HIGH-DENSITY OPERATION ON FRASCATI TOKAMAK UPGRADE

Plasma behaviour in the high density regime has been investigated on t he Frascati Tokamak Upgrade (FTU). The items particularly addressed ar e density limit, MARFE characteristics and fuelling efficiency. With g as puffing, a maximum line average density of 3.2 x 10(20) m(-3) has b een obtained in the ohmic regime at q = 5.1, corresponding to 1.7 time s the Greenwald limit; while with pellet injection, a value of 3.7 x 1 0(20) m(-3) has been reached at q = 3.5. The density limit appears to be connected with the impurity content and edge parameters, so the bes t results are obtained with very clean plasmas and peaked electron den sity profiles. The MARFE phenomenon always appears beyond a critical d ensity that depends on the total input power and the effective charge: emissivities in the range of 2 to 12 MW/m(3) have been measured in th is highly radiating region. The fuelling efficiency, starting from a v alue of about 50% at low density, progressively deteriorates, falling to 10% near the density limit: this behaviour is interpreted as being due to a decrease of the scrape-off layer transparency to incoming neu trals.