Transcript Slide 1

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
Frequency shift during cooldown. Linear representation is
given in function of Y, where Y
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
= (1-(T/TC)4)-1/2
Case study introduction
2
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
kHz
Case study 5
RF cavities: superconductivity and thin films, local
defect…
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 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
behaviour
Welds on the equator
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction
5
Case study 5
RF cavities: superconductivity and thin films, local
defect…
Bulk Niobium: local defects
After 40 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
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
After 150 µm etching
Case study introduction
6
Case study 5
RF cavities: superconductivity and thin films, local
defect…
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
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
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction
7
Case study 5
RF cavities: superconductivity and thin films, local
defect…
Bulk Niobium: local defects: steps @ GB
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction
8
Case study 5
RF 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?
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)
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction
9
Case study 5
RF cavities: superconductivity and thin films, local
defect…
Steps @ GB w. realistic
dimension
RF + thermal
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction
10
Case study 5
RF cavities: superconductivity and thin films, local
defect…
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
Superconductivity for Accelerators, Erice, Italy, 25 April - 4 May, 2013
Case study introduction
11