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The reason has to be known to extrapolate to deuterium-tritium reactors.","posed_since":"","precise":"Local gyro-Bohm scaling predicts energy confinement time $\\tau_E$ proportional to A^-0.5 ($A=\\text{ion mass number}$), while experiments find tau_E increasing with A, more strongly in H-mode. Identify the mechanism (E x B shear, collisional electron dynamics, pedestal dependence, zonal flows) and predict the D-T to D confinement ratio for ITER to within 15 percent.","problem_ref":null,"references":"","settled_by":"Multiscale gyrokinetic and pedestal modeling that reproduces measured H, D and D-T confinement on JET and another device.","status_note":"JET D-T campaigns (2021-2023) confirmed better confinement in D-T than D; no single mechanism is accepted.","title":"Why heavier hydrogen isotopes confine better than theory 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