Superconducting thin films for SRF applications

A competitive award for "Investigation of Proof-Of-Principle Test Samples of Thin Multi-Layer Superconductor/Insulator Systems capable of Sustaining Large Field Gradients" has been awarded to Professor R. A. Lukaszew (VMEC Assoc. Professor at the Applied Science and Physics Departments), as a result of Broad Agency Announcement (BAA) HDTRA1-08-10-BRCWMD-BAA, Research and Development Enterprise, Basic and Applied Sciences Directorate, Basic Research for Combating Weapons of Mass Destruction (C-WMD). The full project will span five years (2010-2015) and will be fully funded with $1,050,000 already committed for the first 3 years, with delivery dates 4/15/2010-13, and provision for two supplementary years at $350,000 level per year with delivery dates 4/14/2013-14 and 4/14/2014-15, with a grand total of $1,750,000. This project complements an existing DOE funded project for SRF-TF research (2009-2012) where W&M (led by Lukaszew) and JLab investigate thin films useful for SRF cavities used in research accelerators.

The broader impact of the proposed research relates to compact high-energy linear accelerators (linacs), which are fundamental for the production of sources for nuclear material security applications. A key component of any linac is the electromagnetic cavity resonating at a microwave frequency imparting energy gain to the charged particle. Due to the advantages of superconducting materials, superconducting cavities are one of the most successful applications of Superconducting-Radio-Frequency (SRF) technology.

In recent years, SRF cavity performance has approached the theoretical limit for bulk niobium (Nb), therefore new research efforts must concentrate on alternative materials and fundamental work is needed to understand the actual correlation between detailed material characteristics and the consequent SRF performance. Nb has been the material of choice so far, yielding a maximum accelerating gradient of less than 50 MV/m and therefore significant material development is required to attain the desired performance of >100 MV/m needed for compact accelerators useful in Defense applications. To maximize accelerating gradients, a high thermodynamic critical field and a high lower critical field are necessary. An interesting alternative has been theorized using a Superconductor-Insulator-Superconductor (SIS) multilayer approach. The key aspect of this approach is the recognition that vortex entry is inhibited if the surface layer thickness of the superconductor is less than the penetration depth. More promising superconductor materials than Nb, such as Niobium nitride (NbN), will be investigated in this novel SIS scheme.

Thus, a team of researchers from Applied Science and Physics Departments at W&M (Lukaszew, PI) in collaboration with scientists at Jefferson National Accelerator Facility (Jlab) with strong and complementary expertise on thin film growth and materials for RF cavities will investigate multilayer coatings possibly leading to gradients >100 MV/m that are of interest for compact accelerators capable of Sensing Fissile Materials at Long Range.