Characteristics of pellet and neutral-beam injected single null divertor discharges of the JFT-2M tokamak

When a single pellet is injected before the H-mode (high confinement mode) transition of neutral-beam heated single null divertor discharges, three different types of discharges are classified by the increment of line-averaged electron density (Δn-barep) at the pellet injection time. One of these three types of the discharges is obtained only in the transient phase, however, the very high confinement state of the neutral-beam heated plasma is discovered. The characteristics of this type of the discharge are as follows; when Δn-barep is greater than about 4 x 1019 m-3, the energy confinement time exceeds that of ohmically heated discharges and volume averaged toroidal beta, <βt>, reaches close to the β-limit by only the neutral-beam power of 0.69 MW (<βt> ∼ 1.6 % at IP = 0.22 MA, a = 0.25 m, BT = 1.2 T and qψ ∼ 2.5). (author)