Surface processing for bulk niobium superconducting radio frequency cavities

Abstract

The majority of niobium cavities for superconducting particle accelerators continue to be fabricated from thin-walled (2–4 mm) polycrystalline niobium sheet and, as a final step, require material removal from the radio frequency (RF) surface in order to achieve performance needed for use as practical accelerator devices. More recently bulk niobium in the form of, single- or large-grain slices cut from an ingot has become a viable alternative for some cavity types. In both cases the so-called damaged layer must be chemically etched or electrochemically polished away. The methods for doing this date back at least four decades, however, vigorous empirical studies on real cavities and more fundamental studies on niobium samples at laboratories worldwide have led to seemingly modest improvements that, when taken together, constitute a substantial advance in the reproducibility for surface processing techniques and overall cavity performance. This article reviews the development of niobium cavity surface processing, and summarizes results of recent studies. We place some emphasis on practical details for real cavity processing systems which are difficult to find in the literature but are, nonetheless, crucial for achieving the good and reproducible cavity performance. New approaches for bulk niobium surface treatment which aim to reduce cost or increase performance, including alternate chemical recipes, barrel polishing and ‘nitrogen doping’ of the RF surface, continue to be pursued and are closely linked to the requirements for surface processing.

Electropolishing valve metals with a sulfuric acid-methanol electrolyte at low temperature

“…In many of these devices, it is critical for the metals to have smooth surfaces. Ti medical implants require low surface roughness for better contact, while polished Nb is a vital component of high-field superconducting radio frequency cavities [2,[4][5][6][7][8]. Smooth Ti and Nb films also facilitate uniform growth of anodized metal oxide nanostructures, which are used in battery electrodes, gas sensors, electrochromic devices, and photocatalysts [9][10][11][12][13].…”

An ultralow-noise superconducting radio-frequency ion trap for frequency metrology with highly charged ions

“…Mechanical machining of metallic surfaces can contaminate them, limiting the performance of Nb as an RF superconductor. 70 The thickness of this so-called damaged or dirty layer depends on the manufacturing process, and varies between 50 and 200 µm. 70 Therefore, we instead employed non-intrusive electrical discharge machining (EDM) by wire at small material ablation rates to suppress heating.…”

“…70 The thickness of this so-called damaged or dirty layer depends on the manufacturing process, and varies between 50 and 200 µm. 70 Therefore, we instead employed non-intrusive electrical discharge machining (EDM) by wire at small material ablation rates to suppress heating. This avoids gettering of hydrogen, oxygen, and other gases by Nb at temperatures above 200 • C. 71 All parts were cut from the ingot using wire EDM in several steps with decreasing material ablation rate, yielding a well-defined contour with a smooth surface.…”

Ac losses in field-cooled type I superconducting cavities

“…For high-field applications, such as superconducting magnets and RF cavities for particle accelerators, the fluxflow dissipation can significantly impact performance. By pinning vortices, the dissipation can be reduced, and this has led to intense and still ongoing research into ways to pin vortices [5], and into surface treatments [6] to remove defects facilitating vortex penetration into the bulk of superconducting cavities [7].…”