The dependence of plasma energy confinement on minor radius, density and plasma current is described for Ohmically heated near-circular plasmas in Doublet III. A wide range of parameters is used for the study of scaling laws; the plasma minor radius defined by the flux surface in contact with limiter is varied by a factor of 2 (a = 44, 32 and 23 cm) , the line average plasma density, n̄e, is varied by a factor of 20 from 0.5 to 10 × 1013 cm−3 (n̄e R0/BT = 0.3 to 6 × 1014 cm−2·kG−1) and the plasma current, I, is varied by a factor of 6 from 120 to 718 kA. The range of the limiter safety factor, qL, is from 2 to 12. – For plasmas with a = 23 and 32 cm, the scaling law at low n̄e for the gross electron energy confinement time can be written as (s, cm) where qc = 2πa2BT/μ0IR0. For the 44-cm plasmas, is about 1.8 times less than predicted by this scaling, possibly owing to the change in limiter configuration and small plasma-wall separation and/or the aspect ratio change. At high n̄e, saturates and in many cases decreases with n̄e but increases with I in a classical-like manner. The dependence of on a is considerably weakened. The confinement behaviour can be explained by taking an ion thermal conductivity 2 to 7 times that given by Hinton-Hazeltine's neoclassical theory with a lumped-Zeff impurity model. Within this range the enhancement factor increases with a or a/R0. The electron thermal conductivity evaluated at half-temperature radius where most of the thermal insulation occurs sharply increases with average current density within that radius, but does not depend on a within the uncertainties of the measurements.

Large-amplitude sawtooth oscillations have been observed in the soft-X-ray emission, central electron temperature, neutron production rate, radiated power, Dα emission, fast-neutral flux and one-turn voltage during beam-heating experiments in the Doublet III tokamak. The necessary conditions for the appearance of giant sawteeth seem to be operation at low qa and high βT. Many features of the giant sawteeth have been simulated with a transport code to which a sawtooth mixing model and several diagnostic models were added. Substantial redistribution of particles and energy evidently occurs over the plasma interior. Global energy confinement, however, is only moderately degraded by the sawteeth.

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).

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.