The neutral-ablation model describing ablation of solid hydrogen fuel pellets in thermo-nuclear plasmas has been extended to determine the species composition of the ablatant consistent with the fluid profiles, and has been used to assess pellet shielding mechanisms. Aside from neutral-ablation shielding, none of the shielding mechanisms studied is very effective. Dissociation of hydrogen molecules extends pellet lifetimes by 10-20%. Ionization reduces the temperatures in the ablation by about a factor of three, but extends pellet lifetimes by only a few per cent. Excitation of atoms is not important because the fraction of excited atoms does not exceed a few per cent anywhere in the ablation. Bremsstrahlung and recombination radiation are negligible energy losses. Where emitted abundantly, line radiation is trapped within the optically thick ablatant. Incident electron heat is reduced by less than 10% in overcoming the electric potential of the pellet. Magnetic shielding is treated elsewhere.

Observation of the intensity of the recycling particle flux at the main plasma edge for various limiter and divertor discharges indicates that the gross energy confinement of beam-heated discharges is closely related to the intensity of the edge particle flux. In limiter discharges, the global particle confinement time and the energy confinement time τE show many similarities: 1) linear Ip dependence at Ip < 600 kA, 2) no BT dependence, and 3) deterioration against injection power. Improvement of τE by increasing Ip, for example, is associated with high temperatures at the plasma edge region accompanied by reduced particle recycling. – Divertor discharges with low particle recycling around the main plasma show better energy confinement than limiter discharges at high plasma densities. The improvement of τE is primarily originated in the reduction of heat transport at the main plasma edge region, which is associated with the reduction of recycling particle flux at the main plasma edge. Under certain operation condition, for example, excessive cold-gas puffing, the discharge shows relatively high scrape-off plasma density and strong particle recycling between the main plasma and the limiter. The energy confinement time of these discharges degrades somewhat or reduces completely to that of the limiter discharge. – In low-recycling divertor discharges, the central electron and ion temperature is proportional to the injection power, and the plasma stored energy is proportional to n̄ePabs (scales as INTOR scaling). With ≈ 4 MW beam injection, high-temperature and high-density plasmas were obtained (stored energy up to 280 kJ, Te(0) ≈ Ti(0) ≈ 2.5–3.0 keV at n̄e ≈ (6–7) × 1013 cm−3, τE* ≈ 70 ms).

No abstract

Using a neutral-beam injection power of 3.4 M W, volume-averaged toroidal betas of up to ⟨βT⟩ = 4.5% have been obtained in low-toroidal-field, low-qψ, vertically elongated discharges in the Doublet III tokamak. This level of ⟨βT⟩ is above the minimum level required for a tokamak reactor, thus demonstrating that reactor level values of ⟨βT⟩ are possible in a tokamak device. The observed enhancement of ⟨βT⟩ with vertical elongation lends confidence in the design of future devices which rely on vertical elongation.

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; n̄e :(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 n̄e 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 n̄eτE ≈ 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.