Change in high field Q-slope by baking and anodizing

The near-surface nanostructure of niobium determines the performance of superconducting microwave cavities. Subtle variations in surface nanostructure lead to yet unexplained phenomena such as the dependence of the quality factor of these resonating structures on the magnitude of rf fields-an effect known as the ''Q slopes''. Understanding and controlling the Q slopes is of great practical importance for particle accelerators. Here we investigate the mild baking effect-120 C vacuum baking for 48 hours-which strongly affects the Q slopes. We used a hydrofluoric acid rinse alternating with oxidation in water as a tool for stepwise material removal of about 2 nanometers=step from the surface of superconducting niobium cavities. Applying removal cycles on mild baked cavities and measuring the quality factor dependence on the rf fields after one or several such cycles allowed us to explore the distribution of lossy layers within the first several tens of nanometers from the surface. We found that a single HF rinse results in the increase of the cavity quality factor. The low field Q slope was shown to be mostly controlled by the material structure within the first six nanometers from the surface. The medium field Q slope evolution was fitted using linear (/ B peak surface magnetic field) and quadratic (/ B 2 ) terms in the surface resistance and it was found that best fits do not require the quadratic term. We found that about 10 nanometers of material removal are required to bring back the high field Q slope and about 20-50 nanometers to restore the onset field to the prebaking value.

Section: High Field Q Slopesupporting

confidence: 91%

Section: High Field Q Slopementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Romanenko

Grassellino

Barkov

et al. 2013

“…This could give some indications of the depth affected by the low-temperature baking. This experiment was recently done at Cornell University [18] on two polycrystalline Nb single cells treated by BCP and electropolishing. After the cavities were baked at 100 C for 48 h, the Q-drop reoccurred by anodizing at 30 V. The following paragraphs describe our experimental results.…”

Section: Anodizationmentioning

confidence: 99%

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

Ciovati

Kneisel

Gurevich

2007

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.

“…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.…”

Section: Introductionmentioning

confidence: 99%

Muon spin rotation studies of niobium for superconducting rf applications

Beard

Kolb

Laxdal

et al. 2013

In this work we investigate superconducting properties of niobium samples via application of the muon spin rotation/relaxation (SR) technique. We employ for the first time the SR technique to study samples that are cut out from large and small grain 1.5 GHz radio frequency (rf) single cell niobium cavities. The rf test of these cavities was accompanied by full temperature mapping to characterize the rf losses in each of the samples. Results of the SR measurements show that standard cavity surface treatments like mild baking and buffered chemical polishing performed on the studied samples affect their surface pinning strength. We find an interesting correlation between high field rf losses and field dependence of the sample magnetic volume fraction measured via SR. The SR line width observed in zero-field-SR measurements matches the behavior of Nb samples doped with minute amounts of Ta or N impurities. A lower and an upper bound for the upper critical field H c2 of these cutouts is found.