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Superconductivity and Thin Films: Case Study on RF Cavities

This case study focuses on superconductivity and thin films in RF cavities, particularly exploring the behavior of thin film niobium and local defects. Topics covered include the calculation of penetration depths, experimental observations and conclusions, and the influence of defect dimensions. The study also discusses the thermal breakdown of the cavity and the limitations of the model used.

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Superconductivity and Thin Films: Case Study on RF Cavities

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  1. CERN Accelerator SchoolSuperconductivity for AcceleratorsCase study 5 Group: Be Free Vicky Bayliss Mariusz Juchno Masami Iio Felix Elefant Erk Jensen

  2. Case study 5RF cavities: superconductivity and thin films, local defect… Thin Film Niobium: penetration depth Frequency shift during cooldown. Linear representation is given in function of Y, where Y = (1-(T/TC)4)-1/2 Case study introduction

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

  4. kHz

  5. Case study 5RF cavities: superconductivity and thin films, local defect… Thin Film Niobium: local defect * • 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 introduction • 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 Bpeak is proportional to Eacc (Bpea~42Oe/MV/m)x Eacc). G = 270W and Rs in normal state = 2mW

  6. Case study 5RF cavities: superconductivity and thin films, local defect… After 40 µm etching Bulk Niobium: local defects After 150 µm etching Case study introduction • 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

  7. Case study 5RF cavities: superconductivity and thin films, local defect… Bulk Niobium: steps @ GB 2D RF model Case study introduction 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 invesly2 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

  8. Case study 5RF cavities: superconductivity and thin films, local defect… Bulk Niobium: local defects: steps @ GB Case study introduction

  9. Case study 5RF cavities: superconductivity and thin films, local defect… Steps @ GB w. realistic dimension RF only 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? Case study introduction 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 Pmax 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)

  10. Case study 5RF cavities: superconductivity and thin films, local defect… Steps @ GB w. realistic dimension RF + thermal Case study introduction

  11. Case study 5RF cavities: superconductivity and thin films, local defect… Case study introduction • Q9 Comment these figures. What will happen if we introduce thermal variation of k, and/or RS. 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

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