Electrodeposition of copper on stainless steel radio frequency cavity of Indus-1

Radio frequency cavity of Indus-1 is a normal conducting cavity having 840 mm internal diameter and 600 mm length. The cavity structure is reentrant type with two capacitor disks of diameter 592 mm each. The preferred material of construction is copper, while stainless steel(304 L) was chosen considering the in house facilities available for fabrication and electro deposition of copper. In storage ring, the operating frequency of the RF system is 31.613 MHz and the skin depth for copper at this frequency is 11 microns. To ensure that most of the current flows in copper, the stainless steel cavity should have a copper coating which is five to eight times thicker than the skin depth. The surface roughness should be better than 1.6 microns to avoid radio frequency break down and for reducing power loss. Stainless steel forms a thin invisible oxide film by reacting with the ambient medium. The resistance to corrosion is due to this ever present oxide film, which acts as a barrier between the metal and its environment. This protective film makes stainless steel a difficult to plate metal and an adherent electro deposit can be obtained after removing this film by employing a series of chemical cleaning methods. Nickel is used as an intermediate strike coating to improve adhesion of copper over stainless steel. In this paper, the selection of suitable cleaning methods, nickel and copper plating solutions to produce an adherent, smooth and uniform deposit will be discussed. (author)