Enhanced Field Emission and Emitter Activation on Flat Dry-ice Cleaned Cu Samples

EDark currents caused by enhanced field emission (EFE) are considered as major origin of breakdown events limiting performance of pulsed normal conducting Cu accelerating structures. Measurements on diamond - turned, flat Cu samples (R a = 126 nm) showed first EFE before (after) dry ice cleaning (DIC) at an activation field E act = 130 (190) MV/m. The number density of emitters was significantly reduced by DIC from N = 52.0 cm - 2 to N = 12.0 cm - 2 at 190 MV/m. Furthermore, EFE of four diamond - turned and chemical ly etched Cu samples (Ra = 150 nm) started at 14 0 MV/m after DIC. Locally measured I(V) curves of the activated emitters yielded onset fields between 20 and 240 MV/m and field enhancement factors up to 350. SEM/EDX investigation s revealed surface defects (57%) and few particulates (12%, Al, Si, Sn, W) as origin of the EFE. Moreover, a strong emitter activation effect was observed. A possible breakdown mechanism based on this activation will be discussed.