Methods for the design and analysis of higher-order mode couplers applied to superconducting accelerating structures

Higher-order modes (HOMs) may affect beam stability and refrigeration requirements of superconducting proton linacs such as the SPL which is being studied at CERN. One option being considered to limit these effects is the use of coaxial HOM couplers. In this work, potentially dangerous modes are analyzed and corresponding damping requirements derived. The design process of coaxial HOM couplers is examined under new aspects. Several prototypes are elaborated and compared in terms of mode damping, thermal loads, structural deformations, mechanical tolerances, and multipacting.Moden höherer Ordnung (HOMs) können beträchtlich die Strahldynamik und Kühlanforderungen supraleitender Linearbeschleuniger, wie dem am CERN untersuchten SPL, beeinflussen. Koaxiale HOM Koppler sind eine Möglichkeit, um Auswirkungen entsprechender Moden zu begrenzen. Im Rahmen dieser Arbeit wurden potentiell gefährliche Moden analysiert und Dämpfungsanforderungen abgeleitet. Der Kopplerentwurf wurde unter neuen Gesichtspunkten aufgegriffen. Verschiedene Prototypen wurden bezüglich Modendämpfung, thermisches Verhaltens, strukturmechanischer Verformungen, Toleranzen und Multipactings verglichen

Methods for the Design and Analysis of Higher-Order Mode Couplers applied to Superconducting Accelerating Structures

Higher-order modes (HOMs) may affect beam stability and refrigeration requirements of superconducting proton linacs such as the SPL which is being studied at CERN as a driver for future neutrino programs. One option being considered to limit these effects is the use of coaxial HOM couplers mounted on the cutoff tubes of the five-cell cavities. In this work, potentially dangerous modes are analyzed and corresponding damping requirements defined, primarily on the basis of longitudinal beam dynamics and eigenmode simulations. A novel approach is worked out to characterize entire HOM spectra based on truncated wake potentials and an extended formulation of vector fitting. The design process of coaxial HOM couplers is examined under new aspects. The first contribution to systematically design coaxial microwave filters on the basis of filter functions is elaborated. Prior to this are empirical studies of certain microwave structures that have not yet been well understood. Furthermore, the problem of transmission zeros inherent to cutoff tubes is formulated. Finally, coaxial HOM couplers are discussed in a more general context. Besides the characteristic frequency response, the designs of several HOM couplers applied to SPL cavities are investigated and compared in terms of mode damping, mechanical tolerances, thermal loads, structural deformations, and multipacting. Some of the computational analyses are validated by prototype measurements

Design of a RF Quadrupole Resonator for Landau Damping in HL-LHC

The design and optimization of a quadrupole resonator for transverse Landau damping in the High Luminosity Large Hadron Collider (HL-LHC) is presented. Two different cavity types are considered whose shape is determined by quadrupolar strength, surface peak fields, and beam coupling impedance. The lower order and higher order mode (LOM and HOM) spectra of the optimized cavities are investigated and different approaches for their damping are proposed. Along an example, the required RF power and optimal external quality factor for the input coupler is derived

HOM Couplers for CERN SPL Cavities

Higher-Order-Modes (HOMs) may affect beam stability and refrigeration requirements of superconducting proton linacs such as the SPL, which is studied at CERN as the driver for future neutrino facilities. In order to limit beam-induced HOM effects, CERN considers the use of HOM couplers on the cut-off tubes of the 5-cell superconducting cavities. These couplers consist of resonant antennas shaped as loops or probes, which are designed to couple to modes of a specific frequency range. In this paper the design process is presented and a comparison is made between various design options for the medium and high-beta SPL cavities, both operating at 704.4 MHz. The RF characteristics and thermal behaviour of the various designs are discussed

Comparison of coaxial higher order mode couplers for the CERN Superconducting Proton Linac study

