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\subsubsection{Transverse polarization}  Transverse beam polarization builds up naturally in a storage ring by the Sokolov-Ternov effect. A transverse polarization in excess of 5-10\%, which was obtained for beam energies up to 61 GeV per beam at LEP, is sufficient for beam energy calibration purposes. It is generally accepted that this upper limit is determined by the energy spread, which becomes commensurate with the fractional part of the spin-tune $\nu_s = E_{\rm beam} {\rm [GeV]} /0.440665$. Given that the energy spread scales as $E^2_{\rm beam} / \sqrt{\rho}$, where $\rho$ is the ring bending radius, it is expected that beam polarization sufficient for energy calibration should be readily available up to and above the WW threshold (i.e., 81~GeV per beam) at TLEP. A new machine with a better control of the orbit should, however, be able to increase this limit. For example, a full 3D spin tracking simulation of the electron machine of the Large Hadron-electron Collider (LHeC) project in the 27 km LHC tunnel predicts 20\% polarization at a beam energy of 65 GeV for typical machine misalignments~\cite{1206.2913}. At the Z peak, the natural polarization building time amounted to five hours at LEP. It is predicted to increase like the third power of the ring bending radius and reach 150 hours at TLEP, which is unpractical. Asymmetric "polarization" wigglers were in use in LEP, and their effect on beam energy spread, polarization time and depolarizing effects is analyzed in Ref.~\cite{cite:Blondel-Jowett-LEP606}. Such polarization wigglers could be used to reduce the polarization time, but care should be taken not to increase the energy spread excessively. As an example, the use of the LEP polarization wigglers in TLEP with a central pole field of 0.6 T would reduce the polarization time to 18 hours while keeping the beam energy spread below 48 MeV -- a value at which polarization could routinely be obtained in LEP at 55GeV. In these conditions, a level of polarization sufficient to perform the resonant depolarization could be reached in a couple hours, following which energy calibrations could be performed every 10 minutes if at least 12 bunches of each sign were kept "single" in the machine. At 80 GeV beam energy the polarization time is 9 hours in TLEP, and the use of wigglers should not be necessary.