Ergodic divertor experiments on the route to steady state operation of Tore Supra

Ergodic divertor operation on Tore Supra is characterized by good performance in terms of divertor physics. Control of particle recirculation and impurity screening are related to the symmetry, both poloidally and toroidally, of the shell of open field lines and to its radial extent, Δr ≈ 0.16 m. Feedback control of the divertor plasma temperature has led to controlled radiative divertor experiments. In particular, good performance is obtained when the plasma is controlled to be at a temperature comparable to the energy involved in the atomic processes (15-20 eV). For standard discharges with 5 MW total power and ICRH heating, the low parallel energy flux ≈ 10 MW m-2 is reduced to ≈ 3 MW m-2 with nitrogen injection. This is achieved at a modest cost in core dilution, ΔZeff ≈ 0.3. Despite the large volume of open field lines ( ≈ 36%), the ergodic divertor does not reduce the possible current in the discharge since stable discharges are achieved with qsep ≈ 2. It is shown that the reorganization of the current profile in conjunction with a transport barrier in the electron temperature on the separatrix stabilizes the (2,1) tearing mode. Confinement follows the standard L mode confinement. In a few cases at high density and with no gas injection (wall fuelled discharges), `RI-like' modes are reported with a modest increase in confinement ( ≈ 40%). Despite the lack of core fuelling on Tore Supra, high densities during ICRH pulses can be achieved with Greenwald fractions fG ≈ 1. Compatibility with both ICRH and LH is demonstrated. In particular, long pulse operation with a flat-top in excess of 20 s is achieved with LHCD.