Routine characterization of 3D profiles of SRF cavity defects using replica techniques

Abstract

Abstract. Recent coordination of thermometry with optical images has shown that obvious defects at specific locations produce heat or even quench superconducting radio frequency (SRF) cavities, imposing a significant limit on the overall accelerating gradient produced by the cavity. Characterization of the topography at such locations provides clues about how the defects originated, from which schemes for their prevention might be devised. Topographic analyses also provide understanding of the electromagnetic mechanism by which defects limit cavity performance, from which viability of repair techniques might be assessed. In this article we discuss how a variety of two-component siliconebased room-temperature vulcanizing agents can be routinely used to make replicas of the cavity surface and extract topographic details of cavity defects. Previously, this level of detail could only be obtained by cutting suspect regions from the cavity, thus destroying the cavity. We show 3-D profiles extracted from several different 1.3 GHz cavities. The defect locations, which were all near cavity welds, compelled us to develop extraction techniques for both equator and iris welds as well as from deep inside long 9-cell cavities. Profilometry scans of the replicas yield micrometer-scale information, and we describe various curious features, such as small peaks at the bottom of pits, which were not apparent in previous optical inspections. We also discuss contour information in terms of electromagnetic mechanisms proposed by others for local cavity heating. We show that production of the replica followed by high-pressure rinsing dose not adversely affect the cavity RF performance.

High-Frequency Nonlinear Response of Superconducting Cavity-Grade Nb Surfaces

“…The high-accelerating gradient performance of Nb SRF cavities is often limited by breakdown events below the intrinsic limiting surface fields of Nb. These breakdowns are often caused by defects at discrete locations inside the cavity [4,5]. Although cavities with high-quality surfaces with excellent rf properties had been achieved, there is a lack of detailed understanding of the causal links among surface treatments, defects, and ultimate rf performance at low temperatures with many theoretical models being proposed to address the issue [6][7][8][9].…”

Atom-Probe Tomographic Analyses of Hydrogen Interstitial Atoms in Ultrahigh Purity Niobium

“…The central issue concerning niobium SRF cavities is their lack of reproducible performance for large-scale production, which means their yield is low and hence their cost for using them for linear accelerators becomes extremely expensive. One of the symptoms of poor performance at the high electric-field gradients, 35 MV/m, specified by the International Linear Collider project and a next-generation accelerator project, Project X, is the appearance of large pits at the edges of the heat-affected zones near the equator welds (Ge et al, 2011). Typical pit dimensions are 0.1–1.0 mm width and 10–100 mm depth (Cooley et al, 2011).…”

Accuracy and Precision of Silicon Based Impression Media for Quantitative Areal Texture Analysis

“…Analysis and quantification of natural and manufactured surfaces at micrometric and sub-micrometric scales is becoming widespread. Applications range from engineering 1 and superconductor technologies in particle accelerators 1 2 3 4 , to archaeology 5 6 7 , human skin surface topography 8 9 , and biomimetics (e.g. antifouling properties of bivalve shells 10 ).…”