SUB-RADIANCE AND ENHANCED-RADIANCE OF UNDULATOR RADIATION FROM A CORRELATED ELECTRON BEAM

The radiant intensity of Synchrotron Undulator Radiation (UR) depends on the current noise spectrum of the electron beam injected into the wiggler. The current noise spectrum and intensity can be controlled (suppressed or enhanced relative to the shot-noise level) by the effect of collective longitudinal space charge interaction in drift and dispersion sections [1]. This new control lever is of significant inter- est for possible control of SASE in FEL, since UR is the incoherent seed of SASE. Thus, control of spontaneous UR is a way to enhance the coherence of seeded FEL [2, 3], or alternatively, obtain enhanced radiation from a cascade noise-amplified electron beam [4]. The dependence of UR emission on the current noise is primarily a result of the longitudinal correlation of the e-beam distribution due to the longitudinal space charge effect. However, at short wave- lengths, 3-D effects of transverse correlation and effects of emittance disrupt the proportionality relation between the UR intensity and e-beam current noise. We present analy- sis and simulation of UR sub-radiance/enhanced-radiance under various ranges of beam parameters, and compare to recent experimental observations [1].