JET Hybrid Scenarios with Improved Core Confinement
The data from the Hybrid scenario experiments has been incorporated into a large database that contains more than 40 plasma and scenario parameters. Analysis of the database has revealed that there is a spectrum of temperature gradient scale lengths that exceed the critical level considered at JET to indicate the presence of an ITB. In the best example (BT = 2.5T, Ip = 2.1MA, PNBI = 13.4MW, PICRH = 2.5MW, 1 < q(0) < 1.5) it was R/(LTi) ~14 and R/(LTe) ~11 in a wide region (Dr ~ 0.2m) of the plasma core (r/a ~0.4). The central ion and electron temperatures were Ti(0) ~17keV and Te(0) ~ 8keV. In the initial phase of the heating pulse the edge temperatures had been quite low (Ti ~ Te ~2keV), but suddenly they increased to ~ 4keV while, at the same time, the edge density decreased providing a more or less constant edge pressure. The discharge was essentially without ELMs, although the additional heating power was well above the usual Hmode power threshold. The global performance was comparable to, or slightly better than that of an equivalent standard Hybrid discharge with type I ELMs (bN ~ 2, H89 ~ 2.2). The plasma energy associated with the pedestal was Wped ~ 37% in this discharge, whilst the part within the steep gradient region was Wcore ~ 23%. Despite the relatively low density the toroidal rotation was only about half that of a typical JET ITB plasma. Nevertheless, the E¥B shearing rate is calculated to be very large. An analysis using the gyrokinetic code, KINEZERO shows that, in the improved confinement region, ITG-TEM wavelength instabilities should be stable. An important factor in the plasmas with improved core confinement appears to be the density, which was higher in the case of similar Hybrid plasmas without core confinement improvement. Transport analysis of discharges with and without core confinement improvement will be presented and compared.