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.Comment: 16 pages, 28 figures. arXiv admin note: substantial text overlap with arXiv:2008.0920

CLIC Wake Field Monitor as a detuned Cavity Beam Position Monitor: Explanation of center offset between TE and TM channels in the TD26 structure

The Wake Field Monitor (WFM) system installed on the CLIC prototype accelerating structure in CERN Linear Accelerator for Research (CLEAR) has two channels for each horizontal/vertical plane, operating at different frequencies. When moving the beam relative to the aperture of the structure, a disagreement is observed between the center position of the structure as measured with the two channels in each plane. This is a challenge for the planned use of WFMs in the Compact Linear Collider (CLIC), where they will be used to measure the center offset between the accelerating structures and the beam. Through a mixture of simulations and measurements, we have discovered a potential mechanism for this, which is discussed along with implications for improving position resolution near the structure center, and the possibility determination of the sign of the beam offset.Comment: 16 pages, 20 figure

High-Power Test of Two Prototype X-band Accelerating Structures Based on SwissFEL Fabrication Technology

This article presents the design, construction, and high-power test of two $X$-band radio frequency (RF) accelerating structures built as part of a collaboration between CERN and the Paul Scherrer Institute (PSI) for the compact linear collider (CLIC) study. The structures are a modified 'tuning-free' variant of an existing CERN design and were assembled using Swiss free electron laser (SwissFEL) production methods. The purpose of the study is two-fold. The first objective is to validate the RF properties and high-power performance of the tuning-free, vacuum brazed PSI technology. The second objective is to study the structures' high-gradient behavior to provide insight into the breakdown and conditioning phenomena as they apply to high-field devices in general. Low-power RF measurements showed that the structure field profiles were close to the design values, and both structures were conditioned to accelerating gradients in excess of 100 MV/m in CERN's high-gradient test facility. Measurements performed during the second structure test suggest that the breakdown rate (BDR) scales strongly with the accelerating gradient, with the best fit being a power law relation with an exponent of 31.14. In both cases, the test results indicate that stable, high-gradient operation is possible with tuning-free, vacuum brazed structures of this kind

Study of Novel Devices for Compact Accelerating Structures

Recent developments in high gradient acceleration and compact electron linacs have demonstrated the capability to reach relatively high electron beam energy in a small space by considering the budget and realization issues. In this context, it is also necessary to manufacture and operate other accelerator components while maintaining a small accelerator footprint but without compromising the final performance. In this thesis, two of these components were studied; wakefield monitors (WFMs) and radio frequency (RF) compact loads. Both components have been designed and developed as part of the Compact Linear Collider (CLIC) study and work in X-band. The capability to measure accurately and correct the position of the beam with respect to accelerating structures is crucial for the accelerator studies to reach aimed design parameters. The WFM feasiblity experiments with beam have been going on for a while at the CERN Linear Electron Accelerator for Research (CLEAR) facility at CERN. The simulations and WFM signal analysis were performed to explore the possible reasons of the minimum position differences of the two wakefield modes measured during the experiments. The results were obtained as the combined effects of the antenna location, total charge per bunch, position jitter along the train and the asymmetric attenuation on the WFM data acqusition chain. The use of WFMs as a beam position instrumentation requires careful design and very tight control of the acquisition chain while expectancies need to be revised against realistic conditions and beam control. In the second part of the thesis, the compact RF spiral load was studied. It is one of the common components in accelerator studies based on traveling wave type linacs and drives the cost accordingly. The last iteration in this design is the optimization of the internal waveguide of the load to allow stackable design to decrease material loss and increase cost savings. A new generation of spiral, dry loads have been designed by alternating between RF and mechanical parametric load models. After additive manufacturing of the prototypes from two different vendors, low-power RF tests were performed and compared to the ideal case. In general, the new generation loads work with similar parameters as the old generation and reflection values well below -20 dB. However, some considerations need to be taken into account when working with additive manufacturing technology like final roughness, remaining dust or material choice. Keywords: Wakefield monitors, Additive manufacturing, RF design, optimization, X-Ban

Simulations and Measurements of X-Band Accelerating Structures of The CLIC Project

The Compact Linear Collider (CLIC) Project is 50 km long e−, e+ linear collider planned to be built in three stages with 3 TeV center-of-mass energy at its last stage. The CLIC accelerator includes around 140000 accelerating structures. The cost optimisation process is ongoing before the production stage. In order to obtain the CLIC design requirements, the accelerating structures are machined out of OFE copper and ultra precision turning and milling with single diamond tool. The required precision on the order of the micron makes the final product relatively expensive. The assembly of these copper parts is done by electron beam welding of two halves or diffusion bonding of disc stacks. In this thesis we investigate the potential geometrical correlation between the imperfection of the accelerating structure discs inner radii and their individual frequency deviations before and after bonding. Following the results of this work, the tolerance study of the machining and bonding effects will be understood and manufacturing process will be optimised for cost saving

