Full Bootstrap Discharge Sustainment in Steady State in the TCV Tokamak

The Advanced Tokamak scenario, one of the modes of operation being considered for ITER, relies on the attainment of a high bootstrap current fraction. This scenario is typically characterized by an internal transport barrier (ITB). An internal feedback loop then governs the current profile, which strongly affects the confinement and thus the properties and location of the high-gradient region, where in turn the bootstrap current component is localized. The bootstrap current fraction can reach 100% only if the bootstrap current profile can be exactly and stably aligned with the high-gradient region it engenders. Recent work on the TCV tokamak has shown that such an alignment is indeed possible. We have produced discharges in which the current is entirely self-generated by the plasma in conditions of intense electron cyclotron heating (ECH, up to 2.7 MW), by employing two different methods. In one scenario, high-power, second-harmonic ECH waves are launched with no current drive component, during the initial plasma current ramp-up. A strong ITB is generated by the transient negative central magnetic shear that develops in the current penetration phase. The Ohmic flux swing is zeroed immediately after breakdown, cutting off the external plasma current source. The plasma can then evolve spontaneously towards a stationary and quiescent state, characterized by a narrow ITB with a confinement enhancement of 2.5-3 over Lmode. The discharge remains stationary over the time scale of a TCV pulse (1-2 s), which is significantly longer than a typical resistive current redistribution time (~150-300 ms) and orders of magnitude longer than the confinement time (~3-6 ms). Following a different approach, we have also succeeded in achieving a 100% bootstrap fraction by annulling the total EC-driven current in pre-existing stationary conditions. Standard stationary noninductive eITBs are first generated by off-axis co-ECCD; counter-ECCD is then added gradually until the total driven current density is nominally zero everywhere. In some cases a quasi-stationary state is indeed established. In this case the barrier remains broad with a standard confinement enhancement factor of the order of 4-5.