Summary of a Meeting on Secondary Emission at Low Incident Energies, Backscattering Measurements, and Possible LHC Contributions from the ALS 30.01.2003 present: John Byrd, Roberto Cimino, Ian Collins, Francesco Ruggiero, Frank Zimmermann ---------------------------------------------------- Ian and Roberto pointed out several observations related to the simulation model: (1) the initial energy spread of the photoelectrons is far too wide in our simulations; it is about 20 eV, while it should be 2 eV. (2) the simulation shows that an overwhelming fraction of electrons hits the surface at low energies; the correct model of secondary emission at low energies is thus critical. (3) the measurements show a minimum in the secondary emission yield at about 10 eV (varying between 5 eV in Noel's data and 15 eV in Ian&Roberto's); at lower energies the yield increases and reaches 1, apparently independent of delta_max; this rise is not included in our present model. The maximum secondary emission yield after scrubbing which they obtain is about 1.0, which looks somewhat lower than for the other data. This lower yield would be more consistent with our simulations. The scrubbing depends strongly on the energy of the incident electrons. Since most of the electrons hitting the surface have low energies they will not contribute to the scrubbing. Future planned or suggest study items of Roberto's apparatus are: a) scrubbing efficiency (and yield curves) for different incident electron energies b) scrubbing and yields in cold conditions c) vacuum aspects, number of monolayers, their reduction, etc. d) heat load and UV emission etc. for different incident energies The results obtained so far are posted on the web site of the Daresbury workshop DR2003. Roberto will measure backward photon reflection on a sawtooth surface next week at ELETTRA. Future measurements could also be done at the ALS, if funds become available. At Trieste/ELETTRA monochromaic light is used at wavelengths corresponding to 5-200 eV. In 1996 meaurements at BESSY employed 'white light' properly filtered (by angle adjustment) to approximate the LHC photon spectrum. Francesco asked for the evidence of photon scrubbing. This was indeed observed at BESSY. The photo-emission yield dropped by a factor 2. The effect on secondary emission has not been quantified, but should be closely related. Photon scrubbing could be studied at Trieste and/or LBNL. The measurement of secondary emission on a surface that was scrubbed by photons would be important for the LHC commissioning strategy. Roberto will inform the management of the vacuum group about this discussion, and, if no objections are raised, send us a set of data with low-energy secondary electron yields, i.e., electron-reflectivities. John Byrd reported that in the framework of the US-CERN LHC collaboration LBNL is looking for items where the ALS can contribute. The electron cloud studies are a natural subject, since it is also a problem for LC damping rings and also since Miguel Furman was already collaborating with us on this issue. LBNL is in charge of designing the NLC damping rings. An independent measurement of backward reflection at ALS appears to be very useful, even if measurements are done at Trieste first. Roberto pointed out that the chemistry of the surface conditioning is not well understood. Auger analysis attempted in the past is an invasive measurement with insufficient resolution. Photon spectroscopy at a light source like ALS should give a much clearer picture of the conditioning phenomenon. This would be interestng not only for LHC but for the entire community. Another topic of interest would be to compare TiN coated surfaces and NEG coatings as foreseen for all warm parts of the LHC. There may be different types of TiN (of yellow and black color). John needs to write a page of motivation for the DOE. The ECLOUD'02 summary by Francesco might be a good reference.