arXiv subjects

Alexej Grudiev

Publications and source records attributed to Alexej Grudiev.

15 recordsLinked to original sources

arXiv2025

Influence of an external static magnetic field on prebreakdown electron emission and heating

High magnetic fields can increase the occurrence of vacuum arcing, suggesting that both electric and magnetic fields can play a role in the vacuum arcing process. The mechanism of vacuum arcing in high magnetic fields is believed to involve both the cathode and the anode, with the cathode serving as the originator of field-emitting nanoprotrusions or tips, while the anode serves a secondary role. Significant heating of the anode surface can be achieved by magnetic focusing of the emitted electron beam, leading to increased heat flux due to greater current density. We simulated the emitted electron beam in different configurations of the electric and magnetic fields using the particle-in-cell (PIC) and finite element methods (FEM). The heating caused by the impacting electron beam was simulated for magnetic fields ranging from 0 T to 30 T. The directions of the electric and magnetic fields were found to play a major role in the focusing of the electron beam. We found that a sufficient temperature increase on the anode surface for evaporation can be reached at magnetic fields on the order of 10-30 T, suggesting the possibility of plasma initiation on the anode side.

physics.acc-ph↗

arXiv2025

The Compact Linear e$^+$e$^-$ Collider (CLIC)

The Compact Linear Collider (CLIC) is a TeV-scale high-luminosity linear e$^+$e$^-$ collider studied by the international CLIC and CLICdp collaborations. CLIC uses a two-beam acceleration scheme, in which normal-conducting high-gradient 12 GHz accelerating structures are powered via a high-current drive beam. CLIC is foreseen to be built and operated in stages. The initial 380 GeV stage, with a site length of 11 km, optimally combines the exploration of Higgs and top-quark physics, including a top threshold scan near 350 GeV. A higher-energy stage, still using the initial single drive-beam complex, can be optimised for any energy up to 2 TeV. Parameters are presented in detail for a 1.5 TeV stage, with a site length of 29 km. Since the 2018 ESPPU reporting, significant effort was invested in CLIC accelerator optimisation, technology developments and system tests, including collaboration with new-generation light sources and free-electron lasers. CLIC implementation aspects at CERN have covered detailed studies of civil engineering, electrical networks, cooling and ventilation, scheduling, and costing. The CLIC baseline at 380 GeV is now 100 Hz operation, with a luminosity of 4.5$\times 10^{34}$ cm$^{-2}$s$^{-1}$ and a power consumption of 166 MW. Compared to the 2018 design, this gives three times higher luminosity-per-power. The new baseline has two beam-delivery systems, allowing for two detectors operating in parallel. The cost estimate of the 380 GeV baseline is approximately 7.2 billion CHF. The construction of the first CLIC energy stage could start as early as ~2034/35 with first beams following a decade later. This report summarises the CLIC project, its implementation and running scenarios, with emphasis on new developments and recent progress. It concludes with an update on the CLIC detector studies and on the physics potential in light of the improved accelerator performance.

physics.acc-ph↗

arXiv2024

Beam-cavity interactions in the rapid cycling synchrotron chain of the future muon collider

The International Muon Collider Collaboration (IMCC) is engaged in a design study for a future facility intended to collide muons. Subsequent to the initial linear acceleration, the counter-rotating muons and anti-muons are accelerated in a chain of rapid cycling synchrotrons (RCS) up to the multi-TeV collision energy. To maximise the number of muons available in the collider, it is essential to exploit the time dilation of the muon lifetime by employing a large accelerating gradient. The 1.3 GHz TESLA cavity serves as the baseline for the RCS chain. Considering the high bunch population and the small aperture of the cavity, the resulting beam-induced voltage per bunch passage is considerable, resulting in a substantial perturbation of the cavity voltage for subsequent bunch passages. In this contribution, the effects of beam loading during the acceleration cycle on the muons are calculated with the objective of determining the optimum parameters for minimising the cavity voltage transients. The interaction of the induced voltages, considering the counter-rotating beams, is studied.

physics.acc-ph↗

arXiv2023

Dielectric Assist Accelerating Structures for Compact Linear Accelerators of Low Energy Particles in Hadrontherapy Treatments

Dielectric Assist Accelerating (DAA) structures based on ultralow-loss ceramic are being studied as an alternative to conventional disk-loaded copper cavities. This accelerating structure consists of dielectric disks with irises arranged periodically in metallic structures working under the TM$_{02}$-$π$ mode. In this paper, the numerical design of an S-band DAA structure for low beta particles, such as protons or carbon ions used for Hadrontherapy treatments, is shown. Four dielectric materials with different permittivity and loss tangent are studied as well as different particle velocities. Through optimization, a design that concentrates most of the RF power in the vacuum space near the beam axis is obtained, leading to a significant reduction of power loss on the metallic walls. This allows to fabricate cavities with an extremely high quality factor, over 100 000, and shunt impedance over 300 M$Ω$/m at room temperature. During the numerical study, the design optimization has been improved by adjusting some of the cell parameters in order to both increase the shunt impedance and reduce the peak electric field in certain locations of the cavity, which can lead to instabilities in its normal functioning.

