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.

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.

The empirical scaling of the electron thermal diffusivity, Xe, is investigated for more than 100 beam-heated discharges. These discharges include two major features: 1) high-temperature and highdensity plasma (T e (O)«Tj(O)« 2.5-3 keV at n ^ (5-7) X 10 13 cm" 3 , P NB i £ 4 MW), which will be the basis for the breakeven experiments in the next-generation tokamaks, 2) three types of discharges, i.e. good and poor confinement divertor discharges and limiter discharges. -All kinds of discharges (good heating and poor heating divertor discharges, limiter discharges) have fhe same functional form in x e within ~ 40% at 0.25 a < r <0.65 a, where Xe in the simplest expression scales as Xe a Vn e -The * on t n e r m a l diffusivity, Xi, is consistent with the assumption that the neoclassical Xi can be applicable to our three kinds of discharges. For discharges with different heating efficiency, there is no systematic difference, in the adjustable multiplier, to the neoclassical theory by Hinton-Hazeltine for an ion collisionality of v*= 0.02-0.5.

The fusion neutron yield during hydrogen neutral beam injection into deuterium plasmas is examined and compared with the neoclassical model, using the Doublet III data obtained in 1983. This provides scalings of thermonuclear fusion neutron yields for the discussion of ion energy transport in tokamak devices. – Experimental data show that fusion neutron yield appears to have a scaling of in low recycling divertor discharges (so called H mode). The neoclassical theory for ion heat conduction satisfactorily described the scaling of fusion neutron yield. With a neutral injection power of Pinj = 4.6 MW, the Doublet III deuterium plasma yields thermonuclear neutrons at a rate of 1.2 × 1013 n·s−1 in low recycling divertor discharge, which is equivalent to a D+ T+ plasma with a thermonuclear fusion power multiplication factor of Q = 7.8 × 10−5 or a beam driven T+ plasma with Q = 8.6 × 10−2. A limiter discharge yields fewer neutrons.