Industrialization of the nitrogen-doping preparation for SRF cavities for LCLS-II

Surface characterization of nitrogen-doped high purity niobium coupons compared with superconducting rf cavity performance

“…Radio-frequency testing of the cavity made from fine-grain Nb showed a moderate increase in quality factor in medium field range (<20 MV=m) over the baseline EP surface treatment. The large-grain cavity showed a higher increase in Q 0 compared to the fine-grain niobium with the same amount of material removal via BCP, comparable to fine-grain EP cavities [11]. The enhancement in Q 0 in the EP treated (20N0) cavity is much higher than for the BCP treated (2N6) FG cavities, which is about 40% on average at B p ∼ 67 mT (E acc ¼ 16 MV=m) based on recent LCLS-II cavities production [11].…”

“…The large-grain cavity showed a higher increase in Q 0 compared to the fine-grain niobium with the same amount of material removal via BCP, comparable to fine-grain EP cavities [11]. The enhancement in Q 0 in the EP treated (20N0) cavity is much higher than for the BCP treated (2N6) FG cavities, which is about 40% on average at B p ∼ 67 mT (E acc ¼ 16 MV=m) based on recent LCLS-II cavities production [11]. BCP may be a particularly attractive postprocessing option for after doping of complex shaped cavities where the EP process is harder to achieve.…”

“…The sample L-04 was subjected to BCP with HF∶HNO 3 ∶H 3 PO 4 ¼ 1∶1∶2 by volume mixture to remove ∼6 μm while keeping acid temperature below 12°C with etch rate ∼1 μm= min, whereas sample L-06 was subjected to EP with HF∶H 2 SO 4 ¼ 1∶9 by volume at ∼30 C with etch rate 0.5 μm= min to remove ∼6 μm from sample, respectively [9]. Two single cell TESLA shaped 1.3 GHz cavities (B p =E acc ¼ 4.2 mT=ðMV=mÞ) [10] named RDTTD-01 and TD-05 were separately prepared with baseline measurement being ∼30 μm EP and the same nitrogen doping protocol [11] was used before the cavities were subjected to BCP 1∶1∶1 for inner surface removal. To precisely characterize the material removal from BCP, a fresh batch of acid was prepared and the etch rate was calibrated on a flat Nb sample.…”

Role of magnetic flux expulsion to reach Q0>3×1010 in superconducting rf cryomodules

“…The LCLS-II linac will operate in CW (continuous wave) mode. There was an R&D phase in preparation for cryomodule production which included studies of the nitrogen-doping process itself to select the "2/6" implementation [11] as well as industrialization of the treatment at the cavity vendors RI and Zanon [12]. In parallel with the R&D was design of the 1.3 GHz main linac cryomodules at Fermilab, based on the TeSLA cryomodule framework, with modifications for CW operation.…”

Two-pass two-way acceleration in a superconducting continuous wave linac to drive low jitter x-ray free electron lasers

“…Moreover, SC cavities typically sustain standing waves that allow the two-fold acceleration of the beam in the L1 linac as described before, at the nominal accelerating gradient here considered. This technology is now mature and it finds applications in many modern accelerators for FEL applications [26,27].…”