3-D computer simulations of EM field sin the APS vacuum chamber. Part 2: Time-domain analysis

Abstract

3-D Computer Simulations of EM Fields in the APS Vacuum ChamberPart 2: Time-Domain AnalysisIn Ref.[l], we analyze the RF modes of the 1-meter-long sector of the APS vacuum chamber in the frequency-domain. This note is a parallel analysis in the time-domain.There are quite a few measurements completed on this 1-meter-long sector.[2] In order to understand these experimental results, in particular, the cause of the strong peak around 4 GHz observed in the narrow gap, the 3-D real-the computer simulations are carried out using MAFIA. [3] In these simulations, the vacuum chamber is approximated by the geometry shown in Fig. 1. The 1-inch-diameter beampipes, which are attached at both ends of the beam chamber, are infinitely long (so-called the open boundary condition). A Gaussian-distributed rigid bunch traverses this structure along (or slightly above) the a x i s of the beampipes and generates wakefields, which are computed by the time-domain solver T3. Five probes are placed inside the top of the vacuum chamber along the horizontal direction. One is in the beam chamber, three in the narrow gap and one in the antechamber, see Fig. l(b). These probes record the E and H fields at each location as a function of the wall clock time t. The E(t) and H(t) are then Fouriertransformed and the spectra are compared with that obtained from measurements.The resolution of our Fourier transform is about 0.17 GHz. This means that there are about 60 points in the region from 0 to 10 GHz, within which the real and imaginary parts of the Fourier transform of E(t) and H(t) are plotted. The power spectra, which are the sum of the squares of the two parts, is not plotted; but its peaks should be easily located in the figures below.Several types of simulations have been performed, each using the upper half of the geometry but with different configurations and boundary conditions. 4. Same as 3, but the beam holes are displaced horizontally by 1 cm.