SUPERCONDUCTING NB FILM FOR RF APPLICATIONS

Copper RF cavities coated with thin Nb (Nb) films are an interesting possible alternative to bulk-Nb ones because copper is cheaper than Nb, has higher thermal conductivity and better mechanical properties. Unfortunately, the observed degradation of the quality factor (Q) with increasing electric field shown by Nb-sputtered cavities makes them unsuitable for future very high energy linear accelerators needing gradients higher than ≈15MV/m. We are therefore developing an alternate deposition technology, based on a cathodic arc system working in UHV conditions. Its main advantages compared to standard sputtering are the ionized state of the evaporated material, the absence of gases to sustain the discharge, the much higher energy of atoms reaching the substrate surface and, possibly, higher deposition rates. To ignite the arc we use a Nd-YAG pulsed laser focused on the cathode surface which proves a reliable and ultraclean trigger. Recent results on the characterization of Nb film samples produced under different conditions are presented proving that the technique can in principle produce bulk-like films suitable for RF superconducting applications