JET multi-pellet injection experiments

The multiple injection of deuterium pellets into JET plasmas under various scenarios for limiter and X-point discharges with currents up to 5 MA with pure ohmic, neutral beam and RF heating has been undertaken in a collaborative effort, between JET and an USDOE team under the umbrella of the EURATOM-USDOE (US Department of Energy) Fusion Agreement on Pellet Injection using an ORNL built 3-barrel, repetitive multi-pellet launcher. The best plasma performance with pellet injection and additional heating so far has been obtained by injecting early into 3 MA, 3.1 T pulses while centrally depositing the pellet mass, with neo initially well in excess of 1020m-3. Subsequent central heating of this dense and clean core by ion cyclotron resonance heating (ICRH) with H and 3He minorities in the 10 MW range yields Teo up to 12 keV and Tio up to more than 10 keV, while neo is decreasing (within up to 1.5s) decaying to 0.6x1020m-3, suggesting an enhanced central energy confinement in limiter discharges with only modestly improved global L-mode confinement. In this plasma core electron pressures of more than 1 bar with gradients in the order of 4 bar*m-1 have been reached with the total pressure approaching ballooning stability limits. The resulting total neutron rate from D-D reactions of up to 4.5*1015s-1 so far increases strongly with RF power and can exceed that of similar non-enhanced shots by factors of 3 to 5. nD(0)*Ti(0)*τE(a) products in the range of 1 to 2*1020m-3keVs are obtained but combined power with neutral beams (up to 28 MW total) generally degrades the performance though leading to higher neutron rates of up to 7*1015s-1. (author). 10 refs, 10 figs