JLAB, Newport News, Virginia
ODU, Norfolk, Virginia
We are pursuing the use of low cost, environmentally friendly dry etching of SRF cavities in an Ar/Cl2 discharge. We have successfully demonstrated on flat samples that etching rates are comparable to the commonly used wet processes, such as BCP or EP. The geometry of bulk Nb SRF cavities involves the use of an asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of driven and grounded electrodes creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the plasma sheath attached to the cavity surface. To study these asymmetric discharges, a single cell cavity has been specially designed with 20 holes symmetrically placed over the cell. This setup can be used for both diagnostics and sample etching purposes. Radially and spectrally resolved profiles of optical intensity of the discharge are combined with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions, and sequences.
ODU, Norfolk, Virginia
JLAB, Newport News, Virginia
Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate non-superconductive pollutants in the penetration depth region and to remove mechanically damaged surface layer improving surface roughness. Here we show that plasma treatment of bulk Nb presents surface preparation method alternative to the commonly used BCP and EP methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on the flat samples. We have achieved etching rates of 1.5 micro-m/min using only 3% Cl2 in the reactive mixture. Combining fast etching step with the moderate one, we have improved a surface roughness without exposing fresh sample surface to the environment. We will apply optimized experimental conditions to single cell cavities, in pursuing improvement of their RF performance.