Resistive Wall Heating of the Undulator in High Repetition Rate Fels*

In next generation high repetition rate FELs, beam energy loss due to resistive wall wakefields will produce significant amount of heat. The heat load for a superconducting undulator (operating at low temperature), must be removed and will be expensive to remove. In this paper, we study this effect in an undulator proposed for a Next Generation Light Source (NGLS) at LBNL. We benchmark our calculations with measurements at the LCLS and carry out detailed parameter studies using beam from a start-to-end simulation. Our preliminary results suggest that the heat load in the undulator is about 2 W/m or lower with an aperture size of 6 mm for nominal NGLS preliminary design parameters. ENERGY LOSSES FORM RESISTIVE WALL WAKEFIELDS Resistive wall wakefields due to finite conductance of the vacuum pipe can cause significant loss of electron beam energy. Such an energy loss inside an undulator can heat up the vacuum pipe and also induce energy chirp along the beam that will limit the performance of the undulator and the final FEL radiation. In reference [1], the relative energy change due to the resistive wall wakefields was studied at LCLS for a room-temperature normal conducting undulator using a double-horn beam distribution from the LCLS linac. The heating effect to the undulator wall is small at LCLS due to the maximum 120 Hz low repetition rate. For a high repetition rate FEL light source (1 MHz or higher), this effect could be significant. In this paper, we will study both the energy loss to the wall and the induced energy spread inside the electron beam using a beam distribution from the proposed high repetition NGLS at LBNL [2]. Given a wake function w(z) across the electron beam, the total energy loss per meter from a single bunch of electron beam is given by dz z dL z dE dL dEbeam ) ( ) (   