Johann Heller

Comparison of techniques for uncertainty quantification of superconducting radio frequency cavities

This study examines the influence of uncertainty in the geometry parameters on the properties of superconducting cavities considering the 1.3 GHz TESLA mid-cell cavity. Monte Carlo simulation as well as uni-variate and multi-variate generalized Polynomial Chaos expansions are applied to estimate these stochastic properties. These stochastic methods are compared based on their computational efficiency and suitability for the uncertainty quantification in the given computational model. For this purpose, crucial geometry parameters are identified and used to reduce the random model parameter space. A central result comprises that the generalised Polynomial Chaos approach is highly applicable for the quantification of uncertainty in single-cell cavities.

Time-Domain Absorbing Boundary Terminations for Waveguide Ports Based on State-Space Models

Absorbing boundary conditions for waveguide ports in time domain are important elements of transient approaches to treat RF structures. A successful way to implement these termination conditions is the decomposition of the transient fields in the absorbing plane in terms of modal field patterns. The absorbing condition is then accomplished by transferring the wave impedances (or admittances) of the modes to time domain, which leads to convolution operations involving Bessel functions and integrals of Bessel functions. This paper presents a new alternative approach: the convolution operations are approximated by appropriate state-space models whose system responses can be conveniently computed by standard integration schemes. These schemes are indispensable for transient simulations anyhow. Sufficiently far away from the cutoff frequency, a wideband match is achieved.

Erratum: Eigenmode Compendium of the Third Harmonic Module of the European X-ray Free Electron Laser [Phys. Rev. Accel. Beams 20 , 042002 (2017)]

We regret the oversight of not acknowledging the significant amount of work by several colleagues in the field (see [1–4] and references therein). In particular, we would like to highlight the work performed at the Istituto Nazionale di Fisica Nucleare (INFN) for the realization of the third harmonic system of the European XFEL. We are also grateful to W.-D. Möller and E. Vogel from DESY and H. Edwards and T. Khabiboulline from FNAL for facilitating rapid access to the technical drawings of the FLASH third harmonic module ACC39. Based on drawings of the FLASHmodule and articles on the module for the European XFEL such as [5], the CADmodel of the third harmonic cavity string, which is available via [6], was assembled. Furthermore, wewould like to thank the colleagues from INFN and DESY, in particular P. Pierini, E. Vogel and the XFEL operation team for supporting and facilitating our measurements at the third harmonic module during the busy injector commissioning period. We also would like to express our appreciation to J. Chen from INFN for sharing his experience in measured and simulated rf properties of the third harmonic cavities, especially for the modes with field energy strongly localized at the region of the first cell, the main coupler and the nearby beam pipe (see modes with the indices 105 to 112 in the compendium [7]).

Analysis of higher order modes in large superconducting radio frequency accelerating structures

Superconducting radio frequency cavities used for accelerating charged particle beams are commonly used in accelerator facilities around the world. The design and optimization of modern superconducting RF cavities requires intensive numerical simulations. Vast number of operational parameters must be calculated to ensure appropriate functioning of the accelerating structures. In this study, we primarily focus on estimation and behavior of higher order modes in superconducting RF cavities connected in chains. To calculate large RF models the state-space concatenation scheme, an efficient hybrid method, is employed.