Simple method to suppress the fundamental in a harmonic free electron laser

Preliminary studies of harmonic lasing have shown significant promise as a method to produce radiation at higher photon energies for a given electron energy and for a given undulator. The basic idea is to suppress radiation at the fundamental resonant wavelength, and allow radiation at a specific harmonic to grow exponentially without being driven by nonlinear processes at the fundamental. This has several potential benefits: higher photon energies for the same undulator field, plus significantly more power and smaller bandwidth compared to extracting nonlinear radiation at the harmonic after the fundamental has reached saturation. In this paper, we use beam parameters from the current design of LCLS-II to take a critical look at the challenge of suppressing radiation at the fundamental wavelength and to evaluate how much of an improvement in terms of photon energy reach and brightness can be achieved through harmonic lasing. For undulators with adjustable magnetic fields, a scheme is presented which can delay the onset of saturation at the fundamental wavelength by a factor of 2. Performance characteristics and especially spectral brightness are compared to self-seeded beam lines as an alternative method to reduce bandwidth, as well as with more conventional SASE beam lines.