An apparatus for the continuous measurement of thickness during the electropolishing of superconducting cavities

An apparatus allowing continuous acquisition of thickness measurements during electropolishing of superconducting cavities is described. The instrument is based on the ultrasound thickness measurement technique and allows the connection of up to six probes. The apparatus has been employed to monitor the surface treatment of PIP-II low beta single cell prototypes developed and manufactured by LASA-INFN and specifically to measure surface removal at different points of interest on the cavity surface. The apparatus facilitated the development and optimization of electropolishing parameters for incorporation into the cavity manufacturing process.

Defect detection inside superconducting 1.3 GHz cavities by means of x-ray fluorescence spectroscopy

X-ray fluorescence probe for detection of foreign material inclusions on the inner surface of superconducting cavities has been developed and tested. The setup detects trace element content such as a few micrograms of impurities responsible for thermal breakdown phenomena limiting the cavity performance. The setup has been customized for the geometry of 1.3 GHz TESLA-type niobium cavities and focuses on the surface of equator area at around 103 mm from the centre axis of the cavities with around 20 mm detection spot. More precise localization of inclusions can be reconstructed by means of angular or lateral displacement of the cavity. Preliminary tests confirmed a very low detection limit for elements laying in the high efficiency spectrum zone (from 5 to 10 keV), and a high angular resolution allowing an accurate localization of defects within the equator surface.

Generation of GW radiation pulses from a VUV free-electron laser operating in the femtosecond regime

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30-100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95-105 nm.