At JET the reference confinement scenario, used for extrapolation to a burning fusion plasma, is based on the so-called H-mode, which exhibits a transport barrier at the plasma edge. The H-mode (for "High confinement") is an enhanced confinement mode obtained above a power threshold (but not still as good as in ohmic operation) in contrast to the confinement mode obtained below the power threshold, called L-mode (for "Low confinement"). It enables improvement of confinement times by a factor of nearly 2 in comparison with L-mode. The discovery of this mode of enhanced confinement, on the ASDEX machine in the eighties, was crucial to thermonuclear fusion. These Confinement modes with internal transport barriers (ITB), also referred to as advanced tokamak scenarios, are envisaged to be the key to steady state operating regime of ITER. However, in these regimes transport barriers bring the plasma at the boundaries of the stability limits and edge phenomena challenge the technical feasibility of plasma facing materials that must ensure resistance to local heat, particle loads and neutron damage.
In Ignitor, a high density L-regime has been chosen as the reference for the most advanced regime of operation, since it ensures a relatively high value of the confinement parameter, n0τE, and high degrees of plasma purity. In particular, the absence of particle transport barriers prevents the accumulation of ash and impurities in the plasma core.
The recently investigated I-regime also represents a promising alternative to the high-density L-regime, as it would ensure relatively high confinement times as well as the ejection of impurities out of the main body of the plasma column. However, it requires further studies in order to understand under which conditions it can be maintained for extended times .
1) B. Coppi et al, Overview Paper OV/P-02, Proceedings of the 24 th IAEA Fusion Energy Conference, San Diego, US, 8-13/10/2012