physics.acc-ph↗

arXiv2023

Non-resonant ultra-fast multipactor regime in dielectric-assist accelerating structures

The objective of this work is the evaluation of the risk of suffering a multipactor discharge in an S-band dielectric-assist accelerating (DAA) structure for a compact low-energy linear particle accelerator dedicated to hadrontherapy treatments. A DAA structure consists of ultra-low loss dielectric ciylinders and disks with irises which are periodically arranged in a metallic enclosure, with the advantage of having an extremely high quality factor and very high shunt impedance at room temperature, and it is therefore proposed as a potential alternative to conventional disk-loaded copper structures. However, it has been observed that these structures suffer from multipactor discharges. In fact, multipactor is one of the main problems of these devices, as it limits the maximum accelerating gradient. Because of this, the analysis of multipactor risk in the early design steps of DAA cavities is crucial to ensure the correct performance of the device after fabrication. In this paper, we present a comprehensive and detailed study of multipactor in our DAA design through numerical simulations performed with an in-house developed code based on the Monte-Carlo method. The phenomenology of the multipactor (resonant electron trajectories, electron flight time between impacts, etc.) is described in detail for different values of the accelerating gradient. It has been found that in these structures an ultra-fast non-resonant multipactor appears, which is different from the types of multipactor theoretically studied in the scientific literature. In addition, the effect of several low electron emission coatings on the multipactor threshold is investigated. Furthermore, a novel design based on the modification of the DAA cell geometry for multipactor mitigation is introduced, which shows a significant increase in the accelerating gradient handling capabilities of our prototype.

physics.acc-ph↗

arXiv2023

Wakefield damping in a distributed coupling LINAC

The number of cells in a $π$-mode standing wave (SW) accelerating structure for the Compact linear Collider (CLIC) project is limited by mode overlap with nearby modes. The distributed coupling scheme avoids mode overlap by treating each cell as independent. Designs of cells suitable for distributed coupling with strong wakefield damping have not previously been studied. In this paper we develop a SW cell to be used in a distributed coupling structure that can satisfy the CLIC transverse wakepotential limit. From the middle cell of the CLIC-G* travelling wave (TW) structure, a SW cell is designed. The cell is adapted to be suitable for distributed coupling. Its wakepotentials in an ideal case of open boundaries are reduced to satisfy the wakepotential threshold. An electric boundary is added to the model to simulate total reflection at the distribution network. A horizontal coupler cell that connects to the distribution network such that the reflected wakefields remain similar to the open boundary case is simulated. A triplet module which takes advantage of cell-to-cell coupling to reduce reflected wakepotential is presented.

physics.acc-ph↗

arXiv2022

Local power coupling as a predictor of high-gradient breakdown performance

A novel quantity for predicting the high-gradient performance of radio frequency accelerating structures is presented. The quantity is motivated, derived and compared with earlier high-gradient limits and experiments. This new method models a nascent RF breakdown as a current-carrying antenna and calculates the coupling of the antenna to an RF power source. With the help of an electron emission model to describe a nascent breakdown, the antenna model describes how a breakdown modifies the local surface electric field before it fully develops in any given structure geometry. For the structure geometries that this method was applied to, it was found that the calculated breakdown-loaded electric field was well-correlated with observed spatial breakdown distributions, and gave consistent values for the maximum breakdown-limited accelerating gradient between different geometries.

physics.acc-ph↗

arXiv2021

Design, fabrication and low-power RF measurement of an X-band dielectric-loaded accelerating structure

Dielectric-loaded accelerating (DLA) structures are being studied as an alternative to conventional disk-loaded copper structures to produce the high accelerating gradient. This paper presents the design, fabrication and low-power RF measurement of an externally-powered X-band DLA structure with a dielectric constant epsilon_r=16.66 and a loss tangent tan_delta=0.0000343. A dielectric matching section for coupling the RF power from a circular waveguide to an X-band DLA structure consists of a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60 degree, resulting in a broadband coupling at a low RF field which has the potential to survive in the high-power environment. Based on simulation studies, a prototype of the DLA structure was fabricated. Results from bench measurements and their comparison with design values are presented. The detailed analysis on the fabrication error which may cause the discrepancy between the RF measurements and simulations is also discussed.

physics.acc-ph↗

arXiv2021

750 MHz radio frequency quadrupole with trapezoidal vanes for carbon ion therapy

High-frequency linear accelerators are very suitable for carbon ion therapy, thanks to the reduced operational costs and the high beam quality with respect to synchrotrons, which are presently the only available technology for this application. In the framework of the development of a new linac for carbon ion therapy, this article describes the design of a compact 750 MHz Radio Frequency Quadrupole (RFQ) with trapezoidal vanes. A new semi-analytic approach to design the trapezoidal-vane RFQ is introduced together with the relevant beam dynamics properties. The RFQ is split into two decoupled rf cavities, both of which make use of a novel dipole detuning technique by means of length adjustment. The splitting is described both from the rf and the beam dynamics point of view. The paper concludes with the rf design of the full structure, including maximum surface field and thermal studies.

physics.acc-ph↗

arXiv2020

rf measurements and tuning of the 1-meter-long 750 MHz radio-frequency quadrupole for artwork analysis

