Estimate of Joule Heating in a Flat Dechirper

Abstract

We have performed Joule power loss calculations for a flat dechirper. We have considered the configurations of the beam on-axis between the two plates-for chirp control-and for the beam especially close to one plate-for use as a fast kicker. Our calculations use a surface impedance approach, one that is valid when corrugation parameters are small compared to aperture (the perturbative parameter regime). In our model we ignore effects of field reflections at the sides of the dechirper plates, and thus expect the results to underestimate the Joule losses. The analytical results were also tested by numerical, time-domain simulations. We find that most of the wake power lost by the beam is radiated out to the sides of the plates. For the case of the beam passing by a single plate, we derive an analytical expression for the broad-band impedance, and-in Appendix B-numerically confirm recently developed, analytical formulas for the short-range wakes. While our theory can be applied to the LCLS-II dechirper with large gaps, for the nominal apertures we are not in the perturbative regime and the reflection contribution to Joule losses is not negligible.With input from computer simulations, we estimate the Joule power loss (assuming bunch charge of 300 pC, repetition rate of 100 kHz) is 21 W/m for the case of two plates, and 24 W/m for the case of a single plate.

Joule heating in a flat dechirper

“…We present more details of this case, since the analysis of it is a relatively new topic. Note that in [11] expressions for the wakes for a short beam passing by a single plate of a dechirper are obtained, and that these expressions are numerically verified in [12].…”

“…However, when used as a fast kicker, with the beam passing close to one plate, the second plate no longer influences the beam nor needs to be in the problem. Note, however, that analytical fitting formulas for the short-range wakes for this case have also been recently derived [11] and numerically verified [12].…”