Wuppertal 2015 – scientific programme

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AKBP: Arbeitskreis Beschleunigerphysik

AKBP 6: SC Resonators

AKBP 6.9: Talk

Tuesday, March 10, 2015, 15:45–16:00, BZ.08.06 (HS 1)

High-Q cavity operation: Study on the thermoelectrically induced contribution to RF surface resistance — •Julia-Marie Vogt, Oliver Kugeler, and Jens Knobloch — Helmholtz-Zentrum Berlin

We present a study concerning the operation of a superconducting RF cavity in horizontal testing with the focus on understanding the thermoelectrically induced contribution to the surface resistance. Starting a few years ago, we suggested a means of reducing the residual resistance by warming up a cavity after initial cooldown to about 20K and cooling it down again, a thermal cycle. In subsequent studies we demonstrated a manipulation of the residual resistance by more than a factor of 2. We postulated that thermocurrents during cooldown generate additinal trapped magnetic flux that impacts the cavity quality factor. Since serveral questions remained open, we present here a more extensive study including measurement of two additional passband modes of the 9-cell cavity (8/9 π and 1/9 π) that confirms the effect. We also discuss simulations that substantiate the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters result in a nonsymmetric current distribution. Hence a significant amount of magnetic flux can be generated at the RF surface resulting in an increased surface resistance.