for using BAS as a load, power flow in accelerating section should be in opposite direction to the beam propagation, i.e. negative group velocity. We studied dependencies of the main electrodynamics characteristics of BTW, such as shunt impedance rsh, group velocity gr, Q-factor, attenuation coefficient and normalized accelerating gradient 1/2 as a function of phase shift per cell and normalized to the wavelength aperture radius . Shunt impedance group velocity is ~1%. Table 1. shows, that shunt impedance rises with smaller aperture radius. We decided to and avoid beam losses in accelerator walls. Table 1: BTW electrodynamics parameters dependence stant group velocity and 2856 MHz operating frequency. 0.06 0.08 0.1 rsh, MOhm/m 82.9 71.2 62.1 gr, % 1.3 1.2 1.2 Q 1250

The results of the electrodynamical and multipactor discharge simulations of the medium betta superconducting cavity for New Superconducting Injector Linac for Nuclotron-NICA are presented. Different designs of CH and Spoke cavities are compared and the optimal one is chosen. INTRODUCTION Development of accelerating structures for the SC Linac Nuclotron-NICA injector is underway by collaboration of Russian and Belorussian research institutions JINR, NRNU MEPhI, INP BSU, PTI NASB, BSUIR and SPMRC NASB. According to the concept of the SC Linac Nuclotron-NICA [1] second group of resonators is designed for frequency of 324 MHz, velocity of 0.21с and 7.7 MV/m accelerating gradient. Three types of cavities could fit the required parameters: HWR, СH and Spoke. The results of RF design optimization for CH and Spoke geometries for desired parameters is presented. Comparative analysis of these cavities was done. The results of this work will be used for cavity type choice for the second section of the SC Linac Nuclotron NICA. СH-CAVITY IAP, HIM and GSI (Germany) are the experts in development, fabrication and application of SC CH-type resonators. The 7-cell 325 MHz CH structure [2] is the closest match for our requirements and it was chosen as a prototype for the CH cavity geometry for the SC Linac Nuclotron NICA. The simulations resulting in optimized cell geometry and increased transit factor along with lower surface peak fields were carried out for infinite periodic model with periodic boundaries. Finite model was used for solving eigenmode problem in order to optimize end-cells geometry. According to the beam dynamics calculations [1], the number of cells should be less than seven. Thus, the geometry of 5-cell 324 MHz and 0.21 beta CH-cavity was developed (Fig. 1). Figure 1: 324 MHz and 0.21 beta CH resonator geometry. The initial (“reference”) cavity design [2] parameters along with ones after our modification are presented in Table 1. Table 1: RF Parameters Parameter Reference After optimization f, MHz 325 324 β 0.16 0.21 R, mm 175 204 L, mm 505 500 Ep/Eacc 5 5.1 Bp/Eacc, [mT/(MV/m)] 13 7.5 G, [Ω] 66 84 Ra/Q 1260 615 The results show that this type of resonator could reach required accelerating gradient only in case of the maximum surface peak field exceeding 35 MV/m limit. Compared to the prototype structure, overall dimensions are increased by 14%. For the optimized structure, a multipactor discharge simulation was done. At Fig. 2 the number of electrons growth rate dependence vs. accelerating gradient is presented. The calculation was carried out using the threeThis project is supportd in part by the MEPhI 5/100 Program of the Russian Academic Excellence Project * * Proceedings of IPAC2017, Copenhagen, Denmark MOPVA088 07 Accelerator Technology T07 Superconducting RF ISBN 978-3-95450-182-3 1061 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs dimensional code for multipactor discharge simulations MultP-M [3]. Figure 2: Number of electrons growth (N) in cavity at different accelerating gradient for CH-cavity. At low field levels in CH-cavity multipactor electrons trajectories are mainly localized at the cavity outer surface. Single trajectories are detected at the operating field strength level of 7.7 MV / m in the area between the side wall and the pylon Fig.3. Figure 3: Areas multipactor trajectories localization at the operating field strength. Detailed study of multipactor shows that the trajectories could be maintained only for about 30 RF periods, after that all trajectories are damped. Nevertheless the optimization of the specified area is required. SPOKE-CAVITY The most closely matching geometry of the Spoke type is SSR1 cavity developed, which is fabricated and tested at Fermilab [4 6]. It was taken as a prototype for the 324 MHz 0.21 beta Spoke cavity design proposed. The cavity geometry which satisfies our requirements is presented on Fig. 4. Figure 4: 324 MHz 0.21 beta Single Spoke Resonator geometry. Numerical simulations for this geometry were performed in order to diminish peak surface electric and magnetic fields and increase shunt impedance of the structure. Simulations results are shown in Table 2. Table 2: RF Parameters Parameter Reference After optimization f, MHz 325 324 β 0.22 0.21 R, mm 205 215 L, mm 251 250 Ep/Eacc 7.84 4.0 Bp/Eacc, [mT/(MV/m)] 9.81 6.3 G, [Ω] 84 79 Ra/Q 242 253 For the optimized structure multipactor discharge simulation was performed. In Fig. 5, electrons population growth for the different accelerating gradient is presented. Figure 5: Number of electrons growth (N) in cavity for the different accelerating gradient. It can be seen that in the Spoke structure multipactor trajectories at the operating field level are not detected. A MOPVA088 Proceedings of IPAC2017, Copenhagen, Denmark ISBN 978-3-95450-182-3 1062 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs 07 Accelerator Technology T07 Superconducting RF 40 RF periods long simulation showed at low field levels the trajectories decay. GEOMETRIES COMPARISON The optimization results f 324 MHz 0.21 beta CH and Spoke cavities are summarized in Table 3. The maximum accelerating field strength values are given with respect to the limitation of the peak electric field at the surface of 35 MV/m and the magnetic field of 75 mT. Table 3: RF Parameters Parameter СH Spoke f, MHz 324 324 β 0.21 0.21 R, mm 204 215 L, mm 500 250 Ep/Eacc 5.1 4.0 Bp/Eacc, [mT/(MV/m)] 7.5 6.3 G, [Ω] 84 79 Ra/Q 615 253 The multicell CH cavity is preferable from the point of overall dimensions and necessary auxiliary equipment: cryostats, tuners, power couplers. The Spoke geometry benefits higher accelerating gradient and Q-factor values. CONCLUSION Comparative analysis of CH and Spoke type cavities for the second group of resonators for SC Linac Nuclotron NICA injector was performed in cooperation with JINR and SPMRC NASB. Obtained results show that multicell CH structure is more convenient, but Spoke type cavity better satisfies desired parameters, so this geometry is the most preferable. Thorough multipator and mechanical simulation and analysis are planned.