CERN Accelerator School Superconductivity for Accelerators Case study 5
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CERN Accelerator School Superconductivity for Accelerators Case study 5 Group: Be Free Vicky Bayliss Mariusz Juchno Masami Iio Felix Elefant Erk Jensen
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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…. Thin Film Niobium: penetration depth. - PowerPoint PPT Presentation
Transcript of CERN Accelerator School Superconductivity for Accelerators Case study 5
CERN Accelerator School
Superconductivity for Accelerators
Case study 5
Group: Be FreeVicky Bayliss
Mariusz JuchnoMasami Iio
Felix ElefantErk Jensen
Case study 5RF 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/TC)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 5RF 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 cavityHysteresis ?
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
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 behaviourWelds on the equator
Case study 5RF cavities: superconductivity and thin films,
local defect…
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction 6
Bulk Niobium: local defectsAfter 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 multipactorLater steps due to defectsDifferent defects activated as the position of the maximum magnetic field changesQ-switches show hysteretic behaviourReproducible in the same conditions
Case study 5RF cavities: superconductivity and thin films,
local defect…
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction 7
Bulk Niobium: steps @ 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 enhancementFor L and F the increase is not that bigFor H there is a strong increase which saturates
The influence of the curvature radius? The radius is invesly2 proportional to the increase of the fieldSharp 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 5RF 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: steps @ GB
Case study 5RF 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?
Steps @ GB w. realistic dimension
RF only
The calculation is coherent with conclusions from the previous modelSimilar behaviour of the field enhancement with R and LThe cavity will always quench at about 0.95 of Hc due to the defectPmax linear with the L (harder to stabilise)One defect can dissipate more power than the whole cavity in S.C. stateThe model does not take into account the thermal diffusion or the local magnetic field perturbationThe defect becomes so large that it affects the behaviour of the whole cavity (oversimplification)
Case study 5RF cavities: superconductivity and thin films,
local defect…
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction 10
Steps @ GB w. realistic dimension
RF + thermal
Case study 5RF 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 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 stableBut even a small increase of the field (0.1mT) can trigger a quenchIf the purity increases the thermal conductivity increases so the stabilisation is betterBetter thermal conductance but worse surface resistance -> outcome not easy to predict
