\subsection{Beamstrahlung}
\label{sec:beamstrahlung}

Beamstrahlung is an issue for $\epem$ rings~\cite{Yokoya,TelnovB}, as its effects may cause ether the beam lifetime to become prohibitively small, or the beam-energy spread and bunch length to become unacceptably large. Indeed, the continuous loss of even a tiny fraction of the beam at each collision reduces the beam lifetime at the higher TLEP beam energies, and cumulative increases in the energy spread result in significant bunch lengthening, especially at the lower energies. Solutions to mitigate these effects are well known, and are described in Ref.~\cite{TelnovB,cite:1305.6498}. Steadily improved simulations and analytical calculations show that, with a momentum acceptance of 2.0\% and a ratio of vertical to horizontal emittances of 0.2\%, the luminosity drops by 10\% every minute. With a top-up rate of once per minute, the average luminosity amounts to 95\% of the peak luminosity. Beamstrahlung effects are, on the other hand, benign for the physics performance. For example, the beamstrahlung-induced beam energy spread is expected to be smaller than 0.1\%, as shown in Fig.~\ref{fig:beamstrahlung} for $\sqrt{s}=240$ GeV.