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Discharges with improved energy confinement are produced in Doublet III when the major recycling location is shifted away from the main plasma by forming an expanded boundary (XB) divertor configuration with a separatrix to limiter separation Δ > 1.5 cm. Monte Carlo calculations indicate that over a wide range in density the divertor plasma effectively limits the penetration of recycled neutrals to the main plasma. This results in large particle recycling rates at the divertor plates and a reduction of particle recycling near the main plasma. This divertor function apparently plays an important role in obtaining improved energy confinement. Particle containment, as inferred from variations in Dα emission near the main plasma, is found to decrease with n e and BT, and increases rapidly (faster than quadratic) with IP whereas energy confinement is relatively insensitive to variations in n e and BT and increases linearly with IP. This improved energy confinement with the XB divertor configuration is suggestive of particle confinement playing an important role. However, in view of their rather different parametric dependences, the correlation between τP and τE, if any, remains unclear.

Bolometric, thermocouple, Langmuir probe and infrared camera data are used to deduce the power flow in neutral beam-heated ( P input ⩽ 5 MW ), dee-shaped, limiter discharges. Power loss measurements typically account for ⩽ 60% of the total input power, with the majority either deposited on the primary limiter (20–40%) or radiated away (10–25%). Infrared camera measurements indicate that the heat flux to the ion drift side of the primary limiter is significantly greater than that to the electron drift side. Charge exchange neutral analysis, diamagnetic loop data and photodiode observations are consistent with a non-thermal flux of fast, beam produced particles to the limiter. If such an effect is indeed present, it could be partially responsible for the degradation in energy confinement observed during beam heating. Analysis of data comparing neutral beam injection from the two beamlines indicates that there is a variance in the energy cofinement time which is beamline specific; this variance may be correlated to the limiter location relative to the beamlines.

The improved energy confinement of the H-mode is invariably coupled to improved particle confinement, In other experiments, increased particle confinement resulted in impurity accumulation and severe power losses. In DIII-D, similar to the results of other experiments, H-mode confinement is associated with improved particle confinement. But in contrast to other devices, in most DIII-D H-mode plasmas there is no indication of impurity accumulation. In high current shots the impurity content of plasma during the H-mode is lower than the L-mode phase. Two factors most responsible for the low impurity content of DIII-D H-mode plasmas are the nearly flat or hollow electron density profiles and the presence of giant Edge Localized Modes (ELMs). Giant ELMs also cause large amplitude modulations in particle and power flow to the divertor target plates, resulting in large scale changes in the parameters of the the scrape-off layer plasma. Our analysis of temperature and density profiles during ELMs show that particle loss during ELMs is primarily due to perpendicular transport across the field lines.