Slow, even cooling reduces losses in superconducting cavities

Julia-Marie Vogt receives award for the best conference lecture by a junior scientist at the 16th International Conference on Radio Frequency Superconductivity.

The demands researchers place on particle and linear accelerators are rising: interesting experiments require ever higher intensities and beam brilliance. These demands can best be met with the help of superconducting cavities. Dr. Oliver Kugeler and Julia-Marie Vogt from the Helmholtz-Zentrum Berlin have found a mechanism through which these cavities can be significantly improved. By controlled cooling of the superconductor, its losses can be minimised and the investment and operating costs of the required cooling system significantly reduced. Julia-Marie Vogt held a lecture at the 16th SRF Conference (2013) on how this thus-far unexplained effect arises. She received the conference prize for the best lecture given by a junior scientist.

The conference is regarded as the most important forum for advanced development of superconducting accelerator components and took place in Paris, 23-27 September 2013.

In joint investigations with HZB physicist Dr. Oliver Kugeler, Julia-Marie Vogt reached the conclusion that losses by superconducting cavities in accelerator modules can be reduced by almost half if cavities are slowly and uniformly cooled using a special procedure. Superconductors must be cooled down to 1.6 degrees Kelvin in order to work properly. The researchers have minimised the losses by first cooling the superconducting cavities down to this temperature, then warming them up again to the transition temperature of 10 degrees Kelvin – and subsequently cooling them back down to 1.8 degrees Kelvin. This process is called “thermal cycling”.

Julia-Marie Vogt has now been able to explain the cause of this effect. She observed in simulations that thermally induced magnetic fields in the superconductors led to these losses. “In theory, superconductors exclude magnetic fields completely, but real superconductors do not do this entirely. If you cool them down slowly, however, then the magnetic fields can be excluded more completely. This way, the losses are reduced significantly”, says Vogt, a physicist who started work on her doctoral degree at the HZB Institute for SRF – Science and Technology in March 2013. She published this result jointly with Dr. Oliver Kugeler and Prof. Dr. Jens Knobloch from the Helmholtz-Zentrum Berlin in Physical Review Special Topics – Accelerators and Beams on 9 October 2013.

“This effect is very important for advancement of particle accelerator cooling systems. If slow and even cooling can be carried out at the superconduction transition temperature and below, the necessary cooling power during accelerator operations could be significantly reduced”, explains Prof. Dr. Jens Knobloch, head of the HZB Institute for SRF. This might mean that less powerful and lower energy cooling systems would be sufficient for operation. This could achieve savings in investment and operating costs of cooling systems. Alternatively, the beam energy could be increased for the same cryogenic performance according to Knobloch.

For Julia-Marie Vogt, it is the second distinction in a short time. She received a poster prize at the 4th International Particle Accelerator Conference for her initial results in May 2013.

Original publication:
J.-M. Vogt, O. Kugeler, and J. Knobloch: ""Impact of cool-down conditions at Tc on the superconducting rf cavity quality factor”
Phys. Rev. ST Accel. Beams 16, 102002 (2013), DOI: 10.1103/PhysRevSTAB.16.102002

The initial results on thermal cycling were presented in 2009. You can view the contribution to the conference here:
MANIPULATING THE INTRINSIC QUALITY FACTOR BY THERMAL CYCLING AND MAGNETIC FIELDS

Authors: O. Kugeler, W. Anders, A. Frahm, S. Klauke, J. Knobloch, A. Neumann, D. Pflückhahn, S. Rotterdam, M. Schuster, S. Voronenko.