There's a technical discussion of using graphite vs. aluminum vs. copper in this paper: http://flash.desy.de/sites2009/site_vuvfel/content/e403/e1642/e1132/e112

There's a technical discussion of using graphite vs. aluminum vs. copper in this paper: http://flash.desy.de/sites2009/site_vuvfel/content/e403/e1642/e1132/e1129/infoboxContent1791/fel2006-05.pdf http://flash.desy.de/sites2009/site_vuvfel/content/e403/e164... And a comparison of graphite vs. water schemes here: http://tesla.desy.de/new_pages/TESLA_Reports/2001/pdf_files/tesla2001-04.pdf http://tesla.desy.de/new_pages/TESLA_Reports/2001/pdf_files/...