Higher order modes (HOMs) may affect beam stability and refrigeration requirements of superconducting proton linacs such as the Superconducting Proton Linac, which is studied at CERN. Under certain conditions beam-induced HOMs can accumulate sufficient energy to destabilize the beam or quench the superconducting cavities. In order to limit these effects, CERN considers the use of coaxial HOM couplers on the cutoff tubes of the 5-cell superconducting cavities. These couplers consist of resonant antennas shaped as loops or probes, which are designed to couple to potentially dangerous modes while sufficiently rejecting the fundamental mode. In this paper, the design process is presented and a comparison is made between various designs for the high-beta SPL cavities, which operate at 704.4 MHz. The rf and thermal behavior as well as mechanical aspects are discussed. In order to verify the designs, a rapid prototype for the favored coupler was fabricated and characterized on a low-power test-stand

Coupled RF-Thermo-Structural Analysis of CLIC Traveling Wave Accelerating Structures

Temperature changes in the CLIC accelerating structure lead to unwanted deformation. One of the aims of the CLIC module team is to optimize the necessary cooling system. The extended thermal program of the module was complemented by these thermal studies on real RF structures tested in X-band high-power test stands at CERN, Geneva (CH). The impact of thermal expansion on the RF characteristics of CLIC traveling wave accelerating structures is studied. The simulation setup in COMSOL is described in much detail. The necessary geometry simplifications are pointed out, mesh configurations are discussed by means of convergence studies, as well as important post-processing quantities are introduced which are common for characterizing traveling wave structures. The present report treats two different prototypes of the TD26_R05 design, a damped traveling wave accelerating structure using 26 cells and two matching cells, with a bending radius of 0.5 mm being applied. One of these prototypes is currently being conditioned in the Xbox2 test stand

Multiphysics Simulations of the Wide Opened Waveguide Crab-cavity

In the frame of a FCC study a first prototype of a compact superconducting crab-cavity, using Nb-on-Cu-coating technique is being manufactured and investigated. The design, which is based on the ridged waveguide resonator, is subjected to multipacting and pressure sensitivity simulations. First results of theses simulations are presented and compared to those of other SRF cavities. Furthermore, several aspects related to the design of the fundamental mode coupler and HOM dampers are presented

Use of RF quadrupole structures to enhance stability in accelerator rings

The beams required for the high luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN call for efficient mechanisms to suppress transverse collective instabilities. In addition to octupole magnets installed for the purpose of Landau damping, we propose to use radio frequency (rf) quadrupole structures to considerably enhance the aforementioned stabilising effect. By means of the PyHEADTAIL macroparticle tracking code, the stabilising mechanism introduced by an rf quadrupole is studied and discussed. As a specific example, the performance of an rf quadrupole system in presence of magnetic octupoles is demonstrated for HL-LHC. Furthermore, potential performance limitations such as the excitation of synchro-betatron resonances are pointed out. Finally, efforts towards possible measurements with the CERN Super Proton Synchrotron (SPS) are discussed aiming at studying the underlying stabilising mechanisms experimentally

RF Quadrupole Structures for Transverse Landau Damping in Circular Accelerators

The beams required for the high luminosity upgrade of the Large Hadron Collider (HL-LHC) and other potential future circular colliders (FCC) call for efficient mechanisms to suppress transverse collective instabilities. In addition to octupole magnets installed for the purpose of Landau damping in the transverse planes, we propose to use radio frequency (rf) quadrupole structures to considerably enhance the aforementioned stabilising effect. By means of the PyHEADTAIL macroparticle tracking code as well as analytical studies, the stabilising mechanism introduced by an rf quadrupole is studied and explained. It is, furthermore, compared to the influence of the second order chromaticity on transverse beam stability

Mode sensitivity analysis of 704.4 MHz superconducting RF cavities

Due to the large variety of beam patterns considered for the superconducting proton linac (SPL) at CERN it is likely that the frequencies of some HOMs are close to machine lines during operation. Hence, in the interest of developing a method to shift HOM frequencies away from machine lines, we study the influence of cavity detuning and re-tuning (e.g. by Lorentz forces, field flatness tuning, frequency tuning during operation) on HOMs. The sensitivity of HOMs with respect to the fundamental mode was studied for a mono-cell and for 5-cell high-beta SPL cavities operating at 704.4 MHz. First, the variation of the HOMs during the flat-field tuning was measured. In this process, several detuning and re-tuning cycles were made to estimate the range of possible HOM frequency shifts. Secondly the effect of the frequency tuner on the HOMs is presented and finally the frequency shifts of all modes due to the cool down