Role of magnetic flux expulsion to reach Q0>3×1010 in superconducting rf cryomodules

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&D 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 Q0 of ~3×10 10 .

Nonlinear dynamics and dissipation of a curvilinear vortex driven by a strong time-dependent Meissner current

“…Part of this R s comes from the exponentially small quasiparticle BCS contribution R BCS ∝ ω 2 exp(−∆/T ) but another one is a weakly-temperature dependent residual resistance R i which can account for 20% of R s in Nb 11 and 50% for Nb 3 Sn at 2K 33 . Much of this contribution to R i comes from trapped vortices generated during the cavity cool down through T c at which the lower critical field H c1 (T ) vanishes [34][35][36][37][38][39][40][41] . In this case even small stray fields H > H c1 (T ) such as a few % of the earth magnetic field can produce vortices in the cavity.…”

A White Paper on Design and Fabrication of SRF Deflecting Cavities for Elletra-2

“…Note that we estimate the surface magnetic field to be below 20 mT, and the typical ambient magnetic field is about 10 mG. The latter has been demonstrated in cryomodules developed for LCLS-II using magnetic shielding around the accelerating cavity [11]. According to the tests of the 3-GHz elliptical cavity performed at JLab [12], one can expect to achieve a surface resistance of Nb at 2 K and 3 GHz in the range 35-70 nΩ.…”

Impact of geometry on the magnetic flux trapping of superconducting accelerating cavities

“…The flux trapping efficiency coefficient mag is typically evaluated with magnetic sensors installed in close proximity with a cavity [9]. The sensor probes the magnetic field distribution altered by the diamagnetic property of the Meissner state when the material goes through superconducting transition.…”

“…Recent technical advances on mechanical process and surface treatment have reduced R0, and therefore, understanding and controlling Rmag becomes of critical importance in state-of-the-art superconducting RF cavities for many applications [4]. Previous studies [5,6,7,8,9] mainly focused on the specific elliptical cavities dedicated to high-energy electron accelerators. In this paper, we report on three more general shaped cavities developed for proton and heavy ion linear accelerators.…”