CERN Accelerator School Superconductivity for Accelerators Case study 5 Group: Be Free Vicky Bayliss Mariusz Juchno Masami Iio Felix Elefant Erk Jensen

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 2 Thin Film Niobium: penetration depth Frequency shift during cooldown. Linear representation is given in function of Y, where Y = (1-(T/T C ) 4 ) -1/2

Thin Film Niobium L (bluk Nb) =36 nm Q2. Calculation the penetration depths of the film. Make the comparison with bulk value. How can you explain the difference

kHz

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 5 Thin Film Niobium: local defect * Q3 : Explain qualitatively the experimental observations. First drops due to defects Then the overall heating/behaviour of the cavity Hysteresis ? Q4 : Deduce the surface of the defect. (For simplicity, one will take the field repartition and dimension from the ESS cavity shown on the right. Note the actual field B peak is proportional to E acc (B pea~ 42Oe/MV/m)x Eacc). G = 270W and Rs in normal state = 2mW Q5: If the hot spot had been observed 7.3 cm from the equator, what conclusion could you draw from the experimental data ? As the field is higher there, a smaller defect can cause such behaviour Welds on the equator

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 6 Bulk Niobium: local defects After 40 µm etching After 150 µm etching Q6 : Regarding the previous questions, and the field distribution in these cavities, how can you explain the multiple observed Q-switches ? First recovered switch due to multipactor Later steps due to defects Different defects activated as the position of the maximum magnetic field changes Q-switches show hysteretic behaviour Reproducible in the same conditions

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 7 Bulk Niobium: GB 2D RF model Q7. What conclusion can we draw about: The influence of the lateral dimensions of the defect? Its height ? The higher the dimensions, the higher the field enhancement For L and F the increase is not that big For H there is a strong increase which saturates The influence of the curvature radius? The radius is invesly 2 proportional to the increase of the field Sharp step is worse than a shallow step The behaviour at high field? The field enhancement saturates with an increase of H/R What happens if the defect is a hole instead of bump (F<<L) ? F/L is zero, so there is no enhancement

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 8 Bulk Niobium: local defects: GB

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 9 Q8.- do these calculation change the conclusion from the precedent simplified model ? - what prediction can be done about the thermal breakdown of the cavity? - why is this model underestimating the field enhancement factor and overestimating the thermal dissipations? GB w. realistic dimension RF only The calculation is coherent with conclusions from the previous model Similar behaviour of the field enhancement with R and L The cavity will always quench at about 0.95 of Hc due to the defect P max linear with the L (harder to stabilise) One defect can dissipate more power than the whole cavity in S.C. state The model does not take into account the thermal diffusion or the local magnetic field perturbation The defect becomes so large that it affects the behaviour of the whole cavity (oversimplification)

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 10 GB w. realistic dimension RF + thermal

Case study 5 RF cavities: superconductivity and thin films, local defect… Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013 Case study introduction 11 Q9 Comment these figures. What will happen if we introduce thermal variation of k, and/or R S. What happen if we increase the purity of Nb ? As the thermal conductivity increases we can put more power in the cavity and it is still stable But even a small increase of the field (0.1mT) can trigger a quench If the purity increases the thermal conductivity increases so the stabilisation is better Better thermal conductance but worse surface resistance -> outcome not easy to predict