Optimization of fusion power density in the two-energy-component tokamak reactor

Fusion power density P_f is a critical parameter for certain fusion reactor applications such as fissile breeding. This paper considers the optimal plasma conditions for maximizing P_f in a beam-driven D-T tokamak reactor. Given T_e=T_i and fixed total plasma pressure, there is an optimal n_eτ_E for maximizing P_f, i.e. n_eτ_E = 4 × 10¹² to 2 × 10¹³ cm⁻³ · s for T_e = 3 – 15 keV and 200-keV D beams. The corresponding = (beam pressure/bulk-plasma pressure) is 0.96 to 0.70. P_fmax increases as T_e is reduced and can be an order of magnitude larger than the maximum P_f of a thermal reactor of the same beta, at any temperature. A lower practical limit to T_e may be set by requiring a minimum beam power multiplication Q_b. For the purpose of fissile breeding, the minimum Q_b ∼ 0.8, requiring T_e⪆ 4 keV if Z = 1.

The optimal operating conditions for obtaining P_fmax in a beam-driven reactor are considerably different from those for enhancing Q_b. Maximizing P_f requires restricting both T_e and n_eτ_E, maintaining a bulk plasma markedly enriched in tritium, and spoiling confinement of fusion alphas. Provided that beam penetration is satisfactory, considerable impurity content can be tolerated without seriously degrading P_fmax.