Optimisation of ITER operational space for long-pulse scenarios

The EPED1 model with boundary conditions from SOLPS predicts no degradation of pedestal pressure with decreasing density in ITER. Modelling of core transport with 1.5D transport models carried out with pedestal parameters predicted by EPED1+SOLPS indicate that there is a large operational space for long pulse plasma operation with high fusion gain Q ≥ 5. Reducing the plasma density to n_e∼5-6 10¹⁹m^-3 leads to an increased plasma temperature (similar pedestal pressure) which reduces the loop voltage increases the duration of the burn phase to Δt_burn ∼ 1000 s with Q ≥ 5 for I_p ≥ 13 MA at moderate normalised pressure, β_N ∼ 2 in ITER. These ITER plasmas require the same level of additional heating power as the reference Q = 10 inductive scenario at 15 MA (33 MW NBI and 17 - 20 MW EC heating and current drive power). However, unlike the 'hybrid' scenarios considered previously, these H-mode plasmas do not require specially shaped q profiles nor improved confinement in the core for the majority of the transport models considered in this study. In addition, the neutron fluence, F = 0.8 Pfus tburn for these long-pulse scenarios is larger than that of the Q = 10 inductive scenario although the margin above the L-H threshold power is lower than for the reference Q = 10 inductive operation (Fig. 4). Thus, these medium density H-mode plasma scenarios with I_p ≥ 13 MA present an attractive alternative to hybrid scenarios to achieve ITER's long pulse Q ≥ 5 and deserve further analysis and experimental demonstration in present tokamaks.