Nitrogen-Doped 9-Cell Cavity Performance in a Test Cryomodule for LCLS-II

TL;DR

This paper reports on the performance of nitrogen-doped 9-cell superconducting RF cavities tested in a cryomodule, demonstrating they meet LCLS-II specifications and providing insights into their magnetic sensitivity and operational stability.

Contribution

First tests of nitrogen-doped 9-cell cavities in a cryomodule, showing they meet LCLS-II performance requirements and analyzing magnetic and thermal effects.

Findings

Cavities achieved an average quench field of 17 MV/m.

Average Q0 of 3x10^10 was obtained, exceeding specifications.

Residual resistance sensitivity to magnetic field was 0.5 nOhm/mG.

Abstract

The superconducting RF linac for LCLS-II calls for 1.3 GHz 9-cell cavities with an average intrinsic quality factor Q0 of 2.7x10^10 at 2 K 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 3x10^10. The sensitivity of the cavities' residual resistance to ambient magnetic field was determined to be 0.5 nOhm/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 cryomodule, which provide critical information for the LCLS-II project.