  In the High Energy LHC (HE-LHC), a beam energy of about 16.5 TeV is currently
contemplated. The beam screen issues linked to the use of 20 T dipole magnets
instead of 8.33 T are discussed, with a particular emphasis on two mechanisms,
the magneto-resistance and the anomalous skin effect, assuming the nominal
machine and beam parameters. The magneto-resistance effect always leads to an
increase of the material resistivity (as the mean free path in the presence of
a transverse magnetic field becomes smaller). As concerns the anomalous skin
effect, the anomalous increase of surface resistance of metals at low
temperatures and high frequencies is attributed to the long mean free path of
the conduction electrons: when the skin depth becomes much smaller than the
mean free path, only a fraction of the conduction electrons moving almost
parallel to the metal surface is effective in carrying the current and the
classical theory breaks down.
