High-density divertor operation in JET

Operating the JET Mark I divertor at high density and with high volume losses is shown to have many beneficial effects on divertor performance. On the other hand, combining high-density operation with high-confinement (H-mode) operation is difficult. In particular, the density at which the plasma naturally operates in ELMy H-mode discharges is difficult to vary. A scaling for how this natural density depends on toroidal field, plasma current, and input power suggests that high density may be achievable in reactors but requires a dedicated multi-machine database, in particular to verify size scalings. Experiments with and without active pumping in the divertor show that the confinement varies with the neutral pressure in the main chamber rather than the fuelling rate per se, making operation with a closed divertor attractive. Such a divertor, Mark IIA, has recently been installed at JET and is now operational. First results are presented demonstrating that the operating regimes relevant to JET's programmes have been re-established.