Confinement of Fast Tritium Beam Ions in Current Hole Plasmas in JET
In Current-Hole (CH) JET plasmas the toroidal current density falls to near-zero values over a significant central region, typically 1/3 of the minor radius. Such plasmas exhibit a transport barrier at the border of the current hole. On JET these conditions are routinely achieved by the application of Lower Hybrid Current Drive in the early breakdown phase, but it has also been demonstrated on JET and other machines that current holes can be formed as a result of bootstrap current effects. Flat profiles of high temperature within the current hole make the regime interesting for tokamak physics in general and for quasi-steady state operation of ITER in particular. Current holes can be predicted to have both beneficial and adverse effects. The presence of a transport barrier offers the prospect of improved plasma energy confinement, but this has to be set against the poor confinement of fast ions inside a current hole due to the very low poloidal field in the plasma core. This can significantly affect a fast ion distribution due to a modification in the fast ion orbit topology. In a reactor, alpha particles born within the current hole have orbits with larger radial excursion than would occur in the current hole's absence, leading to enhanced collisional losses and overall reduction in alpha heating. The distribution in both velocity and spatial coordinates of tritons injected by the Neutral Beam Injection (NBI) heating system is also significantly affected by the presence of a current hole. To study these issues and to benchmark our simulation codes in the current hole regime, the injection of tritium beam ions into CH plasmas was carried out on JET.