Optimization of JET-DT and ITER Operation by Developing an Understanding of the Role of Low-Z Impurity on the H-Mode Pedestal

Full text: Impurity seeding via Ne or N injection will be mandatory in ITER Q=10 reference scenario to reduce inter-ELM divertor power load to within limits. The challenge is achieving the requirements of H₉₈(y, 2)=1, β_N=1.8, n/n_GW=0.85, δ=0.4, with a high radiative divertor. These conditions require a high pedestal temperature, leading the pedestal to play a key role in this challenging integration. Unravelling the mechanism that, in the absence of C in the plasma, leads to a decrease in the pedestal temperature is critical in predicting the pedestal pressure in ITER. It is important to learn how to use the extrinsic impurity to optimize the pedestal temperature in high radiative scenarios. This paper aims at. 1) reviewing our understanding of the effect of C,N and Ne-seeding on the pedestal pressure and temperature; 2) assessing whether the peeling ballooning stability limits the pedestal pressure; and 3) determining which instabilities are causing heat and particle transport. In JET-ILW the limitation on the pedestal temperature is alleviated with the injection of N or C in low and high-β_N plasmas. Seeding Ne can result in opposite behaviour on the pedestal density depending on the collisionality v_e* e and β_N, but in all cases seeding Ne does not lead to a temperature increase, unlike N or C. A detailed analysis of the differences in the electron and ion pedestal profiles in high-β_N plasmas indicates that the difference between C and Ne seeding can be down to the value of collisionality v_e* e , but also the E x B shear considering the difference in ∇T_i,αmax and ∇Ω_tor,αmax at the position of the maximum normalized pressure gradient. Similarly, seeding C₂D₄ in the low-β_N plasmas increases ∇T_i,αmax and ∇Ω_tor,αmax. Detailed analysis with the GENE code will clarify which instability is at the origin of the difference in the pedestal temperature. The peeling ballooning stability has been assessed with MINERVA-DI code. The plasmas considered have the operational points (OP) of the high and low-β_N plasmas within 20% of the stability boundary. (author)