SLAC, Menlo Park, California, USA
LBNL, Berkeley, California, USA
Echo-enabled harmonic generation (EEHG) is one of most promising approaches to seeding of soft x-ray FELs. It allows one to obtain beam bunching at high harmonics (of order of 100) of the laser frequency at a level of a few percent. In this paper we demonstrate that using the second and third harmonics of the laser radiation one can substantially increase the beam bunching: for a cold beam one can obtain values approaching 0.4 in the range of harmonic numbers 100~200. Such bunching factors are close to those achieved at saturation in the FEL process, which means that one can eliminate the lasing process and use coherent radiation of the pre-bunched beam in the undulator-radiator as a bright source of x-rays. We also discuss an option of using nonlinear dispersive elements to increase the bunching factor.
SLAC, Menlo Park, California, USA
Echo-enabled harmonic generation (EEHG) for FEL seeding uses two undulator-modulators and two chicanes to introduce a fine structure into the beam longitudinal phase space which, at the end of the system, transforms into high harmonic modulation of the beam current. As a result of this phase space manipulation, after the first chicane, the energy distribution function becomes a rapidly modulated function of energy, with the scale of the modulation of the order of the initial energy spread of the beam divided by the EEHG harmonic number. Small-angle Coulomb collisions between the particles of the beam (also known as intrabeam scattering) tend to smear out this modulation and hence to suppress the beam bunching. In this paper we calculate the EEHG bunching factor with account of the collisions and derive a simple scaling relation for the strength of the effect. Our estimates show that collisions become a limiting factor in EEGH seeding for harmonic numbers roughly exceeding 100.
WETUI1
SLAC, Menlo Park, California, USA
In this tutorial I will focus on several topics which are sometimes neglected, or shadowed, in textbooks, but are essential for a deeper understanding of radiation processes in accelerator physics. They include the notions of longitudinal and transverse formation lengths in the process of radiation, coherent versus incoherent radiation, effect of correlations of particles' positions on radiation, fluctuations of the spectrum, and radiative reaction forces. Importance of these concepts is illustrated with analysis of synchrotron, undulator and transition radiation. I try to avoid complicated equations throughout the lecture relying instead on order of magnitude estimates.
