• 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.

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