  For synchrotron light sources, the brightness of user X-ray beams is
primarily determined by the electron beam emittance and energy spread at
operational intensity. A common feature of fourth-generation synchrotrons is
the short length of electron bunches combined with a very small transverse beam
size. Consequently, the particle density is much higher than in machines of
previous generations, leading to strong collective effects that significantly
increase the emittance and limit the achievable brightness at operational beam
intensity. In this article, we summarize our studies of the emittance scaled
with the beam energy and intensity, taking into account the effects of
intrabeam scattering, beam-impedance interaction, and bunch lengthening
provided by higher-harmonic RF systems, to identify optimal combinations of
machine and beam parameters.
