Abstract
Plasma based surface modification of niobium is a promising alternative to wet etching of superconducting radio frequency (SRF) cavities. The development of the technology based on Cl2/Ar plasma etching has to address several crucial parameters which influence the etching rate and surface roughness, and eventually, determine cavity performance. This includes dependence of the process on the frequency of the RF generator, gas pressure, power level, the driven (inner) electrode configuration, and the chlorine concentration in the gas mixture during plasma processing. To demonstrate surface layer removal in the asymmetric non-planar geometry, we are using a simple cylindrical cavity with 8 ports symmetrically distributed over the cylinder. The ports are used for diagnosing the plasma parameters and as holders for the samples to be etched. The etching rate is highly correlated with the shape of the inner electrode, radio-frequency (RF) circuit elements, chlorine concentration in the Cl2/Ar gas mixtures, residence time of reactive species and temperature of the cavity. Using cylindrical electrodes with variable radius, large-surface ring-shaped samples and d.c. bias implementation in the external circuit we have demonstrated substantial average etching rates and outlined the possibility to optimize plasma properties with respect to maximum surface processing effect.
Highlights
The etching rate is highly correlated with the shape of the inner electrode, radio-frequency circuit elements, gas pressure, rf power, chlorine concentration in the Cl2=Ar gas mixtures, residence time of reactive species, and temperature of the cavity
To improve the rf performance of superconducting radio frequency (SRF) niobium cavities, the cavity surface must be prepared by a process that enhances surface smoothness, removes impurities, and creates less sharp grain boundaries
In view of the complex technological challenges facing the development of plasma-assisted SRF surface etching, we have adopted a multistep approach to transition from flat coupons to the full scale treatment of large cavity surfaces
Summary
To improve the rf performance of superconducting radio frequency (SRF) niobium cavities, the cavity surface must be prepared by a process that enhances surface smoothness, removes impurities, and creates less sharp grain boundaries. Buffered chemical polishing or electropolishing [1] are currently used technologies based on the use of hydrogen fluoride (HF) in liquid acid baths, which poses major environmental and personal safety concerns. HF-free plasma-based (“dry”) technologies are much more controllable, less expensive, and more environment friendly. To the best of our knowledge, we present the first results for three-dimensional plasma etching of the inner surface of bulk niobium cavities. As a proof of concept, we developed an experimental setup for etching small flat niobium samples [2,3]. The results were very encouraging [3], with etching rates up to 1.7 μm= min. The surface roughness of plasma etched samples was equal or lower than the chemically etched samples
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