A power step-down approach leads to extended, high performance operation of deuterium plasmas in the hot ion edge localized mode (ELM)-free H mode of the JET tokamak. ELM-free discharges in JET, when heated steadily at the maximum power available, are usually transient and terminate with a variety of MHD phenomena. With power stepdown, instabilities are delayed or avoided, and nearly constant plasma conditions at up to 10 MJ of stored energy are maintained by 10 MW of heating power for up to 1 s, about an energy confinement time, at 3.5 MA and 3.4 T. No large transient corrections are required for confinement calculations in this hot ion regime, and the energy confinement time scaling is found to be similar to global ELM-free H mode confinement scaling. The confinement times can increase after the transition to quasi-steady conditions, to some 10-25% above the scaling law predictions. Code simulations of the experimentally observed neutron rate and the lack of a major discontinuity in this rate before and after beam power stepdown confirm that neutron production is predominantly (>60%) from thermal fusion. The scaling of fusion rates from deuterium plasmas to deuterium-tritium (DT) mixtures yields a fusion Q approaching 1. After power stepdown, the edge pressure gradient stops increasing and helps delay the onset of instabilities. Giant ELMs still occur, associated with "outer mode" activity and continuously rising plasma density. The plasma remains in an ELM-free H mode even when the heating power after power stepdown is lower than the L-H transition power. Extended operation optimization experiments indicate that stored energy and neutron production are maximized and the density rise minimized by preferentially injecting high energy 140 keV beams after power stepdown