The helium ash exhaust function of a divertor has been experimentally demonstrated. Helium atoms accumulate in the divertor region as the electron density of the main plasma increases. With a helium concentration of ~ 1.6% of electron density in the main plasma, neutral helium pressure at the divertor region is as high as 1.0 x 10" 4 Torr. This experiment indicates the possibility of helium ash exhaust in an a-particleheated diverted tokamak with use of pumping ducts of a practical size.

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).

A centrifuge injector that repetitively fires 1.3 mm deuterium pellets (1 torr • L per pellet) at a rate of 32 pellets per second was used to build up and maintain a Doublet III 2.4 MW neutral-beam-heated limiter discharge at a line-averaged density of 1 X 10 14 cm" 3 . When compared to a conventional gas-fuelled plasma at similar density, the pelletfuelled plasma was characterized by a factor-of-three reduction in edge neutral density and limiter recycling, a centrally peaked profile, a 70% increase in global energy confinement, and a tenfold increase in the fusion reaction rate.

As a result of the vertical elongation of dee-shaped plasmas, natural single-null poloidal-divertor equilibria have been stably obtained in the upper half of the Doublet III vacuum chamber with a plasma current of 320 kA, a major radius of 140 cm, an average minor radius of ∼45 cm, and a vertical elongation of 1.3–1.4 in the plasma cross-section. Without employing any particular divertor chamber, this simple divertor reduces the influx of metallic impurities to the main plasma and the re-cycling of charged particles in the periphery of the main plasma. The divertor reduces the radiation loss power and improves the energy confinement time.

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 successful operation of a single-null poloidal divertor in Doublet-Ill has demonstrated several new advantages of a diverted tokamak in addition to the suppression of impurity influx as demonstrated in DIVA: 1) The impurity contamination and radiation loss of the main plasma has been reduced by an open divertor geometry, i.e. without a divertor chamber; 2) The radiative cooling and formation of a dense and cold (n e >5Xl0 13 crn~3, T e <7 eV, P H , < 4 X 10' 3 torr) divertor plasma have been observed. -Up to 50% of the Ohmic input power is radiated in the divertor region, thus cooling the plasma in front of the divertor plate down to several eV. This remote radiative cooling greatly reduces the heat load on the divertor plate without cooling the main plasma. -The feasibility of remote radiative cooling in INTOR was studied by use of a volume integration technique of the radiation power along the field line.

Results from the JFT−2a (DIVA) experiments made with a separatrix magnetic surface are described. The main conclusions are: (1) A separatrix magnetic surface is stably located inside the material surface. (2) A plasma enclosed in a separatrix magnetic surface is similar to that of a conventional tokamak as far as the magnetohydrodynamic properties are concerned. (3) Measured parameters of the main plasma column are consistent with those expected from a conventional tokamak. (4) The electron density and temperature in the scrape-off layer are about ten times less than the values at the center of the main plasma column. (5) Heat and particle fluxes to the divertor region are axisymmetric and several times less than those of the total loss fluxes from the main plasma column. (6) Runaway electrons are well guided to the divertor region.

Preliminary results of the experimental determination of the particle loss aperture sizes of the spindle cusp magnetic bottle for laser-produced high-beta deuterium plasmas are presented. The fully ionized dense plasma (with maximum density ∼1016 cm−3 and ion energy ∼200 V) is produced at the null-field center of the magnetic bottle, from a freely falling isolated deuterium ice pellet by a focused giant laser pulse. In preparation, the free expansion of laser-produced plasma has been studied experimentally, and the results indicate that the plasmas produced can maintain the fully ionized condition with prescribed plasma parameters until it expands to a certain prescribed volume if the combination of the physical parameters is properly chosen by considering the electron-ion recombination as one of the important factors. The experimental results of cusp confinement for the plasma thus produced suggest that the loss aperture size of the ring cusp is much smaller than the local ion gyroradius, in sharp contrast with other results thus far reported. Although the determination of the point cusp aperture radius is less complete, the rough estimates show that it is also smaller than the ion gyro-radius.