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.

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

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

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

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

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

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

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

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

Discussion: Reason for Rres variation Rres development: 13.2 nW 5.4 nW 7.4nW 5.3 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

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

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

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

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

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

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