A new approach to XUV: the ϱ cooling technique

Abstract

We demonstrate that on resonance and for a fixed energy and wavelength, there is a maximum value of the fundamental FEL parameter ϱ when the wiggler parameter is one. This circumstance maximizes the efficiency and minimizes the high gain “effective energy spread” Δγ/ϱγ. Furthermore we show the possibility of “effective” beam cooling in the proposed of two wiggler scheme for coherent harmonic generation in the exponential gain regime. This consists of tuning the second wiggler to the nth harmonics while changing simultaneously B w and λ w so that ϱ increases and leads to a decrease of Δγ/ϱγ in the second wiggler. This “ϱ-cooling” technique can be relevant in many applications and, in particular, (i) it allows, in principle, the tapering of the second wiggler so that one can reach an efficiency larger than 10% for the harmonic generation; (ii) it allows one to relax the beam emittance requirement with a proper generalization of the Kim-Pellegrini criterion.