The 750 MHz PIXE-RFQ (radio-frequency quadrupole), developed and built at CERN, provides 2 MeV protons over a length of one meter for proton-induced X-ray emission analysis (PIXE) of cultural heritage artwork. In this paper, we report low-power rf measurements and tuning of the PIXE-RFQ, which have been completed mid-2020. Using a novel algorithm based on direct measurements of the response matrix, field and frequency could tuned at the same time in only two steps to satisfying accuracy. Additionally, we report measurements of single modules, quality factors, and coupling strength. In all cases, very good agreement between rf measurement and design values was observed.

physics.acc-ph↗

arXiv2020

A compact, low field, broadband matching section for externally-powered X-band dielectric-loaded accelerating structures

It has been technically challenging to efficiently couple external radiofrequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures, especially when the DLA structure has a high dielectric constant. This paper presents a novel design of matching section for coupling the RF power from a circular waveguide to an X-band DLA structure with a dielectric constant epsilon_r=16.66 and a loss tangent tan_delta=0.0000343. It consists of a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60 degree, resulting in a broadband coupling at a low RF field which has the potential to survive in the high-power environment. To prevent a sharp dielectric corner break, a 45 degree chamfer is also added. A microscale vacuum gap, caused by metallic clamping between the thin coating and the outer thick copper jacket, is also studied in detail. Through optimizations, more than 99% of RF power is coupled into the DLA structure, with the maximum electromagnetic fields located at a DLA structure. Tolerance studies on the geometrical parameters and mechanical design of the full-assembly structure are also carried out as a reference for realistic fabrication.

physics.acc-ph↗

arXiv2020

Investigations into X-band dielectric disk accelerating structures for future linear accelerators

Dielectric disk accelerating (DDA) structures are being studied as an alternative to conventional disk-loaded copper structures. This paper investigates numerically an efficient X-band DDA structure operating at a higher order mode of TM02-π. This accelerating structure consists of dielectric disks with irises arranged periodically in a metallic enclosure. Through optimizations, the RF power loss on the metallic wall can be significantly reduced, resulting in an extremely high quality factor Q_0=111064 and a very high shunt impedance R_shunt=605 MΩ/m. The RF-to-beam power efficiency reaches 46.6% which is significantly higher than previously-reported CLIC-G structures with an efficiency of only 28.5%. The optimum geometry of the regular and the end cells is described in detail. Due to the wide bandwidth from the dispersion relation of the accelerating mode, the DDA structure is allowed to have a maximum number of 73 regular cells with a frequency separation of 1.0 MHz, which is superior to that of conventional RF accelerating structures. In addition, the DDA structure is found to have a short-range transverse wakefield lower than that of the CLIC-G structure.

physics.acc-ph↗

arXiv2019

Traveling wave method for simulating geometric beam coupling impedance of a beamscreen with pumping holes

In particle accelerators, pumping holes in a vacuum chamber can be a source of unwanted broadband coupling impedance, leading to beam instabilities. Analytical methods have been previously developed to estimate the impedance of holes in circular-like chambers e.g. the beamscreen of the Large Hadron Collider (LHC). More sophisticated chamber designs like that of the High Energy LHC (HE-LHC) and the Future Circular Collider (FCC-hh) call for a different way to calculate the impedance. We propose using decomposition of the wakefield into synchronous traveling waves and employing a numerical solver to find the impedance of each wave. This method is compared to the direct time domain wakefield calculation method and its greater sensitivity to small impedances is shown.

physics.acc-ph↗

arXiv2018

Longitudinal phase space reconstruction simulation studies using a novel X-band transverse deflecting structure at the SINBAD facility at DESY

A transverse deflecting structure (TDS) is a well-known device for characterizing the longitudinal properties of an electron bunch in a linear accelerator. The standard use of such a cavity involves streaking the bunch along a transverse axis and analysing the image on a screen downstream to find the bunch length and slice properties along the other transverse axis. A novel X-band deflecting structure, which will allow the polarization of the deflecting field to be adjusted, is currently being designed as part of a collaboration between CERN, DESY and PSI. This new design will allow bunches to be streaked at any transverse angle within the cavity, which will open up the possibility of new measurement techniques, which could be combined to characterize the 6D phase space distribution of bunches. In this paper, a method is presented for reconstructing the longitudinal phase space distribution of bunches by using the TDS in combination with a dipole. Simulations of this technique for the SINBAD-ARES beamline are presented and the key limitations related to temporal resolution and induced energy spread are discussed.

physics.acc-ph↗

arXiv2012

Beam Coupling Impedance of the LHC TCTP Collimators

As part of an upgrade to the LHC collimation system, 8 TCTP and 1 TCSG collimators are proposed to replace existing collimators in the collimation system. In an effort to review all equipment placed in the accelerator complex for potential side effects due to collective effects and beam-equipment interactions, beam coupling impedance simulations are carried out in both the time-domain and frequency-domain of the full TCTP design. Particular attention is paid to trapped modes that may induce beam instabilities and beam-induced heating due to cavity modes of the device.

physics.acc-ph↗