Role of particle recycling in beam heated expanded boundary divertor discharges in D-III

Discharges with improved energy confinement are produced in Doublet III when the major recycling locations 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 Dsub(α) emission near the main plasma, is found to decrease with anti nsub(e) and Bsub(T), and increases rapidly (faster than quadratic) with Isub(p) whereas energy confinement is relatively insensitive to variations in anti nsub(e) and Bsub(T) and increases linearly with Isub(p). 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 tausub(p) and tausub(E), if any, remains nuclear. (orig.)