Cornell Nb 3 Sn Update Daniel Hall Matthias Liepe Recent focus Thin film regions Regions of a thin Nb 3 Sn layer on the order of the RF penetration depth that cause increased RF losses Losses from trapped flux ID: 764082
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Cornell Nb3Sn Update Daniel Hall Matthias Liepe
Recent focus Thin film regions Regions of a thin Nb 3 Sn layer, on the order of the RF penetration depth, that cause increased RF losses Losses from trapped flux Sources of trapped flux include both ambient magnetic fields and those generated by thermoelectric gradients during cooldown from the bimetallic interface
Thin film regions ERL1-5: A cavity that showed significant heating on one of its half-cells Cavity performance Green : thickness >> λ Red : thickness ≈ λ
Thin film regions in good samples These regions are also seen in samples from our best performing cavities SEM image of sample White regions: thickness ≈ λ
Pre-anodisation Red : not pre-anodised Blue : pre-anodised substrate Growing the oxide layer of the niobium substrate before coating with Nb 3 Sn appears to suppress the formation of thin film regions
Pre-anodisation of a cavity Pre- anodisation has only a minimal impact on cavities that already performed well (thin film region coverage of 10% or less)
Minimising residual resistance At 2.0 K, the cavity performance is dominated by residual resistance At 4.2 K, a BCS of 7 n Ω accounts for roughly 50% of our surface resistance We have 6-8 n Ω of residual resistance in our best performing cavities
Sources of residual resistance
Measuring sensitivity to trapped flux A linear fit is made at a given RF field
Sensitivity is a function of RF field As the RF field increases, the impact of 1 mG of trapped flux increases At 2 MV/m, 1 mG ≈ 0.6 n Ω At 10 MV/m, 1 mG ≈ 1.4 n Ω
Optimising cooldown To achieve the highest possible quality factors at higher accelerating gradients, it is important to trap as little flux as possible + Optimized cool-down: ∆T = 100 mK /m, B amb = 1mG 2.0 nΩ at 5 MV/m 3.9 nΩ at 15 MV/m
Conclusions Thin film regions can be mitigated using pre- anodisation if necessary, but are not the limiting factor in our best performing cavities The sensitivity to trapped flux is a function of the applied RF field, which may cause issues at high gradients if not accounted for