Abstract
Dark 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 (Ra = 126 nm) showed first EFE before (after) dry ice cleaning (DIC) at an activation field Eact = 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 chemically etched Cu samples (Ra = 150 nm) started at 140 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 investigations 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.
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