The effects of neutrals near the X-point of tokamaks on the L/H transition are examined. Four mechanisms for bipolar losses, i.e. the loss cone loss, the bulk viscosity loss, the charge-exchange (CX) loss between ions and neutrals and the anomalous loss, occur simultaneously in high-temperature plasmas. Neutral particles near the X-point are found to trigger the H-mode transition. They cause an additional ion loss cone loss via collisions. The lower limit of the neutral density near the X-point for the onset of the H-mode transition is estimated. Neutral particles at the main plasma are shown to prevent the H-mode transition because of CX loss.
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