A THEORETICAL-MODEL OF H-MODE TRANSITION TRIGGERED BY CONDENSED NEUTRALS NEAR X-POINT

The effects of neutrals near the X-point of tokamaks on the L/H transi tion are examined. Four mechanisms for bipolar losses, i.e. the loss c one loss, the bulk viscosity loss, the charge-exchange (CX) loss betwe en ions and neutrals and the anomalous loss, occur simultaneously in h igh-temperature plasmas. Neutral particles near the X-point are found to trigger the H-mode transition. They cause an additional ion loss co ne loss via collisions. The lower limit of the neutral density near th e 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 transiti on because of CX loss.