1. HZB High-Q0 Optimization by thermal cycling
Axel Neumann for Oliver Kugeler, Julia Vogt, Jens Knobloch, Sarah Aull
TTC CW-SRF meeting Ithaca, NY
Thank you for the introduction. As the title of my talk suggests I‘m going to present you with a simple method to possibly increase the Q0 in SRF cavities almost for free.
This is work that has been done at HZB to a great deal by two of our master students who are also present at this conference, Julia Vogt and Sarah Aull.

2. A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY
Introduction
CW operation of SRF cavities
Cryogenics = cost driver
Minimize cryogenic load
We found a way to reduce trapped flux
physics
originates to great fraction from trapped vortices (incomplete Meissner effect)
O. Kugeler et al., SRF 2009
But first, let me give you a brief introduction:
We are dealing with the CW operation of superconducting cavities as required by many present-day accelerators.
As you all know, in CW the cost driver is the cryogenics.
What we want to do in a CW accelerator is minimize the cryogenic load – or the disspated power in a cavity.
This ios proportional to the surface resistance, or inversely proportional to Q0.
From now on I‘ll be talking in terms of surface resistance.
The surface resistance consists of a BCS part – which is physics and fixed.
A second contribution – are residual losses. These losses originate from normal conducting regions at the surface of the cavity. Even tiniest nc regions are sufficient to spoil the superconductor, since the contribution due to skin effect is 10^6 times higher.
Furthermore, the region where superconductivity is spoiled can serve as pinning centers for magnetic fields. This trapped flux forms vortices with a nc core, that again .
As a matter of fact, at low fields 100% of the ambient field is trapped even in RRR300 material
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

3. A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY
Q0 measurements
HoBiCaT test facility
Temperatures down to 1.5 K
Horizontal, fully equipped cavity weld into Helium tank
Near bc=1
TESLA cavity, standard BCP, fine grain
Inner magnetic shield including end-caps
I‘d like to point out that the measurements I am presenting have been obtained under accelerator like conditions.
It‘s one matter to demonstrate a high Q in a vertical test with a naked cavity, but it‘s an entirely different matter to reproduce these results on a fully dressed horizontal cavity.
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

4. Cavity cool down procedure
heater
Temperature sensors
Fast transition through (150 K – 50 K)
Large temperature differences DT
Large spatial gradients dT/dz
Large temporal gradients dT/dt
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

5. Rsurf after initial cooldown
Rres = 13.2 nW
4 MV/m
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

6. Thermal cycling procedure
Start with superconducting cavity
Turn off Helium supply (JT valve)
Evaporate Helium in tank
Wait. Make sure cavity is just above Tc and normal conducting.
Restart cryo plant
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

7. Influence of thermal cycling on Rsurf
Q0=1.5·1010
Q0=2.7·1010
Rres = 13.2 nW
Rres = 5.4 nW
The result of this simple procedure is presented here, we managed to increase Q0 by almost 100% which cuts the required cryo power in two.
The extracted residual losses are depicted as well and we can see that we are approaching the BCS limitation
4 MV/m
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

8. A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY
Effect reversible ?
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

9. Influence of thermal cycling on Rsurf
13.2 nW
7.4 nW
5.4 nW
5.3 nW
4 MV/m
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

10. Discussion: Reason for Rres variation
Rres development: 13.2 nW nW nW nW
Cycling leads to decrease … increase … decrease
Efficacy of magnetic shielding? No! Permeability measurements of shield yielded no temperature dependance in relevant region AND Rres increase should not be possible
Chemistry? Adsorbate removal? No! Rres increase should not be possible.
But! Increase could have been caused by Q-disease in heavier cycling run. No! Subsequent Rres decrease should not be possible.
Thermocurrents due to temperature gradients
Possible. Performed measurements in model system
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

11. A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY
Thermocurrents
Thermoelectric effect:
Voltage due to material and
temperature dependent
charge carrier velocity
S are Seebeck coefficients
Set up model experiment
Cavity-tank system as „thermoelement“ Close circuit to obtain thermocurrent.
Master thesis Julia Vogt,
see poster WEPWO004
for further details
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

12. Trapped flux and temperature gradient
Findings:
Thermocurrents could be measured (mA range)
Thermocurrents create a magnetic field and this field can be trapped as frozen flux
Linear correlation between trapped flux and temperature gradient
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

13. Trapped flux and temperature gradient
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

14. Flux expulsion at different cooling rates
Measurement: Keep rod isothermal and cool through Tc
Logarithmic dependence of expelled flux from cooling rate
Interpretation:
Meissner state = energetically lowest state
Flux expulsion not instantaneous (unlike Meissner transition)
Mobility of flux lines highest near Tc
The less time is available in the high mobility region, the less field is expelled from the sc
Oliver Kugeler: Pathway to a post processing increase of Q0 in SRF cavities, IPAC 2013, Shanghai, China

15. A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY
Conclusion
Improve Q0 by thermal cycling
Factor of 2 improvement is demonstrated
It appears that thermal currents are responsible for extra flux trapping
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY

16. A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY
Acknowledgement
Julia Vogt (Master thesis on flux dynamics)
Sarah Aull (Diploma thesis on trapped flux)
Jens Knobloch
Michael Schuster, Sascha Klauke, Andre Frahm, Dirk Pflückhahn, Stefan Rotterdam (Experimental support)
Hans-Peter Vogel, Research Instruments (for providing Niobium rod)
A. Neumann, TTC CW-SRF meeting, U Cornell, Ithaca, NY