Martin Timm

A practical guide to 3-D simulation

This article is intended to give design engineers an overview over some properties of numerical methods used in todays most relevant commercial electromagnetic (EM) simulation tools. It cannot and does not want to be a rigorous analysis of the methods themselves nor a concise description of their history.

MAFIA version 4

MAFIA Version 4.0 is an almost completely new version of the general purpose electromagnetic simulator known since 13 years. The major improvements concern the new graphical user interface based on state of the art technology as well as a series of new solvers for new physics problems. MAFIA now covers heat distribution, electro-quasistatics, S-parameters in frequency domain, particle beam tracking in linear accelerators, acoustics and even elastodynamics. The solvers that were available in earlier versions have also been improved and/or extended, as for example the complex eigenmode solver, the 2D–3D coupled PIC solvers. Time domain solvers have new waveguide boundary conditions with an extremely low reflection even near cutoff frequency, concentrated elements are available as well as a variety of signal processing options. Probably the most valuable addition are recursive sub-grid capabilities that enable modeling of very small details in large structures.

The short range wakefields in the SBLC linac

The short-range longitudinal and transverse wakefields of a point charge in the SBLC linac are obtained using a modal summation technique. Simple functional fits to these wakes are given, which can be used as Green functions in beam dynamics simulations of bunches. These results, however, are valid only after the beam has traversed a critical number of cells N/sub crit/. Using time domain computations with Gaussian bunches we have obtained results that are consistent with N/sub crit/ varying as /spl alpha/a/sup 2//(L/spl sigma//sub z/), with a the iris radius and L the period length of the structure, /spl sigma//sub z/ the bunch length and /spl alpha/ a constant on the order of 1. For the loss per cell to reach to within a few per cent of the asymptotic value we find that /spl alpha//spl sim/0.5-1.0.

A proposal for the surface roughness wake field measurement at the TESLA Test Facility

The wake fields due to the rough surface of the vacuum chamber have a major influence on the beam dynamics in linear colliders and free electron lasers. These wake fields mainly consists of the fundamental tube mode, modified by the rough boundary condition, which decreases its phase velocity to the speed of light. Its wavelength is proportional to the square root of the tube radius and the roughness depth and is comparable to the FEL bunch length, i.e. in the order of 10-30 micron. To study this effect with the beam available at the TESLA Test Facility (TTF) the use of tubes with an artificially increased roughness is suggested. The expected pulse power of these wakes, caused by the 250 micron bunch carrying a charge of 1 nC, can reach up to several MW. In this paper a preliminary design of an installation of a surface roughness wake field experiment at the TESLA Test Facility is presented.

Experimental Setup to Measure the Wake Fields excited by a Rough Surface

Abstract The wake fields generated by electron bunches of high charge density in an undulator vacuum chamber might become sufficiently large to affect the FEL amplification. Recent theoretical and numerical calculations show that, with decreasing the bunch length the dominant contribution to the wake fields comes from the surface roughness of the chamber. The excited radiation field should be detectable outside the vacuum chamber. At the TTF linac an experiment is in preparation to measure the radiation power spectrum excited in beam pipes with an artificially increased roughness on the inner surface. Particularly, the dependence on the radius of the pipe and the shape of the surface will be studied in this experiment. In this paper we describe the experimental setup, detection schemes for the radiation and estimates of systematic background.

Simultaneous computation of higher eigenmodes in loss-free waveguides and cavities

In some applications (e.g. gyrotrons, particle accelerators) the higher modes of waveguides and resonant structures are of interest. The computation is difficult since most of the methods calculate either extremal eigenvalues or the inversion of the system matrix is necessary. The presented method extends the subspace iteration with polynomial acceleration to non extremal eigenvalues by using special polynomials. This enables the simultaneous iteration of a group of eigenvalues inside an arbitrary frequency range during one computation run.

Simulation of Linac Operation Using the Tracking Code L

In linear accelerators, misalignments of the machine elements can cause considerable emittance growth due to wake fields, dispersion and other effects. Hence, tight limits are imposed on machine tolerances, design parameters and methods of machine operation. In order to simulate the beam dynamics in linacs, the tracking code L has been developed. Including both singleand multi-bunch effects, the behaviour of the beam in the machine can be simulated and adjustments on parameters of the machine elements up to complete correction techniques and operation procedures can be applied. Utilization of the program is facilitated by a graphical user interface. In this paper we will give an overview over the capabilities of this code and demonstrate its efficiency at attacking the problems associated with large linear accelerators. Introduction