STUDY OF RECOMBINING GAS TARGETS

JET high-density discharges, when approaching detachment, typically sh ow a gradual drop of the ion saturation current J(sat) (rollover), an increase of the D-alpha signal and an increase of the divertor neutral density as the main plasma density is ramped up, even in cases with c onstant net input power. This has been found difficult to explain by i on-neutral collisions alone. This problem has been studied with a view to the potential role of volume recombination using the B2-Eirene scr ape-off layer simulation code package. The main conclusion is that vol ume recombination indeed plays a role in the parameter range under con sideration and is essential to explain the experimental findings.