First DIII-D-WEST hybrid scenario similarity experiments for ITER-relevant long-pulse operation

For the first time, similarity experiments between DIII-D and WEST¹⁴14WEST, stands for ‘Tungsten (chemical symbol ‘W’) Environment in Steady-state Tokamak were performed in the ITER ‘hybrid-like’ regime during dedicated campaigns in April and May 2025. The matched parameters include elongation, triangularity, ion ∇B drift direction toward the X-point, q-profile, and core normalized physics quantities in terms of normalized pressure, normalized gyroradius, electron collisionality, ratio of ion to electron temperature, T_i/T_e. Core transport physics is explored with different aspect ratio (R/a) values (typically 3 at DIII-D and 5 on WEST). DIII-D explored high-beta conditions (electromagnetic effect) with low torque injection (∼0 ± 0.5 N·m) using high heating power (up to 6 MW NBI and 2 MW electron cyclotron resonance heating powers), while scanning the heating mix (ion vs electron), beta, T_i/T_e, core radiation via controlled tungsten injection using the laser blow-off system. WEST extended operation toward long-duration pulses using its actively cooled tungsten divertor, achieving dominated electron heating regimes with reduced tungsten contamination. Boron impurity injection were scanned on WEST to control edge conditions and core performance. It is found that core confinement improves—manifested by higher electron temperature, total energy content, neutron rate, and ion temperature—under conditions of low separatrix density, consistent with previous observations Bourdelle et al (2023 Nucl. Fusion 63 056021). Conditions for H-mode access and for ion heating in electron-dominated regimes in both WEST and DIII-D are discussed and compared. The ratio of the thermal energy confinement time (τ_E) to the volume-averaged electron–ion collisional heat exchange time (τ_e–i) is a key parameter to enhance ion heating and potentially facilitate H-mode access in electron-heated regimes. These first-of-a-kind coordinated DIII-D and WEST experiments provide a unique multi-machine dataset to validate predictive models and to optimize ITER hybrid-scenario performance under diverse core and edge conditions.