Possibility of achieving Q > 1 by tritium pellet injection into hot ion mode deuterium plasmas in a large tokamak

Abstract

In order to achieve a ratio of total fusion output to injected power (Q) of more than one, with minimum tritium consumption in reacting plasma experiments, tritium pellet injections into deuterium plasmas heated by neutral deuterium beams with E beam > 100 keV are promising. In the computation presented, a tokamak transport code is combined with a pellet ablation code based on a neutral shielding model including fast ion effects in the ablation process. Q > 1 is realized during half a second by injecting a pellet with a radius of 2 mm and a velocity of 5 km • s" 1 into a plasma of Tj(O) = 25-30 keV, T e (0) = 9.5 keV and n e (0) = 8.5 X 10 13 cm" 3 , i.e. a 'hot ion mode' plasma. The tokamak parameters are: a (minor radius) = 0.9 m, R o (major radius) = 3.0 m, B t (toroidal magnetic field) = 4.5 T, and I p = 2.7 MA. The anomalous electron transport is assumed to follow Alcator scaling and the ion transport is assumed to be faster than neoclassical. The total neutral beam power is 17 MW. The total energy confinement time is approximately 0.5 s and the average electron density is around 6.5 X 10 13 cm" 3 during Q > 1. For larger pellet radii, smaller injection velocities appear feasible.