Joule heating in a flat dechirper

Abstract

We perform Joule power loss calculations for a flat dechirper. We consider 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). 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 broadband impedance. Our analytical results are also tested by numerical, time-domain simulations. 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. With input from computer simulations, we estimate the Joule power loss for nominal apertures (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.