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

Ergodic divertor operation on Tore Supra is characterized by good performan ce in terms of divertor physics. Control of particle recirculation and impu rity screening are related to the symmetry, both poloidally and toroidally, of the shell of open field lines and to its radial extent, Deltar approxim ate to 0.16 m. Feedback control of the divertor plasma temperature has led to controlled radiative divertor experiments. In particular, good performan ce is obtained when the plasma is controlled to be at a temperature compara ble 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 approximate to3 MW m(-2) with nitrogen inje ction. This is achieved at a modest cost in core dilution DeltaZ(eff) appro ximate to 0.3. Despite the large volume of open field lines (approximate to 36%), the ergodic divertor does not reduce the possible current in the dis charge since stable discharges are achieved with q(sep) approximate to 2. I t is shown that the reorganization of the current profile in conjunction wi th a transport barrier in the electron temperature on the separatrix stabil izes the (2,1) tearing mode. Confinement follows the standard L mode confin ement. In a few cases at high density and with no gas injection (wall fuell ed discharges), 'RI-like' modes are reported with a modest increase in conf inement (approximate to 40%). Despite the lack of core fuelling on Tore Sup ra, high densities during ICRH pulses can be achieved with Greenwald fracti ons f(G) approximate to 1. Compatibility with both ICRH and LH is demonstra ted. In particular, long pulse operation with a flat-top in excess of 20 s is achieved with LHCD.