Apparatus and method for plasma processing of SRF cavities

Abstract

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.

Argon metastable and resonant level densities in Ar and Ar/Cl2 discharges used for the processing of bulk niobium

“…The experimental setup closely resembles that of the previous work reported by Upadhyay et al [3][4][5][6][7][8][9] The main differences are upgrades in the pumping system. The plasma chamber is evacuated using a combination of a Pfeiffer/Adixen ATH-500MT turbomolecular pump and a Leybold Trivac D65 BCS PFPE rough pump.…”

“…IARIE techniques have entered competitively into the development of resonant superconductive radio-frequency (SRF) niobium (Nb) cavities used in high energy particle accelerators. [3][4][5][6][7][8][9] The aim is to complement or replace the "wet" acid processes that are commonly used to remove surface impurities that limit the performance of the bulk superconductive medium and to create a smooth surface with the reduction of grain boundaries and pits. 10 By adopting the same basic principles used on the semiconductor nanotechnology, IARIE shows promise in the preparation of SRF cavities for use in particle accelerators by potentially achieving similar or better superconducting properties as compared to current methods while decreasing costs, processing time, and manpower.…”

“…Much of the work done in the adaptation of the IARIE technology to SRF cavities has been to prove that this process is viable and controllable for etching concave cylindrical surfaces. [3][4][5][6][7][8][9] While it has shown some success, the work has left significant information missing about the plasma parameters and dynamics within the etching reactor. The plasma etch process uses a mixture of Ar and Cl 2 (85%/15%) to remove surface layers from bulk Nb.…”

Secondary Electron Emission from Plasma Processed Accelerating Cavity Grade Niobium

“…Plasma processing was proven as an effective method for processing the surfaces in research areas of semiconductor and medical applications. Recently developed technology might lead to an efficient method of processing the accelerating cavities [2]. Although the effect of plasma on the various accelerator materials has been studied so far, further experimental research is needed to determine the SEE from the cavity surface in order to provide comprehensive data for simulations of cavity performance.…”

“…Recently, two research groups have reported their progress in the application of plasma processing techniques for accelerating cavities. The first group focused on developing the experimental setup that could possibly replace the BCP and ECP methods in surface etching of the niobium SRF cavity[2]. They reported using the ionized Ar/Cl2 gas mixture to remove significant quantities of niobium from ring type samples.…”