Role of magnetic flux expulsion to reach Q0&gt;3×1010 in superconducting rf cryomodules

S Posen,D A Sergatskov,A Palczewski,D Gonnella,A Romanenko,G Wu,E Harms,T Peterson,N Solyak,A Grassellino,O S Melnychuk

doi:10.1103/physrevaccelbeams.22.032001

Abstract

When a superconducting radiofrequency cavity is cooled through its critical temperature, ambient magnetic flux can become ``frozen in'' to the superconductor, resulting in degradation of the quality factor. This is especially problematic in applications where quality factor is a cost driver, such as in the cw linac for LCLS-II. Previously, it had been unknown how to prevent flux from being trapped during cooldown in bulk niobium cavities, but recent R studies showed near-full flux expulsion can be achieved through high temperature heat treatment and cooling cavities through the superconducting transition with a spatial thermal gradient over the surface. In this paper, we describe the first accelerator implementation of these procedures, in cryomodules that are currently being produced for LCLS-II. We compare the performance of cavities under different conditions of heat treatment and thermal gradient during cooldown, showing a substantial improvement in performance when both are applied, enabling cryomodules to reach and, in many cases, exceed a ${Q}_{0}$ of $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{10}$.

Highlights

For high energy SRF linacs operating with high duty factor, cryogenic infrastructure and operation can be a major cost driver of the accelerator
With heat load in the low temperature circuit dominated by the dynamic load of the cavity rf heating, the cavity Q0 specification can be key to determining the size of the required cryogenic system
We present measurements of an LCLS-II prototype cryomodule to show what thermal gradients are achievable during cooldown and compare this to measurements of flux expulsion as a function of thermal gradient for several single cell cavities made using the same niobium that would be used in LCLS-II cavity production

Summary

INTRODUCTION

For high energy SRF (superconducting radiofrequency) linacs operating with high duty factor, cryogenic infrastructure and operation can be a major cost driver of the accelerator. Studies on bare cavities in the vertical test at Fermilab in 2013 established for the first time that careful control of cooldown has an impact on flux trapping behavior in cavities and on quality factor [8,9] In these studies, a magnetic field was applied to single cell 1.3 GHz cavities using Helmholtz coils, and the magnetic field enhancement during cooldown was measured using fluxgate magnetometers. To vary the cooldown conditions, different values of mass flow of cold helium were used during cooldown through the critical temperature, resulting in different spatial thermal gradients across the cavity during transition as measured by temperature sensors on the cavity These studies established an apparent dependence of the flux expulsion ratio. This was a crucial finding of the R&D program as a way to “cure” cavities with poor flux expulsion behavior

ACHIEVING HIGH MASS FLOW IN THE LCLS-II MAIN LINAC CRYOMODULES

DISCUSSION

CONCLUSIONS