Abstract
A recently discovered modified low-temperature baking leads to reduced surface losses and an increase of the accelerating gradient of superconducting TESLA shape cavities. We will show that the dynamics of vacancy-hydrogen complexes at low-temperature baking lead to a suppression of lossy nanohydrides at 2 K and thus a significant enhancement of accelerator performance. Utilizing Doppler broadening Positron Annihilation Spectroscopy, Positron Annihilation Lifetime Spectroscopy and instrumented nanoindentation, samples made from European XFEL niobium sheets were investigated. We studied the evolution of vacancies in bulk samples and in the sub-surface region and their interaction with hydrogen at different temperature levels during in-situ and ex-situ annealing.
Highlights
The influence of hydrogen on the rf losses of cavities and the necessity of outgassing has been known for quite some time
The observable changes happen in a sub-surface region with thickness of about ≈190 nm
All samples show an increase of the S parameter in the sub-surface region during the baking for 4 h up at 120 °C, which is an indication of increased open volume and/or concentration of defects
Summary
The influence of hydrogen on the rf losses of cavities and the necessity of outgassing has been known for quite some time. The so-called ‘hydrogen Q-disease’ marks an increase of the surface losses at cryogenic temperatures which sets in even at low values of the applied accelerating field[1,2] and it works as follows: the operating temperature of superconducting accelerating cavities is 2–4 K, and while crossing the temperature range of 200–50 K during cool down, different phases of niobium hydrides form on the surface and in the rf penetration layer, causing increased losses as niobium hydrides are only superconducting below 1.3 K3 To prevent this Q-disease from happening, cavities need to be annealed at 800–900 °C for 3 h in a vacuum furnace at pressures below 10−5 mbar to purify the material (by hydrogen removal), preventing the formation of hydrides during cool down. Saving substantial fractions of the planned investments is highly desirable
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