Recent JET results and fast wave issues in divertor plasmas

The interior of the JET tokamak has been extensively modified and a pumped divertor has been installed to study effective methods of power exhaust and impurity control. Due to its large size and proximity to reactor conditions, JET aims at providing physics support to ITER in the areas of divertor studies, energy confinement and fusion performance. The JET auxiliary equipment including heating and current drive systems permit a wide variety of operational modes including, most notably, the operation in D-T mixture and advanced tokamak operation with high βp, high boot-strap regimes with a central seed current. For the pumped divertor configuration, completely new ICRH antennas (A2) have been designed and installed to match the changed plasma profile. Also, current straps are grouped into 4-modules of 4-straps each to improve the current drive capability. This required a new ICRH plant control electronics for automatic matching and phasing and features such as an immunity to cross-talk between different antenna modules have also been incorporated. In contrast with the earlier 2-strap A1-antenna operation, the heating efficiency of the 4-strap A2-antenna appears to depend on the phasing. Another major issue in divertor plasmas is to deliver ICRH power to ELMY H-mode discharges since an ELM can produce a large excursion of loading resistance in 100 micros and the power source cannot withstand such fast variations. New techniques for providing matching under such conditions are discussed