Superconducting properties of niobium after electron beam welding

Prakash N Potukuchi,D Kanjilal,S B Roy,M K Chattopadhyay,L S Sharath Chandra,Amit Roy

doi:10.1103/physrevstab.14.122001

Abstract

One of the major criteria for designing superconducting niobium resonant cavities is to minimize the peak surface electric and magnetic fields to maximize the achievable accelerating electric gradient. Even after addressing the extrinsic effects adequately, a large number of cavities perform below the theoretical gradient limit. The peak magnetic field for the first flux-line penetration in the superconducting state of niobium, which either severely degrades the cavity quality factor or results in complete thermal breakdown, is an important limitation. The flux-line penetration is known to depend on the microstructural properties of niobium which may get altered in the process of cavity fabrication. The most common technique of fabricating niobium cavities is to form their components using standard sheet metal techniques and join them by electron beam welding in vacuum. We present results of a study on the superconducting response through magnetization measurements in the electron beam welded region of niobium to understand the limitations (if any) posed by the welding in achieving the highest gradient. We also present and discuss results on the performance of niobium quarter wave resonators incorporating such electron beam welds in the high magnetic field region.

Highlights

Superconducting niobium resonant cavities are used in a wide range of modern accelerators for accelerating electrons, protons, and heavy ions
The performance of a niobium superconducting radio frequency (SCRF) cavity would critically depend on the response of the niobium materials to such magnetic fields
The superconducting transition temperature (TC) for the electron beam welding (EBW) step joint and butt joint niobium materials estimated from the onset of diamagnetism are $9:28 and $9:26 K, respectively

Summary

Introduction

Superconducting niobium resonant cavities are used in a wide range of modern accelerators for accelerating electrons, protons, and heavy ions. The low velocity TEM class resonators are used for accelerating heavy ions [1], they are being increasingly designed for accelerating protons [2]. The medium and high velocity TM class cavities are generally used for accelerating electrons and protons They are designed for particle velocities 1⁄4 0:5 all the way up to the speed of light and operate in the frequency range 350–3000 MHz. To reduce the BCS surface resistance of superconducting niobium at these frequencies to an economically tolerable value, the TM class cavities are operated at 2 K. Several variants of the TEM and TM class structures have been designed and employed in particle accelerators [3,4,5]

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