ITER Ramp Up and Ramp Down Scenarios Studies in Helium and Deuterium Plasmas in JET

Recently, attention is given to documenting the current ramp up and current ramp down phase of tokamak discharges in preparation for ITER. Until 2008, the experimental data from JET were been obtained in deuterium. The latest JET experiments in helium reproduce the experimental conditions of previous deuterium experiments at 2.66MA and 2.36T. Helium neutral beam injection is used together with argon frosting on the divertor cryo-pumps to provide effective pumping of helium. Results show that flux consumption for helium discharges during plasma initiation is higher compared to deuterium. For the current rise phase, matched ohmic discharges show little difference in the plasma inductance values (li(3)) at the end of the current rise. The data in helium have been extended to plasma densities ~<ne>/nGW~0.5, producing li(3)~0.92 at the end of the current rise, surprisingly lower than ohmic discharges at ~<ne>/nGW~0.2 and li(3)~0.98. In helium, a current rise phase in Hmode can produce li(3)~0.75, however, in heated L-mode discharges li(3) is systematically higher compared to the deuterium references cases. TRANSP analyses of these L-mode cases shows that although the total heating power is comparable, the electron heating in JET is reduced in the centre for helium leading to lower temperatures and higher values for li(3). For the current ramp down it is important to maintain a divertor configuration to the lowest possible plasma current. Studies without additional heating (ohmic), show that the flux consumption and li(3) excursion can be controlled during the ramp down using a (strong) reduction in plasma elongation, although at a slow current ramp down rate. When additional heating is available, it is important to stay in H-mode during the current ramp down phase allowing operation at li(3)<1.3. Discharges in L-mode still observe an increase of li(3) comparable to ohmic reference cases. Overall, deuterium and helium discharges have similar plasma inductance evolution and flux consumption during ramp down.