  An apparatus and a method are described for plasma etching of the inner
surface of superconducting radio frequency (SRF) cavities. Accelerator SRF
cavities are formed into a variable-diameter cylindrical structure made of bulk
niobium, for resonant generation of the particle accelerating field. The etch
rate non-uniformity due to depletion of the radicals has been overcome by the
simultaneous movement of the gas flow inlet and the inner electrode. An
effective shape of the inner electrode to reduce the plasma asymmetry for the
coaxial cylindrical rf plasma reactor is determined and implemented in the
cavity processing method. The processing was accomplished by moving axially the
inner electrode and the gas flow inlet in a step-wise way to establish
segmented plasma columns. The test structure was a pillbox cavity made of steel
of similar dimension to the standard SRF cavity. This was adopted to
experimentally verify the plasma surface reaction on cylindrical structures
with variable diameter using the segmented plasma generation approach. The pill
box cavity is filled with niobium ring- and disk-type samples and the etch rate
of these samples was measured.
