CERN, Geneva, Switzerland
HZB, Berlin, Germany
University of Siegen, Siegen, Germany
JLab, Newport News, Virginia, USA
JLab, Newport News, Virginia, USA
The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
CERN, Geneva, Switzerland
ANL, Argonne, Illinois, USA
The RF performance of bulk Nb cavities has continuously improved over the years and is approaching the intrinsic limit of the material. Although some margin seems still available with processes such as N surface doping, long term solutions for SRF surfaces efficiency enhancement need to be pursued. Over the years, Nb/Cu technology, despite its shortcomings, has positioned itself as an alternative route for the future of superconducting structures used in accelerators. Significant progress has been made in recent years in the development of energetic deposition techniques such as Electron Cyclotron Resonance (ECR) plasma deposition. Nb films with very high material quality have then been produced by varying the deposition energy alluding to the promise of performing SRF films. This paper presents RF measurements, correlated with surface and material properties, for Nb films showing how, by varying the film growth conditions, the Nb film quality and surface resistance can be altered and how the Q-slope can be eventually overcome.
The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
Current superconducting radio frequency (SRF) technology, used in various particle accelerator facilities is reliant upon bulk Nb. Due to technological advancements in the processing of bulk Nb cavities, the facilities have reached accelerating fields very close to material-dependent limits, i.e. ~50 MV/m for bulk Nb. One possible solution to overcome this limit proposed by A. Gurevich consists of the deposition of alternating thin layers of superconducting and insulating materials on the interior surface of the cavities which may prevent early field penetration and thus delay high field breakdown*. Some candidate materials proposed for this scheme are NbN and NbTiN. Here we present experimental results correlating film microstructure and surface morphology with superconducting properties on coupon samples made with NbN and NbTiN. We have achieved thin films with close to bulk-like lattice parameters and transition temperatures, while achieving Hc1 values larger than bulk for films thinner than their London penetration depths. We compare results from samples grown utilizing NbTi targets with different stoichiometries and we will show RF measurements from 2” coupon samples.
*A. Gurevich, Appl. Phys. Lett. 88, 012511 (2006).
JLab, Newport News, Virginia, USA
For developing accelerator cavity materials, superconducting transition temperature (TC), transition width (ΔTC), and residual resistivity ratio (RRR), are useful parameters to correlate with SRF performance and fabrication processes of bulk, thin film, and novel materials. The RRR gauges the purity and structure of the superconductor based on the temperature dependence of electron scattering in the normal conducting state. Combining a four point probe delta pulse setup with a switch allows multiplexing of the electrical measurements to 32 samples per cooldown cycle. The samples are measured inside of an isothermal setup in a liquid helium (LHe) dewar. The isothermal setup is required for a quasistatic warmup of the samples through TC. This contribution details the current setup for collecting RRR and TC data, the current standard of throughput, measurement quality of the setup, and the improvements underway to increase the system’s resolution and ease of use.