High-Power Testing Results of X-Band RF-Window and 45 Degrees Spiral Load

The X-Band test facilities at CERN have been running for some years now qualifying CLIC structure prototypes, but also developing and testing high power general-purpose X-Band components, used in a wide range of applications. Driven by operational needs, several components have been redesigned and tested aiming to optimize the reliability and the compactness of the full system and therefore enhancing the accessibility of this technology inside and outside CERN. To this extent, a new high-power RF-window has been designed and tested aiming to avoid unnecessary venting of high-power sections already conditioned, easing the interventions, and protecting the klystrons. A new spiral load prototype has also been designed, built, and tested, optimizing the compactness, and improving the fabrication process. In these pages, the design and manufacturing for each component will be shortly described, along with the last results on the high-power testing

High power conditioning of X-band variable power splitter and phase shifter

The three X-band test facilities currently at CERN aim at qualifying CLIC structures prototypes but are also extensively used to qualify X-band components operation at high power. In order to upgrade one of the facilities from a single test line to a double test line facility, a high power variable splitter and variable phase shifter have been designed and manufactured at CERN. They have been power tested, first in a dedicated test and also in their final configuration, to ensure stable power operation before installing them together with an accelerating structure. In this paper, we broadly describe the RF and mechanical design, manufacturing and low power measurements agreement with simulations. We report the high power qualification of both components and their suitability to be used in existing and planned X-band facilities

X-Band RF Spiral Load Optimization for Additive Manufacturing Mass Production

The CLIC main linac uses X-band traveling-wave normal conducting accelerating structures. The RF power not used for beam acceleration nor dissipated in the resistive wall is absorbed in two high power RF loads that should be as compact as possible to minimize the total footprint of the machine. In recent years, CERN has designed, fabricated and successfully tested several loads produced by additive manufacturing. With the current design, only one load can be produced in the 3D printing machine at a time. The aim of this study is optimizing the internal cross-section of loads in order to create a stackable design to increase the number of produced parts per manufacturing cycle and thus decrease the unit price. This paper presents the new design with an optimization of the internal vacuum part of the so-called RF spiral load. In this case, RF and mechanical designs were carried out in parallel. The new cross section has showed good RF reflection reaching less than -30 dB in simulations. The final load is now ready to be manufactured and high-power tested. This new load will not only provide cost saving but also faster manufacturing for mass production

CLIC Wake Field Monitor as a detuned Cavity Beam Position Monitor: Explanation of center offset between TE and TM channels in the TD26 structure

The Wake Field Monitor (WFM) system installed on the CLIC prototype accelerating structure in CERN Linear Accelerator for Research (CLEAR) has two channels for each horizontal/vertical plane, operating at different frequencies. When moving the beam relative to the aperture of the structure, a disagreement is observed between the center position of the structure as measured with the two channels in each plane. This is a challenge for the planned use of WFMs in the Compact Linear Collider (CLIC), where they will be used to measure the center offset between the accelerating structures and the beam. Through a mixture of simulations and measurements, we have discovered a potential mechanism for this, which is discussed along with implications for improving position resolution near the structure center, and the possibility determination of the sign of the beam offset

Preserving Micrometre Tolerances Through the Assembly Process of an X-band Accelerating Structure

The CLIC structures are designed for operating at X-Band, $2 \pi /3$ traveling wave mode with a loaded 100 MV/m gradient. Mechanical tolerances, at the submicron level, are required to satisfy the RF design constraints and beam dynamics and are reachable using ultra-precision diamond machining. However, inherent to the manufacturing process, there is a deviation from the nominal specifications and as a result; incorrect cavity dimensions produce a less efficient linac. Moreover, the assembly process increase the difference from the original geometry. As part of a cost and manufacturability optimization of the structures for mass production, this study aims to identify a correlation between frequency deviations and geometrical errors of the individual discs of the accelerating structures caused by the production process. A sensitivity analysis has been carried out to determine the most critical parameters. Cell frequency deviations have been monitored by bead pull measurements before and after bonding. Several accelerating structure prototypes have been tested to determine our assumptions and to assess if the assembly process preserves the tight tolerances achieved by machining