FR 17 Dec 2002: Following your request, I enclose some technical information about collimator coatings for impedance and/or vacuum requirements: - in case of CERAMIC COLLIMATORS (e.g. Boron-Nitride as for the TDI), a metallic coating is very important to reduce the inductive impedance seen by the beam. Luc Vos has estimated that a Cu coating of about 100 mu-m would be sufficient. Thinner coatings may be negotiated, depending on the number of collimators. - in case of GRAPHITE COLLIMATORS (only primaries?), the resistive impedance would have modest implications for beam stability, but there would be some heating. Luc estimates a beam induced heating of 240 W/m, for ultimate LHC beam intensity and collimators around 6 sigma at injection. A 100 mu-m Cu coating of the graphite would reduce the beam induced heating to about 8 W/m. This power scales with the square of the beam intensity and approximately with the inverse of the collimator aperture. Therefore in collision the above figures are multiplied by a factor 4. I understand that, depending on the heat deposition, the graphite would outgas and may create vacuum problems, in addition to dust. Therefore the vacuum group will probably require some metallic coating. - will a metallic coating resist to beam impact/losses and for how long? I guess this is an important and difficult question that may have to be addressed by beam tests. Also impedance/heating implications of small scratches caused by the beam in the metallic coating for either type of collimator will require further studies.