H. Yokomizo

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).

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.

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.

Stabilization of the rf system at the SPring-8 linac

Abstract Beam energy variation of the SPring-8 linac was 1% or more at the start of beam commissioning. Depending on fluctuation, beam transmission efficiency from the linac to the booster synchrotron was significantly affected, and beam intensity in the booster synchrotron changed 20–30%. This caused delay of optimization of the various parameters in the booster synchrotron. More problematic, the beam intensities stored in each rf (radio frequency) bucket of the storage ring at SPring-8 were all different from each other. The users utilizing synchrotron radiation requested that the beam intensity in each rf bucket be as uniform as possible. It was thus a pressing necessity to stabilize the beam energy in the linac. Investigation of the cause has clarified that the various apparatuses installed in the linac periodically changed depending on circumstances and utilities such as the air conditioner, cooling water and electric power. After various improvements, beam energy stability in the linac of

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.

Equilibrium and axisymmetric stability of dee-shaped plasmas in Doublet III

Stable plasmas with surface elongations of up to 1.8 (aspect ratio 3.4) have been produced in the upper lobe of Doublet III with the use of both passive and active controls. The growth rate of the vertical instability has been measured at various values of elongation by disabling the feedback circuit of the vertical position control power supply. A rigid-shift analysis of growth rates indicates that the passive stabilization effect of the field-shaping coils plays a key role in achieving a high elongation of 1.8. Experiments have demonstrated that the maximum stable elongation is determined by the strength of the passive stabilization effect even with active feedback control. The dee-shape is found to be preferable to an elliptical shape because the triangularity reduces the absolute value of the decay index required to produce a given elongation.

Reduction of divertor plate heat load in Doublet III

The surface temperature of a divertor plate during discharges in the Doublet III tokamak is measured by an infrared scanning camera with a time resolution of 60 ms. The maximum rise in surface temperature during a discharge is around 120°C in a discharge with a mean electron density of 1.5 X 1013 cm-3 and a plasma current of 360 kA. The e-folding width of the temperature rise profile is ~ 4 cm. By increasing the electron density of the main plasma, the temperature rise of the divertor plate is decreased and the vertical temperature rise profile is broadened. The radiation loss in the divertor region plays an important role in the decrease in power transferred to the divertor plate.