The plasma system of RFX

Abstract

The “plasma system” of RFX includes the first wall, the vacuum vessel, the stabilizing shell, the vacuum and gas injection system, the remote handling system and the relevant auxiliary plants (cooling, baking, glow discharge cleaning, etc.). Since no limiters are provided, the full energy and particle flux from the plasma must be absorbed by the first wall. Taking into account the significant power losses from the plasma in nominal conditions, the high peaking factors due to “dynamo” effects, field errors and plasma shift, and possible sudden losses of configuration, the first wall was designed to be able to withstand high thermal loads. Also the need to use a low Z plasma-facing material led to the choice of pure, microcrystalline graphite. The wall is subdivided into 2016 tiles, individually clamped to the vacuum vessel by means of a special bayonet system suitable for remote handling. The remote handling system, which was designed to facilitate the replacement and inspection of all 2016 graphite tiles, comprises an arm with five degrees of freedom, two grippers for different tasks, two television cameras, and a control system for automatic or teleoperated motions. The vacuum vessel is a stiff sandwich structure with 72 solid poloidal rings bearing the first wall tile supports. It is cooled by CO 2 flowing through the interspace; baking up to 350°C is also provided by means of heating cables attached to the outer surface. The vessel is closely surrounded by a shell, giving passive stabilization against magnetohydrodynamic modes; the shell is made of aluminium alloy, 65 mm thick, and, to allow fast field penetration, it is electrically subdivided into four sections, by means of epoxy-glass layers. The RFX vacuum system is a combination of 12 cryopumps and 12 turbomolecular pumps, giving a total pumping speed at the vessel of 4500 1 s −1 for nitrogen. Because of the short plasma pulse duration, the gas injection system must produce high gas throughputs (up to 700 Pa m 3 s −1) with very short response times (some milliseconds). The glow discharge cleaning system utilizes four anodes toroidally equispaced and located at the centre of the vessel minor cross-section, is r.f. assisted and allows wall conditioning in hydrogen, helium or methane. In this paper we present the basic concepts underlying the main design choices and then, for the main components and plants, we describe the significant design features, as well as the methods and results of the structural, thermal and electromagnetic calculations. We also deal with preliminary tests on prototypes and with the most important manufacturing technologies, production cycles, and assembly procedures. Finally, the preliminary design of the control system for the plasma main electromagnetic parameters is presented.