Surface Issues for Solid Niobium SRF Accelerator Cavities

Abstract

The world-wide physics community looks forward to a slate of accelerator projects of unprecedented magnitude and diversity. Certainly its sheer size makes the International Linear Collider the most visible to the public eye, with 16,000 solid niobium cavities performing at historically high gradient, and built (and operated) for historically low unit cost. Net performance makes superconducting radiofrequency (SRF) technology the approach of choice. Solid niobium is the material most widely used for construction of SRF cavities because it has the highest critical transition temperature (Tc = 9.2 K) of the pure metals, sufficiently high critical magnetic field (Hc > 2 k Oe) for SRF applications, and metallurgical properties adequate for fabrication and service load. Studies of the SRF performance of niobium cavities began to be reported more than 30 years ago and continue now with the application of improved experimental techniques. Niobium metal superconductivity is a nanoscale, near-surface phenomenon because of the shallow RF penetration. Considerable evidence indicates that cavity interior surface chemistry and topography strongly impact SRF accelerator performance, motivating investigation of how they are affected by post-fabrication treatments. Current status and prospects are discussed with respect to accelerator needs and opportunities. Authored by Jefferson Science Associates LLC under US DOE Contract De-AC05-06OR23177