Measurement of the high-fieldQdrop in a high-purity large-grain niobium cavity for different oxidation processes

Abstract

The most challenging issue for understanding the performance of superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio >200) niobium is due to a sharp degradation (''Q-drop'') of the cavity quality factor Q 0 B p as the peak surface magnetic field (B p) exceeds about 90 mT, in the absence of field emission. In addition, a low-temperature (100-140 C) in situ baking of the cavity was found to be beneficial in reducing the Q-drop. In this contribution, we present the results from a series of rf tests at 1.7 and 2.0 K on a single-cell cavity made of high-purity large (with area of the order of few cm 2) grain niobium which underwent various oxidation processes, after initial buffered chemical polishing, such as anodization, baking in pure oxygen atmosphere, and baking in air up to 180 C, with the objective of clearly identifying the role of oxygen and the oxide layer on the Q-drop. During each rf test a temperature mapping system allows measuring the local temperature rise of the cavity outer surface due to rf losses, which gives information about the losses location, their field dependence, and space distribution. The results confirmed that the depth affected by baking is about 20-30 nm from the surface and showed that the Q-drop did not reappear in a previously baked cavity by further baking at 120 C in pure oxygen atmosphere or in air up to 180 C. These treatments increased the oxide thickness and oxygen concentration, measured on niobium samples which were processed with the cavity and were analyzed with transmission electron microscope and secondary ion mass spectroscopy. Nevertheless, the performance of the cavity after air baking at 180 C degraded significantly and the temperature maps showed high losses, uniformly distributed on the surface, which could be completely recovered only by a postpurification treatment at 1250 C. A statistic of the position of the ''hot spots'' on the cavity surface showed that grain boundaries are not the preferred location. An interesting correlation was found between the Q-drop onset, the quench field, and the low-field energy gap, which supports the hypothesis of thermomagnetic instability governing the Q-drop and the baking effect.

Effect of mild baking on superconducting niobium cavities investigated by sequential nanoremoval

“…Reversing an increase in R res and keeping the benefit in R BCS should allow minimizing the total surface resistance. We believe the observed Q 0 increase may stem from the decrease of the residual resistance by a single HF rinse consistent with earlier literature reports [5]. This means that the increase in R res caused by mild baking is due to the changes in either the oxide or in the first 1.5-2 nm of niobium underneath it.…”

“…It was reported that a single HF rinse applied on mild baked cavities does not bring the HFQS back [26] but there is no data in the literature for multiple HF rinse cycles. Anodizing experiments [4,5] showed that when about 20-30 nm of niobium are converted to oxide the HFQS reappears. This thickness corresponds to about 10-15 HF rinses in our experiments.…”

“…First, by oxipolishing experiments the change in BCS surface resistance after 145 C 45 hours bake was found to extend down to about 300 nanometers from the surface [3]. Second, it was demonstrated by cavity anodizing experiments [4,5] that modifications introduced by mild baking at 100-120 C, which are affecting the high field Q slope, are confined to only $20-30 nanometers from the surface. It was also shown that vacuum conditions for mild baking may not be crucial and Ar or even atmospheric air can be used [6].…”

Muon spin rotation studies of niobium for superconducting rf applications

“…Specifically, a 120 C bake in UHV for 48 hours is known to often eliminate or mitigate high field rf losses, by pushing the onset of the HFQS to a higher peak surface magnetic field [6], and also to increase the residual component and decrease the BCS component of the rf surface resistance. The effect of the 120 C bake on a high field Q slope is also known to be confined to the first tens of nm from the surface, and in the first $hundred for BCS surface resistance [7][8][9], so a procedure like BCP completely reverses the benefit of the bake. By repeating the SR measurements before and after such treatments for the same samples, we measure the relative change in magnetic field penetration and magnetic volume fraction due exclusively to the effect of surface treatments and free from any geometrical effect.…”

Superconducting Radio-Frequency Technology R&D for Future Accelerator Applications

“…The value of the energy gap obtained from fits of R s (T) at low RF field with BCS theory numerical calculations clearly showed a correlation between high gap values and high maximum B p -values [122]. Surface treatments such as BCP, EP, anodization, LTB do affect ∆ and therefore the ultimate field in the cavity.…”