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(OV2/3) Ergodic Divertor Experiments on the Route to Steady-state Operation of Tore Supra

P. Ghendrih¹⁾, TORE SUPRA Team
 
¹⁾ Association Euratom-CEA sur la Fusion, CEA Cadarache, 13108 St Paul lez Durance, France

Abstract.  Ergodic Divertor operation on Tore Supra is characterised 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, $\Delta$r $\sim$ 0.16m. 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 a temperature comparable to the energy involved in the atomic processes, (15 to 20 eV). For standard discharges with 5 MW total power and ICRH heating, the low parallel energy flux $\sim$ 10 MW m^{- 2} is reduced to $\sim$ 3MWm^{- 2} with nitrogen injection. This is achieved at a modest cost in core dilution, $\Delta$Z_eff $\sim$ 0.3. Despite the large volume of open field lines ( $\sim$ 36%) the Ergodic Divertor does not reduce the possible current in the discharge since stable discharges are achieved with q_sep $\sim$ 2. It is shown that the reorganisation of the current profile in conjunction with a transport barrier in the electron temperature on the separatrix stabilises 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 modest increase in confinement ( $\sim$ 40%). Despite the lack of core fuelling on Tore Supra, high densities during ICRH pulses can be achieved with Greenwald fractions f_G $\sim$ 1. Compatibility with both ICRH and LH is demonstrated. In particular long pulse operation with flat top in excess of 20 s are achieved with LHCD.

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