Neutral beam heating and current drive systems

Several Neutral Beam Injection (NBI) systems are now capable of injection of powers greater than 20 MW, some for pulse lengths in excess of 5 s. These high power NBI systems have demonstrated the flexibility and utility of neutral beam heating; coupling easily to many plasma configurations; providing heating, fuelling and current drive capability and allowing access to new, improved plasma confinement regimes. All the high power NBI systems have now achieved routinely high reliability and system availability (> or approx.80-90%). The current status of NBI systems is reviewed and successful solutions to the technological problems encountered in design of high power beams, ranging from high heat transfer elements to large area cryopumps, are discussed. The interface to tokamak operations such as the drift duct, stray field cancellation and fast interlock mechanisms are presented, emphasising the considerable progress in these areas. The upgrading of NBI systems for tritium injection on JET and TFTR is reviewed. The possible parameters for a 'Next Step' injector are presented, showing the choice of negative ion beams as an attractive option. With reference to the existing conceptual designs for ITER/NET it is shown that the main areas of uncertainty exist in the source and accelerator stack designs. The beamlines themselves should be able to take advantage of the properties of negative ions to lead to some marked simplifications. (orig.)