New configuration for efficient and durable copper coating on the outer surface of a tube

A well-adhered copper coating on stainless steel power coupler parts is required in superconducting radio frequency (SRF) accelerators. Radio frequency power coupler parts are complex, tubelike stainless steel structures, which require copper coating on their outer and inner surfaces. Conventional copper electroplating sometimes produces films with inadequate adhesion strength for SRF applications. Electroplating also requires a thin nickel strike layer under the copper coating, whose magnetic properties can be detrimental to SRF applications. Coaxial energetic deposition (CED) and sputtering methods have demonstrated efficient conformal coating on the inner surfaces of tubes but coating the outer surface of a tube is challenging because these coating methods are line of sight. When the substrate is off axis and the plasma source is on axis, only a small section of the substrate’s outer surface is exposed to the source cathode. The conventional approach is to rotate the tube to achieve uniformity across the outer surface. This method results in poor film thickness uniformity and wastes most of the source plasma. Alameda Applied Sciences Corporation (AASC) has developed a novel configuration called hollow external cathode CED (HEC-CED) to overcome these issues. HEC-CED produces a film with uniform thickness and efficiently uses all eroded source material. The Cu film deposited on the outside of a stainless steel tube using the new HEC-CED configuration survived a high pressure water rinse adhesion test. HEC-CED can be used to coat the outside of any cylindrical structure.

Figure 1

Concept of the HEC-CED coating configuration.

Figure 2

Schematic of a section of the ${\mathrm{v}}_z$ measurement arrangement in the HEC-CED configuration.

Figure 3

Typical data from an arc pulse: (a) the data from the biased rod, and (b) from the biased loop. The inset in (a) is a two-point average of the rod current data around the peak at 845 ms with peak fitting.

Figure 4

Coated mock WIC_2: (a) next to a TTF WIC, before the HPR test, and (b) after the test.

Figure 5

Illustration of the concept for using a shaped cathode in HEC-CED to achieve a uniform film thickness on an arbitrarily shaped substrate.

Figure 6

(a) The diameter profiles of a straight cathode (blue), a shaped cathode (red) and the TTF WIC part (black). (b) Thickness variation (normalized) for a straight cathode (blue) vs a variable cathode (red).

Figure 7

(a) Arrangement for coating the outer surface of a tube with CED, (b) coating the inside surface of a tube with CED, (c) coating the outer surface of a tube with CED, and (d) coating of the outer surface of a tube with HEC-CED.