Divertor and edge plasma characteristics of beam-heated D-111 discharges

In high-power beam-heated divertor discharges with Psub(IN) approx.= 7.5 MW, the central radiative power density from nickel can be suppressed to less than 10% of the heating power density by gas puffing. The radiative loss in the divertor increased as the main electron density was increased. 24% of the absorbed power was observed to be radiated in the divertor in high-density, H-mode discharges. During a density scan, the radiative power from the main plasma remained constant. With the increase of the main plasma density, the electron temperature at the divertor plate decreased to 8 eV (4.5 MW absorbed power). The electron density at the divertor plate was 3x1014cm-3. Measurements of electrostatic potential in the divertor suggest that there is a possibility of a significant difference in the plasma potential between a good and a poor heating mode. Divertor configuration experiments with a small mirror ratio (approx. 1.2) suggest that a magnetic mirror might not be important in creating good-confinement discharges. (author)