A regime of improved energy confinement in beam-heated expanded-boundary discharges in Doublet III

Neutral-beam-heated expanded-boundary (XB) divertor discharges have been obtained in Doublet III with high heating efficiency for wide ranges of plasma parameters (Ip: 300–800 kA; BT: 8–24kG; e :(2–10) × 1013cm−3; Pb < 4.5 MW, βp ≤ 1.6, βt ≤ 2.8%). The improved heating efficiency is well correlated with a configurational change from limiter discharges to XB discharges. The beam-heated, fully diverted expanded-boundary discharges with a limiter/separatrix distance greater than 1.5 cm exhibit an improvement of up to a factor of two in energy confinement time. The τE increases approximately linearly with Ip, but is insensitive to variations of a factor of two in e and BT. Over the inner two-thirds of the plasma radius (r/a ≤ 0.7), the shape of the Te profile for XB discharges is similar to that for limiter discharges. Hence, the improvement of the global energy confinement is consistent with a reduction of thermal conductivity over most of the plasma radius. With 2 MW of neutral-beam injection into a high-current (Ip = 750 kA) XB discharge, energy confinement times τE ≈ 115 ms and eτE ≈ 1 × 1013 cm−3 · s−1 have been obtained. At high beam power (Pb > 3 MW), a mild deterioration of the energy confinement time has been observed.