Beam intensity limitations in future multibend achromat light sources

We show that the emittance of fourth-generation 6 GeV machines such as PETRA IV is close to what is theoretically achievable due to beam intensity limitations from space charge and intrabeam scattering. By investigating these limitations, in particular their scaling with the bare lattice emittance and the beam energy, we argue that achieving further significant emittance reduction is only possible by increasing the beam energy. We show further that electron energy increase is also the most efficient path toward higher radiation brightness.