REPAIR SRF CAVITIES BY RE-MELTING SURFACE DEFECTS VIA HIGH POWER LASER TECHNIQUE

As field emission is gradually under control in recent SRF activities, cavity performance is limited by hard quench in most cases. Surface defect has been identified as one of main sources caused cavity quench, scattering cavity accelerating gradient from 12MV/m to 40MV/m. Laser re-melting technique is able to re-shape sharp pit rim to be a flat and smooth surface. In Fermilab, a sophisticated laser repair system has been developed for 1.3GHz quench-limited cavities. A pit in a 1.3GHz singlecell cavity was re-melted by high power laser pulse, after the laser processing the cavity took 30µm light Electropolishing. The cavity gradient achieved 39MV/m in initial run; after another 30µm Electropolishing, it reached 40MV/m. An improved laser repair system, which is able to re-melt surface defects in one meter-long 9-cell SRF cavity, has been developed at Fermilab after the success on single-cell cavity. It successfully re-melted a pit in 9-cell SRF cavity TB9ACC017.