Abstract
An apparatus and a method for etching of the inner surfaces of superconducting radio frequency (SRF) accelerator cavities are described. The apparatus is based on the reactive ion etching performed in an Ar/Cl2 cylindrical capacitive discharge with reversed asymmetry. To test the effect of the plasma etching on the cavity rf performance, a 1497 MHz single cell SRF cavity was used. The single cell cavity was mechanically polished and buffer chemically etched and then rf tested at cryogenic temperatures to provide a baseline characterization. The cavity’s inner wall was then exposed to the capacitive discharge in a mixture of Argon and Chlorine. The inner wall acted as the grounded electrode, while kept at elevated temperature. The processing was accomplished by axially moving the dc-biased, corrugated inner electrode and the gas flow inlet in a step-wise manner to establish a sequence of longitudinally segmented discharges. The cavity was then tested in a standard vertical test stand at cryogenic temperatures. The rf tests and surface condition results, including the electron field emission elimination, are presented.
Highlights
Superconducting radio frequency (SRF) cavities are integral components of accelerators used in nuclear and high energy physics research
We presented the experimental setup and procedure to etch a single cell SRF cavity in an rf plasma discharge and the rf test results of the first plasma etched SRF cavity at cryogenic temperature
The test results suggest that there is a possibility that the plasma-etched cavity would perform as good as a chemically etched cavity if the components
Summary
Superconducting radio frequency (SRF) cavities are integral components of accelerators used in nuclear and high energy physics research. The inner surfaces of SRF cavities are chemically treated (etched or electro-polished) to remove impurities, mechanically damaged layers and reduce the surface resistance of the superconducting surface, achieve a favorable RF performance These technologies are based on the use of hydrogen fluoride (HF) in liquid acid baths,[1,2,3,4,5,6] which poses a major environmental and personal safety concern. The concept of surface enhancement of the inner electrode to partially overcome sheath voltage asymmetry was applied and various structures were tested.[9] The optimum, corrugated structure for reversal of the asymmetry has been determined.[9] A stainless steel pillbox cavity was chosen with the aim of studying the plasma processing effect on varied-diameter structures, as uniform plasmasurface interaction is a challenging task.[10] The lesson learned from the pill box cavity experiment was that the etch rate in the beam tube is extremely high compared to the equatorial locations. The same cavity was plasma etched and retested at cryogenic temperature
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