Comparative surface studies on fine-grain and single crystal niobium using xps, aes, ebsd and profilometry

Abstract

As the surface magnetic field in niobium cavities approaches the theoretical critical field, RF losses grow sensitive to increasingly subtle features of the material and the surface. A striking example is the familiar onset of the high field Q-slope, where RF losses increase exponentially with field. A surprising feature of the high field Q-slope is its positive response to a mild baking at 100-120 • C. But the Q-slope returns after the first 20 nm of the niobium metal surface is converted to loss-less pentoxide via anodization. The latter result suggests that the cause of the fast growing losses resides in the first 20 nm of the RF surface. Although there are several propositions, the exact mechanism for the high field Q-slope is not yet fully understood and demands further research. We are conducting surface analytic studies with optical profilometry, EBSD, XPS, Auger and SIMS to shed light on the mechanism of the high field Q-slope. We are comparing the behavior of fine-grain niobium with single crystal niobium, buffered chemical polishing (BCP) treatments with electropolishing (EP) treatments and properties before and after 110 • C bake. Our approach is based on identifying lossy regions, dissecting of these regions and range of analysis.

Muon spin rotation studies of niobium for superconducting rf applications

“…Several investigations have been done on so-called ''hot'' (higher losses) versus ''cold'' (lower losses) samples, and baked versus unbaked HFQS-limited cutout samples, to examine differences in the microscopic structure. Some of the techniques employed include electron back-scattered diffraction and positron annihilation spectroscopy [13]. The results of some of these sample studies [14,15] show a higher density of lattice defects in unbaked compared to baked samples.…”

Elastic recoil detection studies of near-surface hydrogen in cavity-grade niobium

“…The LE1-37 cavity was limited by the HFQS and the cutout represents an area with high RF losses in the HFQS regime (hot spot). Details on LE1-37 cavity tests and surface analytical techniques applied on cutouts are reported elsewhere [5,7,8]. The TE1AES004 cutout represents an area without any RF losses (cold spot) up to the highest field reached, which was determined by the localized quench at a different location.…”

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

“…Before the development of replica techniques, suspect locations had to be cut out of the cavity in order to get quantitative data [12,13]. Unfortunately, the need to sacrifice an expensive cavity through destructive evaluation techniques (cavities are made from high-purity niobium requiring a significant investment in fabrication and processing effort) has historically limited opportunities to pursue many materials studies.…”