High-density operation in tokamaks

Conclusion

In this paper we have examined various aspects regarding high-density operation in tokamaks and in particular the density limit, the plasma detachment, the MARFE formation and the fuelling efficiency. As regarding the density limit, both experimental findings and theoretical model indicate that the plasma current and the total input power are relevant in limiting the edge density that can be sustained in a tokamak discharge: radiation losses and SOL momentum and energy conservation are the underlying mechanisms. In the latest divertor experiments, operating in the detached regime, the influence of the input power seems to vanish or even disappear. Edge phenomena such as plasma detachment, occurring beyond a density threshold that can be lowered by means of impurity injection, can lead to the almost complete exhausting of the heating power by radiation which is greatly helpful for the design of the divertor plates. The compatibility of H-mode operation with this regime is still under investigation. The MARFE phenomenon, sometimes precursor of a major disruption, is now understood in terms of a radiation induced thermal instability. Finally, experiments performed in order to investigate the fuelling efficiency of the gas puffing technique have shown that at high density this technique becomes rather inefficient, thus indicating that pellet injection still remains an essential requirement to fuel the reactor plasma. The drop of the fuelling efficiency of gas-puffing at high density can be accounted for by collision phenomena taking place in the SOL.