Plasma Characteristics and Gas Transport in the Single-Null Poloidal Divertor of the International Tokamak Reactor

Plasma behavior in the scrapeoff and divertor regions of the single-null configuration of the International Tokamak Reactor (INTOR) has been predicted by means of a one-dimensional model of transport through the plasma sheath at the divertor target. Electron capture during ion/surface collisions is the principal mechanism for the production of neutral gas, which recycles to the target within the divertor chamber. This recycling is analyzed using a model for neutral particle transport through a divertor plasma channel of simple geometry and uniform density that overlays the target; the exhaust of those atoms that can escape through the plasma and enter the pumped region of the chamber is assessed by means of a gas transport model that embraces the characteristics of both chamber and pumping ducts. Estimates are made of the power dissipated by atomic line radiation and by transport of fast atoms to the walls of the chamber; sputtering of both target and walls is assessed. Data are evaluated for the specific case when INTOR is operated under “standard conditions, ” i.e., transport by charged particles of 75 MW to the throats of the divertor through a scrapeoff plasma of average density 5 × 10¹⁹ m⁻³. Under these conditions, the temperature of the scrapeoff plasma is predicted to be ∼90 e V, the flow of ions to each divertor target is ∼1.3 × 10²⁴ s⁻¹, the plasma density adjacent to the target sheath ∼9 × 10¹⁹ m⁻³, and the corresponding plasma temperature ∼25 e V. Fusion reactors in INTOR produce 2 × 10²⁰ alpha particle ⋅ ⁻¹ and the exhaust rate of helium gas must be adequate to maintain a low concentration of helium ions in the plasma, i.e., (n_He/n_D-T) ≈0.05. The model predicts that this can be achieved with a gas exhaust rate of ∼10⁵ l⋅s⁻¹ (referred to helium atoms and deuterium-tritium molecules at 300 K) and the corresponding burnup fraction is −25%. The operational lifetime of the divertor specified for INTOR will probably be limited by erosion of the stainless steel walls of the chamber, which is estimated to occur at a rate of −1 cm⋅yr⁻¹ of cyclic operation.