Development of RF based capacitively coupled plasma system for deposition of tungsten nano layers on graphite

In fusion grade machines, the issues of high heat load handling capabilities, low fuel retention properties and low sputtering yields pose stringent demands on the choice of materials of plasma facing components. Choice of suitable material still remains a challenge and open topic of research. Carbon fiber composites (CFC), Beryllium (Be), and Tungsten (W) are being considered as first runners for the first wall components of future fusion machines. Tungsten is considered to be one of the suitable materials for the job because of its superior properties than carbon like low physical sputtering yield and high sputter energy threshold, high melting point, fairly high re-crystallization temperature, low fuel (tritium) retention capabilities, low chemical sputtering with hydrogen and its isotopes and most importantly the reparability with various plasma techniques both ex-situ and in-situ. However, tungsten is a costly material, it is also very heavy and difficult to machine. Hence, fabrication of composite first wall of any fusion grade tokamak with pure tungsten is not a cost effective measure. This obstacle can be overcome by using of tungsten coated graphite tiles, which are lighter, cost effective and easier to machine. The replacement of such components will cost enormous machine operational time and may not be economical. The remedy is to repair the affected part in situ without taking it out of the machine. Again the in-situ coating techniques can be used to repair the damaged tiles without removing them from the machine. Tungsten coatings on various substrates have been successfully attempted in various laboratories worldwide using different techniques. In this paper we describe the designing and development of the coating reactor in detail for production of tungsten coated graphite tiles [1]. We also report initial characterization results of the tungsten coatings produced on a graphite substrate.