A. Kitsunezaki

Impurity reduction and remote radiative cooling with single-null poloidal divertor in Doublet-III

The successful operation of a single-null poloidal divertor in Doublet-III 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 (ne≥5 × l013 cm−3, Te≤7 eV, 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.

Energy confinement of beam-heated divertor and limiter discharges in Doublet III

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 ePabs (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 e ≈ (6–7) × 1013 cm−3, τE* ≈ 70 ms).

Improvement of energy confinement time by continuous pellet fuelling in beam-heated Doublet III limiter discharges

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 × 1014 cm−3. When compared to a conventional gas-fuelled plasma at similar density, the pellet-fuelled 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.

Observation of very dense and cold divertor plasma in the beam-heated doublet III Tokamak with single-null poloidal divertor

A Langmuir probe array in the divertor plate has been used to investigate the dense, cold divertor plasma associated with remote radiative cooling in neutral-beam-heated, single-null open-divertor discharges in Doublet-Ill. With injected powers of up to 1.2 MW, the divertor plasma becomes denser and colder as the main plasma line-averaged density e increases, reaching ned= 2.8 X 1014 cm−3. Since the electron temperature drops to Ted = 3.5 eV under these conditions, this cold, dense plasma can provide a solution to the problem of wall erosion.

Achievement of high fusion triple product in the JT-60U high βp H mode

Improvement of an enhanced confinement state in a high poloidal beta (βp) regime without sawtooth activity has been achieved in JT-60U. A confinement mode has been demonstrated where both the edge and the core confinement are improved. The attainable βp was also extended to higher values in this improved mode, because of its broader pressure profile. As a result of the improvement in confinement and in attainable βp, the highest value of the fusion triple product has been extended by a factor of 2.5 over that achieved in the 1992 experiments; it has reached (1.1 ± 0.3) × 1021 m-3.s.keV with a central ion temperature of about 37 keV. The D-D neutron emission rate has also been doubled in these experiments and has reached (5.6 ± 0.6) × 1016 s-1

Divertor studies in high-power beam heated discharges in Doublet-III

Abstract In high-power beam heated divertor discharges with P IN ~ 7.5 MW , the central radiative power density from nickel can be suppressed to less than 10% of the heating power density. The radiative loss in the divertor increased as the main electron density was increased. 24% of the absorbed power was observed to be radiated in the divertor in high-density, H-mode discharges. During a density scan, the radiative power from the main plasma remained constant. The radiative power can be increased by injection of argon gas to 75% of the absorbed power which resulted in a significant decrease in the heat load of the divertor plate. The electron temperature and the particle flux were measured at the divertor plate. With the increase of the main plasma density, the electron temperature decreased to 8 eV (4.5 MW absorbed power). The electron density at the divertor plate was 3×10 14 cm −3 . All these experimental results indicate the feasibility of the single-null open divertor to a fusion reactor in suppressing central radiative loss, reducing the heat load and erosion of the divertor plate and facilitating ash exhaust.

High density, single-null poloidal divertor results in doublet III☆

Abstract Experimental results of divertor characteristics on impurity suppression, helium ash exhaust and remote radiative cooling are reviewed. In a throatless (open) divertor such as the single-null poloidal divertor in Doublet III, a dense and thick scrape-off layer provides strong particle shielding in high density divertor discharges, which results in a high neutral pressure and radiative cooling of the divertor even in an impurity-free condition. The feasibility of employing an open divertor in a reactor-grade device like INTOR is discussed. A preliminary result of divertor experiments with a 1.6 MW neutral-beam injection is presented.

Head load reduction of the divertor plate by remote radiative cooling in D-III beam-heated divertor discharges

Abstract The heat load on the divertor plate has been measured by a 28 thermocouple array and an infrared camera both measuring the divertor area. The sum of the radiative power from the main plasma and the divertor along with the power to the divertor plate accounts for ∼ 80% of the input power. The radiative power from the divertor plasma increases as the density of the main plasma increases, which results in the reduction of the heat load on the divertor plate by 50%. This result is obtained with a neutral beam injection power of 2 MW.

Energy confinement of Ohmically heated D-shaped plasmas in Doublet III

Energy confinement properties were compared for D-shaped and circular-cross-section plasmas with an identical horizontal minor radius as functions of plasma current, electron density, and vertical elongation under a wide range of discharge conditions. The property of gross energy confinement can be explained by the combination of an electron energy confinement proportional to eq*((n?eBT/Ip)(1+K2)/2) and neoclassical ion energy confinement proportional to The high-current operation capability of D-shaped plasmas produces an improvement in energy confinement at high density due to the reduction of neoclassical heat loss in the ions. The highest energy confinement time (75 ms) is obtained for high-current and high-density D-shaped discharges.

High-density, low-q discharges with D-shaped and circular cross-sections in Doublet III

The maximum plasma current in Doublet III is found to be limited by disruptions when the limiter safety factor is approximately two. However, because of the strong toroidal and shaping-field effects on the rotational transform at the outer plasma edge associated with a vertical elongation of 1.5 D-shape, the safety factor estimated from simple geometric considerations for D-shaped plasmas corresponds to values as low as 1.5. These discharges operate stably with considerably higher plasma current than most reactor design studies assume. These low-q discharges show excellent plasma performance: very flat spatial electron temperature profiles, high-density operation with eR/BT up to 7.8, and good energy confinement producing a volume-average β of up to 1% with Ohmic heating only. This operational regime appears to be applicable to future high-β tokamaks with D-shaped cross-sections.