• C. Yin Vallgren, A. Ashraf, S. Calatroni, P. Chiggiato, P. Costa Pinto, H.P. Marques, H. Neupert, M. Taborelli, W. Vollenberg, I. Wevers, K. Yaqub
  CERN, Geneva

Elec­tron-cloud is one of the main lim­i­ta­tions for par­ti­cle ac­cel­er­a­tors with pos­i­tive­ly charged beams of high in­ten­si­ty and short bunch spac­ing, as SPS at CERN. The Sec­ondary Elec­tron Yield (SEY) of the inner sur­face of the vac­u­um cham­ber is the main pa­ram­e­ter gov­ern­ing the phe­nomenon. The ef­fect could be elim­i­nat­ed by coat­ing the mag­nets vac­u­um cham­bers with a ma­te­ri­al of low SEY, which does not re­quire bake-out and is ro­bust against air ex­po­sure. For such a pur­pose amor­phous car­bon coat­ings were pro­duced by mag­netron sput­ter­ing of graphite tar­gets. They ex­hib­it max­i­mum SEY be­tween 0.9 and 1.1 after air trans­fer to the mea­sur­ing in­stru­ment. After 1 month air ex­po­sure the SEY rises to val­ues be­tween 1.1 and 1.4. Stor­age under ni­tro­gen or by pack­ag­ing in Al foil makes this in­crease neg­li­gi­ble. The coat­ings have a sim­i­lar XPS C1s spec­trum for a large set of de­po­si­tion pa­ram­e­ters and ex­hib­it an en­larged line-width com­pared to pure graphite. The stat­ic out­gassing with­out bake-out de­pends on de­po­si­tion pa­ram­e­ters and is in a range from 1 to 10 times high­er than that of stain­less steel. In­stead, elec­tron stim­u­lat­ed out­gassing is lower than for stain­less steel and is dom­i­nat­ed by CO.

Slides

• A.E. Gustafsson, S. Calatroni, W. Vollenberg
  CERN, Geneva
• R. Seviour
  Cockcroft Institute, Lancaster University, Lancaster

Nio­bi­um thin film cav­i­ties have shown good and re­li­able per­for­mance for LEP and LHC, al­though there are lim­i­ta­tions to over­come if this tech­nique should be used for new ac­cel­er­a­tors such as the ILC. New coat­ing tech­niques like High Power Im­pulse Mag­netron Sput­ter­ing (HiP­IMS) has shown very promis­ing re­sults and we will re­port on its pos­si­ble im­prove­ments for Nb thin film cav­i­ty per­for­mance. Cur­rent ma­te­ri­als used in ac­cel­er­a­tor SRF tech­nolo­gies op­er­ate at tem­per­a­tures below 4 K, which re­quire com­plex cryo­genic sys­tems. Re­searchers have in­ves­ti­gat­ed the use of High Tem­per­a­ture Su­per­con­duc­tors (HTS) to form RF cav­i­ties, with lim­it­ed suc­cess*. We pro­pose a new ap­proach to achieve a high-tem­per­a­ture SRF cav­i­ty based on the su­per­con­duct­ing 'prox­im­i­ty ef­fect'**. The su­per­con­duct­ing prox­im­i­ty ef­fect is the ef­fect through which a su­per­con­duct­ing ma­te­ri­al in close prox­im­i­ty to a non-su­per­con­duct­ing ma­te­ri­al in­duces a su­per­con­duct­ing con­den­sate in the lat­ter. Using this ef­fect we hope to over­come the prob­lems that have pre­vent­ed the use of HTS for ac­cel­er­at­ing struc­tures so far. We will re­port the pre­lim­i­nary stud­ies of mag­netron sput­tered thin films of Cu on Nb.

* E. J. Minehara et al, Superconductivity 3, p277 (1990)
** R. Seviour et al, Superlattices and Microstructures, 25, p647 (1999)