Anna Grassellino

Nitrogen-doped 9-cell cavity performance in a test cryomodule for LCLS-IIa)

The superconducting RF linac for Linac Coherent Light Source-II calls for 1.3u2009GHz 9-cell cavities with an average intrinsic quality factor Q0 of 2.7u2009×u20091010 at 2.0u2009K and 16 MV/m accelerating gradient. Two niobium 9 cell cavities, prepared with nitrogen-doping at Fermilab, were assembled into the Cornell Horizontal Test Cryomodule (HTC) to test cavity performance in a cryomodule that is very similar to a full LCLS-II cryomodule. The cavities met LCLS-II specifications with an average quench field of 17 MV/m and an average Q0 of 3u2009×u20091010. The sensitivity of the cavities residual resistance to ambient magnetic field was determined to be 0.5 nΩ/mG during fast cool down. In two cool downs, a heater attached to one of the cavity beam tubes was used to induce large horizontal temperature gradients. Here, we report on the results of these first tests of nitrogen-doped cavities in a cryomodule, which provide critical information for the LCLS-II project.

Frequency dependence of trapped flux sensitivity in SRF cavities

In this letter, we present the frequency dependence of the vortex surface resistance of bulk niobium accelerating cavities as a function of different state-of-the-art surface treatments. Higher flux surface resistance per amount of trapped magnetic field - sensitivity - is observed for higher frequencies, in agreement with our theoretical model. Higher sensitivity is observed for N-doped cavities, which possess an intermediate value of electron mean-free-path, compared to 120 C and EP/BCP cavities. Experimental results from our study showed that the sensitivity has a non-monotonic trend as a function of the mean-free-path, including at frequencies other than 1.3 GHz, and that the vortex response to the rf field can be tuned from the pinning regime to flux-flow regime by manipulating the frequency and/or the mean-free-path of the resonator, as reported in our previous studies. The frequency dependence of the trapped flux sensitivity to the amplitude of the accelerating gradient is also highlighted.